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Laser Doppler velocimeter measurements of separated shear layers on bluff bodies
Journal of Wind Engineering and Industrial Aerodynamics 74—76 (1998) 455—461
Laser Doppler velocimeter measurements of separated shear layers on bluff bodies R.E. Akins!,*, T.A. Reinhold" ! Department of Physics and Engineering, Washington and Lee University, Lexington, VA 24450, USA " Department of Civil Engineering, Clemson University, Clemson, SC 29631, USA
Abstract Greater understanding of the mechanism of separation and reattachment of shear layers around bluff bodies and the separated regions associated with these shear layers is necessary to advance the knowledge of fluid—structure interactions. These mechanisms are particularly important in wind engineering in determining design pressures on structures. In almost all instances the maximum design pressure occurs in regions of separated flow. The details of the flow in these separated regions and the criteria for physical modeling of the flows in these regions are not well understood. Measurements have been conducted using a laser Doppler velocimeter (LDV) to examine the structure of separated shear layers and the associated regions of separated flow. A LDV is an important tool for this application. A LDV system is capable of measuring reversing flows and consequently high local turbulent intensities. Thermal anemometry does not allow measurements under these conditions. When making measurements with a LDV in air, it is necessary to seed the flow with particles. Seeding is a challenge in applying these measurement techniques to flows in boundary layer wind tunnels. Efforts to understand the basic mechanisms affecting pressure fluctuations and associated extreme pressures on a bluff body will be advanced by the ability to measure the flow in separated shear layers and within separated regions associated with these shear layers. The use of a LDV system provides a additional tool to explore this complex region. ( 1998 Elsevier Science Ltd. All rights reserved. Keywords: Bluff body; Separated flow; Laser doppler velocimeter; Laser doppler anemometer
1. Introduction Many aspects of the flow past bluff bodies must be inferred from measures of the incident flow and a resultant effect on the structure. Force and pressure coefficients
* Corresponding author. 0167-6105/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 8 ) 0 0 0 4 1 - 5
456
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
are generally based on an upstream reference wind speed and turbulence properties. Substantial work has been undertaken to relate surface pressures and associated forces with upstream flow properties [1,2]. Efforts to relate peak pressures often associated with critical design loads to incident flow properties have met with less success [3]. Clearly the physical mechanisms which cause these extreme pressures are a function of the incident flow, of how this flow interacts with the structure, and of the turbulence introduced by the effects of the structure. Any new insights into the mechanism of fluid structure interaction in a turbulent flow must come from a better understanding of the details of the separated flow, the properties of the separated shear layers and the flow in the regions between these shear layers and the structure. While computational methods may help address these issues in the future, at present there is not sufficient knowledge about the details of such flows to even develop a strategy to validate numerical models. Indeed, the ability of a wind-tunnel simulation to investigate and insure adequate simulation of the details of separated and reattached flow is uncertain. This paper describes an initial effort to use a laser Doppler velocimeter (LDV) system to measure the details of the flow in the region of a separated shear layer developed at the leading edge of a surface-mounted prism located in a wind tunnel used to develop a simulated atmospheric flow. Measurements of the mean and fluctuating velocities near the separation were completed using the LDV system. 2. Background Minson [4] and Marwood [5] have used a LDV system to examine details of the flow around model buildings. Minson [4] concentrated on the details of the mean flow near details of the building. Marwood [5] investigated the conical roof vortices often observed near the leading edge of low-rise structures with oblique incident wind. Neither of these investigations attempted to measure the details of the separated shear layers nor the overall properties of the flow in a separated regions. Marwood was able to relate the peak pressures associated with the conical vortices with the LDV measurements of the vortex. The thrust of this work is to document the structure of the separated flow in sufficient detail to develop a strategy to determine the effects of the upstream flow and building geometry on the flow within the shear layer and in the separated region below the shear layer. Recent efforts to correlate incident flow with pressures in separated regions reported by Hajj and Tieleman [3] used a reference velocity measured upstream of the model. Any mechanism which involves the effects of any flow properties caused by the separation of the flow and the subsequent behavior of the shear layer are not included in the approach used by Hajj and Tieleman. 3. Approach The LDV system allows measurement of reversing flows and locally large turbulent intensities. The probe is nonintrusive and the measuring region is not disturbed. In
