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Study of mechanical disturbances effects on the laminar separation bubble by means of infrared thermography
International Journal of Thermal Sciences 50 (2011) 2091e2103
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International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Study of mechanical disturbances effects on the laminar separation bubble by means of infrared thermographyq R. Ricci, S. Montelpare*, E. Renzi Università Politecnica delle Marche, Dip. Energetica, Via Brecce Bianche, 60121 Ancona, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2009 Received in revised form 16 May 2011 Accepted 16 May 2011 Available online 16 July 2011
The aim of this paper is to show the possibility to increase small wind turbine performances using a Micro Electro Mechanical System (M.E.M.S.) placed inside the blade. In particular its effects on the Laminar Separation Bubble (LSB) phenomenon are presented. This is a local boundary layer separation, that may occur on aerodynamic bodies operating at low Reynolds numbers as the root blade sections of small horizontal axis wind turbine rotors. Its presence induces an aerodynamic efficiency drop-off due to a drag increase and a lift decrease. Tests are performed on an airfoil designed for the root section of a small wind turbine having a 10 kW nominal power. A M.E.M.S. was placed internally the wing section to produce mechanical disturbances directly inside the boundary layer; M.E.M.S.’ lower face is clamped to the wing structure, while the upper one is glued to a PVC movable strip aligned with the airfoil upper surface. M.E.M.S. was electrically supplied at different amplitudes and frequencies in order to vary vibration modes. Preliminary measurements, performed by using a pressure distribution analysis, were carried out to qualitatively locate the LSB presence. Laminar separation, transition and turbulent reattachment points positions are quantitatively detected by means of a thermographic approach: the heated thin foil technique is used to observe temperature distribution on the airfoil surface and a numerical energy balance approach was employed to evaluate the local convective heat transfer coefficient h. Afterward the local Stanton number distribution was calculated and a first and second derivatives analysis allowed to localize the LSB characteristic points positions. Results showed a very effective M.E.M.S. action for frequencies about 175 Hz and a Reynolds number of 105. A lift increases about 50%, was showed with a consequent increase of the wind turbine Cp values. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Wind turbine Laminar separation bubble Wind tunnel IR thermography M.E.M.S. Renewable energy
1. Introduction Increased interest for renewable energies is driving many industries to design small wind turbines having a nominal power rate up to 100 kW. Economical feasibility and payback time are greatly related to the local annual average wind velocity and to wind turbines technical specifications. Moreover lands characterized by a complex terrain show strong influences on the wind resources due to the speed-up effects and are very often characterized by low average wind velocities. These problems drive the applied research toward the design of new small wind turbines having lower rate velocities, about 9 m/s, and using boundary layer active control devices for airfoils operating at low Reynolds q This document is a collaborative effort. All authors contributed equally to this work. * Corresponding author. Tel.: þ39 0 71 280 42 39. E-mail addresses: [email protected] (R. Ricci), [email protected] (S. Montelpare), [email protected] (E. Renzi). 1290-0729/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2011.05.013
numbers. The present paper suggests an active control method that relies on mechanical actuators (M.E.M.S.) in order to locally destabilize the laminar boundary layer and so to increase airfoil performances. Present study analyzes the behavior of an airfoil designed for the blade root section of a 10 kW small wind turbine. This airfoil could often operate at low Reynolds numbers due to low tangential velocities, at the blades root, and/or to low wind velocities. Moreover this airfoil has a large maximum thickness (19%chord) for structural design requirements and this makes it particularly subjected to the presence of laminar separation bubble. A laminar bubble induces a modified surface pressure distribution with a wide constant pressure area located inside a pressure recovery zone. The LSB presence causes an airfoil efficiency decrease that reflects in a reduced energy production. An infrared thermographic approach used for a qualitative and quantitative evaluation of mechanical disturbances effects on the Laminar Bubble phenomenon is described in this paper. Disturbances are induced by a M.E.M.S. placed inside the blade section in a way that its movable upper surface is aligned with the airfoil extrados.
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Nomenclature Cd Cl Cp F12 Re St
a b d 3 L
l f j r s sS.B. 4 0
00 000
2D Drag Coefficient 2D Lift Coefficient Pressure Coefficient View Factor Reynolds Number Stanton number wave number dumping factor Boundary Layer Thickness Surface Emissivity Polhausen Parameter Thermal conductivity [W/mK] Angle between rotation plane and Vrel [rad] Streamfunction Air Density [kg/m3] Wind rotor solidity StefaneBoltzmann Constant [W/m2 K4] disturbance amplitude function First Derivative Second Derivative Third Derivative
c A Asurr B c D G h L M N p r T t Ti,j Tsurr U, V, W u, v, w u0 , v0, w0 Ue,s VN
phase velocity of small disturbance [m/s] Axial Force [N] Surrounding Area [m2] Number of Blades Airfoil Chord [m] Drag Force [N] Volumetric heat flux [W/m2] Convective heat transfer coefficient [W/m2 K] Lift Force [N] Wind Turbine Mechanical Torque [Nm] Normal Force [N] Pressure [Pa] Blade section distance from hub [m] Wind Turbine Thrust Load [N] Time [s] Generic Node Temperature [K] Surrounding Temperature [K] Wind Velocity Mean Components [m/s] Wind Velocity Components [m/s] Wind Velocity Fluctuating Components [m/s] Curvilinear Boundary Layer Outer Velocity [m/s] Airfoil incoming relative velocity [m/s]
Fig. 1. The laminar separation bubble.
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A sinusoidal power supply allows to obtain alternative vertical displacements controlled both in amplitude and frequency. The wing section is coated with a 20 mm thin aluminum foil in order to have an electrically conductive surface; constant AC electric current is supplied to produce a uniform heating boundary condition with a temperature increase of few degrees than the environment. Temperature distribution, which is strictly related to the local convective heat transfer coefficient, is revealed by an IR camera and an inverse heat transfer problem is resolved to recognize the fluid dynamic field [1].
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possible to localize multiple turbulence spots that growth up to coalesce into a fully turbulent state.
u ¼ U þ u0
v ¼ v0
w ¼ 0
p ¼ P þ p0
jðx; y; tÞ ¼ 4ðyÞeiðaxbtÞ
(2)
1 000 4 2a2 40 þ a4 4 ðU cÞ 400 a2 4 U 0 4 ¼ aRe
(3)
2. The physical phenomenon The laminar separation bubble (Fig. 1(a)) is a local boundary layer separation phenomenon that may occur on aerodynamic bodies operating at low Reynolds numbers, lesser than one million [2]. It occurs in presence of some conditions that are briefly described: at first it should be a separation of the laminar boundary layer due to an adverse pressure gradient; subsequently a flow transition of the laminar separated flow should occurs in the separated shear layer; finally, if the transition point is not too far above airfoil surface, a turbulent flow reattachment should occur downstream the separation point. Under these conditions, it forms an area characterized by slow recirculating flow and constant pressure (Fig. 1(b)): i.e. the laminar separation bubble (LSB). Normally this is a threedimensional phenomenon, but in the experiments presented here, since an infinite wing flow condition was reproduced inside the wind tunnel (there was no clearance between the blade section ends and the test section lateral walls), the laminar bubble is assumed as a bidimensional phenomenon (independent of the wing span position). The presence of a LSB introduces two main classes of problems: an airfoil efficiency decrease, mainly due to the airfoil drag increase; a pressure pulsation, eventually present in case of bubble bursting. A wind turbine blade having low efficiency, will show lower energy productions and hence economical losses. A bubble bursting phenomenon, normally related to the presence of a LSB near the airfoil leading edge, induces a pre-stall hysteresis that causes cyclic loads reducing the blade lifetime. Many devices are able to reduce or even avoid the LSB presence; they may be subdivided in two categories: passive and active devices. As example, turbulators belong to the first class and are realized with adhesive tapes, having different shapes, glued onto the airfoil surface. Although very cheap they promote turbulent transition for every flow condition resulting in globally worse performances even when flow separation phenomena are not present. Flow blowing or suction are active control methods that allow very good boundary layer control; unfortunately they are very expensive and space consuming. The method proposed in this paper considers the possibility of introducing sinusoidal disturbances directly inside the boundary layer in order to promote transition and reduce or even avoid the LSB onset: the main advantage of this active control method is its ability to disturb the growing boundary layer only in the desired fluid dynamic conditions. 3. Theoretical background 3.1. Stability theory Boundary layer natural transition from laminar to turbulent flow is a very complex mechanism that develops trough subsequent steps. Initially it is possible to observe bi-dimensional waves, named TollmieneSchlichting waves, whereas downstream it is
(1)
Fig. 2. The stability curves.
