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Discussions of bluff body aerodynamics
JOURNALOF
ELSEVIER
Journal of Wind Engineering and Industrial Aerodynamics 67&68 (1997) 959 961
~l~[J~i~
DISCUSSIONS OF BLUFF BODY AERODYNAMICS
Discrete vortex model of flow over a square cylinder Authors: D. Bergstrom, J. Wang George Turkiyyah: Your simulation presumes a known location for the separation point. This is not necessary. If you enforce the traditional no-slip boundary condition by introducing vortex elements on the boundary, you do not need to decide in advance the separation location. Response: This is true. However, one might argue that an advantage of DVM is that it avoids modelling the boundary layer in detail, something perhaps better done with pressure-velocity formulations. For a square cylinder, e.g. bluff body, we know approximately where separation occurs. Therefore the method we used may have the advantage of being simpler for some flow configurations. Camarero: Is the DVM extensible to 3D? Response: Yes it is, perhaps not precisely this methodology, but vortex methods are successfully used for simulating 3D flows. Calculation of the flow past a surface-mounted cube with two-layer turbulence models Authors: D. Lakehal, W. Rodi Leighton Cochran: Experimental mean-velocity data in the separated regions of your plots seem to be consistently larger than the C F D results. If these data were collected by a single hot film then the inability of a film to distinguish direction may be contributing to the higher mean. A single film will yield a mean of the absolute values rather than the mean of a highly-fluctuating, reversing flow. The use of cross-films in these highly turbulent environments to discern direction is not very successful either. Would you comment on this as a partial reason for the mismatch in mean-velocity data? Response: None. 0167-6105/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. P I ! S01 6 7 - 6 1 0 5 ( 9 7 ) 0 0 1 3 6 - 0
960
J. Wind Eng. Ind. Aerodyn. 67&68 (]997) 959 961
Unsteady aerodynamic force prediction on a square cylinder using k-~ turbulence models Author: S. Lee
Ferziger: Lyn and Rodig flow was reported to be periodic but it is not actually periodic. Does this affect the validity of your predictions?
Response: I agree with you in that the experimental flow is not periodic as you commented. But if the signal obtained in the experiment has outstanding frequency where the power is at least orders of magnitude higher than the background (or purely turbulent/stochastic) signals, I think the conventional k e turbulence model can still be used to predict unsteady flows. I also agree with you in that this argument cannot be extended to general turbulent flows without any outstanding signal periodicity. Allan Larsen: Are your calculations 2D or 3D? Response: 2D. Jens Honord Walther: I have conducted simulations on the flow past a circular cylinder at Reynolds number of 3000, and found agreement within 10% of experimental data, using RNG and low Reynolds, number model. Do you have any comment on today's keynote presentation regarding unsteady flow using k-e models?
Response: I think that, in these flow cases, the time scales are so different that the turbulence model is capable of picking up the low transient flow while modelling the high frequency turbulence through "equilibrium" k-z; turbulence models. Finite element modelling of flow around a circular cylinder using LES Author: R.P. Selvam Eddy Willemser: The aerodynamic coefficients of a circular cylinder in the supercritical Reynolds number region strongly depend on the surface roughness. Can you cope with this effect in your numerical simulation? Response: I am modelling for smooth cylinder. This is done by (i) using a smooth surface; (ii) using a proper roughness value in the implementation of the law of the wall.
Lakehal: It sounds that, without talking about turbulence modelling, the finite element method and the control volume approach can produce for the same purpose considerable deviations between results? Response: For the given grid, as I compared the performance of different procedures for the bench mark problem it seems the characteristic FEM is better than CV procedures.
J. ~)'nd Eng. Ind. Aerodyn. 67&68 (1997) 959 ~961
961
Development of a parallel code to simulate skewed flow over a bluff body Authors: T. Thomas, J. Williams
Ted Stathopoulus : H o w long does it take you for a typical run? Response: The present simulation using 239 blocks mapped to 128 modes of a CRAY T3D took about 3 s per time step, and about 24 h for the run. Two-dimensional discrete vortex method for application to bluff body aerodynamics Authors: J.H. Walther, A. Larsen Pavit S. Brar: In deriving aerodynamic derivatives, did you simulate flow for different reduced velocity values?
Response: Yes! Typically aerodynamic derivatives will be calculated for reduced wind speeds, V/fB, ranging between 4 and 16 typically, i.e. V/fB = 4,6,8 ..... 16. B. Bienkiewicz: Did you introduce perturbation to promote flow unsteadiness? Response: None. Numerical simulation of flow around rectangular prisms Authors: D.-H. Yu, A. Kareem R, Panneer Selvam: For Re = 105, the boundary layer depth around the cylinder is approximately ~ x ~ ~< 0.01. But your grid spacing (fine) 1/20 = 0.05 is very coarse. So without the law of the wall, modelling grid the flow is not correct. You should run a much finer of and compare the results.
Response: Thank you for your comments. We are looking into further grid refinement and addition of a wall function. However, at this stage, due to the lack of appropriate wall function for the flow shield around prisms we are not very optimistic regarding its influence in the flow field. Sangsan Lee : If 2D and 3D LES results are similar (even without physical representation in 2D LES), is it ok to say we do not need LES at all? At Re = 105 where you did not use wall functions, what is Ay + adjacent to the wall? Response: None.
