[WilbertS]

Hi everyone,

I am working on a simulation of a valve connected to an discharge pipe, where I try to find the mass flow and velocity profile at the exit of this discharge pipe.
I keep the valve at a certain fixed opening (30%). At the inlet I set up a total pressure of 40 bar and at the outlet I set an opening pressure of 1 bar (absolute).

I initially did a run (Steady State - no Cavitation) with a smaller valve and everything seemed reasonable (reasonable expected massflow and the velocity profile was similar to what I expected and to literature).

With a larger valve, however, I noticed an unrealistic exit velocity profile and large zones with negative (absolute) pressure. As a result I turned on cavitation (water + water vapor) with steady state. With cavitation I get however:

- At the outlet of the discharge pipe (opening type, that allows only inflow of water) an inflow/backflow of water. There is no mass flow going out of the discharge pipe, hence large mass imbalances;
- Very large zones of water vapor (cavitation), especially near the outlet. The amount of water vapor does seem excessive.

I tried turning on total energy, but that did not change anything.

I searched online and in this forum and often I read that the backflow with cavitation can be solved by:
- Setting a lower opening pressure, or;
- Performing a transient simulation;
- I read that putting the outlet further downstream often does not work / resolve the problem.

The problem that I see with a transient simulation is that my model is quite large (discharge tube has a diameter of 2 meters and 8 meters long. The valve is relatively small. Element size 8-12 mm. In total I have 100 million elements. The flow velocity is locally very high, especially at the valve opening speeds can go above 50 m/s) and I read that for cavitation you need small elements and very small timesteps.

So to summarize:
- Does my backflow problem stem indeed from the fact that for accurate simulation of the cavitation you need a transient simulation?
- Using transient flow, I presume I should use adaptive time step. What is a suggested minimum time step or at least starting point for my time step for cavitation (I read someone posting 1E-6 sec, that seems very low)? The total flow-through time of the valve + discharge tube is around 3 seconds.
- Does it make sense to increase the element size (thus reducing the total amount of elements) to be able to run a simulation within a reasonable time? As increase the element size, reduces the accuracy of capturing caviation. Also the valve opening is relatively small, thus I need at least local refinement to capture the flow accurately.

I ran the 100 million steady state model with 100 iterations in 15 hours, so my computer is quite capable of handling a bit of calculations.

I hope I gave enough information. In case something is missing or needs more explanation, please let me know!