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Discussions of bridge aerodynamics
JOURNAl OF
lml,m ,er ELSEVIER
Journal of Wind Engineering and Industrial Aerodynamics 67&68 (1997) 962 965
DISCUSSIONS OF BRIDGE AERODYNAMICS
A fluid mechanicians view of wind engineering: Large eddy simulation of flow past a cubic obstacle Authors: K. Shah, J.H. Ferziger R. Panneer Selvam: In your work, you are using central difference. You refined the grid to reduce oscillations. Did you do any study to know how much the numerical dispersion affects the turbulence or changes the turbulence that you want?
Response: We refined the grid until oscillations disappeared ( < 0.5%). This is a necessary but not sufficient criterion for accuracy. We have also looked at some spectra and found that they are sufficiently resolved. I do not believe that there is any justification for using upwind methods in LES (except at shocks); you will get smooth but quantitatively incorrect results. S. Kate. (1) Do you think it is possible to develop a subgrid model for the coarse grid systems? In the wind engineering field, we always suffer from insufficient mesh dividing systems. (2) What is the difference between the CSC and LES with the coarse grid system?
Response: (1) I believe that it will be possible to do LES on coarse grids. The danger is that the flow may be periodic when it should not be and that is why I think it may be necessary to introduce a random force. (2) They are essentially the same thing. The objective of CSC is to capture the large coherent structures and as little else as necessary. Jens Honord Walther: Back scatter seems to be the problem of the dynamic LES. Do you believe, that the mesh can be refined sufficiently to remove backscatter without going to direct numerical simulation?
Response. No. Roger Pielke, Sr.: You clearly demonstrated the value of the LES approach, in contrast to the Reynold's Averaging Method, to obtain estimates of extreme values 0167-6105/97/$17.00 ',~<~ 1997 Published by Elsevier Science B.V. All rights reserved. P I I S01 6 7 - 6 1 0 5 ( 9 7 ) 0 0 1 3 7 - 2
J. Wind Eng. Ind. Aerodyn. 67&68 (1997) 962-965
963
(e.g. of pressure, skin stress, etc.). This information should be of considerable value for wind engineers. I would like your comments on this capability of the LES method.
Response: I agree that LES can produce information about the pressure fluctuations. In fact, we are currently producing the P D F of the pressure fluctuations and will try to fit it to the standard statistical models. /an Castro: Many people are using unsteady RANS to calculate periodic vortex shedding flows. What is your view of this procedure? Response: 1 do not understand how RANS can be used for unsteady flows. The pressumption of RANS is that all unsteadiness is modeled so if an unsteady flow is predicted this premise is rotated. There is also a danger that a periodic flow will be predicted when the actual flow is not periodic. Fatigue strength design for vortex-induced oscillation and buffetting of a bridge Authors: M. Hosomi, H. Kobayashi, Y. Nitta N. Shiraishk The method presented is the fatigue examination. Have you applied your method for the actual fatigue damaged case of structural members under action of wind such as damaged cases of hangers of arch and others?
Response: We have not applied for actual cases. But, we are going to apply for a girder of a cable stayed bridge. In this study, we checked fatigue strengths of a girder member.
Numerical simulation of flow around a box girder of a long span suspension bridge Author: S. Kuroda AllanLarsen: Re is correct (assumed Re -¢ 3.105). What is the CPU time required for your computations? On which computer were they run?
Response: None. Pavit S, Brar: How do you plan to include body motion for dynamic tests?
Response: In the case there is only one moving body. What I must do is only moving the whole grid rigidly according to the body motion. And the terms related to the grid speed added to the inviscid flux terms. In the case there is relative motion between the bodies, the minor grid is slid on the main grid (when ! use the overlaid grid system).
964
J. Wind Eng. Ind. Aerodyn. 67&68 (1997) 962 965
Aeroelastic analysis of bridge girder sections based on discrete vortex simulations Authors: A. Larsen, J.H. Walther W.W. Yang: (t) How do you consider the cross-correlation of the wind turbulence field spanwise? (2) Is the geometrical nonlinearity of the structure included in the model?
Response: (1) Calculations made are 2D and for a steady onset flow, hence there is no effect of upwind turbulence or its lateral correlation. The effect of turbulence on the bridge structure is calculated by standard buffeting routines for which coefficients calculated by the code are introduced. (2) No. the model only considers a 2D crosssection of the bridge deck. Hiromichi Shirato: (1) How many vortex elements are defined on the body surface? (2) When the free vortex is getting closer to the body surface, the induced wind velocity is sometimes overestimated by the Biot-Savart equation. Could you give us some idea of special treatment for that undesirable phenomenon in your simulation?
Response: (1) The number of vortex panels on a section will typically vary between 100 and 200. At the end of simulations 30.000-60.000 vortices will typically be present in the flow. (2) The Biot-Savart equation is modified by introducing a Gaussian core in which the velocity decreases to 0 at the center of the vortex.
