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Slipstream between marine current turbine and seabed
Energy 68 (2014) 801e810
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Slipstream between marine current turbine and seabed Long Chen, Wei-Haur Lam* Marine Renewable Energy Research Group, Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2013 Received in revised form 18 February 2014 Accepted 20 February 2014 Available online 20 March 2014
The investigation of hydrodynamics near seabed is an initial input to study marine current turbineinduced seabed scour. The authors investigated the slipstream between the seabed and the marine current turbine via OpenFOAM. The axial component of velocity is the dominating velocity of flow below marine current turbine. The maximum axial velocity under the turbine blades is around 1.07 times of the initial incoming flow. The maximum radial and tangential velocity components of the investigated layer are approximately 4.12% and 0.22% of the maximum axial velocity. The slipstream varies in direct proportion to the incoming velocity. A schematic diagram to describe the flow pattern under the marine current turbine has been proposed based on the study. The turbine adopted in current simulation has three blades. The acceleration of flow under the marine current turbine changes seabed boundary layer profile. The height of tip clearance and turbine geometry are the two principal parameters in scour design of the marine current turbine. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Marine current turbine OpenFOAM Slipstream Scour Sediment transport
1. Introduction The energy crisis is one of the major problems that humans have to deal with as people rely heavily on electricity [1]. There are many untapped natural resources in the ocean. These resources are potential to be harnessed and make a contribution to energy supply. Marine current power is one of the potential resources to generate electricity. It has some advantages over other renewable energy sources as it is easier to be predicted and quantified [2]. The tidal or marine current energy has undergone intensive studies in the past decade. A number of large-scale marine current turbines have been tested and deployed around the world [3e5]. It is foreseeable that marine current will be a vital natural resource in future energy supply [6]. Presence of tidal stream device could accelerate the flow in its vicinity and lead to local scour around the marine current turbines (MCTs) [7,8]. Scour around marine structures has been well recognised as an engineering issue which causes structural instability [9]. It is crucial to ensure the structural safety of MCTs during the operation phase to avoid interruption of energy transmission. Moreover, scour-related foundation and scour protection are costly, it takes account 30% of the total cost in analogous wind turbine industry [10]. Repair of coastal structure failure due to scouring is
* Corresponding author. Tel.: þ603 7967 7675; fax: þ603 7967 5318. E-mail addresses: [email protected], [email protected] (W.-H. Lam). http://dx.doi.org/10.1016/j.energy.2014.02.083 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
considerably high, where USD 2e10 million is cost per failure [11]. In addition, the sediment transport in the energy extraction region has negative environmental impacts [8]. This may lead to the change of sea floor topography which could result in adverse consequences for the indigenous marine flora and fauna. Ng et al. [12] reviewed research progress from 2002 to 2012 in horizontal axis marine current turbines. Little information about scouring process around MCTs was found. The published work of Neill et al. [13,14] studied the impact of tidal stream devices on the sediment dynamics. The tidal stream devices cause localised scour and affects the erosion/deposition pattern in global scour. The effects of rotor on local scour and deposition process are the sole considerations in MCTs-induced seabed scour, especially when the turbines are installed close to the seabed. Wang et al. [15] stated that the height of the turbine tip clearance from seabed should be kept at least one turbine diameter distance, in order to avoid adverse effects on the seabed. The investigation of hydrodynamic and wakes of MCT has been studied extensively in the past decade. Sun [16] claimed that there is considerable localised flow acceleration around the device of tidal current energy extraction. Ramos et al. [17] demonstrated the impacts of tidal farm on the hydrodynamics. The velocity is reduced upstream and downstream the farm, and increased beside it. The change of velocity may influence the boundary layer profile which will hit the foundation of support structure. The well documented work regarding parameters affecting wake pattern [18e21] is also able to offer some insights in terms of flow condition below turbine
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rotors for the current study. The close seabed proximity induces a more pronounced gradient. The less tip clearance reduces the reenergisation of fluids along the underside of wake. Chen and Lam [22] highlighted that the clearance between rotor and seabed becomes critical in the MCT induced scour prediction. It further testifies that the height of tip clearance plays a critical role in designing scour-related units of MCT. To the best of authors’ knowledge, the flow pattern between MCT and seabed is not clearly known yet. This paper provides an initial input to study seabed scour and sediment transport process around MCT. Comprehensive understanding of velocity pattern near the seabed enable engineers offer cost effective design on foundation and scour protection. This enables a more affordable marine current energy. The better understanding of sediment transport is also of great importance for the environmental impact assessment of potential MCT sites. The Computational Fluid Dynamic (CFD) method is relatively inexpensive compared to experimental investigations. The open source CFD software adopted in the current study is OpenFOAM (Open Field Operation and Manipulation). The axial, radial, and tangential component of velocity is simulated by solving Reynolds Averaging NaviereStokes (RANS) equations with standard keε turbulence model. The current study also demonstrates the ability of OpenFOAM to investigate the flow profile between MCT and the seabed.
turbulence model. The two equations allow the turbulent velocity and length scales to be independently determined. This model is semi-empirical and solved flows based on the assumption that the rate of production and dissipation of turbulent flows are in nearbalance in energy transfer. The constants in the equations have been given after a wide range of examinations for turbulent flows, where Cm ¼ 0.09; C1ε ¼ 1.44; C2ε ¼ 1.92; sk ¼ 100; sε ¼ 1.30. 2.3. Geometry creation
The application of CFD models is on a case-by-case basis. The CFD package is able to provide a reliable prediction of the velocity field around the rotor of MCT. It is also able to recreate the turbine geometry which is similar to the prototype. The CFD model includes turbine blades creation, grid generation, boundary condition setting and the selection of turbulence model. The simulation was run on an HP Z820 workstation with quad-core processors Intel Xeon @ 2.40 GHZ. The validation of the numerical models will be done by comparing results with published experimental data.
The blade geometry of MCT was generated based on the IFREMER-LOMC configuration as shown in Pinon et al. [26]. The origins of the turbine blade were from Tidal Generation Limited. Table 1 shows the IFREMER-LOMC configuration of the turbine blade. The turbine blades were designed according to a NACA63418 profile which has been widely used. Table 2 demonstrates the detailed blade geometry description of the IFREMER-LOMC configuration, where r is the radial distance measured from the rotational axis, R is the radius of turbine disk, c is the chord length, and t is the maximum thickness as a fraction of the chord. The duplication of the turbine blade was done by using the open source software SALOME. The turbine geometry was created in stereo lithography (STL) format and imported to OpenFOAM. Fig. 1 shows the turbine geometry. The length of the turbine hub in the turbine of current simulation was 0.1m while the hub length of turbine in Pinon et al. [26] is 0.72 m. The purpose of reducing hub length in the current simulation is to speed up the computational progress. The length of hub may influence the wake development behind the turbine. However, it was assumed that the hub’s effects on the velocity profile below turbine blades are negligible. The dimensions of the computational domain are 2.86D and 2.86D, 3.00D in the x-, y- and z-directions, respectively (see Fig. 2). The flow direction is from y to y. The submerge depth of rotor varies to test the effect of confinement on flow pattern. The centre of the rotor located at four different positions (0.9D, 1.1D, 1.3D and 1.5D from Zmin).
2.1. OpenFOAM
2.4. Boundary and initial conditions
The OpenFOAM toolbox is a free, open source CFD software package produced by OpenCFD Ltd [23]. The CFD package should be able to provide a reliable prediction of the velocity field around the rotor discs of MCTs and able to recreate the turbine geometry similar to the prototype. The OpenFOAM users are able to modify the source code to satisfy their respective simulation objectives. This is a great advantage which makes many difficult simulations of engineering issues achievable. The solver employed in the current study is so called pimpleDyMFoam. It is a transient solver for incompressible, flow of Newtonian fluids on a moving mesh. The solving algorithm is PIMPLE, as it is a new algorithm merged from PISO (pressure implicit with splitting of operators) and SIMPLE (semi-implicit method for pressure-linked equations) algorithms [24]. The “DyM” in pimpleDyMFoam stands for dynamic meshes. Hence it is capable to simulate the rotating movement of turbine rotors. It also can simulate the dynamic mesh refinement along the surface of the turbine blades.
