Hi

I have been simulating an underexpanded free jet by solving the EUler equations using a TVD scheme. I have extended the length of the CFD domain to 60D and the jet still appears going 'strong;. I suppose this is due to the underlying inviscid assumption. Also the 'shock cells' within the jets are more widely spaced than those reported in experiments. Not too sure why though.

Any advice is appreciated SHuo

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Shuo could you please tell a little more about the initial jet conditions? like the NPR, the temperature and which TVD scheme are you using?

When you say 'strong' do you mean you see the shock cells up to 60D?

The NPR(P_exit/P_atm) is 1.9 and 2.5: P0 = 365000Pa, 480000Pa, T0 = 288K. I am using a approximate Riemann solver TVD scheme proposed by Yee/Harten. For the lower pressure ratio the shock cells are more closely spaced. Domain is 20D x 8D with 400 x 200 grid points.

Hope I am clearer Shuo

You are right about your problem, the inviscid assumption is very bad. You are not capturing the dominant physics in a jet. The jet decays due to viscous effects, almost assuredly turbulent flow. The shock spacing won't be right because the jet mixing layer grows decaying the size of the inviscid core and changing the "boundary" of the supersonic flow. You need to run viscous and you need to run turbulent with a good turbulence model. For jets, SST or k-epsilon are good.

I have tried the Navier-Stokes equations and there is still problems with boundary conditions. Looks like I need to start reading up on turbulence modelling.

Shuo