cfx two phase semi piercing cylinder

CFD-fellow · April 23, 2014, 07:27

Hi
I am modeling flow over a semi piercing cylinder, which its picture is attached. these are my most important questions:
> Homogeneous or unhomogeneous model should i choose for my multiphase model?
> And free surface or mixture model for water and air?
> Can free surface model predict air bubbles in water like the picture, or may be i should choose air as dispersed fluid in water?

any guidance would be appreciated.

ghorrocks · April 24, 2014, 04:38

The foamy region can be modelled using a very fine mesh with a homogenous model, or a coarser mesh with a inhomogenous model. The inhomogenous model is probably more likely to be managable.

You can use a free surface model with the inhomogenous model. In this case you have air and water as phases, and also air bubbles in water.

CFD-fellow · April 24, 2014, 14:27

Hi Glenn
Thanks for your reply. now i have an important question:
>Imagine such a experimental test have been done on a typical geometry with 30m/s velocity(for example).In experiment they move the body, but in numerical analysis i want to move the water and air over the body.So in this situation i use velocity inlet for inlet and outlet with known static pressure for outlet.

>The question is for outlet and under cavitation condition:we have 0.5*ro*v^2 as dynamic pressure and this value for water is not negligible for 30m/s velocity.What static pressure should i set for outlet?

***If i set 1atm plus static pressure of water height for outlet,does numerical solution(without considering numerical errors and modeling) give the same cavitation bubbles as experiment or in other word would my pressure distribution be like experimental pressure distribution?

Im sorry if my question is long.

Best regards

ARohit · April 25, 2014, 13:52

Respected sir,

I did air water flow simulation. solution converged also but it
" exited without writing result file. with return code zero " why this problem is coming.

I have attached image below.

your obediently
Rohit

ghorrocks · April 26, 2014, 08:29

You never mentioned this flow might cavitate. That makes things a lot more complex (they already were pretty complex). You are trying to model free surface flow with a foamy section and cavitation - this is a very complex model and I hope you have a lot of experience in CFX to get this working. If you are a beginner - then I recommend you try easier models of each of these sub-models by themselves (free surface, foamy flow, cavitation) before you attempt to combine them together.

You last post suggests it converged for a bit then flat lined, then crashed. This FAQ may help: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

CFD-fellow · April 26, 2014, 09:18

Hi glenn
That post which shows a crash after a relative convergency is not mine. Its for Arohit who has not post a new thread. Im a Fluent user, but now im FORCED to model this test with CFX. I will activate cavitation after full convergency, but my question was for pressure distribution.
What static pressure should i set for outlet?(in second post i have described completely)
Regards

ghorrocks · April 27, 2014, 07:31

Have a look at the hydrofoil tutorial example. That is an example of an outlet boundary in free surface flow with cavitation.

CFD-fellow · May 12, 2014, 05:28

Hi Glenn
Ive done the free surface model without cavitation and overcome its problems. All the pressure and velocity distributions are logical and predictable. Now its time to activate cavitation. When I activate cavitation there are a lot of vapor clouds in my domain(even in air) but residuals are converging somehow smoothly. Is it possible to get the write answer by continuing this procedure?

Do have any tricky solution for converging this cavitation problem?
time step:1e-5
iterations per time step: 12
**It is continuing up to 60 time steps and the vapor volume in the domain doesnt decrease.

ghorrocks · May 12, 2014, 08:18

You need very small time steps to converge cavitation models. Also double precision numerics, a good quality mesh and a good initial condition. Also cavitation models often do not converge nicely in steady state and require transient models to converge.

I think there might also be some more subtle settings to help cavitation but you would have to contact ANSYS support for those.

April 24, 2014, 14:27		#3
CFD-fellow Senior Member Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13	Hi Glenn Thanks for your reply. now i have an important question: >Imagine such a experimental test have been done on a typical geometry with 30m/s velocity(for example).In experiment they move the body, but in numerical analysis i want to move the water and air over the body.So in this situation i use velocity inlet for inlet and outlet with known static pressure for outlet. >The question is for outlet and under cavitation condition:we have 0.5rov^2 as dynamic pressure and this value for water is not negligible for 30m/s velocity.What static pressure should i set for outlet? **If i set 1atm plus static pressure of water height for outlet,does numerical solution(without considering numerical errors and modeling) give the same cavitation bubbles as experiment or in other word would my pressure distribution be like experimental pressure distribution? Im sorry if my question is long. Best regards Last edited by CFD-fellow; April 25, 2014 at 17:39.*

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April 24, 2014, 04:38		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	The foamy region can be modelled using a very fine mesh with a homogenous model, or a coarser mesh with a inhomogenous model. The inhomogenous model is probably more likely to be managable. You can use a free surface model with the inhomogenous model. In this case you have air and water as phases, and also air bubbles in water.

April 26, 2014, 08:29		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	You never mentioned this flow might cavitate. That makes things a lot more complex (they already were pretty complex). You are trying to model free surface flow with a foamy section and cavitation - this is a very complex model and I hope you have a lot of experience in CFX to get this working. If you are a beginner - then I recommend you try easier models of each of these sub-models by themselves (free surface, foamy flow, cavitation) before you attempt to combine them together. You last post suggests it converged for a bit then flat lined, then crashed. This FAQ may help: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

April 26, 2014, 09:18		#6
CFD-fellow Senior Member Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13	Hi glenn That post which shows a crash after a relative convergency is not mine. Its for Arohit who has not post a new thread. Im a Fluent user, but now im FORCED to model this test with CFX. I will activate cavitation after full convergency, but my question was for pressure distribution. What static pressure should i set for outlet?(in second post i have described completely) Regards

April 27, 2014, 07:31		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Have a look at the hydrofoil tutorial example. That is an example of an outlet boundary in free surface flow with cavitation.

May 12, 2014, 05:28		#8
CFD-fellow Senior Member Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13	Hi Glenn Ive done the free surface model without cavitation and overcome its problems. All the pressure and velocity distributions are logical and predictable. Now its time to activate cavitation. When I activate cavitation there are a lot of vapor clouds in my domain(even in air) but residuals are converging somehow smoothly. Is it possible to get the write answer by continuing this procedure? Do have any tricky solution for converging this cavitation problem? time step:1e-5 iterations per time step: 12 **It is continuing up to 60 time steps and the vapor volume in the domain doesnt decrease.

May 12, 2014, 08:18		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	You need very small time steps to converge cavitation models. Also double precision numerics, a good quality mesh and a good initial condition. Also cavitation models often do not converge nicely in steady state and require transient models to converge. I think there might also be some more subtle settings to help cavitation but you would have to contact ANSYS support for those.