Parameters have to be determined before the setting of the boundary and initial conditions. The tip speed ratio (TSR) is one of the parameters and defined as:
2. Numerical simulation
2.2. Governing equations The governing equations for the fluid are the RANS equations. The details of the equations can be found in the OpenFOAM source code. The equations are solved in ensemble-averaged form, including appropriate models for the effect of turbulence [23]. The turbulence in the fluid is estimated by the standard keε turbulence model (Lauder and Spalding) [25]. This is a well-established
TSR ¼
VR UN
(1)
where R is the turbine radius in metre, V is the rotational speed in radian per second and UN is the of upstream current velocity in meter per second. The TSR for IFREMER-LOMC case equals to 3.67. According to the radius of turbine and inlet flow velocity 0.8 m/s, it is known that the turbine rotates at constant speed of 480 deg/s. The study also investigated hydrodynamic condition under turbine blades with incoming flow velocity of 2 m/s, 3 m/s. The TSR remained as constant for all computational simulations. The
Table 1 General blades geometry description for IFREMER-LOMC configuration [26]. Description
IFREMER-LOMC
Rotor radius (R) Hub radius Hub length Pitch (set angle) TSR (tip speed ratio) Sense of rotation
350 mm 46 mm 720 mm 0 3.67 Anti-clockwise
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Table 2 Detailed blade geometry description for IFREMER-LOMC configurations [26]. r/R
c/R
Pitch (degree)
t/c (%)
0.13 0.15 0.16 0.20 0.24 0.29 0.33 0.37 0.42 0.46 0.50 0.55 0.59 0.63 0.68 0.72 0.76 0.81 0.85 0.89 0.94 0.98 1.00
0.06 0.06 0.06 0.15 0.25 0.24 0.23 0.21 0.20 0.19 0.18 0.17 0.17 0.16 0.15 0.15 0.14 0.14 0.13 0.13 0.12 0.12 0.07
29.57 29.57 29.57 25.63 22.15 19.30 16.97 15.05 13.46 12.12 10.98 10.01 9.18 8.45 7.82 7.26 6.77 6.34 5.95 5.61 5.29 5.01 4.87
80 100 100 36 21 21 22 22 22 22 23 23 22 22 22 21 21 20 19 19 18 18 25
rotational speeds of turbine for each velocity condition are 1200 deg/s and 1800 deg/s, respectively. The seabed was modelled using the no-slip boundary condition. The no-slip seabed condition allows the viscous effects of seabed faces and lead to the formation of a boundary layer. The setting of noslip condition at the bottom wall is due to a boundary layer occurs at the seabed in real world case. In addition, the free surface interface between the water and air was not included in the numerical model in order to reduce the computational time. The sliding mesh modelling technique was used in the simulation case. The Arbitrary Mesh Interface (AMI) was applied between the rotor and stator subdomains. AMI is a technique that allows the numerical simulation across disconnected mesh domains in adjacent region. The domains can be stationary or move relative to one another [23].
Fig. 2. The computational domain with 1.0D height of tip clearance from seabed.
utility generates 3-D meshes containing hexahedra (hex) and splithexahedra (split-hex) automatically from triangulated surface geometries in STL format. The mesh approximately conforms to the surface by iteratively refining a starting mesh and morphing the resulting split-hex mesh to the surface. The rotor mesh density is higher than the stator mesh density in order to produce a better texture of the turbine blade surface during meshing. The total number of cells is 451,039, which consists of 437,728 hexahedra, 834 prisms, 5 tet wedges and 12,472 polyhedra. The validity of the mesh has been checked through commanding the utility “checkMesh”. The output reported that the mesh quality is satisfactory. The slice section of the mesh is shown in Fig. 3. The number of cells in any given CFD mesh can influence the results due to the approximations and averaging used in RANS equations. The greater number of cells results in more accurate solution to a problem for a specified numerical setting. The increase
2.5. Meshing The meshing of the rotor and stator domain was done by executing the SnappyHexMesh utility code. The snappyHexMesh
Fig. 1. The geometry of turbine blade.
Fig. 3. Slice section of the mesh domain along the Y-axis direction (rotor centre at 1.5D above Zmin).
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of mesh density also requires more computational power. The authors attempt to figure the velocity field accurately with less processing time. The authors conducted another numerical experiment with finer mesh. The cell numbers increased to 526,840. The velocity magnitude at the layer 0.5C (tip clearance C ¼ 1.0D) has been chosen to testify the mesh independence. The results of velocity magnitude are in good agreement for both fine and coarse meshes except the points behind 0.5D downstream. However, the deviations of the results are less than 1%. The mesh with 451,039 cells is adequate to give accurate solution which has advantage of less processing time.
Table 3 Maximum velocity component at each investigated layer. Height of tip clearance
Layer
Axial velocity (Va/V0) %
Radial velocity (Vr/Va) %
Tangential velocity (Vt/Va) %
1.00D
0.50C 0.25C 0.05C
105.15 104.66 100.00
2.05 0.73 0.27
0.03 <0.01 <0.01
0.80D
0.50C 0.25C 0.05C
105.53 104.93 100.07
2.48 0.93 0.30
0.05 0.02 0.01
0.60D
0.50C 0.25C 0.05C
106.00 105.19 84.39
3.12 1.27 0.31
0.08 0.04 0.04
0.40D
0.50C 0.25C 0.05C
106.69 105.30 60.00
4.12 1.80 0.70
0.22 0.11 0.08
3. Validation of numerical models Experimental results and numerical simulation by Pinon et al. [26] for investigating wake of MCT were used in this study to validate the flow model. The velocity profile at the location 1.2D behind the turbine was presented and compared with results obtained by Pinon et al. (see Fig. 4). There is some variability in the region near the rotational axis. The cause of this disagreement might be the effect of the hub. As mentioned previously, the hub length in this study is shorter than the hub incorporated in Pinon et al. [26]. The authors’ computational results have good agreement with their experimental and numerical simulation results. The axial velocity under turbine blades experienced a slight increase. It conforms to Sun’s [16] work that the localised acceleration of flow occurs around the turbine blade. Besides, it indicates that the fluid behind a turbine and near the seabed could have higher velocity. The boundary layer that will hit the foundation of the structure might be changed due to the presence of rotors. There are some restrictions exist in the current numerical model. In sea environments with shallow water, the interaction between air and water may change the hydrodynamic conditions around MCT. The free surface effect has not been addressed in this work. The flume at IFREMER has calm condition and large water depth. It is therefore the numerical results of current study have a quite reasonable agreement with the experimental results obtained at IFREMER.
Fig. 4. Axial velocity profiles behind turbine blades (lateral distance y ¼ 1.2D).
4. Results and discussion The velocity pattern between MCT and seabed is simulated. The velocity near seabed serves to sediment transport under strong tidal flow. The three main velocity components in a propeller jet are the axial, tangential, and radial velocities. Similarly, the three velocity components exist in the fluids passed through or around turbine blades. Plots of the time-averaged axial, radial, and tangential components of velocity of current at different layers close to seabed are shown. The results presented are obtained at the run time of 15 s. The heights of tip clearances of MCT were set at 0.4D, 0.6D, 0.8D, and 1.0D from seabed, respectively. The focus region is half of tip clearance away from seabed in each case. The region is close to the seabed and has less disturbance caused by the tip vortex. The exact investigated layers are 0.05C, 0.25C, and 0.50C from the seabed. Each component of velocity has been nondimensionalised by the respective free stream velocity.
4.1. Axial component of velocity Ship propeller jet-induced scour has been studied for many years. Earlier researchers were focused solely on the axial velocity field within the ship’s propeller jet, due to large contribution made by the axial velocity to seabed scouring. Lam et al. [27] conducted Laser Doppler Anemometry measurement and claimed that the axial component of velocity is the main contributor to the velocity magnitude of the initial plane of a ship’s propeller jet. Liu [28] compared the downstream velocity profile of a propeller and a tidal turbine. It founded that the direction of the downstream velocities of a propeller against that a turbine is opposing. This phenomenon indicates the different function of a tidal turbine and a propeller. The propeller is trying to transfer energy into the fluids which result in the acceleration of inflow. The tidal turbine aims to harness the kinetic energy from fluids and ends up slowing down the inflow. However, ship propeller and tidal/marine current turbine have lots of similarities in terms of hydrodynamic conditions. Table 3 tabulates each maximum component velocity of the slipstream between the marine current turbine and the seabed. It indicates that axial velocity is the biggest contributor to the velocity magnitude, in a same manner as ship’s propeller jets. The maximum axial component of velocity in the investigated layers occurred in 0.5C (C ¼ 0.4D), y/D ¼ 0.4. The maximum value of axial velocity has approximately 1.07 times greater than the initial velocity. The maximum local acceleration of velocity around the current extraction device in Sun’s study [16] is approximately 1.2 times of its initial velocity. The maximum velocity locates at y/D ¼ 1
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and 0.3D away from the tip of the turbine blade. The flow acceleration in Sun’s work is more significant. The experimental instrument applied in her study is actuator disk. It suggests that the turbine characteristic influences the extent of flow acceleration. Furthermore, Sun took the free surface effect for numerical modelling into consideration. The interaction between air and water might increase the velocity around the turbine. Fig. 5 shows the axial velocity of different layers between the MCT and the seabed under different heights of tip clearance. The axial component of velocity in the upper two investigated layers (0.25C and 0.50C above seabed) has a moderate increase. The increment of velocity is approximately 5% of initial incoming flow in average. The axial velocity near seabed (0.05C above seabed) at each tip clearance case is lesser than the initial velocity due to the boundary layer effect. However, the fluids at each layer below and behind the turbine have slight higher axial velocity than that at the same level before turbine. This behaviour is different in the fluids passed through the rotor plane. The fluids passed through the rotor plane region experience velocity reduction and form an expanding wake. The faster moving stream (below rotors) serves to reenergise the wake, breaking it up and increasing the velocity [20]. Moreover, the contraction of MCT increases flow velocity. Based on the Bernoulli equation, the increase of velocity will be balanced by the decrease of pressure. As shown in Fig. 6, the pressure of the fluids behind the turbine blades decreased at each investigated layer. The pressure has been non-dimensionlised by the atmospheric pressure (101.325 kPa). The difference of pressures on the
805
Fig. 6. Dimensionless pressure of fluids at different layers near seabed (height of tip clearance ¼ 1.0D).
