Hi，everbody，

I used the compressible boundary condition for unsteady 2D laminar flow, prescribing the total pressure, the total pressure and the inlet flow direction.It uses for numerical simulations of resonant oscillations in a tube which has one inflow and three walls(no outflow).

But I met two difficulties. First, in certain time-step, the residuals fluctuate dramatically. The code seemed not converge. In most time-step, it was OK. Second, when the inlet velocity specified Uin=u0*sin(wt), I think the velocity wave in the middle of the tube would be the sine wave. Now, in my code, the velocity wave was sine wave when the inlet velocity was positive. When the inlet velocity was negative (3.14+2*k*3.14< t <2*3.14+2*k*3.14,k=1,2,3&hellip;&hellip;.), the velocity was nearly zero.Why？And did I use the right boundary condition?

Thanks a lot!

not sure if I understood it well, but there seem to be conflict: you've prescribed inlet flow direction together with sine wave velocity. I.e. inlet velocity alternates between positive (inflow) and negative (outflow) values. If the inlet flow direction is prescribed inward, negative velocity violates it.

And, imho, sine wave will only proceed into the tube for short portion only and short time - as there's no outlet, the wave will first be deformed (damped) due to rising pressure inside the tube and later due to wave reflections from closed end (-s).

Thanks for your reading!

I think you are right.I cann't prescirbe the inlet flow direction. The periodic boundary condition is inlet velocity alternates between positive and negative values.

My goal is to simulate the resonant oscillations in a tube. When the inlet velocity is positive,the the pressure rises inside. When the velocity is negative, can the inside pressure descend? I.e.the pressure value near outlet could be less than the inlet pressure value? If it could ,why and how to do? In my simulation results, the pressure value near outlet is always more than the inlet pressure value. It dosen't accord with the literature. I think this is the reason why the velocity wave inside is not accord with the literature.

I found when the reflection wave meets with the proceeding wave,the residuals fluctuate dramatically.

Is my boundary conditon not appropriate or something wrong with my code?

Waiting for your advice.

Thanks!