why I can't get Second order convergence
I use fluent to compute a ejecting flow in pipe, the main flow is about 4 mach.I can only make it converge to the first oder upwind scheme,but can't make it converge to the default second order upwind scheme. please give me some advice. thanks.

Re: why I can't get Second order convergence
(1). Are you saying that you are computing the Mach 4 flow in a pipe and you can get converged solution with the firstorder upwind scheme only? (2). What is your Reynolds number?, mesh size and distribution?, turbulence model? boundary conditions? (3). How about the shock structures in the pipe? I think, these will affect the stability of the numerical solution. The firstorder method will give you higher numerical viscosity to stabilize the flow.

Re: why I can't get Second order convergence
If you mean residuals lower than 1E3 with "converged", then you should try to solve the problem with the unsteady solver. This mostly helps due to the instationary behaviour of jets if you have more than one. They reason is that jets are interacting which leads to a periodic phenomenum.

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