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-   -   What does pseudo randomness mean in turbulence (https://www.cfd-online.com/Forums/main/91297-what-does-pseudo-randomness-mean-turbulence.html)

 Nick R August 6, 2011 02:27

What does pseudo randomness mean in turbulence

I read in a CFD book that the randomness in turbulence is pseudo. I don't understand the meaning of the following statement
" The reason for pseudo-randomness is that small perturbations that are constantly added to the solution are enhanced exponentially in time"

-From CFD essentials by Zikanov

Can someone please explain what that statement means

 insane_alien August 6, 2011 05:48

pseudo random means it has all the appearences of being random but if you start from the exact same initial conditions and put it through the exact same process then it will be the exactly the same, ie. it is deterministic.

however, if there is even a slight difference then trial n+1 will differ from trial n by an amount proportional to the length of time the simulation has been running for.

a better way of describing turbulence would be that it is deterministic but highly chaotic.

without a way to accurately measure the position and velocity of all the molecules involved and a similarly accurate way to model their behaviour then the simulation will diverge from reality, the averages can turn out the same but down at the small details there can be big differences.

CFD is and probably always will be an approximation, it can be a pretty darn good approximation however so it is still incredibly useful.

 Nick R August 9, 2011 03:09

I see. Thanks for clarifying it.

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