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 dc February 15, 2008 12:32

cavitation

hi,

does the default cavitation model take care of the effect of turbulence by changing the vapour pressure from that of the saturation pressure? or, do we need to write a udf for it? if so, how to write this udf?

thanks,

dc

 NRD February 16, 2008 14:28

Re: cavitation

Hello,

The cavitation model in FLUENT is based on the model by Singhal et al, Journal of Fluids Engineering, 2002. This model accounts for turbulence by simply raising the phase change pressure threshold. The details are also given in the FLUENT manual.

My understanding is this effect is already accounted in the model.

For the turbulent case that I ran, I had to account for reduction in mixture viscosity due to cavitation using an UDF. If interested let me know and I can share my results.

 dc February 16, 2008 23:16

Re: cavitation

thanks. it'll be great if you can share your results with me. you may mail me directly the same.

dc

 dc February 16, 2008 23:24

Re: cavitation

i have read Singhal's paper as well as the Fluent User Manual. But I am confused about a statement given in the UDF manual of Fluent. I quote it for your consideration.

"The following UDF named user cav rate, is an example of a cavitation model for a multiphase mixture that is di erent from the default model in FLUENT. This cavitation model calculates the cavitation mass transfer rates between the liquid and vapor phase depending on liquid pressure (*p), turbulence kinetic energy (C K(c,t)), and the liquid vaporization pressure (*p v). In general, the existence of turbulence enhances cavitation. In this example, the turbulence e ect is taken into account by increasing the cavitation pressure by 0.195* C R(c,t) * C K(c,t)."

You may note that this UDF is different from that of the default cavitation mass transfer model and here turbulence is taken care of in accordance with Singhal's paper.

Looking forward to hear from you.

Thanks,

dc

 NRD February 17, 2008 00:08

Re: cavitation

Thanks for the note.

I believe the UDF example that is given in the FLUENT manual is similar to Singhal's model (p_turb =0.195*rho*k instead of 0.39*rho*k) which is already included in the model.

I had run FLUENT simulations of cavitation in a converging-diverging geometry using k-eps RNG model. Formation of vapor leads to decrease in mixture viscosity. This is not accounted in the turbulence model. I wrote an UDF which calculates the viscosity as a function of density. When I did not account for this effect, the flow would laminarize.

A detailed explanation can be found in the paper by Coutier-Delgosha et al, Journal of Fluids Engineering, 2003.

 Karim March 8, 2008 23:38

Re: cavitation