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Preprocessing of Turbulent Pipe Flow

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Old   June 19, 2014, 21:46
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Winson Chen
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Quote:
Originally Posted by byrong View Post
Hello Winson,

I assume you already got a proper mesh. However, to obtain a proper perturbation, I kind of cheated because I tried thousands of times using the perturbCylinder, and it did not work. In fact, the sine waves always became laminar. Therefore, what I did was the following:
1. Create a square channel with only two plates at the top and at the bottom which fits within the pipe you have.
2. Generate an initial perturbation using the boxTurb utility. Check a tutorial to use boxTurb first since the mesh must be divided in powers of two in each direction.
3. Run the case with the same velocity and viscosity parameters as the ones you will use in your pipe maybe for 5 to 10 cycles within the flow domain, or until you reach constant turbulence.
4. Once you got the turbulence map the velocity field of the squared channel to the pipe geometry with developed flow as an initial condition (both geometries should have the same length). To do this you require to use the mapFields utility, and create a mapFieldsDict within the system directory (there are plenty of tutorials of this online).
5. Run your pipe case. At the begining, the initial condition will look weird, but as long as the flow develops, you will get some nice turbulence. Run the case through the pipe domain for at least 10 complete cycles. Then use your last time step as a new initial condition, and run your case again for another 5 cycles. By doing this I got really nice statistics. Indeed, my statistics were very close to some DNS statistics that my supervisor gave me.

I hope it helps.

If you still need to use perturbCyl, you might have to write to Eugene de Villers, since he coded that utility, but in my case it did not work. And if you make it work properly, please let me know.


Cheers,

Byron
HI Byron,

Thank you for your quick and detailed reply. I will try your way and let you know the outcome later.

Have a nice weekend.

Winson
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Old   June 23, 2014, 02:47
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  #22
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Winson Chen
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wchen is on a distinguished road
Quote:
Originally Posted by byrong View Post
Hello Winson,

I assume you already got a proper mesh. However, to obtain a proper perturbation, I kind of cheated because I tried thousands of times using the perturbCylinder, and it did not work. In fact, the sine waves always became laminar. Therefore, what I did was the following:
1. Create a square channel with only two plates at the top and at the bottom which fits within the pipe you have.
2. Generate an initial perturbation using the boxTurb utility. Check a tutorial to use boxTurb first since the mesh must be divided in powers of two in each direction.
3. Run the case with the same velocity and viscosity parameters as the ones you will use in your pipe maybe for 5 to 10 cycles within the flow domain, or until you reach constant turbulence.
4. Once you got the turbulence map the velocity field of the squared channel to the pipe geometry with developed flow as an initial condition (both geometries should have the same length). To do this you require to use the mapFields utility, and create a mapFieldsDict within the system directory (there are plenty of tutorials of this online).
5. Run your pipe case. At the begining, the initial condition will look weird, but as long as the flow develops, you will get some nice turbulence. Run the case through the pipe domain for at least 10 complete cycles. Then use your last time step as a new initial condition, and run your case again for another 5 cycles. By doing this I got really nice statistics. Indeed, my statistics were very close to some DNS statistics that my supervisor gave me.

I hope it helps.

If you still need to use perturbCyl, you might have to write to Eugene de Villers, since he coded that utility, but in my case it did not work. And if you make it work properly, please let me know.


Cheers,

Byron

Hi again Byron,

I am using you method to generate some turbulence firstly in a channel. However, I have a couple of questions regarding the channel I need to generate:

1. Is the channel going to have top and bottom planes as walls and inlet and outlet as cyclic? In the boxTurb tutorial 6 planes are all cyclic.
2. In order to run the flow at the same Re in the channel (for my case it is Re_tau = 180), I have to mesh the channel very fine ( even finner than my original pipe to meet the CFL condition). Is it like this in your case too?
3. Do you by any chance know whether grading is allowed for the channel mesh?

Thanks in advance!

Winson
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