Learning Forum

[SneOG]

Hello,

I am trying to perform a density-based CFD simulation of a Laval nozzle. I have set the inlet pressure to 3,000,000 Pa, but after the calculation, the inlet pressure value is always below the value I set. Can someone help me with this issue?

[Rob]

What are you setting, and what are you reporting?

[SneOG]

I set the inlet boundary conditions to a pressure inlet with a gauge total pressure of 3,000,000 Pa and a supersonic/initial gauge pressure of 3,000,000 Pa.

I set the outlet boundary conditions to a pressure outlet with a gauge total pressure of 101,325 Pa.

I chose oxygen as the working fluid and set the density to ideal gas.

For the report, I looked at the static pressure contours and plotted an XY graph along the centerline of the nozzle. I expected to see a pressure of 3,000,000 Pa at the inlet, but the observed value was lower.

Is this enough information?

Thank you in advance.

[Rob]

How much lower, and as the inlet is Total pressure have you checked that? Has the solution converged? Depending on how you initialised the model you may not be seeing the final result for a fairly high number of iterations. 

[SneOG]

 

The static inlet pressure varies with the diameter of the nozzle throat from 100,000 up to 500,000 Pa. Could it be that the inlet pressure I set is the total pressure?

If that’s the case and I don’t know the inlet speed, how am I supposed to determine the correct total pressure for different geometries? The volumetric flow rate and thus the speed, changes depending on the nozzle shape, so I can’t predict which flow rate will prevail at each simulation.

Additionally, my simulation hasn't converged either. I have set the residuals to 1e-6 for convergence.