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
457
order to use an LDV system in a boundary-layer wind tunnel, the flow must be seeded with particles. Previous efforts concentrated on seeding a small region [4,5]. A small aerosol generator was used which produced particles near one micron in diameter. A mixture of water and glycerine provided particles which persisted longer than just water droplets. In this work, the goal was to seed a much larger region in a large, 6 m2 cross-section, boundary layer wind tunnel. Initial efforts to use aerosol generators and humidifiers in conjunction with a manifold on the flow of the tunnel resulted in some regions with seeding adequate to obtain data rates with the LDV system near 500 Hz. This seeding arrangement was not repeatable and the regions of the flow with reasonable data rates was limited. The aerosol was emitted from the manifolds on the floor of the tunnel with a small, but measurable velocity which will have a minor effect on the incident flow. It was difficult to seed separated regions with this arrangement. A commercial theatrical smoke generator which produces an alcohol-based smoke was used at the entrance to the wind tunnel, an open circuit tunnel. This arrangement seeded the entire volume of the tunnel and within a few minutes the entire room containing the wind tunnel. Data rates of 2—3 kHz were consistently attained in the undisturbed flow of the boundary layer. Near models and in separated regions data rates of over 500 Hz were attained. This system was used for all results reported in this paper. Three components of velocity were measured using a two-component probe, TSI Model 9253-350. This probe has a nominal diameter of 0.08 m, a length of 0.46 m and a focal length of 0.35 m. The probe was mounded on a traverse in a horizontal position to measure longitudinal and vertical velocities and in a vertical position to measure longitudinal and lateral velocities. A second probe with a single component was permanently mounted to measure a reference longitudinal velocity. This probe was located 1.3 m upwind of the model and 0.5 m off the centerline of the model. This reference velocity was used to relate all measurements to a common value. Data were collected on the three channels for a period of approximately 30 s based on the data rates. Random sampling was employed such that each component independently measured a velocity when a particle was in the measuring volume. The mean and standard deviation of the signals was computed using FIND [6]. Future work will examine the probability density and spectral density of these signals as well. A LDV system provided data only when particles are in the measuring volume and data reduction algorithms must include provisions to deal with unequally spaced data. LDV measurements are much more difficult to obtain than corresponding measurements with thermal anemometry. It will only be possible to obtain such measurements in a limited range of conditions.
4. Boundary layer simulation These measurements were completed in the boundary layer wind tunnel of the Wind Load Test Facility of the Department of Civil Engineering, Clemson University. The tunnel is open return with the fans in the upwind position. A settling chamber with a grill and screens provides a uniform incident flow. The test section has a cross
458
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
section of 3 m by 2 m and a working length of 16 m. Boundary layers are created with combinations of trips, spires and roughness elements on the floor of the tunnel. The flow used in this study was developed for use in simulations of low-rise structures and is representative of flow over open terrain at a nominal scale of 1 : 50. The measurements reported in this paper are intended to demonstrate a measurement technique, not a specific flow condition. As such this boundary layer should be considered representative of atmospheric flows in general, not a specific flow. The boundary layer had a power-law exponent of 0.16, longitudinal turbulence at the height of the prism of 0.16, and longitudinal integral scale of 0.64 m. Monroe [7] provides a detailed description of this simulation.
