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Applying turbulence model on laminar flow

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Old   April 23, 2010, 11:54
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Excellent! Thanks so very much, agd. I have learned a lot from you in this discussion which I appreciate very much.

I was further thinking. It makes sense that the streamlines are not the best result to look at since they reflect the mean velocity. What variable is plotted when we see pretty pictures showing eddies?

Last edited by Jade M; April 26, 2010 at 10:20.
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Old   April 23, 2010, 12:03
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Hello again, agd and others.

I have been trying to learn about turbulence as fast as I can but my knowledge is still lacking a great deal. Can you briefly describe what the results for eddy viscosity should look like or what the trends should be in the quantitative values, or is this too complicated to describe?

Thanks so much for any further information.

Last edited by Jade M; April 26, 2010 at 10:20.
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Old   April 23, 2010, 14:44
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Hi agd, I have another question for you and everyone. I seem to have consistently heard that SST is a good model. Is there any situation in which it is better to use the k-epsilon or any epsilon-based models? Based on my very limited knowledge, I would think tha the answer to this question is no. However, it is a widely used model ... why?

Thanks so much for any further information!

Last edited by Jade M; April 26, 2010 at 09:26.
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Old   April 23, 2010, 18:54
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@mannobot - normally you will be able to use 1 turbulence model per simulation.

Do
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Old   April 23, 2010, 19:06
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Quote:
Originally Posted by DoHander View Post
@mannobot - normally you will be able to use 1 turbulence model per simulation.

Do

you can chose different turbulence models for different regions if you want in same simulation.
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Old   April 23, 2010, 19:33
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Quote:
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you can chose different turbulence models for different regions if you want in same simulation.
You mean you can use 2 or more different turbulence models in the same simulation in Fluent ? What version of Fluent are you using ???

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Old   April 23, 2010, 19:46
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Quote:
Originally Posted by DoHander View Post
You mean you can use 2 or more different turbulence models in the same simulation in Fluent ? What version of Fluent are you using ???

Do
When you use DES you use two different turbulence models. It does not matter what software i am using, if i am using DES, it is two model simulation.
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Old   April 23, 2010, 19:58
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When you use DES you use two different turbulence models. It does not matter what software i am using, if i am using DES, it is two model simulation.
My answer for mannobot was about using RANS and Fluent, and it does matter what software you use, since Fluent (after my knowledge) does not let you use more then 1 turbulence model per simulation (again I'm talking about RANS).

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Old   April 23, 2010, 20:11
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Quote:
Originally Posted by DoHander View Post
My answer for mannobot was about using RANS and Fluent, and it does matter what software you use, since Fluent (after my knowledge) does not let you use more then 1 turbulence model per simulation (again I'm talking about RANS).

Do

sorry i could not figure out all this from this post:

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@mannobot - normally you will be able to use 1 turbulence model per simulation.

Do
i thought you are talking about general case.
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Old   July 14, 2010, 10:51
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Quote:
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@mannobot

I see two solutions if you know the regions where your flow is turbulent:

1. Geometrically split the calculation domain in more regions, you can do this in Gambit or in your mesh generator or even in Fluent. Say your domain is a rectangle and you know that after some point on the South side (which is a wall) you have turbulent flow, then simply draw a vertical line on the entire domain and split the mesh in two regions. In Fluent enable a turbulence model, after that you will be able from the Boundary Conditions panel to pick a domain and tell Fluent this is a laminar region. This is the approach used in the above mentioned article.

2. Use a UDF to delimit a certain spatial domain in which you can cancel the turbulent viscosity. This method is potentially more flexible then 1, but harder to implement.

Do
Should we do splitting operation before or after meshing?
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Old   July 15, 2010, 12:01
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Hi there,

I do have another problem. I try to understand the effects dominating the problem of a flat plate within a channel. Is it true that I will get a laminar boundary layer starting at the leading edge no matter what kind of turbulence level the flow does have at the inlet? How would I model that case. How do all the NACA investigators model their problems? I would have to model not only the transition point but also the relaminarization at the leading edge.

