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Old   September 2, 2016, 23:29
Question Intermittent Turbulenct FLUENT
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Hi Guys,

Simulation of Intermittent turbulence, straight pipe
Diameter 2.8 mm
Length 0.84 m

Pressure gradient; P1 - P2 = Constant.

Meshing requirements are fine, geometry all set, Boundary Conditions?

I would like to know your thoughts on the Boundary conditions....
All i need is oscillatory velocity profile....



PLEASE HELP!
Thanks
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Old   September 3, 2016, 00:58
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ImperialFluid,
Are you looking for an oscillating velocity profile inlet boundary condition (with time? with dimension?), or are you looking for a fixed dP across the pipe? It's not clear.

To get a fixed dp, you'd simply set an inlet pressure condition and a fixed outlet condition. To get an oscillating velocity profile in the pipe, you can either vary the inlet pressure as a function of time, or change the boundary condition to velocity inlet and vary that as a function of time.

Let us know what you're actually trying to accomplish.

Thanks
ComputerGuy

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Originally Posted by ImperialFluid View Post
Hi Guys,

Simulation of Intermittent turbulence, straight pipe
Diameter 2.8 mm
Length 0.84 m

Pressure gradient; P1 - P2 = Constant.

Meshing requirements are fine, geometry all set, Boundary Conditions?

I would like to know your thoughts on the Boundary conditions....
All i need is oscillatory velocity profile....



PLEASE HELP!
Thanks
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Old   September 4, 2016, 03:29
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Karim Shalash
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I am currently working on similar problem, my boundary condition is a time varying pressure profile.
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Old   September 4, 2016, 16:15
Default Intermittency in the transition to turbulence
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Hi, Firstly thanks for the responses guys!

Clarity;

I am looking at constant pressure drop along a straight pipe. No oscillations in the inlet conditions.
I am looking at creating a system where by the fluid, originally in laminar regime, increases in speed until reaching a critical Re at which point the fluid turns Turbulent and the resistance in the flow created by eddies and such causes the fluid to slow to a laminar phase. I am hoping to get that process repeating (intermittent turbulence). However after countless hours of simulations I have not found any good inlet conditions and i was hoping some insight from you guys.

Would you guys know?

Thankyou,

ImperialF
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Old   September 4, 2016, 22:57
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To restate your problem:
You have a fixed differential pressure across a pipe which does not change with time. You're hoping to identify a differential pressure across the pipe which gives a particular velocity profile within the pipe? Or something like intermittent turbulent behavior?

If you've really spent a huge amount of time searching/iterating on differential pressures and can't see what you're after, perhaps:
1) Your mesh is not fine enough to capture the behavior of interest (have you considered adaptive meshing based on something like vorticity?)
2) Your physics are inappropriate. Have you tried other turbulence models? What are you using now?

Last, have you verified you're searching in a differential pressure solution area which definitely encompasses the Reynolds numbers you're looking for? Instead of posting a picture of a sine wave, perhaps if you give us a reference to a paper you're trying to reproduce, or something which approximates it, we'll be better able to help out.

ComputerGuy

Quote:
Originally Posted by ImperialFluid View Post
Hi, Firstly thanks for the responses guys!

Clarity;

I am looking at constant pressure drop along a straight pipe. No oscillations in the inlet conditions.
I am looking at creating a system where by the fluid, originally in laminar regime, increases in speed until reaching a critical Re at which point the fluid turns Turbulent and the resistance in the flow created by eddies and such causes the fluid to slow to a laminar phase. I am hoping to get that process repeating (intermittent turbulence). However after countless hours of simulations I have not found any good inlet conditions and i was hoping some insight from you guys.

Would you guys know?

Thankyou,

ImperialF
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Old   September 4, 2016, 23:23
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Hi ComputerGuy,

You have stated my desired simulation perfectly. I want to Identify a differential pressure across the pipe which gives a particular velocity profile within the pipe, something like that found in the image attached.

I have attached 2 images, both from the work of Couette 1889-1890. It can be found in Prandtl and Teitjens, Applied Hydro- and Aeromechanics book.

Image1: showing the spouting velocity vs time
Image2: shows the Resistance Coefficient vs Reynolds number
The Reynolds number in image 2 is given as a function of radius, so it needs to be doubled to be in the usual form. So that gives me this phenomena occurring at Re=20,000? which i believed to much too large.

I am currently using SST-Transition model within FLUENT. I have checked my Y+ < 1.

Hopefully this is enough information.

and as always, thank you
Attached Images
File Type: jpg U vs T.jpg (106.4 KB, 2 views)
File Type: jpg re.jpg (66.9 KB, 3 views)
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