5. Results A single isolated rectangular prism (¼"0.13 m, D"0.13 m, H"0.26 m) was mounted with the incident flow normal to the surface. A detailed mapping of the flow in the vicinity of the prism was conduced and this paper reports the mean and fluctuating velocities over the top or roof of the prism. The goal was to map the location of the separated shear layer and to determine the turbulence intensity in the shear layer and in the separated region below the shear layer. A coordinate system with x alongwind, y across wind, and z in the vertical direction was used. The reference probe was located at an elevation of 0.9H, upwind and to the side of the building. This probe provided a reference velocity measured with the same system as the probe used to traverse the region on the roof of the prism. All dimensions are normalized with respect to the width of the prism, ¼. All velocities are normalized with the longitudinal velocity at the reference location. All turbulence intensities are the ratio of the standard deviation of the local velocity to the mean velocity at the reference location. Table 1 includes the mean velocities in the u, v, w (longitudinal, lateral, and vertical directions) for the region above the model and on the centerline of the flow. These values are all normalized with respect to the reference longitudinal velocity measured at the reference location. Measurement locations form a grid from the leading edge to the trailing edge of the model and at five locations above the model, z/¼"0.05, 0.1, 0.2, 0.3, 0.4. The separated shear layer begins at the leading edge of the top of the prism. An arbitrary definition of location of the shear layer is adopted as the ratio of 0.5 of the reference velocity. The shear layer is located at z/¼ from 0.2 to 0.3 at x/¼ of 0.8, near the rear of the prism. For this building shape and incident flow there is clearly no reattachment of the shear layer along the top of the prism although the shear layer is moving closer to the surface toward the trailing edge. Reversed flow is evident near the leading edge at the bottom of the separated region (x/¼"0.2 to 0.4, z/¼"0.05). Table 2 includes the same information at a location y/¼"0.2 off the centerline. The normalized velocities in the longitudinal direction are very similar to those on the centerline, Table 1. A marked difference in the normalized vertical velocity is evident between the two locations. At the leading edge, x/¼"0.0, the normalized vertical velocities are similar. Downstream of the leading edge, the normalized vertical velocities are much less at the y/¼"0.2 location. Based on a normalized longitudinal
459
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
Table 1 Normalized velocities along centerline of prism, y/¼"0.0. All values are normalized with respect to the reference velocity at 0.9H z/¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
1.01 1.00 0.95 0.88 0.80
!0.01 0.00 !0.02 !0.02 !0.01
0.35 0.47 0.62 0.80 1.06
1.06 1.08 1.05 0.31 !0.14
0.04 !0.02 !0.02 0.07 0.06
0.35 0.42 0.48 0.26 0.14
1.17 1.03 0.73 0.17 !0.13
!0.04 0.00 0.05 0.10 0.13
0.24 0.34 0.31 0.19 0.14
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
x/¼"0.8
x/¼"1.0
u
v
w
u
v
w
u
v
w
1.07 0.96 0.70 0.23 0.06
!0.01 0.02 0.07 0.12 0.18
0.16 0.30 0.26 0.21 0.14
1.04 0.93 0.74 0.41 0.30
0.00 0.02 0.07 0.15 0.21
0.14 0.21 0.23 0.18 0.12
1.02 0.87 0.68 0.53 0.47
0.02 0.05 0.07 0.14 0.16
0.14 0.20 0.18 0.17 0.12
Table 2 Normalized velocities, y/¼"0.2. All values are normalized with respect to the reference velocity at 0.9H z¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
1.03 1.01 0.94 0.95 0.86
!0.04 !0.07 !0.07 !0.11 !0.12
0.37 0.42 0.53 0.79 0.97
1.06 1.04 1.03 0.41 !0.02
!0.03 !0.05 !0.07 0.05 0.08
0.15 0.15 0.16 0.06 0.05
1.05 1.00 0.77 0.32 0.03
!0.03 !0.05 0.03 0.11 0.17
0.10 0.10 0.07 0.03 0.02
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
x/¼"0.8
x/¼"1.0
u
v
w
u
v
w
u
v
w
1.04 0.92 0.68 0.42 0.23
0.01 0.02 0.08 0.18 0.02
0.05 0.04 0.02 0.01 0.02
0.96 0.89 0.75 0.49 0.36
0.02 0.06 0.12 0.19 0.23
0.02 0.01 0.01 0.00 0.01
1.04 0.91 0.79 0.63 0.51
0.02 0.05 0.12 0.18 0.20
0.00 0.01 0.01 0.01 0.02
velocity of 0.5 as the location of the shear layer, both data sets show the shear layer closer to the surface at the trailing edge of the model. The turbulence intensities for the same measurement locations are shown in Tables 3 and 4. These turbulence intensities are based on the reference longitudinal
460
