Sharp Edged Pipe with Negative Pressure

Question

I want to calculate the 2D pipe flow with a sharp-edged contraction pipe (see below figure). The fluid is water, the model is steady state k-epsilon. The boundary condition is velocity inlet (8.47ft/s = 2.58m/s) and pressure outlet (0 psi = 0 Pa). Friction is not taken into account on the wall.

The result shows negative pressure at the sharp edge of the contraction. Would you tell me why the negative pressure occurs in this calculation?

I think it's because I don't take cavitation into account at the moment. Do you think this is correct? Mesh size might be an issue, but the result didn't change much when I used more fine mesh. Is there any possibility that the mesh is too fine, maybe with respect to Courant number in the transient calculation?

Answer 1

I guess you are in incompressible flow. When you solve incompressible flow, only the pressure gradient is relevant, not the absolute value of the pressure itself.

The pressure values you are seeing in your solution are relative (see Cell Zone Conditions > Operating Conditions).

As you have set the outlet to a pressure of 0 Pa, if you get negative pressure regions in your domain, what it means is that those regions have lower pressure than the outlet. (And so, the regions showing a positive pressure are regions with a pressure higher than the outlet.)