wave speed

moutazz

Hi, I am trying to model the water hammer in pipelines. For my model the valve should close suddenly within 0.03 seconds causing the velocity and pressure to change rapidly. My solution consist of three steps, in the first step I calculate a steady-state solution as an initial guess for a transient flow prediction, in the second step the transient solution is enabled with enable the valve to close the pipe's end completely by using the sliding mesh capability of fluent .after the valve is closed completely the final step begins to study the pressure and velocity waves which are propagate down stream and upstream of the valve. I have encountered some unexpected results in my model which appeared in the pressure peaks and wave speed. For a point upstream of the valve the second pressure peak is higher than the first pressure peak. Also, resultant wave speed is about 1400 m/s which is exceeded the value I have specified. For my model I used a pvc material with a density of 1420 kg/m3, a constant value of 518 m/s for the speed of sound, a udf is used to specify the density as a function of pressure for the working fluid, a time step size of 0.0005 and 1000 for the number of time step. can any body help me in order to get realistic results. thanks. Moutazz..