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461 Table 3 Turbulence intensities along centerline of prism, y/¼"0.0. All values are normalized with respect to the reference velocity at 0.9H z/¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
0.13 0.12 0.12 0.12 0.15
0.13 0.15 0.13 0.13 0.15
0.11 0.14 0.14 0.15 0.18
0.12 0.12 0.22 0.43 0.25
0.13 0.13 0.15 0.27 0.22
0.15 0.14 0.20 0.23 17
0.14 0.24 0.41 0.41 0.28
0.13 0.16 0.24 0.28 0.27
0.16 0.24 0.25 0.22 0.19
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
u 0.18 0.30 0.42 0.38 0.32
v 0.14 0.19 0.25 0.30 0.33
x/¼"0.8 w 0.20 0.27 0.26 0.23 0.21
u 0.19 0.30 0.41 0.40 0.36
v 0.16 0.20 0.27 0.32 0.34
x/¼"1.0 w 0.23 0.27 0.26 0.23 0.20
u 0.19 0.28 0.36 0.36 0.36
v 0.18 0.20 0.25 0.31 0.32
w 0.24 0.26 0.25 0.23 0.19
Table 4 Turbulence intensities, y/¼"0.2. All values are normalized with respect to the reference velocity at 0.9H z/¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
0.12 0.12 0.13 0.13 0.13
0.14 0.13 0.14 0.15 0.15
0.12 0.15 0.12 0.15 0.13
0.12 0.12 0.20 0.42 0.30
0.16 0.13 0.14 0.27 0.26
0.07 0.08 0.08 0.12 0.10
0.12 0.23 0.37 0.41 0.34
0.13 0.15 0.21 0.28 0.29
0.08 0.08 0.12 0.14 0.13
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
x/¼"0.8
x/¼"1.0
u
v
w
u
v
w
u
v
w
0.17 0.31 0.38 0.42 0.38
0.16 0.17 0.22 0.30 0.31
0.08 0.09 0.12 0.13 0.12
0.23 0.30 0.35 0.40 0.38
0.16 0.19 0.23 0.28 0.30
0.09 0.11 0.12 0.14 0.13
0.23 0.26 0.33 0.38 0.34
0.17 0.19 0.22 0.28 0.27
0.09 0.11 0.12 0.13 0.12
velocity measured in the approach flow at 0.9H. Another possible definition of the shear layer could be the location of the maximum longitudinal turbulence intensity. This definition would locate the shear layer at a location of z/¼ of 0.1 to 0.2 below the location based on the maximum longitudinal velocity. A dramatic increase of all three
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
461
components of turbulence between the leading edge, x/¼"0.0, and the next measurement location, x/¼"0.2, is evident for both data sets. The turbulence intensity increases by a factor of two to three to values exceeding 0.4. This change is very local and the values return to the nominal free-stream values at z/¼"0.4. A marked difference in the vertical values is evident between the two locations, a trend consistent with the mean velocity values.
6. Conclusions A LDV system is capable of measuring the details of the separated shear layers emanating from a bluff body and the structure of the turbulence below the shear layer. The width of the shear layer based on either the mean velocities or the turbulence intensities is of the order of 10% of the width of the prism. The interaction of the mean flow with the shear layer and the effect of the flow beneath the shear layer of the fluctuating pressures on the surface of the bluff body are clearly areas for further study using the LDV system. Techniques to compute spectral density functions of unequally spaced time series may be applied to examine the frequency content of the velocity fluctuations in the separated regions. Wavelet techniques may be applied to determine the correlation between the fluctuating velocity and pressures. These phenomenon will be best studied in an experimental situation. The prospect for computational fluid mechanics to predict behavior in the separated regions are not good. Future experiment using LDV systems will be time consuming and expensive and must be planned with care.
References [1] R.E. Akins, Wind pressures on buildings, Ph.D. Dissertation, Department of Civil Engineering, Colorado State University, USA, 1976. [2] H.W. Tieleman, R.E. Akins, The effect of incident turbulence on the surface pressures on surfacemounted prisms, J. Fluids Struct. 10 (4) (1996) 367—388. [3] M.R. Hajj, H.W. Tieleman, Application of wavelet analysis to incident wind in relevance to wind loads on low-rise structures, J. Fluids Eng., ASME, 118 (4) (1996) 874—875. [4] A.J. Minson, Use of laser doppler anemometer measurements near model buildings to determine wind loading on building attachments, Ph.D. Dissertation, Department of Engineering Science, University of Oxford, UK (1993). [5] R. Marwood, An investigation of conical roof edge vortices, Ph.D. Dissertation, Department of Engineering Science, University of Oxford, UK (1996). [6] TSI, Inc., FIND for Windows, Reference Manual, TSI Incorporated, St. Paul, MN, USA, 1996. [7] J.S. Monroe, Wind tunnel modeling of low rise structures in a validated open country simulation, MS Thesis, Department of Civil Engineering, Clemson University, USA, 1996.
