question about LES

gino briganti

Hi, we are new to CFD thematics and we are trying to apply Fluent 6.2.16 to a test case with a tank filled of water with a top initial vertical stable temperature stratification (capping) 0.2 m thick and 0.5 °C/cm gradient. The domain was an hexahedron, having walls at all the six boundaries with no friction, adiabatic conditions and gravity was included too; the dimension of the domain were about 1m x 1m x 1m, with a grid spacing of 0.02 m. We chose to simulate unsteady conditions with LES viscous model and we noticed that capping inversion was quickly destroyed (after only a couple of seconds). We wish to know the opinion of some expert about this abnormal behaviour; we suspect that turbulence velocity fluctuations used by Fluent to reconstruct large eddies are too large and we don't know how to control them (in absence of boundary inlet conditions). Thanks in advance for your help! gino