Fig. 5. Dimensionless axial component velocity at different layers with various tip clearances from seabed: (a) 1.0D; (b) 0.8D; (c) 0.6D; (d) 0.4D.
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two sides of turbine blades results in a suction phenomenon. The fluids at the high pressure side will be sucked to the low-pressure side. This suction effect further accelerates the fluids below turbine blades on the back side. These aforementioned reasons contributed to the velocity acceleration of the fluids in each investigated layer. A schematic diagram has been proposed to describe the flow under rotors of MCT (see Fig. 7). A low-pressure zone is formed below the boundary of the wake. The distribution of axial component velocity has a similar pattern despite different confinement. The influence of seabed proximity in current study is not evident. It suggests that the wake does not expand remarkably and approaches the seabed. The fluids beneath the rotor disk are enough to re-energise the wake. Moreover, the solidity of turbine blade is low. The fluids pass through the turbine smoothly as the blockage effect is not significant. The flow suppression under the turbine is not pronounced. A turbine with high blade area ratio may have different velocity pattern when the confinement below turbine varies. The flow acceleration below the turbine blades is expected to develop along the downstream of the wake. The domain size of current study is not big enough to capture the flow field further downstream. A greater domain size could help the researchers to investigate the flow field below the turbine blades in far wake region. 4.2. Radial component of velocity McGarvey [29] claimed that the radial component of velocity was approximately 30% of the axial velocity along the face of the propeller. Later on, Lam et al. [27] conducted measurements and stated that the radial axial component of velocity is the third largest contributor to the magnitude of flow filed in the propeller jet, and their measurement of tangential velocity is 14% of the axial velocity, which is lower than the 30% of axial velocity suggested by McGarvey [29]. Table 3 shows the radial component of velocity of the present slipstream is the second largest contributor compared to a ship propeller jet. Tangential velocity is the second largest component of a propeller jet. The maximum radial velocity component is approximately 4.12% of the maximum axial velocity component. It is the second largest contributor of velocity, which is in a different manner as ship propeller jets. The radial velocity acts parallel to the turbine blade axis and represents the lateral expansion of fluids. The fluids diffused downward once they approached the blades due to the blockage effect of turbine
Fig. 7. Schematic diagram of slipstream between seabed and marine current turbine.
blades. The distributions of radial velocity at each layer are symmetric which are in valley shapes. The peak ridged radial velocity occurs either at or near the Z-axis (see Fig. 8). Table 4 tabulates the maximum radial velocity at each investigated layer. The maximum radial velocity located at the 0.5C (C ¼ 0.4D), y/D ¼ 0 (Z-axis), which is 4.4% of the initial velocity. The trend of each component velocity close to the tip of turbine blade might be more complicated due to tip vortex. The radial component velocity is more significant in the region near turbine blades. The radial velocity of the fluids in the investigated layer decreased right after the turbine blades. The radial component velocity is relatively less and may not have pronounced effects on local scour. The radial component of velocity might contribute to the backfilling of scour hole. 4.3. Tangential component of velocity Many researches approved that the tangential component of velocity is the second largest contributor to the resultant field in the ship’s propeller jet. Prosser [30] estimated that the magnitude of maximum tangential velocity is approximately 30% of the maximum axial velocity. The results have been contradicted by Lam et al. [27], they reported that the maximum tangential component of velocity within a ship propeller jet is 82% of the maximum axial velocity. The tangential velocity acts perpendicular to the turbine blade rotating axis. The tangential component results from the rotating nature of turbine blades and it contributes to the rotation of fluids. The fluids below and behind the turbine should not have significant tangential component as the fluids are away from the blades. Fig. 9 shows the distribution of tangential velocity. The distribution of tangential velocity is more complicated compared to the radial velocity. The distribution of tangential velocity varies with height of tip clearance and it is remarkably small. However, Fig. 9d is more predictable and shows seasonal rise and falls of tangential velocity at the layer 0.5C when tip clearance is 0.4D. This might be the cyclic decay of tangential component. Table 5 shows the maximum tangential velocity at each investigated layer. The highest tangential velocity occurred at y/D ¼ 0.09 (C ¼ 0.4D), which is 0.24% of initial incoming flow. It shows that the rotation of fluids is higher in the region near turbine blade. 4.4. Influence of incoming velocity Marine current sites with a current velocity of 2.5 m/s or more are considered to have an exceptionally high energy resource [31]. The incoming flow velocity in the previous simulation is 0.8 m/s. The increase of free stream velocity may influence the velocity pattern. Thus, the hydrodynamic conditions near the seabed with incoming flow velocity of 2 m/s, and 3 m/s have been investigated. The TSR remains as constant for all computational simulations. The rotational speeds of turbine for each velocity condition are 1200 deg/s and 1800 deg/s, respectively. Fig. 10 shows the axial, radial and tangential component of velocity between the seabed and the MCT with 3 m/s incoming flow velocity. It shows that the increment of axial velocity acts in a direct proportion manner with the incoming flow. It means the acceleration turns larger when the incoming flow velocity may become higher. The increases of axial velocities under different incoming flow condition are approximately 5% in average, which is the same as the incoming flow 0.8 m/s (see Fig. 5d). The flow patterns with incoming flow velocity of 2 m/s has the same trend shown in Fig. 11, which the figures of velocities have not been presented in the paper. The blockage effect of the turbine may have a more significant effect on the extent of flow acceleration. The influence of turbine
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Fig. 8. Dimensionless radial component velocity at different layers with various heights of tip clearances from seabed: (a) 1.0D; (b) 0.8D; (c) 0.6D; (d) 0.4D.
geometry towards the velocity pattern under MCT has to be studied in future work. The increase of rotational speed may result in more radial and tangential component. However, there is no much variation of radial and tangential velocities near seabed. The radial and tangential velocities have the same pattern as the one with 0.8 m/s incoming flow. Similarly, the radial and tangential velocities increased due to the larger incoming flow velocity. The interaction of the tangential component of velocity with sediment can be neglected due to its insignificant contribution to the magnitude of velocity. The radial and tangential velocities in the near turbine tip region might be highly influenced by the rotational speed. Turbine rather close to seabed is not applicable in tidal turbine farms due to the seabed proximity of wake recovery. Environment implicated issue associated with short tip clearance is another concern during the development of MCT. However, more logarithmic velocity distribution is existed in real marine environment. Mason-Jones et al. [32] highlighted the importance of local depth-wise velocity on tidal turbine performance. The vertical velocity profile in sea environment is governing by the factors such as local bathymetry and turbulence. The increase of bed roughness results in increasing of shear in bed region. The topography of the real field site may induce coherent flow structures in the incoming flow. The coherent flow structure may possibly impose certain impacts to the hydrodynamic conditions near seabed.