Laser Doppler velocimeter measurements of separated shear layers on bluff bodies R.E. Akins!,*, T.A. Reinhold" ! Department of Physics and Engineering, Washington and Lee University, Lexington, VA 24450, USA " Department of Civil Engineering, Clemson University, Clemson, SC 29631, USA
Abstract Greater understanding of the mechanism of separation and reattachment of shear layers around bluff bodies and the separated regions associated with these shear layers is necessary to advance the knowledge of fluid—structure interactions. These mechanisms are particularly important in wind engineering in determining design pressures on structures. In almost all instances the maximum design pressure occurs in regions of separated flow. The details of the flow in these separated regions and the criteria for physical modeling of the flows in these regions are not well understood. Measurements have been conducted using a laser Doppler velocimeter (LDV) to examine the structure of separated shear layers and the associated regions of separated flow. A LDV is an important tool for this application. A LDV system is capable of measuring reversing flows and consequently high local turbulent intensities. Thermal anemometry does not allow measurements under these conditions. When making measurements with a LDV in air, it is necessary to seed the flow with particles. Seeding is a challenge in applying these measurement techniques to flows in boundary layer wind tunnels. Efforts to understand the basic mechanisms affecting pressure fluctuations and associated extreme pressures on a bluff body will be advanced by the ability to measure the flow in separated shear layers and within separated regions associated with these shear layers. The use of a LDV system provides a additional tool to explore this complex region. ( 1998 Elsevier Science Ltd. All rights reserved. Keywords: Bluff body; Separated flow; Laser doppler velocimeter; Laser doppler anemometer
1. Introduction Many aspects of the flow past bluff bodies must be inferred from measures of the incident flow and a resultant effect on the structure. Force and pressure coefficients
* Corresponding author. 0167-6105/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 8 ) 0 0 0 4 1 - 5
456
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
are generally based on an upstream reference wind speed and turbulence properties. Substantial work has been undertaken to relate surface pressures and associated forces with upstream flow properties [1,2]. Efforts to relate peak pressures often associated with critical design loads to incident flow properties have met with less success [3]. Clearly the physical mechanisms which cause these extreme pressures are a function of the incident flow, of how this flow interacts with the structure, and of the turbulence introduced by the effects of the structure. Any new insights into the mechanism of fluid structure interaction in a turbulent flow must come from a better understanding of the details of the separated flow, the properties of the separated shear layers and the flow in the regions between these shear layers and the structure. While computational methods may help address these issues in the future, at present there is not sufficient knowledge about the details of such flows to even develop a strategy to validate numerical models. Indeed, the ability of a wind-tunnel simulation to investigate and insure adequate simulation of the details of separated and reattached flow is uncertain. This paper describes an initial effort to use a laser Doppler velocimeter (LDV) system to measure the details of the flow in the region of a separated shear layer developed at the leading edge of a surface-mounted prism located in a wind tunnel used to develop a simulated atmospheric flow. Measurements of the mean and fluctuating velocities near the separation were completed using the LDV system. 2. Background Minson [4] and Marwood [5] have used a LDV system to examine details of the flow around model buildings. Minson [4] concentrated on the details of the mean flow near details of the building. Marwood [5] investigated the conical roof vortices often observed near the leading edge of low-rise structures with oblique incident wind. Neither of these investigations attempted to measure the details of the separated shear layers nor the overall properties of the flow in a separated regions. Marwood was able to relate the peak pressures associated with the conical vortices with the LDV measurements of the vortex. The thrust of this work is to document the structure of the separated flow in sufficient detail to develop a strategy to determine the effects of the upstream flow and building geometry on the flow within the shear layer and in the separated region below the shear layer. Recent efforts to correlate incident flow with pressures in separated regions reported by Hajj and Tieleman [3] used a reference velocity measured upstream of the model. Any mechanism which involves the effects of any flow properties caused by the separation of the flow and the subsequent behavior of the shear layer are not included in the approach used by Hajj and Tieleman. 