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L ¼
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d2 dUe;s n dx
(4)
OrreSommerfeld equation for the linear stability theory is obtained under the main hypothesis that disturbances verify NaviereStokes equations too; they prescribe that disturbances injected into a boundary layer are amplified or damped with regard to their wave number and the local Reynolds number based on the thickness displacement [1,3e5].
Stability curves (Fig. 2(a)) show the existence of a particular Reynolds number (defined as neutral or indifference Reind), below which no frequencies are able to destabilize the laminar boundary layer. The figure also shows that above Reind there are different values of the disturbance wave number alpha able to promote the turbulent transition. The presence of an adverse pressure gradient plays a significant role also on the boundary layer instability and consequently it modifies the useful disturbances frequency ranges. Fig. 2(b) illustrates this behavior by means of the Polhausen
Fig. 3. Reind evaluation.
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Fig. 4. HAWT aerodynamics.
parameter, that is expression of the pressure gradient (a negative value corresponds to an adverse pressure gradient). The Reind value for an airfoil operating at different angles of attack may be evaluated by the cited Polhausen parameter (Fig. 3(a)) or alternatively by the Wazzan curve (Fig. 3(b)); the latter [6] reports transition and indifference curves as function of the shape factor H12 and the local Reynolds number based on the curvilinear abscissa (Res). Values for Polhausen parameter, H12 and Res was obtained, in this work, by using the freeware code XFOIL [7]. The ncrit value used in the en transition criteria was set to the turbulence level of the wind tunnel (ncrit ¼ 5.5 for the 0.3% wind tunnel corresponding turbulence value measured by a CTA single wire probe). Tested airfoil curves, obtained for angles of attack of þ2, þ4 and þ6 (Fig. 3(c)), intersect the indifference curve at values corresponding to different chord positions: i.e. for a ¼ 2 the indifference point is located at about 0.3chord from the leading edge, for an angle of 4 at 26%chord ca. and for a ¼ 6 at 20%chord. Because of the M.E.M.S. is placed between 16%chord and 24%chord this theoretical approach suggests that disturbances generated by the M.E.M.S. should be effective only for angles greater than þ4 ; as a matter of fact a disturbance introduced upstream the indifference point will be damped by the favorable pressure gradient.
a single blade element (BEM Theory) it is possible to observe that torque and thrust coefficients are dependent on the lift and drag forces acting on the airfoil constituting the blade element (Fig. 4). The wind speed acting on the airfoil (Vrel) is resulting by the vector sum of the incoming wind velocity VN and the rotational velocity ur. Aerodynamics forces, i.e. Lift and Drag, acting on the airfoil are related to Vrel (Eq. (6)); their vectorial sum can be decomposed along and normally the incoming wind direction, respectively producing the thrust and torque loads on the wind turbine (Eq. (6)).
dT ¼ BNdr
(8)
3.2. Wind turbine aerodynamics
dM ¼ rBAdr
(9)
A horizontal axis wind turbine has a power production directly related to the lift capacity of its blades. Analyzing forces acting on
An active control of the airfoil boundary layer may ensure a lift increase and a drag decrease, so it is possible to derive from Eq. (8),
9 1 2 = L ¼ Cl rVrel cðrÞ > 2 1 2 > cðrÞ ; D ¼ Cd rVrel 2 N ¼ Lcosf þ Dsinf A ¼ Lsinf Dcosf
sðrÞ ¼
(5)
cðrÞB ðwhere B is the number of bladesÞ 2pr
Fig. 5. The UNIVPM aeronautical wind tunnel.
(6)
(7)
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Fig. 6. The IR facility setup.
a lower thrust on the wind turbine and a higher torque at the electric generator shaft. Globally an active boundary layer control method may ensure higher annual energy production. 4. Experimental apparatus WT1 tested airfoil was designed as root blade airfoil for a small wind turbine having a nominal power of 3.5 kW. Airfoil maximum
Fig. 8. Standard deviation analysis.
thickness is 19% of chord length, located at 35%chord. Maximum camber is 3.5%chord and is located at 48%chord. A large airfoil thickness, required for structural needs, typically involves significant adverse pressure gradients that may induce a laminar separation
Fig. 7. M.E.M.S. placement.
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Fig. 9. View factor analysis.
bubble located on the airfoil rear side; besides the airfoil behavior will be characterized by a smooth trailing edge stall.
test section dimensions [cm]: 62(W) 38(H) 150(L) ; inlet test section area contraction ratio: 4.65; max velocity: 38 m/s; mean turbulence in the inlet section: 0.3%; fan power 5.5 kW, inverter controlled.
Both aerodynamic tests and infrared visualizations are carried out in the open circuit subsonic wind tunnel available at the Department of Energetic (Fig. 5). Flow velocity may be varied from 0 to 35 m/s controlling the fan motor RPM with an AC inverter. Turbulence level, measured by means of a hot wire anemometer, is lesser than 0.3%. The WT1 wing section is placed vertically in order to have a ratio between the test section width and the airfoil chord larger than 3:1; this allows to avoid flow blockage even at high angles of attack (WT1 airfoil shows a blockage section lesser than 5% up to angles of attack of 14 ). Velocity and turbulence measurements are carried out with a Dantec CTA system equipped with a pneumatic calibration unit; this unit is used before every measurement session in order to reduce the velocity overall error, estimated as 0.08 m/s. Pressure measurements, used to compare thermographic results and to validate XFOIL results, are carried out with a Druck LPM 9481 differential pressure transducer having an operating range between 10 Pa and 1000 Pa and a typical error less than 0.1% FS BSL. In order to graph the Cp pressure coefficient distribution on the airfoil, this device is sequentially connected to all the pressure taps drilled in the middle section of the WT1 wing. A three axes load balance is used to measure Lift, Drag and Moment values at different angles of attack and Reynolds numbers.
