# Unsymmetric results of velocity profile

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 September 4, 2007, 06:31 Unsymmetric results of velocity profile #1 Michelle Guest   Posts: n/a Hi everyone! I am simulating a cylindrical mixing section with two inlets in the same plane (inner jet is faster than the outer flow and they are concentric). I used k-epsilon turbulence model and incompressible air. My results show that the velocity profile in some sections are not symmetric with respect to the centerline, and sometimes the maximum velocity doesn't lie along the center. As far as I know, it should be symmetric with maximum pt at the center as the flow becomes fully developed. I tried to lower (from 5% to 1%) the intensity of turbulence for the inlet conditions but the results were the same. Do you have any explanation for this guys? The most unsymmetric profile is located in the region where there are sudden change in turbulence kinetic energy(TKE). Is it okay to assign similar inlet TKE eventhough the inlet velocities are not equal? Thank you so much guys for your time! Michelle

 September 4, 2007, 09:37 Re: Unsymmetric results of velocity profile #2 CycLone Guest   Posts: n/a Hi Michelle, Welcome to the non-linear world of fluid dynamics, where the obvious is rarely right! This is a classic problem of instability. A pin standing on its head is symmetric and it should be possible for the pin to remain in the standing position. However, the solution is unstable; the slightest deviation will cause the pin to fall because a pin laying down is at a lower energy level. Another physical example would be that of a sphere sitting on the top of a hill; again symmetric, but unstable. Physical examples such as these are easy to visualize, but fluid dynamics is often less obvious. In the case of an axisymmetric jet within a tube, the slightest movement of the jet towards one of the walls will cause the static pressure on the side nearer the wall to drop, drawing the jet even closer and causing asymmetry in the flow. The solution isn't always steady either. As the jet is pulled to the wall, a recirculation zone may develop upstream due to the blockage. This recirculation zone may push the upstream portion of the jet towards the other side or at least away from the current impinging position. As a result, the jet will wander or fluctuate. This is a good example of unsteady behavior that can lead to convergence problems in steady state solutions; because there is no steady state. It all depends on Reynolds number of course. At low Reynolds numbers, the viscous transport of momentum will keep the jet stable, but as the Reynolds number increases, the advective transport of momentum will dominate and lead to instability. -CycLone

 September 5, 2007, 02:22 Re: Unsymmetric results of velocity profile #3 Michelle Guest   Posts: n/a Hi Cyclone! Thank you so much for your very informative response. Actually there are negative velocities inside the mixing section which imply that there's recirculation/swirling inside. However,I made a refined mesh (reduced the body spacing to half and doubled the no. of inflation layers) and the results were very different though they both converged using the same criterion. Is it right to assume that I have to improve the quality of mesh?I'm using 6inch(diam)x 30ft long cylinder. Do you have any suggestion about the appropriate inflation layers? Also, I've read that there's specific range of yplus for every turbulence model, is the yplus of my mesh equal to the maximum yplus given in cfx post? I'm so sorry for many questions. I'm quite new to cfx. My simulation results is quite far from experiment results. I really appreciate your response and any other recommendations is much appreciated. Regards, Michelle

 September 6, 2007, 06:34 Re: Unsymmetric results of velocity profile #5 Michelle Guest   Posts: n/a Thank you so much Cyclone! Your response is much appreciated. It helps me a lot! I made some mesh refinement and then the results of the velocity profile became more and more symmetric. I followed your advise to refine the mesh until there's no appreciable difference in results, I'm still simulating with some more refined mesh. I hope I'll reach that point of having independent mesh. But what if the results continue to change significantly eventhough the mesh size is very small? would that mean that i really need to simulate it in transient? Thanks again cyclone! I really appreciate it! Best regards, Michelle

 September 7, 2007, 22:39 Re: Unsymmetric results of velocity profile #6 TB Guest   Posts: n/a It really depends on what you are studying and how you are refining your mesh. If your meshing strategy is correct, solver may capture more unsteadiness, as the mesh is refining. The question is whether it will affect what you are studying. If you are solving steady flow problem, don't waste your time doing transient simulation, unless you are sure that it will have significant effect on your problem.

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