3. Approach The LDV system allows measurement of reversing flows and locally large turbulent intensities. The probe is nonintrusive and the measuring region is not disturbed. In
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
457
order to use an LDV system in a boundary-layer wind tunnel, the flow must be seeded with particles. Previous efforts concentrated on seeding a small region [4,5]. A small aerosol generator was used which produced particles near one micron in diameter. A mixture of water and glycerine provided particles which persisted longer than just water droplets. In this work, the goal was to seed a much larger region in a large, 6 m2 cross-section, boundary layer wind tunnel. Initial efforts to use aerosol generators and humidifiers in conjunction with a manifold on the flow of the tunnel resulted in some regions with seeding adequate to obtain data rates with the LDV system near 500 Hz. This seeding arrangement was not repeatable and the regions of the flow with reasonable data rates was limited. The aerosol was emitted from the manifolds on the floor of the tunnel with a small, but measurable velocity which will have a minor effect on the incident flow. It was difficult to seed separated regions with this arrangement. A commercial theatrical smoke generator which produces an alcohol-based smoke was used at the entrance to the wind tunnel, an open circuit tunnel. This arrangement seeded the entire volume of the tunnel and within a few minutes the entire room containing the wind tunnel. Data rates of 2—3 kHz were consistently attained in the undisturbed flow of the boundary layer. Near models and in separated regions data rates of over 500 Hz were attained. This system was used for all results reported in this paper. Three components of velocity were measured using a two-component probe, TSI Model 9253-350. This probe has a nominal diameter of 0.08 m, a length of 0.46 m and a focal length of 0.35 m. The probe was mounded on a traverse in a horizontal position to measure longitudinal and vertical velocities and in a vertical position to measure longitudinal and lateral velocities. A second probe with a single component was permanently mounted to measure a reference longitudinal velocity. This probe was located 1.3 m upwind of the model and 0.5 m off the centerline of the model. This reference velocity was used to relate all measurements to a common value. Data were collected on the three channels for a period of approximately 30 s based on the data rates. Random sampling was employed such that each component independently measured a velocity when a particle was in the measuring volume. The mean and standard deviation of the signals was computed using FIND [6]. Future work will examine the probability density and spectral density of these signals as well. A LDV system provided data only when particles are in the measuring volume and data reduction algorithms must include provisions to deal with unequally spaced data. LDV measurements are much more difficult to obtain than corresponding measurements with thermal anemometry. It will only be possible to obtain such measurements in a limited range of conditions.
4. Boundary layer simulation These measurements were completed in the boundary layer wind tunnel of the Wind Load Test Facility of the Department of Civil Engineering, Clemson University. The tunnel is open return with the fans in the upwind position. A settling chamber with a grill and screens provides a uniform incident flow. The test section has a cross
458
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
section of 3 m by 2 m and a working length of 16 m. Boundary layers are created with combinations of trips, spires and roughness elements on the floor of the tunnel. The flow used in this study was developed for use in simulations of low-rise structures and is representative of flow over open terrain at a nominal scale of 1 : 50. The measurements reported in this paper are intended to demonstrate a measurement technique, not a specific flow condition. As such this boundary layer should be considered representative of atmospheric flows in general, not a specific flow. The boundary layer had a power-law exponent of 0.16, longitudinal turbulence at the height of the prism of 0.16, and longitudinal integral scale of 0.64 m. Monroe [7] provides a detailed description of this simulation.
