A hybrid RANS/LES technique based on Reynolds stress tensor reconstruction

Antonella Abba'
Dipartimento di scienze e tecnologie aerospaziali, Politecnico di Milano
Tuesday 5th April 2016
Aula Seminari III Piano Dipartimento di Matematica, POLITECNICO DI MILANO
Both from industrial and research point of view, very often is required a good description of turbulent flows and their unsteady features. LES is able to represent an instantaneous picture of the flow, ensuring good accuracy, but it is very demanding in term of cost. In a fully resolved LES simulation of a wall bounded flow, the number of the grid points scales with the Reynolds number as N ? Re 1.76 , while for RANS the dependency concerns only the wall normal direction and the number of grid points scales as N ? lnRe. Nevertheless, we remark that RANS can only provide a description of the mean quantities. