How to calculate turulent intensity?

Clau.77 · May 27, 2021, 05:04

Hey guys,

i m wondering how to calculate the turbulence intensity (for k-epsilon model) at the inlet. I have only found equations for pipes, but i have a rectangular, not a circular geometry.

I think it should be a medium intensity, so something between 1 and 5% but i would like to have a equation for it.

Does anyone have an idea?

HPE · May 27, 2021, 07:15

could help? initial conditions for k-epsilon model.

Clau.77 · May 27, 2021, 07:26

i have seen that webside, i want to use it to calculate k and epsilon but what I am looking for is way to calculate the turbulence intensity I.

HPE · May 27, 2021, 15:50

To my knowledge, the magnitude of turbulence intensity is usually "set" by experimental values from similar laboratory or field experiments.

If such magnitude is not known, you simply "estimate" a value based on your engineering judgement and the case at hand.

I don't think the turbulence intensity is a computable variable, like k, which you can find a closed-form expression for its estimation.

Hope somebody else gives a better insight.

Tobermory · May 28, 2021, 09:52

Yes, agreed HPE. When you are asking "what value of turbulent intensity is appropriate?" then you are essentially asking "how turbulent is my flow?", and the answer is of course that it depends on the flow ... and how turbulent it is!

Outside of a boundary layer, you may have a very low intensity (eg. some wind tunnels work very hard to generate low free stream turbulence levels, <<1% intensity), i.e. close to laminar flow. inside a BL, or other shear layer, then values of the order of 5 to 15% are typical, although again this varies widely between flows and within flows (intensity varies across a BL, for example, peaking near the wall and decaying to the free stream value at the edge).

The best idea, as HPE suggests, is to look in the literature for an analogous flow and see what values are present, then pick something appropriate. I would then suggest that you rerun your simulation with a smaller and a larger value, and check to see whether your results are sensitive to the parameter.

Clau.77 · May 28, 2021, 10:00

okay thank you both so much for your input! I will try to find values from similar experiments and then just estimate a value.

May 27, 2021, 05:04	How to calculate turulent intensity?	#1
Clau.77 Member Claudia Join Date: Mar 2021 Posts: 43 Rep Power: 5	Hey guys, i m wondering how to calculate the turbulence intensity (for k-epsilon model) at the inlet. I have only found equations for pipes, but i have a rectangular, not a circular geometry. I think it should be a medium intensity, so something between 1 and 5% but i would like to have a equation for it. Does anyone have an idea?

May 27, 2021, 07:15		#2
HPE Senior Member Herpes Free Engineer Join Date: Sep 2019 Location: The Home Under The Ground with the Lost Boys Posts: 932 Rep Power: 12	could help? initial conditions for k-epsilon model. __________________ The OpenFOAM community is the biggest contributor to OpenFOAM: User guide/Wiki-1/Wiki-2/Code guide/Code Wiki/Journal Nilsson/Guerrero/Holzinger/Holzmann/Nagy/Santos/Nozaki/Jasak/Primer Governance Bugs/Features: OpenFOAM (ESI-OpenCFD-Trademark) Bugs/Features: FOAM-Extend (Wikki-FSB) Bugs: OpenFOAM.org How to create a MWE New: Forkable OpenFOAM mirror

May 27, 2021, 15:50		#4
HPE Senior Member Herpes Free Engineer Join Date: Sep 2019 Location: The Home Under The Ground with the Lost Boys Posts: 932 Rep Power: 12	To my knowledge, the magnitude of turbulence intensity is usually "set" by experimental values from similar laboratory or field experiments. If such magnitude is not known, you simply "estimate" a value based on your engineering judgement and the case at hand. I don't think the turbulence intensity is a computable variable, like k, which you can find a closed-form expression for its estimation. Hope somebody else gives a better insight. __________________ The OpenFOAM community is the biggest contributor to OpenFOAM: User guide/Wiki-1/Wiki-2/Code guide/Code Wiki/Journal Nilsson/Guerrero/Holzinger/Holzmann/Nagy/Santos/Nozaki/Jasak/Primer Governance Bugs/Features: OpenFOAM (ESI-OpenCFD-Trademark) Bugs/Features: FOAM-Extend (Wikki-FSB) Bugs: OpenFOAM.org How to create a MWE New: Forkable OpenFOAM mirror

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May 27, 2021, 07:26		#3
Clau.77 Member Claudia Join Date: Mar 2021 Posts: 43 Rep Power: 5	i have seen that webside, i want to use it to calculate k and epsilon but what I am looking for is way to calculate the turbulence intensity I.

May 28, 2021, 09:52		#5
Tobermory Senior Member Join Date: Apr 2020 Location: UK Posts: 668 Rep Power: 14	Yes, agreed HPE. When you are asking "what value of turbulent intensity is appropriate?" then you are essentially asking "how turbulent is my flow?", and the answer is of course that it depends on the flow ... and how turbulent it is! Outside of a boundary layer, you may have a very low intensity (eg. some wind tunnels work very hard to generate low free stream turbulence levels, <<1% intensity), i.e. close to laminar flow. inside a BL, or other shear layer, then values of the order of 5 to 15% are typical, although again this varies widely between flows and within flows (intensity varies across a BL, for example, peaking near the wall and decaying to the free stream value at the edge). The best idea, as HPE suggests, is to look in the literature for an analogous flow and see what values are present, then pick something appropriate. I would then suggest that you rerun your simulation with a smaller and a larger value, and check to see whether your results are sensitive to the parameter.

May 28, 2021, 10:00		#6
Clau.77 Member Claudia Join Date: Mar 2021 Posts: 43 Rep Power: 5	okay thank you both so much for your input! I will try to find values from similar experiments and then just estimate a value.