pulsating static pressure around zero at pipe flow
 

Hi, I am simulating the unsteady pulsating pipe flow by segregate solver. Very low Re, incompressible, inlet vel=0.1 + 0.04*sin(80pi * t) [m/s], pressure outlet (gauge press=0), PISO, working fluid air (constant density), operating pressure 101325 Pa, pipe dia 0.04m, length 0.2m.

I think this is very simple problem. But the STATIC pressure results along pipe centerline are unreasonable. I expected the results as the static pressure at inlet is larger than that of outlet. And the static pressure at inlet has a phase difference with that at different locations along pipe centerline, as well as with velocity profile. But there are no phase differences, and the static pressures fluctuate sinusoidally around 0 Pa at all centerline positions, that is, the static pressure has MINUS sign, and the static press of inlet is smaller than that of outlet (0Pa). This is very low Mach number flow. Should I use coupled implicit solver and ideal gas law to see the phase change? Any help is highly appreciated, David