4.5. Development of boundary layer The mechanism of horseshoe vortex is of significant importance in the process of scour. The rotation of the incoming flow results in the horseshoe vortex. The boundary layer experiences a threedimensional separation under the influence of the adverse pressure gradient induced by the presence of the support structure. The separated boundary layer accordingly rolls up to from a spiral vortex around the support structure [33]. The incoming boundary layer and pressure gradient induced by the support structure are the two necessary preconditions to generate a horseshoe vortex. Baker [34,35] claimed that the nondimensional quantities describing the horseshoe vortex in the case of steady current mainly depends on the following parameters: d/D, ReD (or alternatively Red Þ, and Pile geometry in which d/ Dp ¼ the ratio of bed boundary layer thickness to the pile diameter, ReD is the pile Reynolds number (Eq. (2)):
ReD ¼
UDp v
(2)
and Red is the bed boundary layer Reynolds number (Eq. (3)):
Red ¼
Ud v
(3)
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Table 4 Maximum radial component of velocity. Height of tip clearance
Layer
Position (y/D)
Radial velocity (Vr/V0) %
1.00D
0.50C 0.25C 0.05C
0.00 0.04 1.37
2.16 0.76 0.27
0.80D
0.50C 0.25C 0.05C
0.00 0.02 1.34
2.62 0.98 0.30
0.60D
0.50C 0.25C 0.05C
0.00 0.00 1.37
3.31 1.34 0.26
0.40D
0.50C 0.25C 0.05C
0.00 0.03 1.40
4.40 1.90 0.42
where U is the velocity at the edge of the bed boundary layer, v is the kinetic viscosity. Fig. 11 shows the current boundary layer profile with rotor (C ¼ 1.0D) in operation in conjunction with measured data from Chilworth flume, University of Southampton [19]. The measured boundary layer profiles are without the presence of energy extraction devices. The boundary layer thickness for smooth bed (d/ D ¼ 0.67) is higher than that (d/D ¼ 0.32) at rough bed. It
demonstrates that the boundary layer thickness is in inverse proportion to bed roughness. The seabed condition incorporated in the current numerical simulation is without roughness. The numerical results indicate that the flow near seabed is increasing and the velocity gradient is reduced compared to experimental data without rotor in operation. The flow acceleration increases the turbulence level in seabed which causes the sediment to transport. The change of boundary layer thickness results in changing of d/D and Red . The change of these two governing parameters definitely alters the horseshoe vortex mechanism. The authors cannot visualise the formation of horseshoe vortex due to the limitation of equipment. Wang et al. [15] stated that ocean turbine may cause sediment movement on seabed if the shaft height from the seabed is less than 1.0D. However, the numerical simulation of current study shows that even the centre of the turbine is 1.5D from seabed (C ¼ 1.0D) changes the boundary layer profile. The changes of flow near to the seabed could possibly affect the bed load and suspended load of sediments. The geometry of turbine blades might be the cause of the contradiction with Wang et al. [15]. The greater acceleration of flow under MCT may further increase shaft height from seabed to avoid severe scour and environmental impact. Giles et al. [36] presented an experimental study investigating the potential benefits of foundation-based flow acceleration structures for marine energy converters. It proved that such structure can bring lots of benefits including increase in device power output, increase in
Fig. 9. Dimensionless tangential component velocity at different layers with various tip clearances: (a) 1.0D; (b) 0.8D; (c) 0.6D; (d) 0.4D.
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Table 5 Maximum tangential component of velocity. Height of tip clearance
Layer
Position (y/D)
Tangential velocity (Vt/V0) %
1.00D
0.50C 0.25C 0.05C
0.20 0.26 0.26
0.03 <0.01 <0.01
0.80D
0.50C 0.25C 0.05C
0.17 0.23 0.23
0.05 0.02 0.01
0.60D
0.50C 0.25C 0.05C
0.11 0.20 0.20
0.09 0.04 0.03
0.40D
0.50C 0.25C 0.05C
0.09 0.11 0.17
0.24 0.12 0.05
Fig. 11. The boundary layer development of rotor in operation (tip of turbine blade located at Z/D ¼ 1.0).
foundation footprint and scour protection. Besides, many other scour protection methods are available in offshore wind farm, such as geotextile containers, concrete armour units, concrete block mattresses etc. [37]. However, the placement of protection materials may influence the performance of MCT. Secondary scour also may occur at the edge of the protection units. The tip clearance and turbine geometry affect scour process of MCT as they alter the boundary layer profile. Many other governing factors are also potential to influence the scour process of MCT, such as sediment materials, water depth, geometry of support structure, etc. Objective measurements are recommended to quantify the effects of these aforementioned parameters on the scour process of MCT. 5. Conclusion
Fig. 10. Dimensionless axial, radial and tangential velocity at different layers with incoming flow velocity 6 m/s (C ¼ 0.4D).
The flow profile between MCT and seabed is still poorly understood compared to the wake characteristics and ship propeller jets. No documented work has been found studying the axial, radial and tangential velocity near seabed in the operation of MCT. In this paper, the velocity pattern near seabed of MCT has been studied through three-dimensional numerical simulation. The outcome of this research work is to give better understanding of the flow pattern between MCT and seabed. OpenFOAM is able to predict the velocity profile under the blade of MCT. The axial component is the largest contributor to the magnitude of velocity. The radial component of the velocity is the second largest contributor to the magnitude of velocity. The tangential component is insignificant so that it suggests that the tangential component of velocity has no impact to the scour and deposition process. The confinement has no effect to the velocity pattern of each velocity component when the tip clearance is large enough. The blockage ratio of MCT is directly proportional to the extent of flow acceleration. The influences of geometry on the magnitude of flow acceleration are suggested to be studied in future work. The presence of rotor changes the boundary layer profile subsequently results in the altering of horseshoe vortex formation. A schematic diagram to describe the flow profile between seabed and MCT has been proposed (Fig. 7). The numerical results show good agreement with experimental results from Pinon et al. [26], which has been demonstrated in Section 3. The promising results testify the
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potential of OpenFOAM to become a powerful engineering tool for designing MCT system. The height of tip clearances and turbine geometry are the two critical parameters in scour design of MCT. Acknowledgement The authors wish to extend their gratitude to the Ministry of Higher Education Malaysia for the financial support received under the UM/MOHE High Impact Research Grant (H-1600-00-D000047). We would like also to thank Mr Chu Yung Jeh for his kind help. References [1] Rocks L, Runyon RP. The energy crisis. New York: Crown Publishers; 1972. [2] Watchorn M, Trapp T, Sayigh AAM. Tidal stream renewable offshore power generation. In: World renewable energy congress VI. Oxford: Pergamon; 2000. pp. 2664e7. [3] Annual report 2010: implementing agreement on ocean energy systems. Lisbon, Portugal: OES-IA; 2011. [4] Annual report 2011: implementing agreement on ocean energy system. Lisbon, Portugal: OES-IA; 2012. [5] Annual report 2012: implementing agreement on ocean energy system. Lisbon, Portugal: OES-IA; 2013. [6] Rourke FO, Boyle F, Reynolds A. Marine current energy devices: current status and possible future applications in Ireland. Renew Sustain Energy Rev 2010;14(3):1026e36. [7] Xia JQ, Falconer RA, Lin B. Impact of different tidal renewable energy projects on the hydrodynamic processes in the Severn Estuary, UK. Ocean Model 2010;32:86e104. [8] Shields MA, Woolf DK, Grist EPM, Kerr SA, Jackson AC, Harris RE, et al. Marine renewable energy: the ecological implications of altering the hydrodynamics of the marine environment. Ocean Coast Manage 2011;54:2e9. [9] Hoffmans GJCM, Verheij HJ. Scour manual. Netherlands: Balkema; 1997. [10] Sumer B. Mathematical modelling of scour: a review. J Hydraul Res 2007;45(6):723e35. [11] Lillycrop WJ, Hughes SA. Scour hole problems experienced by the corps of engineers: data presentation and summary. Vicksburg: CERC-93-2, US Army Engineer Waterways Experiment Station, Coastal Engineering Research Centre; 1993. [12] Ng KW, Lam WH, Ng KC. 2002e2012: 10 years of research progress in horizontal-axis marine current turbines. Energies 2013;6:1497e526. [13] Neill SP, Litt EJ, Couch SJ, Davies AG. The impact of tidal stream turbines on large-scale sediment dynamics. Renew Energy 2009;34:2803e12. [14] Neill SP, Jordan JR, Couch SJ. Impact of tidal energy converter (TEC) arrays on the dynamics of headland sand banks. Renew Energy 2012;37:387e97. [15] Wang D, Atlar M, Sampson R. An experimental investigation on cavitation, noise, and slipstream characteristics of ocean stream turbines. Proc IMchE Part A J PowerEnergy; 2006:221. [16] Sun X. Numerical and experimental investigation of tidal current. PhD thesis. University of Edinburgh; 2008.
