Request for truth on VLES/Unsteady RANS

Nick Georgiadis

My group at NASA Glenn Research Center (formerly Lewis) is beginning work towards developing a Large Eddy Simulation capability - particularly for nozzle and jet aerodynamics simulations. As a newcomer to this field, I have read numerous papers, heard presentations on the LES field, and am trying to make sense of all of it. Recently, I have seen increased interest in what is called Very Large Eddy Simulation and Unsteady RANS. If what I understand about LES is correct (where you "directly" calculate large scales and model the smallest ones) - I am confused as to how VLES and unsteady RANS can work.....specifically, from my past RANS work, I know all eddy viscosity models act as diffusive factors in the NS equations - and as a result, replace or "model" an unsteady-turbulent flow as a steady flow with an effective variable viscosity (from the turbulence model) replacing the turbulent stresses. After this long introduction, my question for those in the field out there is: How can VLES or Unsteady RANS work to still give you a meaningful representation of the large scales? I would think that since you are replacing decent size scales of turbulence with a dissipative model (turbulence model for RANS, SGS model for VLES), your large scales do not have a hope to be useful.

I would appreciate any insight on this topic.

Nick Georgiadis NASA Glenn Research Center tonjg@grc.nasa.gov