ELSEVIER
Journal of Wind Engineering and Industrial Aerodynamics 67&68 (1997) 959 961
~l~[J~i~
DISCUSSIONS OF BLUFF BODY AERODYNAMICS
Discrete vortex model of flow over a square cylinder Authors: D. Bergstrom, J. Wang George Turkiyyah: Your simulation presumes a known location for the separation point. This is not necessary. If you enforce the traditional no-slip boundary condition by introducing vortex elements on the boundary, you do not need to decide in advance the separation location. Response: This is true. However, one might argue that an advantage of DVM is that it avoids modelling the boundary layer in detail, something perhaps better done with pressure-velocity formulations. For a square cylinder, e.g. bluff body, we know approximately where separation occurs. Therefore the method we used may have the advantage of being simpler for some flow configurations. Camarero: Is the DVM extensible to 3D? Response: Yes it is, perhaps not precisely this methodology, but vortex methods are successfully used for simulating 3D flows. Calculation of the flow past a surface-mounted cube with two-layer turbulence models Authors: D. Lakehal, W. Rodi Leighton Cochran: Experimental mean-velocity data in the separated regions of your plots seem to be consistently larger than the C F D results. If these data were collected by a single hot film then the inability of a film to distinguish direction may be contributing to the higher mean. A single film will yield a mean of the absolute values rather than the mean of a highly-fluctuating, reversing flow. The use of cross-films in these highly turbulent environments to discern direction is not very successful either. Would you comment on this as a partial reason for the mismatch in mean-velocity data? Response: None. 0167-6105/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. P I ! S01 6 7 - 6 1 0 5 ( 9 7 ) 0 0 1 3 6 - 0
960
J. Wind Eng. Ind. Aerodyn. 67&68 (]997) 959 961
Unsteady aerodynamic force prediction on a square cylinder using k-~ turbulence models Author: S. Lee
Ferziger: Lyn and Rodig flow was reported to be periodic but it is not actually periodic. Does this affect the validity of your predictions?
Response: I agree with you in that the experimental flow is not periodic as you commented. But if the signal obtained in the experiment has outstanding frequency where the power is at least orders of magnitude higher than the background (or purely turbulent/stochastic) signals, I think the conventional k e turbulence model can still be used to predict unsteady flows. I also agree with you in that this argument cannot be extended to general turbulent flows without any outstanding signal periodicity. Allan Larsen: Are your calculations 2D or 3D? Response: 2D. Jens Honord Walther: I have conducted simulations on the flow past a circular cylinder at Reynolds number of 3000, and found agreement within 10% of experimental data, using RNG and low Reynolds, number model. Do you have any comment on today's keynote presentation regarding unsteady flow using k-e models?
Response: I think that, in these flow cases, the time scales are so different that the turbulence model is capable of picking up the low transient flow while modelling the high frequency turbulence through "equilibrium" k-z; turbulence models. Finite element modelling of flow around a circular cylinder using LES Author: R.P. Selvam Eddy Willemser: The aerodynamic coefficients of a circular cylinder in the supercritical Reynolds number region strongly depend on the surface roughness. Can you cope with this effect in your numerical simulation? Response: I am modelling for smooth cylinder. This is done by (i) using a smooth surface; (ii) using a proper roughness value in the implementation of the law of the wall.
Lakehal: It sounds that, without talking about turbulence modelling, the finite element method and the control volume approach can produce for the same purpose considerable deviations between results? Response: For the given grid, as I compared the performance of different procedures for the bench mark problem it seems the characteristic FEM is better than CV procedures.
J. ~)'nd Eng. Ind. Aerodyn. 67&68 (1997) 959 ~961
961
Development of a parallel code to simulate skewed flow over a bluff body Authors: T. Thomas, J. Williams
Ted Stathopoulus : H o w long does it take you for a typical run? Response: The present simulation using 239 blocks mapped to 128 modes of a CRAY T3D took about 3 s per time step, and about 24 h for the run. Two-dimensional discrete vortex method for application to bluff body aerodynamics Authors: J.H. Walther, A. Larsen Pavit S. Brar: In deriving aerodynamic derivatives, did you simulate flow for different reduced velocity values?
Response: Yes! Typically aerodynamic derivatives will be calculated for reduced wind speeds, V/fB, ranging between 4 and 16 typically, i.e. V/fB = 4,6,8 ..... 16. B. Bienkiewicz: Did you introduce perturbation to promote flow unsteadiness? Response: None. Numerical simulation of flow around rectangular prisms Authors: D.-H. Yu, A. Kareem R, Panneer Selvam: For Re = 105, the boundary layer depth around the cylinder is approximately ~ x ~ ~< 0.01. But your grid spacing (fine) 1/20 = 0.05 is very coarse. So without the law of the wall, modelling grid the flow is not correct. You should run a much finer of and compare the results.
Response: Thank you for your comments. We are looking into further grid refinement and addition of a wall function. However, at this stage, due to the lack of appropriate wall function for the flow shield around prisms we are not very optimistic regarding its influence in the flow field. Sangsan Lee : If 2D and 3D LES results are similar (even without physical representation in 2D LES), is it ok to say we do not need LES at all? At Re = 105 where you did not use wall functions, what is Ay + adjacent to the wall? Response: None.