[17] Ramos V, Carballo R, Alvarez M, Sanchez M, Iglesias G. Assessment of the impacts of tidal stream energy through high-resolution numerical modelling. Energy 2013;61:541e54. [18] Myers LE, Bahaj AS, Germain G. Flow boundary interaction effects for marine current energy conversion devices. In: Proceedings of the 10th world renewable energy congress, Glasglow (UK); 2008. [19] Myers LE, Bahaj AS, Rawlinson-Smith RI, Thomson M. The effect of boundary proximity upon the wake structure of horizontal axis marine current turbines. In: Proceedings of 27th international conference on offshore mechanics and arctic engineering, Estoril, Portugal; 2008. [20] Myers LE, Bahaj AS. Experimental analysis of the flow field around horizontal axis tidal turbines by use of scale mesh disk rotor simulators. Ocean Eng 2010;37:218e27. [21] Myers LE, Bahaj AS. An experimental investigation simulating flow effects in first generation marine current energy converter arrays. Renew Energy 2012;37:28e36. [22] Chen L, Lam WH. Methods for predicting seabed scour around marine current turbine. Renew Sustain Energy Rev 2014;29:683e92. [23] OpenFOAM Foundation. Available from: http://www.openfoam.com [accessed 15.03.13]. [24] Higuera P, Lara JL, Losada IJ. Realistic wave generation and active wave absorption for NaviereStokes models application to OpenFOAM. Coast Eng 2013;71:102e18. [25] Launder BE, Spalding DB. Lectures in mathematical models of turbulence. London, England: Academic Press; 1972. [26] Pinon G, Mycek P, Germain G, Rivoalen E. Numerical simulation of the wake of marine current turbines with a particle method. Renew Energy 2012;46:111e 26. [27] Lam WH, Hamill G, Robinson D, Raghunathan S. Observations of the initial 3D flow from a ship’s propeller. Ocean Eng 2011;37:1380e8. [28] Liu P. A computational hydrodynamics method for horizontal axis turbine e panel method modelling migration from propulsion to turbine energy. Energy 2010;35:2843e51. [29] McGarvey J. The influence of the rudder on the hydrodynamics and the resulting bed scour of a ship’s screw wash. PhD thesis. UK: Queen’s University of Belfast; 1996. [30] Prosser M. Propeller induced scour. Technical report, BHRA project RP A01415. Cranfield: The Fluid Engineering Centre; 1986. [31] Charlier R. A “sleeper” awakes: tidal current power. Renew Sustain Energy Rev 2003;6(7):515e29. [32] Mason-Jones A, O’Doherty DM, Morris CE, O’Doherty T, Byrner CB, Prickett PW, et al. Non-dimensional scaling of tidal stream turbines. Energy 2012;44:820e9. [33] Sumer BM, Fredsøe J. The mechanics of scour in the marine environment. Singapore: World Scientific Press; 2002. [34] Baker C. The laminar horseshoe vortex. J Fluid Mech 1979;95:347e67. [35] Baker C. Vortex flow around the bases of obstacles. PhD thesis. UK: University of Cambridge; 1987. [36] Giles J, Myers LE, Bahaj AS, O’Nians J, Shelmerdine B. Foundation-based flow acceleration structures for marine current energy converters. IET Renew Power Gener 2011;4(5):287e98. [37] Whitehouse RJS, Harris JM, Sutherland J, Rees J. The nature of scour development and scour protection at offshore wind farm foundations. Mar Pollut Bull 2011;62:73e88.
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Slipstream between marine current turbine and seabed Long Chen, Wei-Haur Lam* Marine Renewable Energy Research Group, Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2013 Received in revised form 18 February 2014 Accepted 20 February 2014 Available online 20 March 2014
The investigation of hydrodynamics near seabed is an initial input to study marine current turbineinduced seabed scour. The authors investigated the slipstream between the seabed and the marine current turbine via OpenFOAM. The axial component of velocity is the dominating velocity of flow below marine current turbine. The maximum axial velocity under the turbine blades is around 1.07 times of the initial incoming flow. The maximum radial and tangential velocity components of the investigated layer are approximately 4.12% and 0.22% of the maximum axial velocity. The slipstream varies in direct proportion to the incoming velocity. A schematic diagram to describe the flow pattern under the marine current turbine has been proposed based on the study. The turbine adopted in current simulation has three blades. The acceleration of flow under the marine current turbine changes seabed boundary layer profile. The height of tip clearance and turbine geometry are the two principal parameters in scour design of the marine current turbine. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Marine current turbine OpenFOAM Slipstream Scour Sediment transport
1. Introduction The energy crisis is one of the major problems that humans have to deal with as people rely heavily on electricity [1]. There are many untapped natural resources in the ocean. These resources are potential to be harnessed and make a contribution to energy supply. Marine current power is one of the potential resources to generate electricity. It has some advantages over other renewable energy sources as it is easier to be predicted and quantified [2]. The tidal or marine current energy has undergone intensive studies in the past decade. A number of large-scale marine current turbines have been tested and deployed around the world [3e5]. It is foreseeable that marine current will be a vital natural resource in future energy supply [6]. Presence of tidal stream device could accelerate the flow in its vicinity and lead to local scour around the marine current turbines (MCTs) [7,8]. Scour around marine structures has been well recognised as an engineering issue which causes structural instability [9]. It is crucial to ensure the structural safety of MCTs during the operation phase to avoid interruption of energy transmission. Moreover, scour-related foundation and scour protection are costly, it takes account 30% of the total cost in analogous wind turbine industry [10]. Repair of coastal structure failure due to scouring is
* Corresponding author. Tel.: þ603 7967 7675; fax: þ603 7967 5318. E-mail addresses: [email protected], [email protected] (W.-H. Lam). http://dx.doi.org/10.1016/j.energy.2014.02.083 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
considerably high, where USD 2e10 million is cost per failure [11]. In addition, the sediment transport in the energy extraction region has negative environmental impacts [8]. This may lead to the change of sea floor topography which could result in adverse consequences for the indigenous marine flora and fauna. Ng et al. [12] reviewed research progress from 2002 to 2012 in horizontal axis marine current turbines. Little information about scouring process around MCTs was found. The published work of Neill et al. [13,14] studied the impact of tidal stream devices on the sediment dynamics. The tidal stream devices cause localised scour and affects the erosion/deposition pattern in global scour. The effects of rotor on local scour and deposition process are the sole considerations in MCTs-induced seabed scour, especially when the turbines are installed close to the seabed. Wang et al. [15] stated that the height of the turbine tip clearance from seabed should be kept at least one turbine diameter distance, in order to avoid adverse effects on the seabed. The investigation of hydrodynamic and wakes of MCT has been studied extensively in the past decade. Sun [16] claimed that there is considerable localised flow acceleration around the device of tidal current energy extraction. Ramos et al. [17] demonstrated the impacts of tidal farm on the hydrodynamics. The velocity is reduced upstream and downstream the farm, and increased beside it. The change of velocity may influence the boundary layer profile which will hit the foundation of support structure. The well documented work regarding parameters affecting wake pattern [18e21] is also able to offer some insights in terms of flow condition below turbine
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rotors for the current study. The close seabed proximity induces a more pronounced gradient. The less tip clearance reduces the reenergisation of fluids along the underside of wake. Chen and Lam [22] highlighted that the clearance between rotor and seabed becomes critical in the MCT induced scour prediction. It further testifies that the height of tip clearance plays a critical role in designing scour-related units of MCT. To the best of authors’ knowledge, the flow pattern between MCT and seabed is not clearly known yet. This paper provides an initial input to study seabed scour and sediment transport process around MCT. Comprehensive understanding of velocity pattern near the seabed enable engineers offer cost effective design on foundation and scour protection. This enables a more affordable marine current energy. The better understanding of sediment transport is also of great importance for the environmental impact assessment of potential MCT sites. The Computational Fluid Dynamic (CFD) method is relatively inexpensive compared to experimental investigations. The open source CFD software adopted in the current study is OpenFOAM (Open Field Operation and Manipulation). The axial, radial, and tangential component of velocity is simulated by solving Reynolds Averaging NaviereStokes (RANS) equations with standard keε turbulence model. The current study also demonstrates the ability of OpenFOAM to investigate the flow profile between MCT and the seabed.
turbulence model. The two equations allow the turbulent velocity and length scales to be independently determined. This model is semi-empirical and solved flows based on the assumption that the rate of production and dissipation of turbulent flows are in nearbalance in energy transfer. The constants in the equations have been given after a wide range of examinations for turbulent flows, where Cm ¼ 0.09; C1ε ¼ 1.44; C2ε ¼ 1.92; sk ¼ 100; sε ¼ 1.30. 2.3. Geometry creation
The application of CFD models is on a case-by-case basis. The CFD package is able to provide a reliable prediction of the velocity field around the rotor of MCT. It is also able to recreate the turbine geometry which is similar to the prototype. The CFD model includes turbine blades creation, grid generation, boundary condition setting and the selection of turbulence model. The simulation was run on an HP Z820 workstation with quad-core processors Intel Xeon @ 2.40 GHZ. The validation of the numerical models will be done by comparing results with published experimental data.
The blade geometry of MCT was generated based on the IFREMER-LOMC configuration as shown in Pinon et al. [26]. The origins of the turbine blade were from Tidal Generation Limited. Table 1 shows the IFREMER-LOMC configuration of the turbine blade. The turbine blades were designed according to a NACA63418 profile which has been widely used. Table 2 demonstrates the detailed blade geometry description of the IFREMER-LOMC configuration, where r is the radial distance measured from the rotational axis, R is the radius of turbine disk, c is the chord length, and t is the maximum thickness as a fraction of the chord. The duplication of the turbine blade was done by using the open source software SALOME. The turbine geometry was created in stereo lithography (STL) format and imported to OpenFOAM. Fig. 1 shows the turbine geometry. The length of the turbine hub in the turbine of current simulation was 0.1m while the hub length of turbine in Pinon et al. [26] is 0.72 m. The purpose of reducing hub length in the current simulation is to speed up the computational progress. The length of hub may influence the wake development behind the turbine. However, it was assumed that the hub’s effects on the velocity profile below turbine blades are negligible. The dimensions of the computational domain are 2.86D and 2.86D, 3.00D in the x-, y- and z-directions, respectively (see Fig. 2). The flow direction is from y to y. The submerge depth of rotor varies to test the effect of confinement on flow pattern. The centre of the rotor located at four different positions (0.9D, 1.1D, 1.3D and 1.5D from Zmin).