Sincerely

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Old   July 16, 2010, 06:47
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Hi,

the development of the boundary layer does depend on the turbulence level. The critical Reynolds number is a function of the T.I.. So, in case of relaminarization. How to model that case?


Sincerely

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Old   July 16, 2010, 06:56
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I don't know much but just an idea: define laminar flow zone.
I have never defined a laminar zone, but read about it.
I don't know if it works or not.
Just an idea...
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Old   July 16, 2010, 08:23
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But how to define the zone. Do I have to use boundary layer theory and calculate the thickness of the boundary layer over length?
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Old   July 26, 2010, 17:23
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I was further thinking. It makes sense that the streamlines are not the best result to look at since they reflect the mean velocity. What variable is plotted when we see pretty pictures showing eddies?
For those who are interested, I recently learned the answer to my question! In Post, click Insert > Location > Vertex Core Regions.
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Old   July 26, 2010, 17:30
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It is true that the boundary layer will be laminar starting at the leading edge. As the velocity increases, the Reynolds number will increase. The transition for a flat plate occurs at the position for which the Reynolds number is equal to 5x10^5. The literature will give better numbers at which the boundary layer transitions from laminar to transition and from transition to turbulence, if you wanted to be more precise. If you are using CFX, then SST with the Gamma Theta Model may be the way to go. Good luck!
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Old   July 26, 2010, 17:32
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Originally Posted by mannobot View Post
Hi,

what happens when I do apply a turbulence model on a laminar flow field?

Thank you for your answers

mannobot, I also received some very helpful advice from ANSYS Tech Support that I thought might be of interest to you. Please see below.

In regards to determining when flow if fully turbulent that's sort of a tough question. Most flows have some laminar regions. For example, if you had flow over a flat plate, as the flow initially hits the plate the flow is likely laminar because the characteristic length ( the distance from the leading edge of the plate which is used in Reynolds number calculations) is rather small. As the flow moves along the plate, the characteristic length increases, thereby transitioning the flow from laminar to turbulent. This transition can happen very rapidly and in many cases it won't make a difference if you model the transition or assume the entire flow is turbulent. The question is how do you know when the laminar region is insignificant enough. There are a couple of approaches you can take: - always run with SST with the turbulence transition model - do a turbulence model sensitivity study, which would mean run the same case with k-epsilon, SST, and SST + transition. You will then get a feel for which turbulence model gives you the best results for the least amount of computational overhead and then apply that to future studies. Practically speaking, the vast majority of users today simply start with SST. If they are not getting good results they try the SST with transition enabled. I hope this helps!
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Old   July 27, 2010, 06:29
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Hi Jade M,

thanks for your effort. I do use FLUENT. In the mean time they implemented the 4 eqn SST. I am not sure how precise this is. I will start some benchmark calculations. I found some lturature. And the begin of the transition region does depend on the critical Re but the critical Re is also a function of the turbulent intensity in the initial flow. Therefore very hard to predict.
I have to read about the transitional models. I know that transition is very hard to predict and many groups do work on that issue but I think they mostly do DNS.
So in the end ANSYS gives the advice to just apply the SST and compare the results. No laminar zone stuff...
I thought those results would have to be wrong.
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Old   July 29, 2010, 11:15
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Hi mannobot,

Very interesting. In CFX, there is the option for transition with the SST model. The recommended transition model is the Gamma Theta Model developed by Langtry and Menter, which has been developed for standard bypass transition and low free-stream turbulence. There are other choices in CFX. The CFX documentation says that this model has been widely validated.

Now I understand from an ANSYS report by Menter that there are 4 types of transition, which are Natural Transition, Bypass Transition, Separated Flow Transition, and Wake Induced Transition. I would like to better understand these types of transitions, the modelling of them, and the options within CFX and FLUENT.

It'll be interesting to see/use the ANSYS CFD software in Release 14 where there will be a single CFD program. Two ANSYS engineers have stated that they believe (and hope) that the user will have the freedom to select which solver (FLUENT-based or CFX-based) since they each have their strengths. So I'll be interested in what you learn with using FLUENT since it may be applicable for my use in Release 14, even though I'm currently a CFX user.
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