Pavit S. Brat. I noted the discrepancy in St number between your results and experimental results for the Gibraltar bridge section. Discrete vortex methods are known to have some problems in predicting generation of vorticity in the boundary layer. Do you think this could be a factor in the Strouhal number discrepancy found above?
Response: For all "mono-box" sections investigated St have been in very good agreement with experiments, hence the vortex method is not expected to be inaccurate with respect to prediction of St. At the present stage discrepancy is not clear but computations as well as model experiments will be reviewed. It is noted that the predicted St is almost exactly 1/2x the measured St indicating perhaps that the model frequency which locked on to the vortex shedding frequency was the 2nd structural harmonic. Jae Seok Lee: Your approach is very good to evaluate the aeroelastic behavior of a bridge in a practical point of view. Have you ever performed dynamic analysis of the 3D bridge structural model using the unsteady wind forces obtained by discrete vortex simulation? If you have not performed yet, the structural dynamic analysis of 3D model using unsteady wind forces is highly recommended, because it gives more understanding of the structural response of the bridge under unsteady wind forces.
J. Wind Eng. Ind. Aerodyn. 67&66 ()997) 962 965
965
Response: I have not tried yet. I agree with your comment. Prediction of vortex-induced wind loading on long-span bridges Authors: S. Lee, J. Lee, J. Kim Allan Larsen: Did your calculations include effects of structural motion? Can your code handle moving boundaries and thus accommodate non-linear effects?
Response: None. Karlrnanz Beyer: The assumption of all forces in phase along the bridge deck may lead to either safe or unsafe results, depending on length relations and stiffness distributions. A realistic assumption of coherence should be included. Response: In-phase forcing along the bridge span is the first possible choice, when forces on the 2-D deck model are available. Even though the in-phase forcing is not the extreme choice as commented, it may serve as a good starting point to buildup further ideas. Kangpyo Cho: Did you check the fundamental frequency using spectral analysis from wind tunnel test result? Your first peak of displacement is due to resonance effect. What is the meaning of the second peak? Response: We checked the fundamental frequency of the structure by comparing with the other researchers' models for the same bridge used in structural analysis and in wind tunnel test. The second resonance seems to be from the sub-harmonic resonance, whose frequency is twice the fundamental frequency.
An analysis of vortex induced oscillation of long span bridge Author: S. Lee N. Shiraishi: In connection with the Seohae Bridge, torsional oscillation seems to tend to occur because of cross-sectional shape. Have you any kind of investigation on vortex-induced oscillations of torsional modes?
Response, We computed torsional moments on the bridge, and they are then applied in the structural analysis. Whenever the vertical displacements are small, the torsional motion does not give any undesirably large oscillations.
lml,m ,er ELSEVIER
Journal of Wind Engineering and Industrial Aerodynamics 67&68 (1997) 962 965
DISCUSSIONS OF BRIDGE AERODYNAMICS
A fluid mechanicians view of wind engineering: Large eddy simulation of flow past a cubic obstacle Authors: K. Shah, J.H. Ferziger R. Panneer Selvam: In your work, you are using central difference. You refined the grid to reduce oscillations. Did you do any study to know how much the numerical dispersion affects the turbulence or changes the turbulence that you want?
Response: We refined the grid until oscillations disappeared ( < 0.5%). This is a necessary but not sufficient criterion for accuracy. We have also looked at some spectra and found that they are sufficiently resolved. I do not believe that there is any justification for using upwind methods in LES (except at shocks); you will get smooth but quantitatively incorrect results. S. Kate. (1) Do you think it is possible to develop a subgrid model for the coarse grid systems? In the wind engineering field, we always suffer from insufficient mesh dividing systems. (2) What is the difference between the CSC and LES with the coarse grid system?
Response: (1) I believe that it will be possible to do LES on coarse grids. The danger is that the flow may be periodic when it should not be and that is why I think it may be necessary to introduce a random force. (2) They are essentially the same thing. The objective of CSC is to capture the large coherent structures and as little else as necessary. Jens Honord Walther: Back scatter seems to be the problem of the dynamic LES. Do you believe, that the mesh can be refined sufficiently to remove backscatter without going to direct numerical simulation?
Response. No. Roger Pielke, Sr.: You clearly demonstrated the value of the LES approach, in contrast to the Reynold's Averaging Method, to obtain estimates of extreme values 0167-6105/97/$17.00 ',~<~ 1997 Published by Elsevier Science B.V. All rights reserved. P I I S01 6 7 - 6 1 0 5 ( 9 7 ) 0 0 1 3 7 - 2
J. Wind Eng. Ind. Aerodyn. 67&68 (1997) 962-965
963
(e.g. of pressure, skin stress, etc.). This information should be of considerable value for wind engineers. I would like your comments on this capability of the LES method.