A Peltier refrigerant system is implemented to control load balance extensimeters temperature; this allows to reduce drift errors and to obtain the subsequent overall errors: Lift 0.0377 N; Drag 0.0706 N; Moment 0.0028 Nm. Infrared facilities include a FLIR SC3000 IR camera, a ZnSe IR window and a Low-Voltage High-Current power supplier. A thin aluminum foil is used to metallize the tested wing section (Fig. 6(a)). IR camera is equipped with a 320 240 QWIP sensor array and with a 20 lens that performs a 20 15 field of view with a minimum focus distance of 0.3 m; reported thermal sensitivity is 0.03 C at 30 C.
Fig. 10. The energy balance approach.
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The IR camera is placed outside of the wind tunnel at a distance of 0.59 m from the wing section (Fig. 6(b)): the spatial resolution on the airfoil upper surface is 0.65 mm. ZnSe IR window is 150 mm large, 50 mm width, and 4 mm thick; it allows to observe an area covering all the airfoil length. The IR window transmissivity coefficient, inside the IR camera range 8 O 9 mm, is estimated by using a reference black body and its value is 0.74. Electric power supplier produces controlled AC low voltages (about 2 V) and high currents (about 90 A) used to heat to the wing metallic coating; a NI acquisition system is employed to measure Voltage and Current values during tests. Airfoil aluminum coating, 25 mm thick, allows a uniform Joule heating once AC current is supplied; a dull black paint is applied to the aluminum coating to get a known emissivity value of 0.94. Mechanical disturbances are introduced directly inside the boundary layer by a M.E.M.S. flush mounted on the wing upper surface. The cradle for supporting M.E.M.S. and lining up the vibrating surface to the airfoil
profile is placed between 16%chord and 24%chord (Fig. 7). An HP signal generator controls the M.E.M.S. power supplier in order to generate a variable sinusoidal current; several tests (results are presented in the next pages) show that the most effective frequency is 170 Hz. A very interesting data is the low power consumption used to supply the M.E.M.S.; i.e. a power of 34 mW is used. 5. Thermographic approach Two main hypotheses are introduced to relate the airfoil upper surface temperature distribution to the kinematic boundary layer: forced convection and Prair x 1. Having a boundary condition of fixed heat flow, the temperature distribution is directly related to the convective heat transfer coefficient that in turn depends on the flow condition. In the case of a fully attached boundary layer the following situations could be present:
Fig. 11. Thermographic results quantitative analysis.
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A developing laminar boundary layer: this will be revealed by a downstream increasing temperature. The thickening of the thermal boundary layer induces a temperature wall gradient lowering and with a fixed heat flux wall condition this produces a temperature increase. A transition from a laminar to a turbulent condition: this will be observed as a local temperature decrease due to the higher turbulent convective heat transfer coefficient with respect to the laminar one. A developing turbulent boundary layer: this will have a behavior similar to a developing laminar boundary layer, but it will have different absolute values of the surface temperature and convective heat transfer coefficient. The presence of a LSB modifies the boundary layer growth and so the thermal distribution on the surface; a laminar bubble that, as previously referred, is a 3D volume of slow recirculating flow, has a very low convective heat transfer coefficient. This will be revealed as a zone having a local temperature increase followed by an abrupt temperature decrease corresponding to the flow jet impingement in the reattachment point. A proprietary image analysis camera software was used to acquire IR images and a customized Matlab
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code was subsequently employed to carry out the post processing procedure. Standard deviation analysis, performed for every acquisition and for the whole sensor array (Fig. 8(a)), showed that a camera adjusting must be performed before every measurement to reduce the sensors thermal drift and that at least 50 images (Fig. 8(b)) must be acquired to obtain an almost constant standard deviation value. Thermographic analyses were performed by sampling 100 frames at 50 Hz for every angle of attack and an averaging procedure was employed to obtain the single thermogram to be analyzed. In addition, the presence of the ZnSe IR window made necessary to carry out thermogram subtractions to avoid Narcissus effect and to reduce background thermal noise. A subtraction between recorded thermograms and a corresponding background image, acquired without airfoil heating, was implemented for every airfoil angle of attack; this procedure was used both for tests with M.E.M.S. off and M.E.M.S. on. Airfoil was observed with the IR camera at different angles of attack; this induces variable geometrical view factors and an error evaluation procedure is so implemented inside the MatlabÓ code to quantify these errors. The heating uniformity and the ratio between the real view factor and an equivalent view factor of a flat plate inclined at the same angle were measured. Fig. 9(a)
Fig. 12. M.E.M.S. effect analysis by pressure distribution measurements.
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reports the ratio between the local temperature and the maximum temperature revealed in absence of wind flow: it shows a good heating uniformity both for negative and positive angle of attack. Fig. 9(b) shows instead low view factor ratios near the leading edge (up to 5% of the chord); as a consequence this area is not considered in the subsequent Stanton number analysis. Thermograms obtained by the subtraction procedure were analyzed using an energy balance approach (Fig. 10). This procedure takes into account longitudinal heat conduction, radiative heat transfer, convective heat transfer and volumetric heat generation by Joule effect The main hypotheses are an adiabatic boundary condition under the aluminum coating, that was obtained by filling the wind section body with an insulating material, and a bi-dimensional flow behavior, that allows to neglect the transverse heat conduction. This is guaranteed by the infinite wing section assumption. The unknown convective heat transfer coefficient is obtained by solving numerically the equation:
GDxDys þ
that extends from the middle part of the airfoil downstream toward the trailing edge. Reattachment point is difficult to localize in the case of tests with M.E.M.S. turned off, because the LSB extends up to the airfoil section where there are not present pressure taps (i.e. downstream 80%chord); pressure taps absence is due to the small airfoil thickness in that area, that does not physically allow the insertion of tubes used to sense the static pressure. Cp measurements with M.E.M.S. turned on clearly show its effect on the pressure behavior with a steep pressure recovery downstream the plateau. However the M.E.M.S. efficiency varies sensibly at different angles: for values grater than 4 there is marked suction peak increase that underlines a significant boundary layer change and this causes a lift increase. By referring to the indifference point position, previously discussed in the theoretical background section, it could be better understand why the M.E.M.S. effect is more pronounced for those angles that show an indifferent point position upstream the M.E.M.S. location. These results confirm the possibility to use this approach in
Tðiþ1;jÞ þ Tði1;jÞ 2Tði;jÞ Tði;jþ1Þ þ Tði;j1Þ 2Tði;jÞ þ Dx Dy lsDy lsDx
þ hDxDy TNTi;j þ sS:B:
4 T4 Tsurr ði;jÞ
13 1 1 3surr þ þ 3DxDy F12 þ DxDy 3surr Asurr
¼ 0 ð10Þ
and then it is graphed along with the local Stanton number. This is calculated by using the relation
Stx ¼
hx
rN Cp Ue;s
(11)
where the Ue,s value is obtained by means of the XFOIL code with the inviscid condition input. Pressure distribution analysis confirms thermographic results [8,9]. It is possible to observe (Fig. 11(a) and (b)) that: the minimum h or St values corresponds to the transition point (a zero for the first derivative); the separation point is associated to an inflection point just before the transition (a second derivative null value); the turbulent reattachment point corresponds to the relative maximum (a zero for the first derivative) subsequent the transition point (i.e. the turbulent reattachment behaves like a jet impingement with a high heat exchange). 6. The experimental results 6.1. Pressure distribution analysis Pressure distribution analysis is carried out to verify the LSB presence and to obtain lift and drag coefficients for the tested airfoil. The LSB corresponds to a constant pressure area inside a pressure recovery zone: as example the pressure distribution for an a value of 2 (Fig. 12(a)) shows a plateau starting at 48%chord ca. (separation point) up to 68%chord ca. and a subsequent steep recovery up to 80%chord ca. (turbulent reattachment point). Pressure analysis is useful to study the bubble position but some limits related to the expensive setup arise: these ones are mainly due to the pressure taps realization and to long measurement time needed for the pressure scanning. As a matter of fact low pressure values, typical of low velocities, require a very accurate pressure transducer but the typical simultaneous scanners are not very sensitive; as a result a single sensor is used and the pressure taps are scanned manually and sequentially. Pressure distribution analysis shows also problems into identifying thin LSBs that induce small pressure changes. The WT1 airfoil is tested for angles of attack between 4 and 16 with steps of 2 and for Reynolds between 100 k and 150 k. Fig. 12(a) and (b) show the presence of a large LSB
Fig. 13. M.E.M.S. effects on the airfoil behavior.
