[MaryBau]

Hi everyone;

I am trying to calculate de dissipation of an LES simulation, but I am not getting the correct value.

I am doing a decaying isotropic turbulence, then the derivative of the energy with respect to time should be equal to my -dissipation: dE/dt=-epsilon.

If I calculate dE/dt in my simulations, my results are in agreement with a DNS simulation (A. Wray in the AGARD database), so the flow behaviour is correct. However if I calculate epsilon directly from the filtered velocity field it does not give me the correct value. I do not think is a coding error, probably my formulation of epsilon is not correct. I am calculating my total dissipation (modelled + resolved) as:

epsilon=2*(nu+nuSgs)*(S::S) where S is the strain-rate

Is this valid for decaying isotropic turbulence? Any suggestions or ideas of what I might be doing wrong ? Am I missing something?

Thanks for your help and have a nice day,

Mary

[FMDenaro]

Originally Posted by MaryBau

Hi everyone;

I am trying to calculate de dissipation of an LES simulation, but I am not getting the correct value.

I am doing a decaying isotropic turbulence, then the derivative of the energy with respect to time should be equal to my -dissipation: dE/dt=-epsilon.

If I calculate dE/dt in my simulations, my results are in agreement with a DNS simulation (A. Wray in the AGARD database), so the flow behaviour is correct. However if I calculate epsilon directly from the filtered velocity field it does not give me the correct value. I do not think is a coding error, probably my formulation of epsilon is not correct. I am calculating my total dissipation (modelled + resolved) as:

epsilon=2*(nu+nuSgs)*(S::S) where S is the strain-rate

Is this valid for decaying isotropic turbulence? Any suggestions or ideas of what I might be doing wrong ? Am I missing something?

Thanks for your help and have a nice day,

Mary

of course the expression for dissipation does not depend on the type of flow, it must be general...
Said that, if you assume no production of energy, the balance dE/dt=-epsilon must be true even at the initial condition but that does not hold from your figure.

[MaryBau]

Thanks Filippo, what I ment to say is if my expression for the dissipation is correct?

epsilon=2*(nu+nuSgs)*(S::S)

or what could it be the reason for this discrepancy in my results?

[FMDenaro]

the expression of the total dissipation you wrote is correct, of course coding it can have some errors...

Anyway, check a t=0 both dE/dt and the dissipation ...