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-   -   ~ Cavitation in nozzles ~ (http://www.cfd-online.com/Forums/fluent/41546-cavitation-nozzles.html)

 Leigh June 28, 2006 04:47

~ Cavitation in nozzles ~

Hello all,

I am a beginer at using cavitation in FLUENT, and so I am having trouble in a number of areas.

I have used the "mixture multi-phase model" with two phases of liquid water and air. I am expecting to see the area just after the sharp inlet of the nozzle to display a decrease in desity around the walls to display the effect of cavitation. I cannot seem to get this result, plus my iterations are having difficulty in converging.

Any ideas????

Thanks, Leigh

 fluent June 28, 2006 13:14

Re: ~ Cavitation in nozzles ~

have you turned on compressible gas for the gas phase?

you also need to take in accoutn temperature variation, are you solving for temperature?

fluent

 Leigh June 28, 2006 13:43

Re: ~ Cavitation in nozzles ~

Hello,

I was not aware that you have to turn on the compressable gas, I was just using a incompressable fluids, is this necessary when finding cavitation?

I also thought that cavitation was due to large pressure changes, the change in temperature will be minimum in my model, is it still neccesary to solve for temperature??

 fluent June 29, 2006 15:34

Re: ~ Cavitation in nozzles ~

did not you read theory about cavitation? what is the temperature and pressure rise in the bubbles ?

 mateus June 30, 2006 04:56

Re: ~ Cavitation in nozzles ~

Hi Leigh!

I hope you got my paper. You donīt really need to use compressible fluids when modelling cavitation. I mean if you will consider the vapour to be compressible you really didnīt do much. What you should do (but cannot do in Fluent) is consider the compressibility of mixture of vapour and liquid water. If the mixture is right (for example 50/50) you can get sound speed at as low as 20m/s. You can get arround that using artifficially low turbulnt viscosity (check the paper) - then you can simulate most of the cavitation effect but not shock waves for example... Appart from temperature consideration you can treat cavitation process as isotherm (if youīre not dealing with cryogenic liquids like liquid helium). The heat spent at bubble growth will be produced at bubble collapse. In any case forget about whatīs going on in bubbles since theyīre to small to considere anyway - the model only uses bubble dynamics for mass source and sink terms in vapour fraction transport equation...

I hope my post isnīt to confuzing.

by MATEUS

 I.Kane July 2, 2006 17:05

Re: ~ Cavitation in nozzles ~

like you will predict the correct bubble size and volume displacement and volume fraction if you don't take in acccunt the compressibility of the gas...

 mateus July 3, 2006 00:44

Re: ~ Cavitation in nozzles ~

Hi!

Of course you will not. But you have to draw a line somewhere...it's not possible for example to simulate each bubble, since it's to small...

We do the same thing with turbulence modeling...it's wrong but produces usable results...

mateus

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