R. Ricci et al. / International Journal of Thermal Sciences 50 (2011) 2091e2103
order to choose the better electromechanical actuator position during the wind turbine blades design phase. Analysis of the lift coefficient, derived from the pressure distribution (Fig. 13(a)), confirms the good effect of mechanical disturbances; results obtained without a M.E.M.S. disturbance show a clear curve bending from 0 up to an angle of 12 , followed by a great lift increase at 14 . The curve bending is due to the presence of a large bubble on the airfoil extrados while the abrupt increase at 14 corresponds to a sudden bubble movement and to a bubble length reduction. Once the M.E.M.S. is turned on the Cp distribution analysis shows that the lift coefficient curve, between 0 and 10 , has a lower bending; this is due to a different bubble behavior and corresponds to higher lift values. 6.2. Wake rake analysis M.E.M.S. effects on the airfoil wake and consequently on the drag coefficient, were analyzed by means of the wake rake device. Experimental data clearly show a wake extension decrease and also a down movement of the slowed velocity zone; the former results underline a strong drag decrease resulting from the momentum balance on the control volume. The latter underline an overall change of the flow field around the airfoil section (Fig. 13(b)). 6.3. Thermographic results
analysis; besides formers allow an even more detailed LSB characterization. The transition point locates near the point of higher LSB normal thickness; due to the slow recirculating flow inside the bubble and to the higher bubble vertical extension, the transition point qualitatively corresponds to the higher temperature values inside the warmer zone (Fig. 14(a)) and so to the lower St number and h coefficient values. The laminar separation, corresponding to a boundary layer development singularity, is identified as the inflection point upstream the transition and, in the pressure analysis, this corresponds to the point where the pressure plateau begins. The separation point position could be derived indifferently by the local h coefficient distribution or the local Stanton number distribution even if they have different numerical values. Turbulent reattachment point is instead more difficult to localize if the impingement effect is not significant (as example for thin bubbles). In this case it is worthwhile noting that the local Stanton distribution generally does not show a relative maximum downstream the transition but shows a convexo-concave behavior. The local h coefficient trend shows instead a relative maximum also for a thin bubble and so it is more useful when the reattachment point is difficult to localize. The mechanical disturbance effect, for angles greater than 4 , is extremely clear to identify by observing thermograms reported in Figs. 14(a) and 15. Bubble presence is not avoided but there is a more pronounced wall cooling downstream the flow reattachment point; the effect is a change in the global flow arrangement around the airfoil and a greater pressure suction
As verified in previous research works [8e12], thermographic results are in good agreement with the pressure distribution
Fig. 14. M.E.M.S. effects on the airfoil behavior.
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Fig. 15. M.E.M.S. effects derivative analysis.
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Fig. 16. LSB quantitative analysis.
peak, as revealed by the pressure analysis. Particularly for the 6 angle, the transition point (Fig. 15(a) and (b)) does not move, but there is a delayed laminar separation and consequently a downstream turbulent reattachment point movement; separation delay is associated to the additional energy, supplied to the flow by the M.E.M.S., that allows to better contrast the adverse pressure gradient. Extending thermographic analysis to all the tested angles of attack is possible to characterize the bubble dimension and position. Graphs reported in Fig. 16(a) and (b) substantially show a negligible M.E.M.S. effect for angles less than 2 , where there is only a change in the reattachment point position (the pressure analysis also shows only a change in the pressure recovery
behavior). At 4 there is a little downstream movement of the separation and transition points that does not reflects in a substantial dimension change but that reflects in an appreciable pressure suction peak increase. At 6 there are both a bubble dimension reduction and a great pressure suction peak increase that induce a lift increase. 7. Conclusions The use of mechanical disturbances on airfoils operating at low Reynolds numbers seems to be a good boundary layer active control system. The point where disturbances are to be introduced should be chosen by performing a preliminary analysis of the indifference
R. Ricci et al. / International Journal of Thermal Sciences 50 (2011) 2091e2103
point position: the electromechanical device should be located downstream this point. The thermographic approach performed in addition to classical techniques, such as the pressure distribution analysis, the load balance measurements and the wake analysis, is very useful to characterize the bubble behavior and the effects of this phenomenon on the flow field. With reference to small wind turbine application field, it is possible to say that the use of electromechanical systems is a very interesting approach aimed to the reduction of local separation phenomena and the consequent increase of the wind turbine energy production. It is furthermore worthwhile reminding that M.E.M.S. power consumption in the performed tests is just 0.034 W. Despite the low value of energy at stake, a doubling of the airfoil lift coefficient is evident.
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[4] C. Haggmark, Investigation of Disturbance Developing in a Laminar Separation Bubble Flow Tech. Rep.. Department of Mechanics, S-100 44 Stockholm, Sweden, 2000 [5] W. Saric, H. Reed, E. Kerscen, Boundary layer receptivity to freestream disturbances, Annual Review of Fluid Mechanics 34 (2002) 291e319. [6] A. Wazzan, C. Gazley, A. Smith, H-R/x/ method for predicting transition, AIAA Journal 19 (1981) 810e812. [7] M. Drela, Low Reynolds Number Aerodynamics, , In: Lecture Notes in Engineering, vol. 54. Springer Verlag, 1989. [8] S. Montelpare, R. Ricci, A thermographic method to evaluate the local boundary layer separation phenomena on aerodynamic bodies operating at low Reynolds number, International Journal of Thermal Sciences 43 (2004) 315e329. [9] R. Ricci, S. Montelpare, E. Silvi, Study of acoustic disturbances effect on laminar separation bubble by IR thermography, Experimental Thermal and Fluid Science 31 (2007) 349e359. [10] G. Cesini, R. Ricci, S. Montelpare, E. Silvi, A thermographic method to evaluate laminar bubble phenomena on airfoil operating at low Reynolds number. in: D. Balageas, G. Busse, G.M. Carlomagno, S. Svaic (Eds.), Quantitative InfraRed Thermography 6 (2002), ISBN 953-6313-50-2, pp. 101e107 Dubrovnik, Croatia. [11] R. Ricci, F. Angeletti, S. Montelpare, A. Secchiaroli, Thermographic analysis of acoustic disturbance effects on laminar separation bubble. in: D. Balageas, G. Busse, G.M. Carlomagno, S. Svaic (Eds.), Quantitative InfraRed Thermography 8. ITC-CNR, Padova, Italy, 2006, ISBN 953-6313-50-2, pp. 101e107. [12] R. Ricci, S. Montelpare, Analysis of boundary layer separation phenomena by infrared thermography e use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects, QIRT Journal, Lavoisier Edn. 6 (2009) 101e125.
