help with supersonic flow over duct with backward step
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new to fluent. need help.
2D duct: it is an isolator with test section, symmetric, isolator dimmension is 0.5''X 5'', test section part is 1'' X 15''. The step height is 0.5''. use very fine uniform mesh, the step height has 40 interval count, other lengths has the counts proportional to the length. isolator entrance flow is P0=70 psi, T0=300k, with M=1.56 so static P= 17.47psi. use pressure inlet and pressure outlet BC, operating P=0. inviscid, densitybased steady flow. Courant No. 0.5, Flux Type: ASUM. I monitored the outlet mass flow rate. At the first 3000 iteration, mass flow rate is constant, then suddenly increases and oscillates, till diverge. The residual oscillates too and the strange thing is Y velocity keeps increasing while others decrease. could anyone give me some help? why it happens like this? IF I use the AS viscous flow, with underrelaxation factor of modified turbulent viscosity as 0.2(default is 0.8), after long time iteration, it still diverged. what procedure should I use to solve it? many thanks for any help. 
Anybody has any idea about this? Thanks a lot

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