Cavitiy Length for NPSHi calculation

zacko · October 5, 2022, 04:12

Hello,

is there a way to calculate the cavity length in a pump simulation with the cavitation model ?
I would even like to stop the solver when a certain cavity length has been reached, so I can calculate the NPSHi for a given flow.

I tried to work with IsoClips in CFD Post and use the length expression, but that is not very clear to me, wether this is a correct or suitable approach.
Has anybody done this before ?

Thank you your help!
Daniel

ghorrocks · October 8, 2022, 04:14

If you define an isosurface of volume fraction you can get the volume which has gone to vapour. But you will need to define how you want to convert that volume to a length - possible approaches could be maximum extent in x, y z, or a specific direction; longest extent; cube root of the volume - or maybe some other definition is suitable for your case.

zacko · October 10, 2022, 02:44

Thank you, Glenn!
Indeed it worked with the Volume Fraction.
I created a polyline on the hub and shroud (later additional lines from hub to shroud) and plotted the "Curve Length" of that polyline and calcualted the length where the volume fraction is greater than zero.

October 5, 2022, 04:12	Cavitiy Length for NPSHi calculation	#1
zacko Senior Member Daniel Join Date: Feb 2017 Location: Germany Posts: 147 Rep Power: 9	Hello, is there a way to calculate the cavity length in a pump simulation with the cavitation model ? I would even like to stop the solver when a certain cavity length has been reached, so I can calculate the NPSHi for a given flow. I tried to work with IsoClips in CFD Post and use the length expression, but that is not very clear to me, wether this is a correct or suitable approach. Has anybody done this before ? Thank you your help! Daniel

October 8, 2022, 04:14		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If you define an isosurface of volume fraction you can get the volume which has gone to vapour. But you will need to define how you want to convert that volume to a length - possible approaches could be maximum extent in x, y z, or a specific direction; longest extent; cube root of the volume - or maybe some other definition is suitable for your case. zacko likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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October 10, 2022, 02:44		#3
zacko Senior Member Daniel Join Date: Feb 2017 Location: Germany Posts: 147 Rep Power: 9	Thank you, Glenn! Indeed it worked with the Volume Fraction. I created a polyline on the hub and shroud (later additional lines from hub to shroud) and plotted the "Curve Length" of that polyline and calcualted the length where the volume fraction is greater than zero.