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Study of mechanical disturbances effects on the laminar separation bubble by means of infrared thermographyq R. Ricci, S. Montelpare*, E. Renzi Università Politecnica delle Marche, Dip. Energetica, Via Brecce Bianche, 60121 Ancona, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2009 Received in revised form 16 May 2011 Accepted 16 May 2011 Available online 16 July 2011
The aim of this paper is to show the possibility to increase small wind turbine performances using a Micro Electro Mechanical System (M.E.M.S.) placed inside the blade. In particular its effects on the Laminar Separation Bubble (LSB) phenomenon are presented. This is a local boundary layer separation, that may occur on aerodynamic bodies operating at low Reynolds numbers as the root blade sections of small horizontal axis wind turbine rotors. Its presence induces an aerodynamic efficiency drop-off due to a drag increase and a lift decrease. Tests are performed on an airfoil designed for the root section of a small wind turbine having a 10 kW nominal power. A M.E.M.S. was placed internally the wing section to produce mechanical disturbances directly inside the boundary layer; M.E.M.S.’ lower face is clamped to the wing structure, while the upper one is glued to a PVC movable strip aligned with the airfoil upper surface. M.E.M.S. was electrically supplied at different amplitudes and frequencies in order to vary vibration modes. Preliminary measurements, performed by using a pressure distribution analysis, were carried out to qualitatively locate the LSB presence. Laminar separation, transition and turbulent reattachment points positions are quantitatively detected by means of a thermographic approach: the heated thin foil technique is used to observe temperature distribution on the airfoil surface and a numerical energy balance approach was employed to evaluate the local convective heat transfer coefficient h. Afterward the local Stanton number distribution was calculated and a first and second derivatives analysis allowed to localize the LSB characteristic points positions. Results showed a very effective M.E.M.S. action for frequencies about 175 Hz and a Reynolds number of 105. A lift increases about 50%, was showed with a consequent increase of the wind turbine Cp values. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Wind turbine Laminar separation bubble Wind tunnel IR thermography M.E.M.S. Renewable energy
1. Introduction Increased interest for renewable energies is driving many industries to design small wind turbines having a nominal power rate up to 100 kW. Economical feasibility and payback time are greatly related to the local annual average wind velocity and to wind turbines technical specifications. Moreover lands characterized by a complex terrain show strong influences on the wind resources due to the speed-up effects and are very often characterized by low average wind velocities. These problems drive the applied research toward the design of new small wind turbines having lower rate velocities, about 9 m/s, and using boundary layer active control devices for airfoils operating at low Reynolds q This document is a collaborative effort. All authors contributed equally to this work. * Corresponding author. Tel.: þ39 0 71 280 42 39. E-mail addresses: [email protected] (R. Ricci), [email protected] (S. Montelpare), [email protected] (E. Renzi). 1290-0729/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2011.05.013
numbers. The present paper suggests an active control method that relies on mechanical actuators (M.E.M.S.) in order to locally destabilize the laminar boundary layer and so to increase airfoil performances. Present study analyzes the behavior of an airfoil designed for the blade root section of a 10 kW small wind turbine. This airfoil could often operate at low Reynolds numbers due to low tangential velocities, at the blades root, and/or to low wind velocities. Moreover this airfoil has a large maximum thickness (19%chord) for structural design requirements and this makes it particularly subjected to the presence of laminar separation bubble. A laminar bubble induces a modified surface pressure distribution with a wide constant pressure area located inside a pressure recovery zone. The LSB presence causes an airfoil efficiency decrease that reflects in a reduced energy production. An infrared thermographic approach used for a qualitative and quantitative evaluation of mechanical disturbances effects on the Laminar Bubble phenomenon is described in this paper. Disturbances are induced by a M.E.M.S. placed inside the blade section in a way that its movable upper surface is aligned with the airfoil extrados.
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Nomenclature Cd Cl Cp F12 Re St
a b d 3 L
l f j r s sS.B. 4 0
00 000
2D Drag Coefficient 2D Lift Coefficient Pressure Coefficient View Factor Reynolds Number Stanton number wave number dumping factor Boundary Layer Thickness Surface Emissivity Polhausen Parameter Thermal conductivity [W/mK] Angle between rotation plane and Vrel [rad] Streamfunction Air Density [kg/m3] Wind rotor solidity StefaneBoltzmann Constant [W/m2 K4] disturbance amplitude function First Derivative Second Derivative Third Derivative
c A Asurr B c D G h L M N p r T t Ti,j Tsurr U, V, W u, v, w u0 , v0, w0 Ue,s VN
phase velocity of small disturbance [m/s] Axial Force [N] Surrounding Area [m2] Number of Blades Airfoil Chord [m] Drag Force [N] Volumetric heat flux [W/m2] Convective heat transfer coefficient [W/m2 K] Lift Force [N] Wind Turbine Mechanical Torque [Nm] Normal Force [N] Pressure [Pa] Blade section distance from hub [m] Wind Turbine Thrust Load [N] Time [s] Generic Node Temperature [K] Surrounding Temperature [K] Wind Velocity Mean Components [m/s] Wind Velocity Components [m/s] Wind Velocity Fluctuating Components [m/s] Curvilinear Boundary Layer Outer Velocity [m/s] Airfoil incoming relative velocity [m/s]
Fig. 1. The laminar separation bubble.
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A sinusoidal power supply allows to obtain alternative vertical displacements controlled both in amplitude and frequency. The wing section is coated with a 20 mm thin aluminum foil in order to have an electrically conductive surface; constant AC electric current is supplied to produce a uniform heating boundary condition with a temperature increase of few degrees than the environment. Temperature distribution, which is strictly related to the local convective heat transfer coefficient, is revealed by an IR camera and an inverse heat transfer problem is resolved to recognize the fluid dynamic field [1].
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possible to localize multiple turbulence spots that growth up to coalesce into a fully turbulent state.
u ¼ U þ u0
v ¼ v0
w ¼ 0
p ¼ P þ p0
jðx; y; tÞ ¼ 4ðyÞeiðaxbtÞ
(2)
1 000 4 2a2 40 þ a4 4 ðU cÞ 400 a2 4 U 0 4 ¼ aRe
(3)
2. The physical phenomenon The laminar separation bubble (Fig. 1(a)) is a local boundary layer separation phenomenon that may occur on aerodynamic bodies operating at low Reynolds numbers, lesser than one million [2]. It occurs in presence of some conditions that are briefly described: at first it should be a separation of the laminar boundary layer due to an adverse pressure gradient; subsequently a flow transition of the laminar separated flow should occurs in the separated shear layer; finally, if the transition point is not too far above airfoil surface, a turbulent flow reattachment should occur downstream the separation point. Under these conditions, it forms an area characterized by slow recirculating flow and constant pressure (Fig. 1(b)): i.e. the laminar separation bubble (LSB). Normally this is a threedimensional phenomenon, but in the experiments presented here, since an infinite wing flow condition was reproduced inside the wind tunnel (there was no clearance between the blade section ends and the test section lateral walls), the laminar bubble is assumed as a bidimensional phenomenon (independent of the wing span position). The presence of a LSB introduces two main classes of problems: an airfoil efficiency decrease, mainly due to the airfoil drag increase; a pressure pulsation, eventually present in case of bubble bursting. A wind turbine blade having low efficiency, will show lower energy productions and hence economical losses. A bubble bursting phenomenon, normally related to the presence of a LSB near the airfoil leading edge, induces a pre-stall hysteresis that causes cyclic loads reducing the blade lifetime. Many devices are able to reduce or even avoid the LSB presence; they may be subdivided in two categories: passive and active devices. As example, turbulators belong to the first class and are realized with adhesive tapes, having different shapes, glued onto the airfoil surface. Although very cheap they promote turbulent transition for every flow condition resulting in globally worse performances even when flow separation phenomena are not present. Flow blowing or suction are active control methods that allow very good boundary layer control; unfortunately they are very expensive and space consuming. The method proposed in this paper considers the possibility of introducing sinusoidal disturbances directly inside the boundary layer in order to promote transition and reduce or even avoid the LSB onset: the main advantage of this active control method is its ability to disturb the growing boundary layer only in the desired fluid dynamic conditions. 3. Theoretical background 3.1. Stability theory Boundary layer natural transition from laminar to turbulent flow is a very complex mechanism that develops trough subsequent steps. Initially it is possible to observe bi-dimensional waves, named TollmieneSchlichting waves, whereas downstream it is
(1)
Fig. 2. The stability curves.
