CFD Online Discussion Forums - which b.c. i should apply for this prob.urgent

Hi to all, I am doing transient flow analysis on a pressure vessel.(More or less it is working as a surge tank).At the inlet of surge tank there is compressor. i.e .outlet of compressor is feeding flow to inlet of surge tank. Inlet known conditions at surge tank are velocity and outlet known conditions are velocity and pressure. Inlet velocity is 22 m/sec .and outlet velocity is 22 m/sec(cross sectional area is same) and pressure at outlet is 3.7 bar. Inlet pressure is not known. Now compressor is giving flow to surge tank inlet and this nature of flow is sinusoidal. i.e. pressure is varying sinusoidal at inlet. i.e pressure variation of flow is sinosodiual. The problem may be considered as compressible, but I have simulated it and found that there is little change in density and also little change in pressure .difference of pressure at inlet and outlet is more or less 0.05 bar. My query is which boundary condition I should apply? I have known conditions at inlet is only velocity (22 m/sec).and outlet known conditions is 22 m/sec and pressure is 3.7 bar. Second thing is whether I should consider the flow compressible or incompressible.? I am thing that following boundary conditiona should be applied.

(1) For steady state velocity at inlet and pressure at outlet. (2) For unsteady state velocity at inlet and pressure at outlet. Now for unsteady anything should vary with respect to time. I have varying thing are pressure at inlet and outlet. but I only know pressure variation at outlet. So should I apply that at outlet. Whether it will create divergence flow at outlet due to varying nature or not? that's why I am afraid of that. Please reply me urgent if anybody knows. Thabkibng you all.