2.1. OpenFOAM
2.4. Boundary and initial conditions
The OpenFOAM toolbox is a free, open source CFD software package produced by OpenCFD Ltd [23]. The CFD package should be able to provide a reliable prediction of the velocity field around the rotor discs of MCTs and able to recreate the turbine geometry similar to the prototype. The OpenFOAM users are able to modify the source code to satisfy their respective simulation objectives. This is a great advantage which makes many difficult simulations of engineering issues achievable. The solver employed in the current study is so called pimpleDyMFoam. It is a transient solver for incompressible, flow of Newtonian fluids on a moving mesh. The solving algorithm is PIMPLE, as it is a new algorithm merged from PISO (pressure implicit with splitting of operators) and SIMPLE (semi-implicit method for pressure-linked equations) algorithms [24]. The “DyM” in pimpleDyMFoam stands for dynamic meshes. Hence it is capable to simulate the rotating movement of turbine rotors. It also can simulate the dynamic mesh refinement along the surface of the turbine blades.
Parameters have to be determined before the setting of the boundary and initial conditions. The tip speed ratio (TSR) is one of the parameters and defined as:
2. Numerical simulation
2.2. Governing equations The governing equations for the fluid are the RANS equations. The details of the equations can be found in the OpenFOAM source code. The equations are solved in ensemble-averaged form, including appropriate models for the effect of turbulence [23]. The turbulence in the fluid is estimated by the standard keε turbulence model (Lauder and Spalding) [25]. This is a well-established
TSR ¼
VR UN
(1)
where R is the turbine radius in metre, V is the rotational speed in radian per second and UN is the of upstream current velocity in meter per second. The TSR for IFREMER-LOMC case equals to 3.67. According to the radius of turbine and inlet flow velocity 0.8 m/s, it is known that the turbine rotates at constant speed of 480 deg/s. The study also investigated hydrodynamic condition under turbine blades with incoming flow velocity of 2 m/s, 3 m/s. The TSR remained as constant for all computational simulations. The
Table 1 General blades geometry description for IFREMER-LOMC configuration [26]. Description
IFREMER-LOMC
Rotor radius (R) Hub radius Hub length Pitch (set angle) TSR (tip speed ratio) Sense of rotation
350 mm 46 mm 720 mm 0 3.67 Anti-clockwise
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Table 2 Detailed blade geometry description for IFREMER-LOMC configurations [26]. r/R
c/R
Pitch (degree)
t/c (%)
0.13 0.15 0.16 0.20 0.24 0.29 0.33 0.37 0.42 0.46 0.50 0.55 0.59 0.63 0.68 0.72 0.76 0.81 0.85 0.89 0.94 0.98 1.00
0.06 0.06 0.06 0.15 0.25 0.24 0.23 0.21 0.20 0.19 0.18 0.17 0.17 0.16 0.15 0.15 0.14 0.14 0.13 0.13 0.12 0.12 0.07
29.57 29.57 29.57 25.63 22.15 19.30 16.97 15.05 13.46 12.12 10.98 10.01 9.18 8.45 7.82 7.26 6.77 6.34 5.95 5.61 5.29 5.01 4.87
80 100 100 36 21 21 22 22 22 22 23 23 22 22 22 21 21 20 19 19 18 18 25
rotational speeds of turbine for each velocity condition are 1200 deg/s and 1800 deg/s, respectively. The seabed was modelled using the no-slip boundary condition. The no-slip seabed condition allows the viscous effects of seabed faces and lead to the formation of a boundary layer. The setting of noslip condition at the bottom wall is due to a boundary layer occurs at the seabed in real world case. In addition, the free surface interface between the water and air was not included in the numerical model in order to reduce the computational time. The sliding mesh modelling technique was used in the simulation case. The Arbitrary Mesh Interface (AMI) was applied between the rotor and stator subdomains. AMI is a technique that allows the numerical simulation across disconnected mesh domains in adjacent region. The domains can be stationary or move relative to one another [23].
Fig. 2. The computational domain with 1.0D height of tip clearance from seabed.
utility generates 3-D meshes containing hexahedra (hex) and splithexahedra (split-hex) automatically from triangulated surface geometries in STL format. The mesh approximately conforms to the surface by iteratively refining a starting mesh and morphing the resulting split-hex mesh to the surface. The rotor mesh density is higher than the stator mesh density in order to produce a better texture of the turbine blade surface during meshing. The total number of cells is 451,039, which consists of 437,728 hexahedra, 834 prisms, 5 tet wedges and 12,472 polyhedra. The validity of the mesh has been checked through commanding the utility “checkMesh”. The output reported that the mesh quality is satisfactory. The slice section of the mesh is shown in Fig. 3. The number of cells in any given CFD mesh can influence the results due to the approximations and averaging used in RANS equations. The greater number of cells results in more accurate solution to a problem for a specified numerical setting. The increase
2.5. Meshing The meshing of the rotor and stator domain was done by executing the SnappyHexMesh utility code. The snappyHexMesh
Fig. 1. The geometry of turbine blade.
Fig. 3. Slice section of the mesh domain along the Y-axis direction (rotor centre at 1.5D above Zmin).
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of mesh density also requires more computational power. The authors attempt to figure the velocity field accurately with less processing time. The authors conducted another numerical experiment with finer mesh. The cell numbers increased to 526,840. The velocity magnitude at the layer 0.5C (tip clearance C ¼ 1.0D) has been chosen to testify the mesh independence. The results of velocity magnitude are in good agreement for both fine and coarse meshes except the points behind 0.5D downstream. However, the deviations of the results are less than 1%. The mesh with 451,039 cells is adequate to give accurate solution which has advantage of less processing time.
Table 3 Maximum velocity component at each investigated layer. Height of tip clearance
Layer
Axial velocity (Va/V0) %
Radial velocity (Vr/Va) %
Tangential velocity (Vt/Va) %
1.00D
0.50C 0.25C 0.05C
105.15 104.66 100.00
2.05 0.73 0.27
0.03 <0.01 <0.01
0.80D
0.50C 0.25C 0.05C
105.53 104.93 100.07
2.48 0.93 0.30
0.05 0.02 0.01
0.60D
0.50C 0.25C 0.05C
106.00 105.19 84.39
3.12 1.27 0.31
0.08 0.04 0.04
0.40D
0.50C 0.25C 0.05C
106.69 105.30 60.00
4.12 1.80 0.70
0.22 0.11 0.08
3. Validation of numerical models Experimental results and numerical simulation by Pinon et al. [26] for investigating wake of MCT were used in this study to validate the flow model. The velocity profile at the location 1.2D behind the turbine was presented and compared with results obtained by Pinon et al. (see Fig. 4). There is some variability in the region near the rotational axis. The cause of this disagreement might be the effect of the hub. As mentioned previously, the hub length in this study is shorter than the hub incorporated in Pinon et al. [26]. The authors’ computational results have good agreement with their experimental and numerical simulation results. The axial velocity under turbine blades experienced a slight increase. It conforms to Sun’s [16] work that the localised acceleration of flow occurs around the turbine blade. Besides, it indicates that the fluid behind a turbine and near the seabed could have higher velocity. The boundary layer that will hit the foundation of the structure might be changed due to the presence of rotors. There are some restrictions exist in the current numerical model. In sea environments with shallow water, the interaction between air and water may change the hydrodynamic conditions around MCT. The free surface effect has not been addressed in this work. The flume at IFREMER has calm condition and large water depth. It is therefore the numerical results of current study have a quite reasonable agreement with the experimental results obtained at IFREMER.
Fig. 4. Axial velocity profiles behind turbine blades (lateral distance y ¼ 1.2D).
4. Results and discussion The velocity pattern between MCT and seabed is simulated. The velocity near seabed serves to sediment transport under strong tidal flow. The three main velocity components in a propeller jet are the axial, tangential, and radial velocities. Similarly, the three velocity components exist in the fluids passed through or around turbine blades. Plots of the time-averaged axial, radial, and tangential components of velocity of current at different layers close to seabed are shown. The results presented are obtained at the run time of 15 s. The heights of tip clearances of MCT were set at 0.4D, 0.6D, 0.8D, and 1.0D from seabed, respectively. The focus region is half of tip clearance away from seabed in each case. The region is close to the seabed and has less disturbance caused by the tip vortex. The exact investigated layers are 0.05C, 0.25C, and 0.50C from the seabed. Each component of velocity has been nondimensionalised by the respective free stream velocity.
