[erkampasha]

Dear Users,

I try to perform cfd analysis in creo flow simulation. In demo simulation video, at outlet boundary condition, volumetric flow rate is specified. I did not understand that how can this boundary type can be applied at outlet? As i know that at outlet boundary, the values are extrapolated from domain but in that demo video boundary value is specified. And how do we know the value of outlet boundary at the beginning of the analysis?

I will be really happy if someone help me

[aero_head]

My guess is hand calculations.

[FliegenderZirkus]

For a given centrifugal pump at fixed impeller RPM, you can plot a curve on a chart with Volume flow on the x axis and Head on the y axis. In other words, the pump RPM is not enough to put in your CFD case, you still need to specify either the flow rate or the Head (pressure rise) to get a point on the curve. Whether you specify the volume flow on your inlet or outlet is mostly a matter of where the constant velocity assumption is more representative, but mathematically either is fine. There is lot of nuance in this, especially when the medium is compressible, and I'm no expert in turbomachinery, so this is maybe not 100% exactly put, but I hope this was the key misunderstanding you had, namely that fixing a flowrate doesn't mean you somehow prescribe the result, because the pressure rise (and efficiency) is still unknown.

[erkampasha]

Originally Posted by aero_head

My guess is hand calculations.

I do not think so, because i am sure that the physical phenomenas effects the outlet boundary. At least outlet boundary value should not be uniform. This is how i think it over.

[erkampasha]

Originally Posted by FliegenderZirkus

For a given centrifugal pump at fixed impeller RPM, you can plot a curve on a chart with Volume flow on the x axis and Head on the y axis. In other words, the pump RPM is not enough to put in your CFD case, you still need to specify either the flow rate or the Head (pressure rise) to get a point on the curve. Whether you specify the volume flow on your inlet or outlet is mostly a matter of where the constant velocity assumption is more representative, but mathematically either is fine. There is lot of nuance in this, especially when the medium is compressible, and I'm no expert in turbomachinery, so this is maybe not 100% exactly put, but I hope this was the key misunderstanding you had, namely that fixing a flowrate doesn't mean you somehow prescribe the result, because the pressure rise (and efficiency) is still unknown.

I did not say that outlet boundary is related to our result. I am not expert in turbomachinery but i can guess that we cannot prescribe uniform flux on the outlet boundary. How is that possible? My wonder is that. The flux value on the chart you ve mentioned needs to be uniform.

[FliegenderZirkus]

OK I understand now. You're right that the constant velocity BC is a simplification, but often it's good enough, assuming you place it far enough downstream from where your phenomena of interest take place. Say, the pump volute ends with a circular cross section. Then you might extrude this cross section, forming a straight pipe and apply the BC at the end of that pipe. Similarly at the inlet. There are other options of course, but this one is quite common.

[erkampasha]

Originally Posted by FliegenderZirkus

OK I understand now. You're right that the constant velocity BC is a simplification, but often it's good enough, assuming you place it far enough downstream from where your phenomena of interest take place. Say, the pump volute ends with a circular cross section. Then you might extrude this cross section, forming a straight pipe and apply the BC at the end of that pipe. Similarly at the inlet. There are other options of course, but this one is quite common.

I see. Thank you very much for explaining