TAGGED: ansys-cfd, convergence, flow-separation
April 8, 2023 at 1:12 pmgoktug.yilmazSubscriber
I am conducting an analysis in Fluent software, focusing on flow separation at different angles of attack, such as 6 and 10 degrees. However, I have noticed that the residual values display fluctuating characteristics and the lift and drag coefficients do not converge to a certain value. I have attached an image showing the velocity vectors at 4, 6, and 10 degrees and residuals at 6 degrees for reference. For 4 degrees, I have converged results because there is nearly no separation. But when it separates, residuals start to oscillate shown in the image below.
To obtain a more accurate analysis, I am seeking guidance on how to approach this issue. I have tried reducing the velocity at 6 degrees to decrease the flow separation and subsequently decrease the residual values, resulting in a slight convergence of the lift and drag coefficients. However, my analysis involves a Mach number around 0.8, and I am currently using the kw-SST method.
I would greatly appreciate any suggestions or solutions that you may have regarding this problem. Thank you in advance for your assistance.
April 10, 2023 at 1:45 pmFederico Alzamora PrevitaliSubscriber
I suspect some inherent unsteadiness from the flow separation which prevents the solution to converge to steady-state.
April 10, 2023 at 1:55 pmgoktug.yilmazSubscriber
Thanks for your answer. That is what I also suspect. However I do not know how to solve the problem. I tried to reduce time scale from 1 to 0.4, however it does not work.
April 10, 2023 at 3:28 pmFederico Alzamora PrevitaliSubscriber
You can confirm if this is indeed the issue by inspecting where these larger residuals occur in your domain: #2067529 - How can I post-process my residuals to better understand convergence issues ? (ansys.com).
If the solution cannot converge in steady-state, you might need to switch to transient simulation.
April 11, 2023 at 10:03 amRobAnsys Employee
To add, put some monitor points into the domain in regions of interest and look at (for example) velocity fluctuations on those points. Plot wall shear stress on the upper & lower wing surfaces every few iterations too. Then review the results to see what's changing, and by how much.
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