4.1. Axial component of velocity Ship propeller jet-induced scour has been studied for many years. Earlier researchers were focused solely on the axial velocity field within the ship’s propeller jet, due to large contribution made by the axial velocity to seabed scouring. Lam et al. [27] conducted Laser Doppler Anemometry measurement and claimed that the axial component of velocity is the main contributor to the velocity magnitude of the initial plane of a ship’s propeller jet. Liu [28] compared the downstream velocity profile of a propeller and a tidal turbine. It founded that the direction of the downstream velocities of a propeller against that a turbine is opposing. This phenomenon indicates the different function of a tidal turbine and a propeller. The propeller is trying to transfer energy into the fluids which result in the acceleration of inflow. The tidal turbine aims to harness the kinetic energy from fluids and ends up slowing down the inflow. However, ship propeller and tidal/marine current turbine have lots of similarities in terms of hydrodynamic conditions. Table 3 tabulates each maximum component velocity of the slipstream between the marine current turbine and the seabed. It indicates that axial velocity is the biggest contributor to the velocity magnitude, in a same manner as ship’s propeller jets. The maximum axial component of velocity in the investigated layers occurred in 0.5C (C ¼ 0.4D), y/D ¼ 0.4. The maximum value of axial velocity has approximately 1.07 times greater than the initial velocity. The maximum local acceleration of velocity around the current extraction device in Sun’s study [16] is approximately 1.2 times of its initial velocity. The maximum velocity locates at y/D ¼ 1
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and 0.3D away from the tip of the turbine blade. The flow acceleration in Sun’s work is more significant. The experimental instrument applied in her study is actuator disk. It suggests that the turbine characteristic influences the extent of flow acceleration. Furthermore, Sun took the free surface effect for numerical modelling into consideration. The interaction between air and water might increase the velocity around the turbine. Fig. 5 shows the axial velocity of different layers between the MCT and the seabed under different heights of tip clearance. The axial component of velocity in the upper two investigated layers (0.25C and 0.50C above seabed) has a moderate increase. The increment of velocity is approximately 5% of initial incoming flow in average. The axial velocity near seabed (0.05C above seabed) at each tip clearance case is lesser than the initial velocity due to the boundary layer effect. However, the fluids at each layer below and behind the turbine have slight higher axial velocity than that at the same level before turbine. This behaviour is different in the fluids passed through the rotor plane. The fluids passed through the rotor plane region experience velocity reduction and form an expanding wake. The faster moving stream (below rotors) serves to reenergise the wake, breaking it up and increasing the velocity [20]. Moreover, the contraction of MCT increases flow velocity. Based on the Bernoulli equation, the increase of velocity will be balanced by the decrease of pressure. As shown in Fig. 6, the pressure of the fluids behind the turbine blades decreased at each investigated layer. The pressure has been non-dimensionlised by the atmospheric pressure (101.325 kPa). The difference of pressures on the
805
Fig. 6. Dimensionless pressure of fluids at different layers near seabed (height of tip clearance ¼ 1.0D).
Fig. 5. Dimensionless axial component velocity at different layers with various tip clearances from seabed: (a) 1.0D; (b) 0.8D; (c) 0.6D; (d) 0.4D.
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two sides of turbine blades results in a suction phenomenon. The fluids at the high pressure side will be sucked to the low-pressure side. This suction effect further accelerates the fluids below turbine blades on the back side. These aforementioned reasons contributed to the velocity acceleration of the fluids in each investigated layer. A schematic diagram has been proposed to describe the flow under rotors of MCT (see Fig. 7). A low-pressure zone is formed below the boundary of the wake. The distribution of axial component velocity has a similar pattern despite different confinement. The influence of seabed proximity in current study is not evident. It suggests that the wake does not expand remarkably and approaches the seabed. The fluids beneath the rotor disk are enough to re-energise the wake. Moreover, the solidity of turbine blade is low. The fluids pass through the turbine smoothly as the blockage effect is not significant. The flow suppression under the turbine is not pronounced. A turbine with high blade area ratio may have different velocity pattern when the confinement below turbine varies. The flow acceleration below the turbine blades is expected to develop along the downstream of the wake. The domain size of current study is not big enough to capture the flow field further downstream. A greater domain size could help the researchers to investigate the flow field below the turbine blades in far wake region. 4.2. Radial component of velocity McGarvey [29] claimed that the radial component of velocity was approximately 30% of the axial velocity along the face of the propeller. Later on, Lam et al. [27] conducted measurements and stated that the radial axial component of velocity is the third largest contributor to the magnitude of flow filed in the propeller jet, and their measurement of tangential velocity is 14% of the axial velocity, which is lower than the 30% of axial velocity suggested by McGarvey [29]. Table 3 shows the radial component of velocity of the present slipstream is the second largest contributor compared to a ship propeller jet. Tangential velocity is the second largest component of a propeller jet. The maximum radial velocity component is approximately 4.12% of the maximum axial velocity component. It is the second largest contributor of velocity, which is in a different manner as ship propeller jets. The radial velocity acts parallel to the turbine blade axis and represents the lateral expansion of fluids. The fluids diffused downward once they approached the blades due to the blockage effect of turbine
Fig. 7. Schematic diagram of slipstream between seabed and marine current turbine.
blades. The distributions of radial velocity at each layer are symmetric which are in valley shapes. The peak ridged radial velocity occurs either at or near the Z-axis (see Fig. 8). Table 4 tabulates the maximum radial velocity at each investigated layer. The maximum radial velocity located at the 0.5C (C ¼ 0.4D), y/D ¼ 0 (Z-axis), which is 4.4% of the initial velocity. The trend of each component velocity close to the tip of turbine blade might be more complicated due to tip vortex. The radial component velocity is more significant in the region near turbine blades. The radial velocity of the fluids in the investigated layer decreased right after the turbine blades. The radial component velocity is relatively less and may not have pronounced effects on local scour. The radial component of velocity might contribute to the backfilling of scour hole. 4.3. Tangential component of velocity Many researches approved that the tangential component of velocity is the second largest contributor to the resultant field in the ship’s propeller jet. Prosser [30] estimated that the magnitude of maximum tangential velocity is approximately 30% of the maximum axial velocity. The results have been contradicted by Lam et al. [27], they reported that the maximum tangential component of velocity within a ship propeller jet is 82% of the maximum axial velocity. The tangential velocity acts perpendicular to the turbine blade rotating axis. The tangential component results from the rotating nature of turbine blades and it contributes to the rotation of fluids. The fluids below and behind the turbine should not have significant tangential component as the fluids are away from the blades. Fig. 9 shows the distribution of tangential velocity. The distribution of tangential velocity is more complicated compared to the radial velocity. The distribution of tangential velocity varies with height of tip clearance and it is remarkably small. However, Fig. 9d is more predictable and shows seasonal rise and falls of tangential velocity at the layer 0.5C when tip clearance is 0.4D. This might be the cyclic decay of tangential component. Table 5 shows the maximum tangential velocity at each investigated layer. The highest tangential velocity occurred at y/D ¼ 0.09 (C ¼ 0.4D), which is 0.24% of initial incoming flow. It shows that the rotation of fluids is higher in the region near turbine blade. 4.4. Influence of incoming velocity Marine current sites with a current velocity of 2.5 m/s or more are considered to have an exceptionally high energy resource [31]. The incoming flow velocity in the previous simulation is 0.8 m/s. The increase of free stream velocity may influence the velocity pattern. Thus, the hydrodynamic conditions near the seabed with incoming flow velocity of 2 m/s, and 3 m/s have been investigated. The TSR remains as constant for all computational simulations. The rotational speeds of turbine for each velocity condition are 1200 deg/s and 1800 deg/s, respectively. Fig. 10 shows the axial, radial and tangential component of velocity between the seabed and the MCT with 3 m/s incoming flow velocity. It shows that the increment of axial velocity acts in a direct proportion manner with the incoming flow. It means the acceleration turns larger when the incoming flow velocity may become higher. The increases of axial velocities under different incoming flow condition are approximately 5% in average, which is the same as the incoming flow 0.8 m/s (see Fig. 5d). The flow patterns with incoming flow velocity of 2 m/s has the same trend shown in Fig. 11, which the figures of velocities have not been presented in the paper. The blockage effect of the turbine may have a more significant effect on the extent of flow acceleration. The influence of turbine
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Fig. 8. Dimensionless radial component velocity at different layers with various heights of tip clearances from seabed: (a) 1.0D; (b) 0.8D; (c) 0.6D; (d) 0.4D.
geometry towards the velocity pattern under MCT has to be studied in future work. The increase of rotational speed may result in more radial and tangential component. However, there is no much variation of radial and tangential velocities near seabed. The radial and tangential velocities have the same pattern as the one with 0.8 m/s incoming flow. Similarly, the radial and tangential velocities increased due to the larger incoming flow velocity. The interaction of the tangential component of velocity with sediment can be neglected due to its insignificant contribution to the magnitude of velocity. The radial and tangential velocities in the near turbine tip region might be highly influenced by the rotational speed. Turbine rather close to seabed is not applicable in tidal turbine farms due to the seabed proximity of wake recovery. Environment implicated issue associated with short tip clearance is another concern during the development of MCT. However, more logarithmic velocity distribution is existed in real marine environment. Mason-Jones et al. [32] highlighted the importance of local depth-wise velocity on tidal turbine performance. The vertical velocity profile in sea environment is governing by the factors such as local bathymetry and turbulence. The increase of bed roughness results in increasing of shear in bed region. The topography of the real field site may induce coherent flow structures in the incoming flow. The coherent flow structure may possibly impose certain impacts to the hydrodynamic conditions near seabed.
