CFX or FLuent?
 

hi,

Is CFX good enough for LES? or is fluent better in terms of memory capability and speed?

i heard passing remarks from collegues that fluent was better but nobody was able to substantiate..!!

can any one tell me whether or not CFX is good enough??

Re: CFX or FLuent?
 

Hi,

Neither codes are ideal for LES as, in general, dedicated LES codes are explicit solvers whereas CFX and Fluent are both implicit. As LES uses very small timesteps then you are usually in the region of stability for explicit solvers and they are much faster and lower memory than implicit solvers.

CFX has the SAS turbulence model which can be very useful in some types of the LES simulations, eg bluff bodies. Check it out and see if it is useful for your case.

CFX is likely to use more memory than fluent as it is a coupled solver versus a segregated solver. Whether CFX or fluent is faster for your simulation is hard to say - the only thing for sure is neither codes are ideal.

Glenn Horrocks

Re: CFX or FLuent?
 

hello

glenn ... one sales man from ansys tell me that cfx and fluent are both explicit codes..... and also star cd ...

are you sure about that?

best regards hayate

Re: CFX or FLuent?
 

Yes, Glenn is correct.

Re: CFX or FLuent?
 

Hmm,

Glenn, usually your answers are so well thought out....

Stability for explicit solvers with advection terms only is one thing. Stable courant number scales with 1/dx, where dx is the smallest mesh length scale.

The problem is that stability when including diffusion terms, which is most LES calculations, is much more restrictive than advection. The stable courant # scales with 1/dx^2.

Implicit solvers are insensitive, or possibly far less sensitive to this numerical fact.

So, explicit is not necessarily the best solution. It's probably fine if you run a uniform mesh.

Explicit will kill you thought when:

- you refine the mesh. The required timestep goes down as well 1/dx^2. So, if you make the mesh a factor of 2 finer you need a timestep a factor of 4 smaller, making your calculation a factor of 4 more costly.

- real industrial case meshes are almost never nice and uniform. Mesh spacing varies all over the place so the smallest cell sets the time step requirements.

So, for all intensive purposes implicit solvers are the only solution for practial LES calculations. For academic cases like flow around a cylinder or other simple geometries you can probably get away with an explicit solver.