I would like to check whether CFX is able to capture pressure pulsations, the phenomena so called water hammer or pressure surge. For this, I made a simple 1 m pipe length. At the entrance, I employed high pressure and at the exit I have imposed wall and pressure outlet conditions depending on time. For the few initial transient time steps, the outlet was a wall and then the boundary condition changed to pressure outlet to simulate sudden valve opening. From the results I observed that, if I choose small time steps, I only see a initial sharp pressure decrease and afterward pressure reaches a constant value. But If I choose, larger time steps, after the initial sharp pressure decrease, pressure pulsations were also induced.

For sure, such time step dependency is not correct. Is anybody knows this problem. Or anybody have expierence with simulation pressure surge problem in hydraulic pipelines with CFX?

Hi,

I have done similar simulations in hydraulic oil systems. Yes, CFX can capture these waves. If you want to resolve the wave you need a fine time step. Keep in mind the acoustic velocity of water is something like 5000 m/s so you need fine enough timesteps to resolve waves of that velocity! You can use larger timesteps but the wave effect will be blurred and possibly loose simulation accuracy. Need to check how sensitive your simulation is to it.

Glenn Horrocks

Thank you very much for your reply. Could you please tell me bit more about your simulations. What was the booundary conditions, geometry, material properties you have used and so on.

Thanks in advance

Hi,

It was a hydraulic oil circuit. It had no boundaries (except walls), the fluid motion was created by pistons moving. The geometry had a number of chambers connected by holes. The material properties were defined using a variable density, with the density being a function of pressure. The function was the definition of bulk modulus for a fluid. Search wikipedia for bulk modulus for more info.

Glenn Horrocks