5. Results A single isolated rectangular prism (¼"0.13 m, D"0.13 m, H"0.26 m) was mounted with the incident flow normal to the surface. A detailed mapping of the flow in the vicinity of the prism was conduced and this paper reports the mean and fluctuating velocities over the top or roof of the prism. The goal was to map the location of the separated shear layer and to determine the turbulence intensity in the shear layer and in the separated region below the shear layer. A coordinate system with x alongwind, y across wind, and z in the vertical direction was used. The reference probe was located at an elevation of 0.9H, upwind and to the side of the building. This probe provided a reference velocity measured with the same system as the probe used to traverse the region on the roof of the prism. All dimensions are normalized with respect to the width of the prism, ¼. All velocities are normalized with the longitudinal velocity at the reference location. All turbulence intensities are the ratio of the standard deviation of the local velocity to the mean velocity at the reference location. Table 1 includes the mean velocities in the u, v, w (longitudinal, lateral, and vertical directions) for the region above the model and on the centerline of the flow. These values are all normalized with respect to the reference longitudinal velocity measured at the reference location. Measurement locations form a grid from the leading edge to the trailing edge of the model and at five locations above the model, z/¼"0.05, 0.1, 0.2, 0.3, 0.4. The separated shear layer begins at the leading edge of the top of the prism. An arbitrary definition of location of the shear layer is adopted as the ratio of 0.5 of the reference velocity. The shear layer is located at z/¼ from 0.2 to 0.3 at x/¼ of 0.8, near the rear of the prism. For this building shape and incident flow there is clearly no reattachment of the shear layer along the top of the prism although the shear layer is moving closer to the surface toward the trailing edge. Reversed flow is evident near the leading edge at the bottom of the separated region (x/¼"0.2 to 0.4, z/¼"0.05). Table 2 includes the same information at a location y/¼"0.2 off the centerline. The normalized velocities in the longitudinal direction are very similar to those on the centerline, Table 1. A marked difference in the normalized vertical velocity is evident between the two locations. At the leading edge, x/¼"0.0, the normalized vertical velocities are similar. Downstream of the leading edge, the normalized vertical velocities are much less at the y/¼"0.2 location. Based on a normalized longitudinal
459
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
Table 1 Normalized velocities along centerline of prism, y/¼"0.0. All values are normalized with respect to the reference velocity at 0.9H z/¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
1.01 1.00 0.95 0.88 0.80
!0.01 0.00 !0.02 !0.02 !0.01
0.35 0.47 0.62 0.80 1.06
1.06 1.08 1.05 0.31 !0.14
0.04 !0.02 !0.02 0.07 0.06
0.35 0.42 0.48 0.26 0.14
1.17 1.03 0.73 0.17 !0.13
!0.04 0.00 0.05 0.10 0.13
0.24 0.34 0.31 0.19 0.14
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
x/¼"0.8
x/¼"1.0
u
v
w
u
v
w
u
v
w
1.07 0.96 0.70 0.23 0.06
!0.01 0.02 0.07 0.12 0.18
0.16 0.30 0.26 0.21 0.14
1.04 0.93 0.74 0.41 0.30
0.00 0.02 0.07 0.15 0.21
0.14 0.21 0.23 0.18 0.12
1.02 0.87 0.68 0.53 0.47
0.02 0.05 0.07 0.14 0.16
0.14 0.20 0.18 0.17 0.12
Table 2 Normalized velocities, y/¼"0.2. All values are normalized with respect to the reference velocity at 0.9H z¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
1.03 1.01 0.94 0.95 0.86
!0.04 !0.07 !0.07 !0.11 !0.12
0.37 0.42 0.53 0.79 0.97
1.06 1.04 1.03 0.41 !0.02
!0.03 !0.05 !0.07 0.05 0.08
0.15 0.15 0.16 0.06 0.05
1.05 1.00 0.77 0.32 0.03
!0.03 !0.05 0.03 0.11 0.17
0.10 0.10 0.07 0.03 0.02
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
x/¼"0.8
x/¼"1.0
u
v
w
u
v
w
u
v
w
1.04 0.92 0.68 0.42 0.23
0.01 0.02 0.08 0.18 0.02
0.05 0.04 0.02 0.01 0.02
0.96 0.89 0.75 0.49 0.36
0.02 0.06 0.12 0.19 0.23
0.02 0.01 0.01 0.00 0.01
1.04 0.91 0.79 0.63 0.51
0.02 0.05 0.12 0.18 0.20
0.00 0.01 0.01 0.01 0.02
velocity of 0.5 as the location of the shear layer, both data sets show the shear layer closer to the surface at the trailing edge of the model. The turbulence intensities for the same measurement locations are shown in Tables 3 and 4. These turbulence intensities are based on the reference longitudinal
