Choosing Unsteady RANS model

dskvsp · December 14, 2021, 22:44

Hi,
I would like extract unsteady pressures of a water wave.
I am suggested in another forum to choose URANS (Unsteady Navier stokes) solver to get the needed unsteady pressures.
Can some one let me know how I can choose this solver from fluent tool.
I find several models, k-omega, k-epsilon etc. But hiw can I select them to be capture unsteady pressures.

LuckyTran · December 15, 2021, 18:55

It's in general solver settings. The default is steady. Choose transient.

It's the same place where you choose pressure-based vs density-based. Underneath the scale mesh, check mesh, and report mesh quality.

If you choose transient and then use a RANS model, it becomes URANS.

dskvsp · December 15, 2021, 22:40

Hi Tran, thank for the info,

You have shared some info on unsteady pressures in recent forum but as I understand from the solution using transient and pressure based rans, .
The time derivative of velocity (temporal acceleration) is not being computed in the solution.
It's integral through the displacement along streamlines should give unsteady pressures.
I would like to know how I can capture and extract these pressures.

Pls see the link for image for reference on unsteady pressures.
https://images.app.goo.gl/jtvwRNoxMcg1viLe8

Thank you

LuckyTran · December 15, 2021, 23:07

1) After temporal discretization there are no more terms involving time derivatives in the governing equations. That's why they are not calculated They're not calculated because they're not needed to be calculated. Sometimes there is an exception to this in the energy equation when it is written in a specific form but that's not the case we are talking about here.

2) You have the unsteady pressure from Fluent. The problem is you are asking for more. It's up to you to calculate the time-derivative of arbitrary quantities using your own arbitrary temporal discretization schemes. CFD gives you pressure as a function of time, the rest is up to you.

3) What is the purpose of integrating the velocity to get pressure only for you to differentiate it again to get the time-derivative of pressure. If you just listen to yourself, you know you already have it in the form of velocity. Actually there is an impedence involved but I have to say...

why do ya'll keep trying to find all sorts of workarounds for calculating the time-derivative of pressure instead of just doing just that? And please stop calling it unsteady pressure.

dskvsp · December 31, 2021, 20:00

Hi tran,
I could get things now,
Thank u for the explanation.
And I wish u a happy and prosperous 2022.

December 14, 2021, 22:44	Choosing Unsteady RANS model	#1
dskvsp New Member suresh Join Date: Dec 2021 Posts: 11 Rep Power: 5	Hi, I would like extract unsteady pressures of a water wave. I am suggested in another forum to choose URANS (Unsteady Navier stokes) solver to get the needed unsteady pressures. Can some one let me know how I can choose this solver from fluent tool. I find several models, k-omega, k-epsilon etc. But hiw can I select them to be capture unsteady pressures.

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December 15, 2021, 18:55		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	It's in general solver settings. The default is steady. Choose transient. It's the same place where you choose pressure-based vs density-based. Underneath the scale mesh, check mesh, and report mesh quality. If you choose transient and then use a RANS model, it becomes URANS.

December 15, 2021, 22:40		#3
dskvsp New Member suresh Join Date: Dec 2021 Posts: 11 Rep Power: 5	Hi Tran, thank for the info, You have shared some info on unsteady pressures in recent forum but as I understand from the solution using transient and pressure based rans, . The time derivative of velocity (temporal acceleration) is not being computed in the solution. It's integral through the displacement along streamlines should give unsteady pressures. I would like to know how I can capture and extract these pressures. Pls see the link for image for reference on unsteady pressures. https://images.app.goo.gl/jtvwRNoxMcg1viLe8 Thank you

December 15, 2021, 23:07		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	1) After temporal discretization there are no more terms involving time derivatives in the governing equations. That's why they are not calculated They're not calculated because they're not needed to be calculated. Sometimes there is an exception to this in the energy equation when it is written in a specific form but that's not the case we are talking about here. 2) You have the unsteady pressure from Fluent. The problem is you are asking for more. It's up to you to calculate the time-derivative of arbitrary quantities using your own arbitrary temporal discretization schemes. CFD gives you pressure as a function of time, the rest is up to you. 3) What is the purpose of integrating the velocity to get pressure only for you to differentiate it again to get the time-derivative of pressure. If you just listen to yourself, you know you already have it in the form of velocity. Actually there is an impedence involved but I have to say... why do ya'll keep trying to find all sorts of workarounds for calculating the time-derivative of pressure instead of just doing just that? And please stop calling it unsteady pressure.

December 31, 2021, 20:00		#5
dskvsp New Member suresh Join Date: Dec 2021 Posts: 11 Rep Power: 5	Hi tran, I could get things now, Thank u for the explanation. And I wish u a happy and prosperous 2022.