Hi all,

My questions are

1. What if I use a solver with RANS method to solve a laminar problem, what will happen? can I get the same result with the laminar solver?

2. Since Rans solver also contain the discretization of d/dt, and then we get a series of time step result, just like LES method. So concerning the information of result we get from both RANS and LES, what are their differences? I mean concerning the information of the results.

Any reply would be highly appreciated.

Daniel

If you use a RANS solver, you get a RANS solution. If you use a laminar solver you get a laminar solution. Generally speaking most RANS solvers do not have any transition capability, so regardless of the Reynolds number of the actual flow a RANS solver will produce some eddy viscosity. This eddy viscosity models the effects of turbulence, whether that turbulence is actually there or not. So if it is not supposed to be there, then use a laminar solver.

When you use a RANS solver you are solving the Reynolds averaged equations. One basic assumption is that the time-averaging process that removes the turbulent scales involves time scales much smaller than any gross unsteadiness of the flow. It is the gross unsteadiness of the flow that is captured by the unsteady terms. The most significant difference between RANS and LES is that LES is designed to capture more of the small scall unsteadiness using the grid to resolve at least some of the turbulent structures. Thus the unsteadiness that is simulated includes (ideally) unsteadiness associated with some of the larger turbulent time scales. However you still end up having to model the stuff that falls below the grid resolution.

1. You will not get similar results because RANS solver assumes the flow to be fully turbulent and thus employs eddy viscosity to model the effect of turbulence regardless of the fact that flow under investigation is laminar.

2. In RANS modelling you solve averaged navier stokes equations where as in LES you solve filtered navier stokes equations. Therefore in LES depending on your grid resolution you can directly solve the large eddies. As a result you obtain instantaneous properties of the flow in LES at least for large eddies that you have directly computed which is not the case with RANS modelling.

Another major difference between LES and RANS modelling has to do with the resolution of grid. In LES if you employ infinitely fine grid the application of sub grid turbulence model becomes minimum and as a result you can obtain almost perfect results. On the other hand if you employ infinitely fine grid in RANS modelling (assuming you do) you will get horrible results. Can you figure out why? Answer is in your first question!

Usman