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L ¼
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d2 dUe;s n dx
(4)
OrreSommerfeld equation for the linear stability theory is obtained under the main hypothesis that disturbances verify NaviereStokes equations too; they prescribe that disturbances injected into a boundary layer are amplified or damped with regard to their wave number and the local Reynolds number based on the thickness displacement [1,3e5].
Stability curves (Fig. 2(a)) show the existence of a particular Reynolds number (defined as neutral or indifference Reind), below which no frequencies are able to destabilize the laminar boundary layer. The figure also shows that above Reind there are different values of the disturbance wave number alpha able to promote the turbulent transition. The presence of an adverse pressure gradient plays a significant role also on the boundary layer instability and consequently it modifies the useful disturbances frequency ranges. Fig. 2(b) illustrates this behavior by means of the Polhausen
Fig. 3. Reind evaluation.
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Fig. 4. HAWT aerodynamics.
parameter, that is expression of the pressure gradient (a negative value corresponds to an adverse pressure gradient). The Reind value for an airfoil operating at different angles of attack may be evaluated by the cited Polhausen parameter (Fig. 3(a)) or alternatively by the Wazzan curve (Fig. 3(b)); the latter [6] reports transition and indifference curves as function of the shape factor H12 and the local Reynolds number based on the curvilinear abscissa (Res). Values for Polhausen parameter, H12 and Res was obtained, in this work, by using the freeware code XFOIL [7]. The ncrit value used in the en transition criteria was set to the turbulence level of the wind tunnel (ncrit ¼ 5.5 for the 0.3% wind tunnel corresponding turbulence value measured by a CTA single wire probe). Tested airfoil curves, obtained for angles of attack of þ2, þ4 and þ6 (Fig. 3(c)), intersect the indifference curve at values corresponding to different chord positions: i.e. for a ¼ 2 the indifference point is located at about 0.3chord from the leading edge, for an angle of 4 at 26%chord ca. and for a ¼ 6 at 20%chord. Because of the M.E.M.S. is placed between 16%chord and 24%chord this theoretical approach suggests that disturbances generated by the M.E.M.S. should be effective only for angles greater than þ4 ; as a matter of fact a disturbance introduced upstream the indifference point will be damped by the favorable pressure gradient.
a single blade element (BEM Theory) it is possible to observe that torque and thrust coefficients are dependent on the lift and drag forces acting on the airfoil constituting the blade element (Fig. 4). The wind speed acting on the airfoil (Vrel) is resulting by the vector sum of the incoming wind velocity VN and the rotational velocity ur. Aerodynamics forces, i.e. Lift and Drag, acting on the airfoil are related to Vrel (Eq. (6)); their vectorial sum can be decomposed along and normally the incoming wind direction, respectively producing the thrust and torque loads on the wind turbine (Eq. (6)).
dT ¼ BNdr
(8)
3.2. Wind turbine aerodynamics
dM ¼ rBAdr
(9)
A horizontal axis wind turbine has a power production directly related to the lift capacity of its blades. Analyzing forces acting on
An active control of the airfoil boundary layer may ensure a lift increase and a drag decrease, so it is possible to derive from Eq. (8),
9 1 2 = L ¼ Cl rVrel cðrÞ > 2 1 2 > cðrÞ ; D ¼ Cd rVrel 2 N ¼ Lcosf þ Dsinf A ¼ Lsinf Dcosf
sðrÞ ¼
(5)
cðrÞB ðwhere B is the number of bladesÞ 2pr
Fig. 5. The UNIVPM aeronautical wind tunnel.
(6)
(7)
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Fig. 6. The IR facility setup.
a lower thrust on the wind turbine and a higher torque at the electric generator shaft. Globally an active boundary layer control method may ensure higher annual energy production. 4. Experimental apparatus WT1 tested airfoil was designed as root blade airfoil for a small wind turbine having a nominal power of 3.5 kW. Airfoil maximum
Fig. 8. Standard deviation analysis.
thickness is 19% of chord length, located at 35%chord. Maximum camber is 3.5%chord and is located at 48%chord. A large airfoil thickness, required for structural needs, typically involves significant adverse pressure gradients that may induce a laminar separation
Fig. 7. M.E.M.S. placement.
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Fig. 9. View factor analysis.
bubble located on the airfoil rear side; besides the airfoil behavior will be characterized by a smooth trailing edge stall.
test section dimensions [cm]: 62(W) 38(H) 150(L) ; inlet test section area contraction ratio: 4.65; max velocity: 38 m/s; mean turbulence in the inlet section: 0.3%; fan power 5.5 kW, inverter controlled.
Both aerodynamic tests and infrared visualizations are carried out in the open circuit subsonic wind tunnel available at the Department of Energetic (Fig. 5). Flow velocity may be varied from 0 to 35 m/s controlling the fan motor RPM with an AC inverter. Turbulence level, measured by means of a hot wire anemometer, is lesser than 0.3%. The WT1 wing section is placed vertically in order to have a ratio between the test section width and the airfoil chord larger than 3:1; this allows to avoid flow blockage even at high angles of attack (WT1 airfoil shows a blockage section lesser than 5% up to angles of attack of 14 ). Velocity and turbulence measurements are carried out with a Dantec CTA system equipped with a pneumatic calibration unit; this unit is used before every measurement session in order to reduce the velocity overall error, estimated as 0.08 m/s. Pressure measurements, used to compare thermographic results and to validate XFOIL results, are carried out with a Druck LPM 9481 differential pressure transducer having an operating range between 10 Pa and 1000 Pa and a typical error less than 0.1% FS BSL. In order to graph the Cp pressure coefficient distribution on the airfoil, this device is sequentially connected to all the pressure taps drilled in the middle section of the WT1 wing. A three axes load balance is used to measure Lift, Drag and Moment values at different angles of attack and Reynolds numbers.
