HI Xianwen, This is a very challenging Reynolds number. Have you tried running it at a lower Re to being with until you establish the flow a bit. You might find you need to increase the quadrature points at a fixed polynomial order to help dealias the solution. Attached is a study an MSc student undertook last year using the shock capturing articifical viscosity which may have a few more details about how she set up her runs. I note the Re of this case was much lower. Cheers, Spencer. On 23 Apr 2018, at 02:04, 郭先文 <guoxw@pku.edu.cn<mailto:guoxw@pku.edu.cn>> wrote: Dear all, Recently I'm applying Nektar++ into calculating a 2D case of subsonic flow around a circular cylinder. The flow condition is under Re = 200000 and Ma = 0.75. And NaN in speed of sound keeps appearing within two specific element in the downstream and off-wall region of the cylinder, even after changing of grid size and time step. I wonder if the problem could be solved with the usage of artificial viscosity or smooth shock capture method (Non-smooth didn't work). But the user's guide doesn't cover enough information about those settings in detail. Where could I find some details and settings about those methods? Or is there any other solution to the problem? Thanks very much for your help. Regards, Xianwen. Xianwen Guo College of Engineering Peking University <testc.xml><contour of speed of sound a.png><same region of abnormal pressure.png><zoom in of countour of a.png>_______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users Spencer Sherwin FREng, FRAeS Head, Aerodynamics, Professor of Computational Fluid Mechanics, Department of Aeronautics, Imperial College London South Kensington Campus, London, SW7 2AZ, UK s.sherwin@imperial.ac.uk<mailto:s.sherwin@imperial.ac.uk> +44 (0)20 7594 5052 http://www.imperial.ac.uk/people/s.sherwin/