Learning Forum

[Long Yun]

I have some questions when I read the literatures about the energy spectrum.

When we calculate the turbulent flow with Large eddy simulation, it is important to make sure to resolve the scale in the inertial subrange. I want to see  the Kolmogorov spectrum law (-5/3 law) in my calculation results. While, how to make the spectrum for turbulent kinetic energy ?

[raul.raghav]

Could you provide information regarding the solver you’re working with? Is it Fluent or CFX?

[Long Yun]

CFX.

Thank you

[raul.raghav]

Plotting Energy Spectrum for LES simulations in CFX:

Step 1: Achieve a statistically stable or averageable state, where  turbulent instabilities due to the initial condition effects are convected out of the system. This might be a minimum of 1 through flow cycle.

Step 2: Create the following expressions for TKE.

C = 0.18 #(for Smagorinski, C = 0.18 / WALE, C = 0.5)

delta = (Volume of Finite Volumes)^(1/3)

resTKE = 0.5*((u-u.Trnavg)^2 + (v-v.Trnavg)^2 + (w-w.Trnavg)^2)

unresTKE = ((C*delta*Shear Strain Rate)^2)/0.3

totalTKE = Total TKE = resTKE + unresTKE

Create an Additional Variable > TKE > Variable Type: Unspecified; Units: [m^2 s^-2]; Tensor Type: Scalar.

Inside the domain panel: Domain > Fluid Models > Additional Variable > TKE > Option: Algebraic Equation; Add. Var. Value > totalTKE

Step 3: (In CFX-Pre) Create monitor points at a few locations in the flow field (at specific regions of interest). Monitor TKE at those points.

Step 4: (In CFX-Solver) Monitor TKE at the all points (separately). (i) Workspace > New Monitor > OK; (ii) Plot Lines > USER POINT > TKE > *Monitor Point Location* > OK; (iii) Range Settings > Plot Data By > Simulation Time. 

Step 5: (In CFX-Solver) Run for sufficiently long time so that you get a sharper energy spectrum plot. Once the simulation is complete, right click on the screen to save the monitor plot as a csv file.

Step 6: (In CFD-Post) (i) Insert > Chart > OK; (ii) Data Series > File (Select the saved csv file); (iii) General > Fast Fourier Transform; (iv) X Axis > Frequency > Log Scale; (v) Y Axis > Magnitude > Log Scale; (vi) Line Display > FFT Line Type > Lines; (vii) Apply. This will give you the energy spectrum.

Hope this helps. Let us know if you have any issues.

[douglascemin]

Dear @raul.raghav

How did you get the equation for resTKE ? can you inform the source with the demonstration please? thank you

[joemonjacob]

I am working with Ansys Fluent 17.2 and trying to get spectrum for turbulent kinetic energy. Can you please help me with the energy spectrum in Fluent.

My problem is 'flow past a circular cylinder' and I have the simulation results with 'Data sampling on'. To get turbulent kinetic energy variation along a line, do i need to monitor velocity for all points throughout or can postprocess from the available variables in LES .dat file.