A Peltier refrigerant system is implemented to control load balance extensimeters temperature; this allows to reduce drift errors and to obtain the subsequent overall errors: Lift 0.0377 N; Drag 0.0706 N; Moment 0.0028 Nm. Infrared facilities include a FLIR SC3000 IR camera, a ZnSe IR window and a Low-Voltage High-Current power supplier. A thin aluminum foil is used to metallize the tested wing section (Fig. 6(a)). IR camera is equipped with a 320 240 QWIP sensor array and with a 20 lens that performs a 20 15 field of view with a minimum focus distance of 0.3 m; reported thermal sensitivity is 0.03 C at 30 C.
Fig. 10. The energy balance approach.
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The IR camera is placed outside of the wind tunnel at a distance of 0.59 m from the wing section (Fig. 6(b)): the spatial resolution on the airfoil upper surface is 0.65 mm. ZnSe IR window is 150 mm large, 50 mm width, and 4 mm thick; it allows to observe an area covering all the airfoil length. The IR window transmissivity coefficient, inside the IR camera range 8 O 9 mm, is estimated by using a reference black body and its value is 0.74. Electric power supplier produces controlled AC low voltages (about 2 V) and high currents (about 90 A) used to heat to the wing metallic coating; a NI acquisition system is employed to measure Voltage and Current values during tests. Airfoil aluminum coating, 25 mm thick, allows a uniform Joule heating once AC current is supplied; a dull black paint is applied to the aluminum coating to get a known emissivity value of 0.94. Mechanical disturbances are introduced directly inside the boundary layer by a M.E.M.S. flush mounted on the wing upper surface. The cradle for supporting M.E.M.S. and lining up the vibrating surface to the airfoil
profile is placed between 16%chord and 24%chord (Fig. 7). An HP signal generator controls the M.E.M.S. power supplier in order to generate a variable sinusoidal current; several tests (results are presented in the next pages) show that the most effective frequency is 170 Hz. A very interesting data is the low power consumption used to supply the M.E.M.S.; i.e. a power of 34 mW is used. 5. Thermographic approach Two main hypotheses are introduced to relate the airfoil upper surface temperature distribution to the kinematic boundary layer: forced convection and Prair x 1. Having a boundary condition of fixed heat flow, the temperature distribution is directly related to the convective heat transfer coefficient that in turn depends on the flow condition. In the case of a fully attached boundary layer the following situations could be present:
Fig. 11. Thermographic results quantitative analysis.
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A developing laminar boundary layer: this will be revealed by a downstream increasing temperature. The thickening of the thermal boundary layer induces a temperature wall gradient lowering and with a fixed heat flux wall condition this produces a temperature increase. A transition from a laminar to a turbulent condition: this will be observed as a local temperature decrease due to the higher turbulent convective heat transfer coefficient with respect to the laminar one. A developing turbulent boundary layer: this will have a behavior similar to a developing laminar boundary layer, but it will have different absolute values of the surface temperature and convective heat transfer coefficient. The presence of a LSB modifies the boundary layer growth and so the thermal distribution on the surface; a laminar bubble that, as previously referred, is a 3D volume of slow recirculating flow, has a very low convective heat transfer coefficient. This will be revealed as a zone having a local temperature increase followed by an abrupt temperature decrease corresponding to the flow jet impingement in the reattachment point. A proprietary image analysis camera software was used to acquire IR images and a customized Matlab
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code was subsequently employed to carry out the post processing procedure. Standard deviation analysis, performed for every acquisition and for the whole sensor array (Fig. 8(a)), showed that a camera adjusting must be performed before every measurement to reduce the sensors thermal drift and that at least 50 images (Fig. 8(b)) must be acquired to obtain an almost constant standard deviation value. Thermographic analyses were performed by sampling 100 frames at 50 Hz for every angle of attack and an averaging procedure was employed to obtain the single thermogram to be analyzed. In addition, the presence of the ZnSe IR window made necessary to carry out thermogram subtractions to avoid Narcissus effect and to reduce background thermal noise. A subtraction between recorded thermograms and a corresponding background image, acquired without airfoil heating, was implemented for every airfoil angle of attack; this procedure was used both for tests with M.E.M.S. off and M.E.M.S. on. Airfoil was observed with the IR camera at different angles of attack; this induces variable geometrical view factors and an error evaluation procedure is so implemented inside the MatlabÓ code to quantify these errors. The heating uniformity and the ratio between the real view factor and an equivalent view factor of a flat plate inclined at the same angle were measured. Fig. 9(a)
Fig. 12. M.E.M.S. effect analysis by pressure distribution measurements.
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reports the ratio between the local temperature and the maximum temperature revealed in absence of wind flow: it shows a good heating uniformity both for negative and positive angle of attack. Fig. 9(b) shows instead low view factor ratios near the leading edge (up to 5% of the chord); as a consequence this area is not considered in the subsequent Stanton number analysis. Thermograms obtained by the subtraction procedure were analyzed using an energy balance approach (Fig. 10). This procedure takes into account longitudinal heat conduction, radiative heat transfer, convective heat transfer and volumetric heat generation by Joule effect The main hypotheses are an adiabatic boundary condition under the aluminum coating, that was obtained by filling the wind section body with an insulating material, and a bi-dimensional flow behavior, that allows to neglect the transverse heat conduction. This is guaranteed by the infinite wing section assumption. The unknown convective heat transfer coefficient is obtained by solving numerically the equation:
GDxDys þ
that extends from the middle part of the airfoil downstream toward the trailing edge. Reattachment point is difficult to localize in the case of tests with M.E.M.S. turned off, because the LSB extends up to the airfoil section where there are not present pressure taps (i.e. downstream 80%chord); pressure taps absence is due to the small airfoil thickness in that area, that does not physically allow the insertion of tubes used to sense the static pressure. Cp measurements with M.E.M.S. turned on clearly show its effect on the pressure behavior with a steep pressure recovery downstream the plateau. However the M.E.M.S. efficiency varies sensibly at different angles: for values grater than 4 there is marked suction peak increase that underlines a significant boundary layer change and this causes a lift increase. By referring to the indifference point position, previously discussed in the theoretical background section, it could be better understand why the M.E.M.S. effect is more pronounced for those angles that show an indifferent point position upstream the M.E.M.S. location. These results confirm the possibility to use this approach in
Tðiþ1;jÞ þ Tði1;jÞ 2Tði;jÞ Tði;jþ1Þ þ Tði;j1Þ 2Tði;jÞ þ Dx Dy lsDy lsDx
þ hDxDy TNTi;j þ sS:B:
4 T4 Tsurr ði;jÞ
13 1 1 3surr þ þ 3DxDy F12 þ DxDy 3surr Asurr
¼ 0 ð10Þ
and then it is graphed along with the local Stanton number. This is calculated by using the relation
Stx ¼
hx
rN Cp Ue;s
(11)
where the Ue,s value is obtained by means of the XFOIL code with the inviscid condition input. Pressure distribution analysis confirms thermographic results [8,9]. It is possible to observe (Fig. 11(a) and (b)) that: the minimum h or St values corresponds to the transition point (a zero for the first derivative); the separation point is associated to an inflection point just before the transition (a second derivative null value); the turbulent reattachment point corresponds to the relative maximum (a zero for the first derivative) subsequent the transition point (i.e. the turbulent reattachment behaves like a jet impingement with a high heat exchange). 6. The experimental results 6.1. Pressure distribution analysis Pressure distribution analysis is carried out to verify the LSB presence and to obtain lift and drag coefficients for the tested airfoil. The LSB corresponds to a constant pressure area inside a pressure recovery zone: as example the pressure distribution for an a value of 2 (Fig. 12(a)) shows a plateau starting at 48%chord ca. (separation point) up to 68%chord ca. and a subsequent steep recovery up to 80%chord ca. (turbulent reattachment point). Pressure analysis is useful to study the bubble position but some limits related to the expensive setup arise: these ones are mainly due to the pressure taps realization and to long measurement time needed for the pressure scanning. As a matter of fact low pressure values, typical of low velocities, require a very accurate pressure transducer but the typical simultaneous scanners are not very sensitive; as a result a single sensor is used and the pressure taps are scanned manually and sequentially. Pressure distribution analysis shows also problems into identifying thin LSBs that induce small pressure changes. The WT1 airfoil is tested for angles of attack between 4 and 16 with steps of 2 and for Reynolds between 100 k and 150 k. Fig. 12(a) and (b) show the presence of a large LSB
Fig. 13. M.E.M.S. effects on the airfoil behavior.
