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?