Residuals high for 2D Convergent divergent nozzle simulation
 

Hello, I met problems when simulating a 2D axisymmetric convergent-divergent nozzle using density-based solver in Fluent.

I calculated my inlet and outlet boundary conditions following the 1D analysis of gas dynamics, for throat Mach number 1 and 0.7 respectively. Specified four types of boundary for inlet and outlet:

1. Velocity inlet & Pressure outlet
2. Pressure inlet & outlet
2. Pressure-far-field for inlet & outlet
3. Mass flow inlet & outlet

And I also tried to lower the Courant number to 0.1.

However, all simulations result in a very high residual level. The x-velocity residuals are as high as 50.

I'd appreciate it if anyone can give me some help.