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order to choose the better electromechanical actuator position during the wind turbine blades design phase. Analysis of the lift coefficient, derived from the pressure distribution (Fig. 13(a)), confirms the good effect of mechanical disturbances; results obtained without a M.E.M.S. disturbance show a clear curve bending from 0 up to an angle of 12 , followed by a great lift increase at 14 . The curve bending is due to the presence of a large bubble on the airfoil extrados while the abrupt increase at 14 corresponds to a sudden bubble movement and to a bubble length reduction. Once the M.E.M.S. is turned on the Cp distribution analysis shows that the lift coefficient curve, between 0 and 10 , has a lower bending; this is due to a different bubble behavior and corresponds to higher lift values. 6.2. Wake rake analysis M.E.M.S. effects on the airfoil wake and consequently on the drag coefficient, were analyzed by means of the wake rake device. Experimental data clearly show a wake extension decrease and also a down movement of the slowed velocity zone; the former results underline a strong drag decrease resulting from the momentum balance on the control volume. The latter underline an overall change of the flow field around the airfoil section (Fig. 13(b)). 6.3. Thermographic results
analysis; besides formers allow an even more detailed LSB characterization. The transition point locates near the point of higher LSB normal thickness; due to the slow recirculating flow inside the bubble and to the higher bubble vertical extension, the transition point qualitatively corresponds to the higher temperature values inside the warmer zone (Fig. 14(a)) and so to the lower St number and h coefficient values. The laminar separation, corresponding to a boundary layer development singularity, is identified as the inflection point upstream the transition and, in the pressure analysis, this corresponds to the point where the pressure plateau begins. The separation point position could be derived indifferently by the local h coefficient distribution or the local Stanton number distribution even if they have different numerical values. Turbulent reattachment point is instead more difficult to localize if the impingement effect is not significant (as example for thin bubbles). In this case it is worthwhile noting that the local Stanton distribution generally does not show a relative maximum downstream the transition but shows a convexo-concave behavior. The local h coefficient trend shows instead a relative maximum also for a thin bubble and so it is more useful when the reattachment point is difficult to localize. The mechanical disturbance effect, for angles greater than 4 , is extremely clear to identify by observing thermograms reported in Figs. 14(a) and 15. Bubble presence is not avoided but there is a more pronounced wall cooling downstream the flow reattachment point; the effect is a change in the global flow arrangement around the airfoil and a greater pressure suction
As verified in previous research works [8e12], thermographic results are in good agreement with the pressure distribution
Fig. 14. M.E.M.S. effects on the airfoil behavior.
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Fig. 15. M.E.M.S. effects derivative analysis.
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Fig. 16. LSB quantitative analysis.
peak, as revealed by the pressure analysis. Particularly for the 6 angle, the transition point (Fig. 15(a) and (b)) does not move, but there is a delayed laminar separation and consequently a downstream turbulent reattachment point movement; separation delay is associated to the additional energy, supplied to the flow by the M.E.M.S., that allows to better contrast the adverse pressure gradient. Extending thermographic analysis to all the tested angles of attack is possible to characterize the bubble dimension and position. Graphs reported in Fig. 16(a) and (b) substantially show a negligible M.E.M.S. effect for angles less than 2 , where there is only a change in the reattachment point position (the pressure analysis also shows only a change in the pressure recovery
behavior). At 4 there is a little downstream movement of the separation and transition points that does not reflects in a substantial dimension change but that reflects in an appreciable pressure suction peak increase. At 6 there are both a bubble dimension reduction and a great pressure suction peak increase that induce a lift increase. 7. Conclusions The use of mechanical disturbances on airfoils operating at low Reynolds numbers seems to be a good boundary layer active control system. The point where disturbances are to be introduced should be chosen by performing a preliminary analysis of the indifference
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point position: the electromechanical device should be located downstream this point. The thermographic approach performed in addition to classical techniques, such as the pressure distribution analysis, the load balance measurements and the wake analysis, is very useful to characterize the bubble behavior and the effects of this phenomenon on the flow field. With reference to small wind turbine application field, it is possible to say that the use of electromechanical systems is a very interesting approach aimed to the reduction of local separation phenomena and the consequent increase of the wind turbine energy production. It is furthermore worthwhile reminding that M.E.M.S. power consumption in the performed tests is just 0.034 W. Despite the low value of energy at stake, a doubling of the airfoil lift coefficient is evident.
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[4] C. Haggmark, Investigation of Disturbance Developing in a Laminar Separation Bubble Flow Tech. Rep.. Department of Mechanics, S-100 44 Stockholm, Sweden, 2000 [5] W. Saric, H. Reed, E. Kerscen, Boundary layer receptivity to freestream disturbances, Annual Review of Fluid Mechanics 34 (2002) 291e319. [6] A. Wazzan, C. Gazley, A. Smith, H-R/x/ method for predicting transition, AIAA Journal 19 (1981) 810e812. [7] M. Drela, Low Reynolds Number Aerodynamics, , In: Lecture Notes in Engineering, vol. 54. Springer Verlag, 1989. [8] S. Montelpare, R. Ricci, A thermographic method to evaluate the local boundary layer separation phenomena on aerodynamic bodies operating at low Reynolds number, International Journal of Thermal Sciences 43 (2004) 315e329. [9] R. Ricci, S. Montelpare, E. Silvi, Study of acoustic disturbances effect on laminar separation bubble by IR thermography, Experimental Thermal and Fluid Science 31 (2007) 349e359. [10] G. Cesini, R. Ricci, S. Montelpare, E. Silvi, A thermographic method to evaluate laminar bubble phenomena on airfoil operating at low Reynolds number. in: D. Balageas, G. Busse, G.M. Carlomagno, S. Svaic (Eds.), Quantitative InfraRed Thermography 6 (2002), ISBN 953-6313-50-2, pp. 101e107 Dubrovnik, Croatia. [11] R. Ricci, F. Angeletti, S. Montelpare, A. Secchiaroli, Thermographic analysis of acoustic disturbance effects on laminar separation bubble. in: D. Balageas, G. Busse, G.M. Carlomagno, S. Svaic (Eds.), Quantitative InfraRed Thermography 8. ITC-CNR, Padova, Italy, 2006, ISBN 953-6313-50-2, pp. 101e107. [12] R. Ricci, S. Montelpare, Analysis of boundary layer separation phenomena by infrared thermography e use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects, QIRT Journal, Lavoisier Edn. 6 (2009) 101e125.