[M.Hodali]

Hi every one,
I'm trying in Autodesk CFD 2018 to simulate the body movement such like a piston or desk inside tube or pipe between two different fluids.
body is separating between two fluids zones which one is a liquid and the second one is a gas while the body movement is due to the oscillating pressure in the liquid zone as a result of ultrasonic waves propagation through liquid and suppose that sounic pressure amplitude is pushing the piston forward and backward and cause a pressure oscillating in the gas zone.
Simulation target to study the pressure oscillation pattern in the gas zone .
I have setup the inlet boundary condition into liquid as pressure transient state according to the wave equation P(t)= Pa sin (w*time), and the gas zone outlet boundary condition is zero velocity, while the body movement setted as liner flow driven with a spring displacement +1mm and -1mm, then I run the solver for a transient mode but unfortunately, the simulation won't to run. the error message that simulation can not run two different fluids zones touching together, even though the body thickness is 10 mm and displacement is only 1mm.
I tried to set a resistance surface material for both of liquid and gad zones interfacing the body but simulator not accepted and said that resistance surface is only between two fluids not between fluid and solid zones.
I tried to increase the body thickness but I got the same problem of mixing two different fluids error.
does anyone have any other technique to run this simulation case?. or any advice can help.
thanks .