4.5. Development of boundary layer The mechanism of horseshoe vortex is of significant importance in the process of scour. The rotation of the incoming flow results in the horseshoe vortex. The boundary layer experiences a threedimensional separation under the influence of the adverse pressure gradient induced by the presence of the support structure. The separated boundary layer accordingly rolls up to from a spiral vortex around the support structure [33]. The incoming boundary layer and pressure gradient induced by the support structure are the two necessary preconditions to generate a horseshoe vortex. Baker [34,35] claimed that the nondimensional quantities describing the horseshoe vortex in the case of steady current mainly depends on the following parameters: d/D, ReD (or alternatively Red Þ, and Pile geometry in which d/ Dp ¼ the ratio of bed boundary layer thickness to the pile diameter, ReD is the pile Reynolds number (Eq. (2)):
ReD ¼
UDp v
(2)
and Red is the bed boundary layer Reynolds number (Eq. (3)):
Red ¼
Ud v
(3)
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Table 4 Maximum radial component of velocity. Height of tip clearance
Layer
Position (y/D)
Radial velocity (Vr/V0) %
1.00D
0.50C 0.25C 0.05C
0.00 0.04 1.37
2.16 0.76 0.27
0.80D
0.50C 0.25C 0.05C
0.00 0.02 1.34
2.62 0.98 0.30
0.60D
0.50C 0.25C 0.05C
0.00 0.00 1.37
3.31 1.34 0.26
0.40D
0.50C 0.25C 0.05C
0.00 0.03 1.40
4.40 1.90 0.42
where U is the velocity at the edge of the bed boundary layer, v is the kinetic viscosity. Fig. 11 shows the current boundary layer profile with rotor (C ¼ 1.0D) in operation in conjunction with measured data from Chilworth flume, University of Southampton [19]. The measured boundary layer profiles are without the presence of energy extraction devices. The boundary layer thickness for smooth bed (d/ D ¼ 0.67) is higher than that (d/D ¼ 0.32) at rough bed. It
demonstrates that the boundary layer thickness is in inverse proportion to bed roughness. The seabed condition incorporated in the current numerical simulation is without roughness. The numerical results indicate that the flow near seabed is increasing and the velocity gradient is reduced compared to experimental data without rotor in operation. The flow acceleration increases the turbulence level in seabed which causes the sediment to transport. The change of boundary layer thickness results in changing of d/D and Red . The change of these two governing parameters definitely alters the horseshoe vortex mechanism. The authors cannot visualise the formation of horseshoe vortex due to the limitation of equipment. Wang et al. [15] stated that ocean turbine may cause sediment movement on seabed if the shaft height from the seabed is less than 1.0D. However, the numerical simulation of current study shows that even the centre of the turbine is 1.5D from seabed (C ¼ 1.0D) changes the boundary layer profile. The changes of flow near to the seabed could possibly affect the bed load and suspended load of sediments. The geometry of turbine blades might be the cause of the contradiction with Wang et al. [15]. The greater acceleration of flow under MCT may further increase shaft height from seabed to avoid severe scour and environmental impact. Giles et al. [36] presented an experimental study investigating the potential benefits of foundation-based flow acceleration structures for marine energy converters. It proved that such structure can bring lots of benefits including increase in device power output, increase in
Fig. 9. Dimensionless tangential component velocity at different layers with various tip clearances: (a) 1.0D; (b) 0.8D; (c) 0.6D; (d) 0.4D.
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Table 5 Maximum tangential component of velocity. Height of tip clearance
Layer
Position (y/D)
Tangential velocity (Vt/V0) %
1.00D
0.50C 0.25C 0.05C
0.20 0.26 0.26
0.03 <0.01 <0.01
0.80D
0.50C 0.25C 0.05C
0.17 0.23 0.23
0.05 0.02 0.01
0.60D
0.50C 0.25C 0.05C
0.11 0.20 0.20
0.09 0.04 0.03
0.40D
0.50C 0.25C 0.05C
0.09 0.11 0.17
0.24 0.12 0.05
Fig. 11. The boundary layer development of rotor in operation (tip of turbine blade located at Z/D ¼ 1.0).
foundation footprint and scour protection. Besides, many other scour protection methods are available in offshore wind farm, such as geotextile containers, concrete armour units, concrete block mattresses etc. [37]. However, the placement of protection materials may influence the performance of MCT. Secondary scour also may occur at the edge of the protection units. The tip clearance and turbine geometry affect scour process of MCT as they alter the boundary layer profile. Many other governing factors are also potential to influence the scour process of MCT, such as sediment materials, water depth, geometry of support structure, etc. Objective measurements are recommended to quantify the effects of these aforementioned parameters on the scour process of MCT. 5. Conclusion
Fig. 10. Dimensionless axial, radial and tangential velocity at different layers with incoming flow velocity 6 m/s (C ¼ 0.4D).
The flow profile between MCT and seabed is still poorly understood compared to the wake characteristics and ship propeller jets. No documented work has been found studying the axial, radial and tangential velocity near seabed in the operation of MCT. In this paper, the velocity pattern near seabed of MCT has been studied through three-dimensional numerical simulation. The outcome of this research work is to give better understanding of the flow pattern between MCT and seabed. OpenFOAM is able to predict the velocity profile under the blade of MCT. The axial component is the largest contributor to the magnitude of velocity. The radial component of the velocity is the second largest contributor to the magnitude of velocity. The tangential component is insignificant so that it suggests that the tangential component of velocity has no impact to the scour and deposition process. The confinement has no effect to the velocity pattern of each velocity component when the tip clearance is large enough. The blockage ratio of MCT is directly proportional to the extent of flow acceleration. The influences of geometry on the magnitude of flow acceleration are suggested to be studied in future work. The presence of rotor changes the boundary layer profile subsequently results in the altering of horseshoe vortex formation. A schematic diagram to describe the flow profile between seabed and MCT has been proposed (Fig. 7). The numerical results show good agreement with experimental results from Pinon et al. [26], which has been demonstrated in Section 3. The promising results testify the
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potential of OpenFOAM to become a powerful engineering tool for designing MCT system. The height of tip clearances and turbine geometry are the two critical parameters in scour design of MCT. Acknowledgement The authors wish to extend their gratitude to the Ministry of Higher Education Malaysia for the financial support received under the UM/MOHE High Impact Research Grant (H-1600-00-D000047). We would like also to thank Mr Chu Yung Jeh for his kind help. References [1] Rocks L, Runyon RP. The energy crisis. New York: Crown Publishers; 1972. [2] Watchorn M, Trapp T, Sayigh AAM. Tidal stream renewable offshore power generation. In: World renewable energy congress VI. Oxford: Pergamon; 2000. pp. 2664e7. [3] Annual report 2010: implementing agreement on ocean energy systems. Lisbon, Portugal: OES-IA; 2011. [4] Annual report 2011: implementing agreement on ocean energy system. Lisbon, Portugal: OES-IA; 2012. [5] Annual report 2012: implementing agreement on ocean energy system. Lisbon, Portugal: OES-IA; 2013. [6] Rourke FO, Boyle F, Reynolds A. Marine current energy devices: current status and possible future applications in Ireland. Renew Sustain Energy Rev 2010;14(3):1026e36. [7] Xia JQ, Falconer RA, Lin B. Impact of different tidal renewable energy projects on the hydrodynamic processes in the Severn Estuary, UK. Ocean Model 2010;32:86e104. [8] Shields MA, Woolf DK, Grist EPM, Kerr SA, Jackson AC, Harris RE, et al. Marine renewable energy: the ecological implications of altering the hydrodynamics of the marine environment. Ocean Coast Manage 2011;54:2e9. [9] Hoffmans GJCM, Verheij HJ. Scour manual. Netherlands: Balkema; 1997. [10] Sumer B. Mathematical modelling of scour: a review. J Hydraul Res 2007;45(6):723e35. [11] Lillycrop WJ, Hughes SA. Scour hole problems experienced by the corps of engineers: data presentation and summary. Vicksburg: CERC-93-2, US Army Engineer Waterways Experiment Station, Coastal Engineering Research Centre; 1993. [12] Ng KW, Lam WH, Ng KC. 2002e2012: 10 years of research progress in horizontal-axis marine current turbines. Energies 2013;6:1497e526. [13] Neill SP, Litt EJ, Couch SJ, Davies AG. The impact of tidal stream turbines on large-scale sediment dynamics. Renew Energy 2009;34:2803e12. [14] Neill SP, Jordan JR, Couch SJ. Impact of tidal energy converter (TEC) arrays on the dynamics of headland sand banks. Renew Energy 2012;37:387e97. [15] Wang D, Atlar M, Sampson R. An experimental investigation on cavitation, noise, and slipstream characteristics of ocean stream turbines. Proc IMchE Part A J PowerEnergy; 2006:221. [16] Sun X. Numerical and experimental investigation of tidal current. PhD thesis. University of Edinburgh; 2008.
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