[cfdnewb123]

I am starting on turbulent flow and I am using a DNS code for low speed compressible air flow.
The code is discretization is based on central difference for its 1st order and 2nd order derivative. The code uses the temperature equation to determine the flow dynamic viscosity using Sutherland formula as well as the flow pressure using ideal gas law. The flow is also affected by gravitational along the z-direction. The code is initialized at a flow speed of 1 in each direction and at a starting temperature of 293K. Random noise are also given to speed and temperature to speed up turbulence.

However, after time of 10, the result produces velocity of the thousands which are not possible. I think that the code is still incomplete, in particular the timestep as the timestep is very big at dt=3.3333. May I know what is the suitable timestep for DNS flow? I try using CFL-based timestep but I need to set my CFL=0.001 for the resulting speed to be more reasonable, however this seems rather arbitrary plus, the timestep is very small. Also, the flow Re number is around 0.6.