How can I detect cavitation presence while the simulation is still running?

Stabum · May 16, 2015, 06:53

Hello
I need to design the blades for an axial pump whose flow rate, head and RPM is known.
Obviously the pump should not caviate.

My question is double:
1) Let's suppose I could "probe" the static pressure on the most critical region of the pump blade and that I could monitor this variable during the simulation process. At what residual level (or at what convergence level) do the local static pressure (in case of cavitation of course) become reliable? I mean: should I wait to reach 1E-7 MAX residual or 1E-4 is enough for this kind of information?
2) The blade grid has been created with TGrid. Is there a way I could DYNAMICALLY monitor a variable (like "static pressure" in the most critical region of the blade) during the simulation, so that if I see that the value gets stable (and near to 2500 Pa) I can say: ok, there's cavitation, I need to modify the blades, let's stop the simulation!

I hope I've been "understandable". If not, then please feel free to ask and I'll try to be more clear.

As Always, thanks so much for your help!!!

ghorrocks · May 16, 2015, 19:44

1) the convergence required to achieve a reliable answer is obviously different for different applications. For some people 1% error could be all they can tolerance, but for others being off by 20% is OK. On top of that, a given level of residual does not equate to a universal accuracy number - the relationship between accuracy and residuals is not fixed.

This is a long way of saying you should determine the level of residual required for your application and your accuracy requirement. Do a sweep of convergence tolerance and monitor values of importance to you - then you can select the tolerance which gives you the accuracy you require.

2) Use a monitor point. You could put it at a defined point like you suggest, or you could just use minVal(p)@domain and it will return the minimum pressure in the entire domain.

Stabum · May 17, 2015, 10:59

Thanks so much for your super useful answer. I understand that residuals and convergence are always delicate concepts. Can you tell me something more about the monitor point? How can I locate it? It would be perfect for my case. Thanks again

ghorrocks · May 17, 2015, 20:08

Monitor points are described in the documentation and used in the tutorial examples.

Have a look on the output tab in CFX-Pre.

Stabum · May 18, 2015, 01:57

Ok, thanks. I'm checking out right now. Thank again!!!

Stabum · May 18, 2015, 19:38

I used the monitor points as a probe in given locations on the blade, as you suggested. Everything works so nice! Thanks, ghorrocks, you saved my life!

May 16, 2015, 06:53	How can I detect cavitation presence while the simulation is still running?	#1
Stabum Member Join Date: Aug 2012 Location: Italy Posts: 68 Rep Power: 14	Hello I need to design the blades for an axial pump whose flow rate, head and RPM is known. Obviously the pump should not caviate. My question is double: 1) Let's suppose I could "probe" the static pressure on the most critical region of the pump blade and that I could monitor this variable during the simulation process. At what residual level (or at what convergence level) do the local static pressure (in case of cavitation of course) become reliable? I mean: should I wait to reach 1E-7 MAX residual or 1E-4 is enough for this kind of information? 2) The blade grid has been created with TGrid. Is there a way I could DYNAMICALLY monitor a variable (like "static pressure" in the most critical region of the blade) during the simulation, so that if I see that the value gets stable (and near to 2500 Pa) I can say: ok, there's cavitation, I need to modify the blades, let's stop the simulation! I hope I've been "understandable". If not, then please feel free to ask and I'll try to be more clear. As Always, thanks so much for your help!!!

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May 16, 2015, 19:44		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	1) the convergence required to achieve a reliable answer is obviously different for different applications. For some people 1% error could be all they can tolerance, but for others being off by 20% is OK. On top of that, a given level of residual does not equate to a universal accuracy number - the relationship between accuracy and residuals is not fixed. This is a long way of saying you should determine the level of residual required for your application and your accuracy requirement. Do a sweep of convergence tolerance and monitor values of importance to you - then you can select the tolerance which gives you the accuracy you require. 2) Use a monitor point. You could put it at a defined point like you suggest, or you could just use minVal(p)@domain and it will return the minimum pressure in the entire domain.

May 17, 2015, 10:59		#3
Stabum Member Join Date: Aug 2012 Location: Italy Posts: 68 Rep Power: 14	Thanks so much for your super useful answer. I understand that residuals and convergence are always delicate concepts. Can you tell me something more about the monitor point? How can I locate it? It would be perfect for my case. Thanks again

May 17, 2015, 20:08		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Monitor points are described in the documentation and used in the tutorial examples. Have a look on the output tab in CFX-Pre.

May 18, 2015, 01:57		#5
Stabum Member Join Date: Aug 2012 Location: Italy Posts: 68 Rep Power: 14	Ok, thanks. I'm checking out right now. Thank again!!!

May 18, 2015, 19:38		#6
Stabum Member Join Date: Aug 2012 Location: Italy Posts: 68 Rep Power: 14	I used the monitor points as a probe in given locations on the blade, as you suggested. Everything works so nice! Thanks, ghorrocks, you saved my life!