homogenous turbulence.
 

I have been reading a paper which says that "to ensure similar forcing on all simulations the 64^3 runs are forced on a 32^3 grid; this is non-trivial since the flow is sensitive to energy addition and its distribution. " (An comparative study of sgs models in homogenous turbulence , Fureby et al, 1997. ). Can anyone explain what this means?

Re: homogenous turbulence.
 

They are doing DNS of boxes of turbulence - the first lot talks about the grids used: 64x64x64 cells and 32x32x32 cells, with cyclic boundary conditions. The second lots tells you that if you want to compare the results, you need to introduce the same kind of forcing (otherwise turbulence dies out) - therefore, both simulations use forcing fields generated on the coarser mesh.

Re: homogenous turbulence.
 

Thank you for your help.

What do you mean by "therefore, both simulations use forcing fields generated on the coarser mesh". What is a "forcing field"?

Re: homogenous turbulence.
 

Ok, think about this: you create a box 32x32x32 cells and you want to study homogenous isotropic turbulence. Therefore, you need to create vortices and have them dissipate energy through the energy cascade. However, what you've got as a domain is a box with cyclic boundaries all round, representing infinite space. During the simulation, energy is dissipated when it reaches the bottom of the energy cascade - that is what turbulence does.

You've got two choices: i) you create a box, put some real vortices into it and start solving Navier-Stokes. After a while, all the energy that is contained in the initial field dissipates and you get no velocity. The turbulence has died, which makes for a boring simulation, not to mention that you don't get much useful data. ii) you need a mechanism which will continuously introduce energy into large scales and have it go through the energy cascade until it is dissipated. Much nicer!

The mechanism which introduces the energy is called a forcing field - have a look at the paper to find out how it's done.

Hrv