Learning Forum

[Amruthkiran Hegde]

Hello, in CFX, for my geometry (which is a pipe), I assigned a 'Reference pressure' of 20 psig as 'Total Pressure' at the inlet. For the outlet, I assigned  'Opening' as type of boundary condition and assigned 0 psig as 'Reference pressure'. I then ran the simulation. Although the x,y and z momentum residuals oscillate around 0.0001, the mass residual oscillates around 0.001 after 10000 interations. Nevertheless, I stopped the simulation and measured the total pressure and static pressure at the inlet which were 19.94 psig and 16.09 psig respectively. Since I am assuming that the values of both the pressures should be the same at the inlet, I am wondering whether it is because of the fact that the residual value is not small enough. It would be great to have some insight on this.

[DrAmine]

Why should the value of total pressure and static pressure be same at the inlet?

[Amruthkiran Hegde]

Hello Dr.Amine, I though that the total pressure should be approximately equal to the static pressure at the inlet as the fluid is at rest right at the inlet. 

[DrAmine]

So it is drawing fluid at rest into the domain. They should be approximntely equal but that will be result of the simulatuion and depends on the pressure field evolution. I think you should more try to drive your case into deeper convergence and assess all aspects of numerical errors before evaluating the results :).

The combination of BC's you are using is sensitive to initial guess. 

[Amruthkiran Hegde]

Thank you for the suggestion. Do you think that a finer grid (currently the element size is 0.5 mm) and higher number of inflation layers (currently the number is 10) may resolve the issue?

[DrAmine]

Avoiding large normal gradients at opening is always good. I won't bother much about the values if you are setting the right BC'S and convergence is deep enough. To draw air from outside then Total Pressure Opening is required with same value of pressure as outside pressure.