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-   -   velocity fluctuations for an unsteady simulation (https://www.cfd-online.com/Forums/fluent/86018-velocity-fluctuations-unsteady-simulation.html)

 ajkratos March 11, 2011 17:33

velocity fluctuations for an unsteady simulation

hi
is there any way in fluent to find velocity fluctuations to calculate turbulence kinetic energy for a transient/unsteady simulation? thanks

 xrs333 March 12, 2011 06:31

Hi, you are actually talking about DNS of turbulent flow.

 ajkratos March 12, 2011 07:00

No I am performing LES and in post processing i only get an option for sub grid scale turbulence. So i figured out if i have velocity fluctuations i can calculate TKE using equation 1/2(u'^2+v'^2+w'^2) where u',v' and w' are the fluctuating components of the velocity. I also know that i can get fluctuating components by using u'=u-ubar where u is instanteanous velocty and ubar is the time averaged velocity. I m just not sure how to do it in fluent.

 ajkratos March 12, 2011 07:13

I have 1 idea in mind but not sure if it is correct. So to find velocity fluctuations at a point in the domain...i can turn on the data sampling which will alow me to find average velocity at a specific time and i also have instanteanous velocity at that time, now if i subtract average velocity from the instanteneous velocity am i correct to understand tht i will get fluctuating velocity at tht time at tht point?

 xrs333 March 12, 2011 07:57

if you solve n-s eq. like that, you are performing DNS.
non of the turbulent models, including LES, solves for the instantaneous velocity.

 ajkratos March 12, 2011 08:52

thanks for ur reply. ok i understand wat u mean...but if i want to find mean TKE in LES wat shuld i do n by velocity fluctuations (u', v', w') i meant 'mean' velocity fluctuations. How do i find the fluctuating components of velocity in fluent? sorry if i m bein very confusing, i m confused myself :)

 xrs333 March 12, 2011 17:24

you can use monitor or export during calculation (menu: file -> export -> ...) to collect transient data.

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