[Opaque]

Hydraulic jump in incompressible flow is equivalent to a shock wave in compressible flow. That means, it is a very thin region in the flow direction with rapid changes (if not sudden) of the solution variables.

How good are the current results to predict the start and end of the jump region ? Predicting the height is a very sensitive function of the mesh and the free surface model being used.

Hope the above helps,

[yaseen wsu]

Originally Posted by Opaque

Hydraulic jump in incompressible flow is equivalent to a shock wave in compressible flow. That means, it is a very thin region in the flow direction with rapid changes (if not sudden) of the solution variables.

How good are the current results to predict the start and end of the jump region ? Predicting the height is a very sensitive function of the mesh and the free surface model being used.

Hope the above helps,

thanks
imagine, 7 cm error in jump is very, if only one or two cm error I can say due to mesh size, I think related to other things, in most free surface flow in CFX I think elurian multiphase (homogeneous with standard free surface is the best choice).

[flow_CH]

Originally Posted by yaseen wsu

I used RSM but its result similar to other turbulent models
at the end of channel there is 8 cm difference with experiment, which is very high, I dont know why. is it possible to say CFX (RANS, with turbulent model) does not work in hydraulic jump.
my mesh and all BC are correct only Iam not sure about outlet B.C
I used outlet BC at the end of channel (at drop) with super critical, and at bottom of channel with static pressure = 0

https://drive.google.com/open?id=0B3...0h4VzBLUGJEVlk
https://drive.google.com/open?id=0B3...kM3T3QzZUdqaWs
https://drive.google.com/open?id=0B3...EtpVWZseGVTdUE

Hello.
I want to simulate an spillway with different turbulent models in Fluent but I wanted to know the differences of volume fraction of water and air with changing the turbulence models, in advance. Are there any difference in water surface when you change turbulence models? I must say I use VOF multiphase model.
Thank you

[ghorrocks]

Note this is the CFX forum. Try the Fluent forum for questions on Fluent. Having said that I note your question is a generic question on turbulence models which is applicable to either code.

[flow_CH]

Originally Posted by ghorrocks

Note this is the CFX forum. Try the Fluent forum for questions on Fluent. Having said that I note your question is a generic question on turbulence models which is applicable to either code.

Ok. I will ask it on fluent forum. But as you said this is a generic question, if I get the answer from yaseen wsu, I can start my simulation in fluent because it is difficult to find someone in forum who has simulated a free surface flow like hydraulic jump with different turbulence models. Because, usully most people simulate their project with only one turbulence model.

[yaseen wsu]

Originally Posted by flow_CH

Ok. I will ask it on fluent forum. But as you said this is a generic question, if I get the answer from yaseen wsu, I can start my simulation in fluent because it is difficult to find someone in forum who has simulated a free surface flow like hydraulic jump with different turbulence models. Because, usully most people simulate their project with only one turbulence model.

Hydraulic jump is a complex phenomenon, turbulence models have great influence on the accuracy of the results, in my view RANS with turbulence model, cant accurate predict of free surface at hydraulic jump

[flow_CH]

Originally Posted by yaseen wsu

Hydraulic jump is a complex phenomenon, turbulence models have great influence on the accuracy of the results, in my view RANS with turbulence model, cant accurate predict of free surface at hydraulic jump

Thanks for your reply.
I do not mean the accuracy of flow surface. I want to know that is there any difference for flow free surface for a specific section of model when you change turbulence model? For example when you change from RNG to STl or any other turbulence models, is there any difference in volume fraction for that section? Or they are exactly the same? My aim is to compare free surface of flow for different turbulence models.

[ghorrocks]

You seem to be talking at crossed purposes. Yaseen said that in his opinion RANS turbulence models cannot predict the free surface at the hydraulic jump accurately (I am no expert on hydraulic jump modelling but this sounds likely)

Whereas Saeed said if you change the turbulence model does it change the free surface location (I am sure the answer to this is yes, especially for grossly different models like RNG versus Reynolds Stress models)

[flow_CH]

Originally Posted by ghorrocks

You seem to be talking at crossed purposes. Yaseen said that in his opinion RANS turbulence models cannot predict the free surface at the hydraulic jump accurately (I am no expert on hydraulic jump modelling but this sounds likely)

Whereas Saeed said if you change the turbulence model does it change the free surface location (I am sure the answer to this is yes, especially for grossly different models like RNG versus Reynolds Stress models)

I was not absolutely sure that by changing the turbulence model whether the surface will be changed or not but I had to ask that because I wanted to be quite sure as the numerical time cost is high.

[ghorrocks]

If you want to be "quite sure" then why are you asking people you have never met on an internet forum? How can you trust their answers? Isn't the only way to be "quite sure" to run the models yourself, or to read about them in a respected journal?

[flow_CH]

Originally Posted by ghorrocks

If you want to be "quite sure" then why are you asking people you have never met on an internet forum? How can you trust their answers? Isn't the only way to be "quite sure" to run the models yourself, or to read about them in a respected journal?

I tried journals in advance, but there was no information about my question. At least, maybe I was not able to find.
Did I have any other choice when I cannot start the simulation except other people's experience? If we do not trust the research conducted in literature by other people, we would not continue to make progress the current knowledge.
I am sorry if I chose uncommon way. Maybe I should review the way I look at people.

[ghorrocks]

I don't mean to cause disrespect and certainly there is nothing to apologise for. I was just trying to point out that answers given on online forums can be wrong and there are no checks and balances to correct mistakes. Sometimes forums work well and you get good answers, sometimes you don't - and it can be hard to tell at first glance whether you got a good answer or not.