[vysje]

Hi,

I have done some numerical simulations in Ansys CFX on a high speed flow (7 m/s) in a small nozzle (mm scale). The pressure of the water in the nozzle is of importance. However, I am not sure how to interpret the results.

For the simulations without cavitation the pressure in the throat gets to -5 bar. A have read that a negative liquid pressure is physical (liquid tension) and is related to the onset to cavitation. However, when including cavitation in the simulation, the pressure profile is very different. Not only at the location where cavitation takes place (after the throat), but also before and at the throat the pressure is much higher. I would expect the pressure profile to be the same for the locations where there is no cavitation. It is almost like Ansys is not 'allowed' to present negative pressures when there is a gas in the system and therefore it shifts the graph upwards.

I have included graphs showing the pressure profile for the simulation with and without cavitation, the vapor fraction and a colormap showing the nozzle shape used.

So to summarize my question: if a negative water pressure is a physical phenomenon, why wouldn't Ansys allow it in a system where cavitation is included?

I have always considered negative pressure to be a non-equilibrium situation, thus could it be that there is a negative pressure in the throat, but that the formation of bubbles takes so much time that there is only cavitation after the throat?

Thank you a lot for your help!

greetings,
Joy

PS. the nozzle with the same conditions as in the simulation was used in experiments, however, it was not possible to measure any pressure data. It was attached to tap water, which would suggest an inlet pressure closer to 3 than to 8 bar.