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-   -   Turbulence postprocessing (http://www.cfd-online.com/Forums/fluent/92610-turbulence-postprocessing.html)

 Mohsin September 19, 2011 21:05

Turbulence postprocessing

Hello

In fluent Post processessing, I need to show the turbulence inside the cylinder. How much is the turbulence in the domain?

There are 3 parameters to determine it:

1. Turbulence intensity
2. Turbulence Kinetic Energy
3. Turbulence disspation rate

The general discription of the afforementioned parameters is as follows: (Please correct me if I am wrong)

-->Turbulence intensity is the turbulence level. (what is the intensity of turbulence in the flow)
-->Turbulence Kinetic Energy determines the energy in the turbulence. (the mean kinetic energy per unit mass associated with eddies in turbulent flow).
-->Turbulence dissipation rate determines the scale of turbulence (With what rate turbulence is dissipated in the flow)

Looking at the above definitions, turbulence intensity seems to be a better option for post processing to determine the turbulence through out the domain. It can show the intensity of turbulence in the domain. In my case, the turbulence intensity contours value doesn't exceed 50 throughout the domain. Does it mean that there is very less turbulence in the domain?? Or can i use any other turbulence paramter to determine turbulence.

Regards

 Hossein Mirabi October 2, 2016 02:56

Hello dear Mohsin

Turbulence intensity shows the ratio of fluctuating velocity to mean velocity. So can you see which place has more deviation.
Turbulence dissipation rate shows how much turbulent energy dissipate in time. in dissipation state, scales are in kolmogorov microscale. the size that viscosity force and momentum are equal.
So I believe Turbulence Production term and Turbulence Kinetic Energy term can indicate the amount of Turbulence. Energetic regions have more fluctuating activity in compare with other regions. This leads to Turbulence Intensity term.
Turbulence Production→Turbulence Kinetic Energy→Turbulence Intensity

 LuckyTran October 3, 2016 14:18

Unfortunately, turbulence is so much more easy to discuss subjectively & qualitatively than objectively & quantitatively.

If you consider turbulence as a phenomenon (fluctuating velocities) then turbulent kinetic energy is the most straightforward way to discuss it in the same way that velocity magnitude is a good descriptor of how fast something is flowing.

Turbulence intensity is just a normalized turbulent kinetic energy.

However, it is generally not conducive to discuss turbulent kinetic energy simply because nobody has a good feel for typical values of k is w.r.t. to certain flow scenarios. You can't tell anybody that flow has 200 m^2/s^2 because how much this is depends no geometric length and velocity scales of the problem (it's like saying 200 m/s flow but without discussing other contexts like Reynolds number or Mach number).

On the other hand, if you tell someone that I have a turbulence intensity of 1&, 10% or 100%, they do appreciate that one has much more fluctuating velocity than another (in the same sense that Re and Mach number really help puts everything in the same context). This is because most people do appreciate that a typical turbulence intensity is only a few percent. The only caveat is that turbulence intensity isn't a similarity variable.

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