Response: I agree that LES can produce information about the pressure fluctuations. In fact, we are currently producing the P D F of the pressure fluctuations and will try to fit it to the standard statistical models. /an Castro: Many people are using unsteady RANS to calculate periodic vortex shedding flows. What is your view of this procedure? Response: 1 do not understand how RANS can be used for unsteady flows. The pressumption of RANS is that all unsteadiness is modeled so if an unsteady flow is predicted this premise is rotated. There is also a danger that a periodic flow will be predicted when the actual flow is not periodic. Fatigue strength design for vortex-induced oscillation and buffetting of a bridge Authors: M. Hosomi, H. Kobayashi, Y. Nitta N. Shiraishk The method presented is the fatigue examination. Have you applied your method for the actual fatigue damaged case of structural members under action of wind such as damaged cases of hangers of arch and others?
Response: We have not applied for actual cases. But, we are going to apply for a girder of a cable stayed bridge. In this study, we checked fatigue strengths of a girder member.
Numerical simulation of flow around a box girder of a long span suspension bridge Author: S. Kuroda AllanLarsen: Re is correct (assumed Re -¢ 3.105). What is the CPU time required for your computations? On which computer were they run?
Response: None. Pavit S, Brar: How do you plan to include body motion for dynamic tests?
Response: In the case there is only one moving body. What I must do is only moving the whole grid rigidly according to the body motion. And the terms related to the grid speed added to the inviscid flux terms. In the case there is relative motion between the bodies, the minor grid is slid on the main grid (when ! use the overlaid grid system).
964
J. Wind Eng. Ind. Aerodyn. 67&68 (1997) 962 965
Aeroelastic analysis of bridge girder sections based on discrete vortex simulations Authors: A. Larsen, J.H. Walther W.W. Yang: (t) How do you consider the cross-correlation of the wind turbulence field spanwise? (2) Is the geometrical nonlinearity of the structure included in the model?
Response: (1) Calculations made are 2D and for a steady onset flow, hence there is no effect of upwind turbulence or its lateral correlation. The effect of turbulence on the bridge structure is calculated by standard buffeting routines for which coefficients calculated by the code are introduced. (2) No. the model only considers a 2D crosssection of the bridge deck. Hiromichi Shirato: (1) How many vortex elements are defined on the body surface? (2) When the free vortex is getting closer to the body surface, the induced wind velocity is sometimes overestimated by the Biot-Savart equation. Could you give us some idea of special treatment for that undesirable phenomenon in your simulation?
Response: (1) The number of vortex panels on a section will typically vary between 100 and 200. At the end of simulations 30.000-60.000 vortices will typically be present in the flow. (2) The Biot-Savart equation is modified by introducing a Gaussian core in which the velocity decreases to 0 at the center of the vortex.
Pavit S. Brat. I noted the discrepancy in St number between your results and experimental results for the Gibraltar bridge section. Discrete vortex methods are known to have some problems in predicting generation of vorticity in the boundary layer. Do you think this could be a factor in the Strouhal number discrepancy found above?
Response: For all "mono-box" sections investigated St have been in very good agreement with experiments, hence the vortex method is not expected to be inaccurate with respect to prediction of St. At the present stage discrepancy is not clear but computations as well as model experiments will be reviewed. It is noted that the predicted St is almost exactly 1/2x the measured St indicating perhaps that the model frequency which locked on to the vortex shedding frequency was the 2nd structural harmonic. Jae Seok Lee: Your approach is very good to evaluate the aeroelastic behavior of a bridge in a practical point of view. Have you ever performed dynamic analysis of the 3D bridge structural model using the unsteady wind forces obtained by discrete vortex simulation? If you have not performed yet, the structural dynamic analysis of 3D model using unsteady wind forces is highly recommended, because it gives more understanding of the structural response of the bridge under unsteady wind forces.
J. Wind Eng. Ind. Aerodyn. 67&66 ()997) 962 965
965
Response: I have not tried yet. I agree with your comment. Prediction of vortex-induced wind loading on long-span bridges Authors: S. Lee, J. Lee, J. Kim Allan Larsen: Did your calculations include effects of structural motion? Can your code handle moving boundaries and thus accommodate non-linear effects?
Response: None. Karlrnanz Beyer: The assumption of all forces in phase along the bridge deck may lead to either safe or unsafe results, depending on length relations and stiffness distributions. A realistic assumption of coherence should be included. Response: In-phase forcing along the bridge span is the first possible choice, when forces on the 2-D deck model are available. Even though the in-phase forcing is not the extreme choice as commented, it may serve as a good starting point to buildup further ideas. Kangpyo Cho: Did you check the fundamental frequency using spectral analysis from wind tunnel test result? Your first peak of displacement is due to resonance effect. What is the meaning of the second peak? Response: We checked the fundamental frequency of the structure by comparing with the other researchers' models for the same bridge used in structural analysis and in wind tunnel test. The second resonance seems to be from the sub-harmonic resonance, whose frequency is twice the fundamental frequency.
An analysis of vortex induced oscillation of long span bridge Author: S. Lee N. Shiraishi: In connection with the Seohae Bridge, torsional oscillation seems to tend to occur because of cross-sectional shape. Have you any kind of investigation on vortex-induced oscillations of torsional modes?
Response, We computed torsional moments on the bridge, and they are then applied in the structural analysis. Whenever the vertical displacements are small, the torsional motion does not give any undesirably large oscillations.