Dear Bruno, Thank you for your suggestions. I am going to take a look at Lethe and compare with my implementation. In stabilized formulation, I used quadrilateral element, instead of P2 P1 Taylor-Hood element. The used element is only C0 element. I also did not expect such a discontinuity between elements. Although I use P2 P1 Taylor-Hood element without stabilization terms, the discontinuity is still there. Probably I made mistakes somewhere in setup. I also suspect that my solution is not converged. After taking a small relative tolerance 1e-8, the discontinuity still appears. As Professor Wolfgang suggested, I am currently checking the convergence rate of this formulation. Thank you for your suggestions. If I cannot resolve this issue, I will update in the forum again.
Best D. ________________________________ From: dealii@googlegroups.com <dealii@googlegroups.com> on behalf of Bruno Blais <blais.br...@gmail.com> Sent: Tuesday, January 14, 2020 9:53 PM To: deal.II User Group <dealii@googlegroups.com> Subject: Re: [deal.II] discontinous contour over elements Dear David, How are you calculating the vorticity? As Wolfgang and Praveen have mentioned, if you are using the DataPostProcessor, then this will use your shape functions to calculate the vorticity. However, your P2-P1 elements are only C0 continuous. Consequently, your vorticity can possibly be inherently discontinuous at the element edges. However, I am surprised that you obtain such strong discontinuity. In our code based on deal.ii (https://github.com/lethe-cfd/lethe) we implement stabilized formulations for the NS equations and the vorticity results for such cases are very smooth (even when represented using discontinuous shape functions. Have you established the convergence of your code using manufactured solution? This is the first thing I would suggest. You can look at the applications_tests of lethe for some examples of easy manufactured solutions for the Incompressible Navier-Stokes equations. There are also common books that treat this issue (for instance : https://www.amazon.ca/Verification-Validation-Scientific-Computing-Oberkampf/dp/0521113601) Please feel free to reach out if you have more questions. Best Bruno On Monday, 13 January 2020 20:29:53 UTC-5, David Eaton wrote: Thanks Wolfgang and Praveen for providing suggestions. I have tried to debugging this code for a while. I have attached this simple code on this email. I followed the instructions in tutorial closely. Hopefully, anyone could give some suggestions. Best D. ________________________________ From: dea...@googlegroups.com <dea...@googlegroups.com> on behalf of Wolfgang Bangerth <bang...@colostate.edu> Sent: Tuesday, January 14, 2020 6:24 AM To: dea...@googlegroups.com <dea...@googlegroups.com> Subject: Re: [deal.II] discontinous contour over elements On 1/12/20 9:17 PM, David Eaton wrote: > My inflow condition is uniform. This formulation and mesh is tested in a > simple C++ code without library. The large mesh near the inflow does not give > this problem. > Yes. I am using C0 element. I did calculation using tecplot. However, the > results from a my C++ code does not give this issue either. Just now, I check > the formulation again. Although I use Q2Q1 Taylor-Hood element without any > stabilization, these issues are still happening. David -- we don't really know what formulation you are using, how you are implementing it, what you are comparing against, and a number of other factors. If you have a formulation that computes u,p, and you are plotting the vorticity, you need to expect that the isocontours are discontinuous for the reasons Praveen already stated. If you are getting results that make no sense to you, then the first step would be to ensure that your program is converging as expected. To do this, choose a solution that you know and compute the error norm; then ensure that the program yields error norms that decrease as expected with mesh refinement. Best W. -- ------------------------------------------------------------------------ Wolfgang Bangerth email: bang...@colostate.edu www: http://www.math.colostate.edu/~bangerth/ -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. 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