460
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461 Table 3 Turbulence intensities along centerline of prism, y/¼"0.0. All values are normalized with respect to the reference velocity at 0.9H z/¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
0.13 0.12 0.12 0.12 0.15
0.13 0.15 0.13 0.13 0.15
0.11 0.14 0.14 0.15 0.18
0.12 0.12 0.22 0.43 0.25
0.13 0.13 0.15 0.27 0.22
0.15 0.14 0.20 0.23 17
0.14 0.24 0.41 0.41 0.28
0.13 0.16 0.24 0.28 0.27
0.16 0.24 0.25 0.22 0.19
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
u 0.18 0.30 0.42 0.38 0.32
v 0.14 0.19 0.25 0.30 0.33
x/¼"0.8 w 0.20 0.27 0.26 0.23 0.21
u 0.19 0.30 0.41 0.40 0.36
v 0.16 0.20 0.27 0.32 0.34
x/¼"1.0 w 0.23 0.27 0.26 0.23 0.20
u 0.19 0.28 0.36 0.36 0.36
v 0.18 0.20 0.25 0.31 0.32
w 0.24 0.26 0.25 0.23 0.19
Table 4 Turbulence intensities, y/¼"0.2. All values are normalized with respect to the reference velocity at 0.9H z/¼
x/¼"0.0
x/¼"0.2
x/¼"0.4
u
v
w
u
v
w
u
v
w
0.4 0.3 0.2 0.1 0.05
0.12 0.12 0.13 0.13 0.13
0.14 0.13 0.14 0.15 0.15
0.12 0.15 0.12 0.15 0.13
0.12 0.12 0.20 0.42 0.30
0.16 0.13 0.14 0.27 0.26
0.07 0.08 0.08 0.12 0.10
0.12 0.23 0.37 0.41 0.34
0.13 0.15 0.21 0.28 0.29
0.08 0.08 0.12 0.14 0.13
z/¼
x/¼"0.6
0.4 0.3 0.2 0.1 0.05
x/¼"0.8
x/¼"1.0
u
v
w
u
v
w
u
v
w
0.17 0.31 0.38 0.42 0.38
0.16 0.17 0.22 0.30 0.31
0.08 0.09 0.12 0.13 0.12
0.23 0.30 0.35 0.40 0.38
0.16 0.19 0.23 0.28 0.30
0.09 0.11 0.12 0.14 0.13
0.23 0.26 0.33 0.38 0.34
0.17 0.19 0.22 0.28 0.27
0.09 0.11 0.12 0.13 0.12
velocity measured in the approach flow at 0.9H. Another possible definition of the shear layer could be the location of the maximum longitudinal turbulence intensity. This definition would locate the shear layer at a location of z/¼ of 0.1 to 0.2 below the location based on the maximum longitudinal velocity. A dramatic increase of all three
R.E. Akins, T.A. Reinhold/J. Wind Eng. Ind. Aerodyn. 74—76 (1998) 455–461
461
components of turbulence between the leading edge, x/¼"0.0, and the next measurement location, x/¼"0.2, is evident for both data sets. The turbulence intensity increases by a factor of two to three to values exceeding 0.4. This change is very local and the values return to the nominal free-stream values at z/¼"0.4. A marked difference in the vertical values is evident between the two locations, a trend consistent with the mean velocity values.
6. Conclusions A LDV system is capable of measuring the details of the separated shear layers emanating from a bluff body and the structure of the turbulence below the shear layer. The width of the shear layer based on either the mean velocities or the turbulence intensities is of the order of 10% of the width of the prism. The interaction of the mean flow with the shear layer and the effect of the flow beneath the shear layer of the fluctuating pressures on the surface of the bluff body are clearly areas for further study using the LDV system. Techniques to compute spectral density functions of unequally spaced time series may be applied to examine the frequency content of the velocity fluctuations in the separated regions. Wavelet techniques may be applied to determine the correlation between the fluctuating velocity and pressures. These phenomenon will be best studied in an experimental situation. The prospect for computational fluid mechanics to predict behavior in the separated regions are not good. Future experiment using LDV systems will be time consuming and expensive and must be planned with care.
References [1] R.E. Akins, Wind pressures on buildings, Ph.D. Dissertation, Department of Civil Engineering, Colorado State University, USA, 1976. [2] H.W. Tieleman, R.E. Akins, The effect of incident turbulence on the surface pressures on surfacemounted prisms, J. Fluids Struct. 10 (4) (1996) 367—388. [3] M.R. Hajj, H.W. Tieleman, Application of wavelet analysis to incident wind in relevance to wind loads on low-rise structures, J. Fluids Eng., ASME, 118 (4) (1996) 874—875. [4] A.J. Minson, Use of laser doppler anemometer measurements near model buildings to determine wind loading on building attachments, Ph.D. Dissertation, Department of Engineering Science, University of Oxford, UK (1993). [5] R. Marwood, An investigation of conical roof edge vortices, Ph.D. Dissertation, Department of Engineering Science, University of Oxford, UK (1996). [6] TSI, Inc., FIND for Windows, Reference Manual, TSI Incorporated, St. Paul, MN, USA, 1996. [7] J.S. Monroe, Wind tunnel modeling of low rise structures in a validated open country simulation, MS Thesis, Department of Civil Engineering, Clemson University, USA, 1996.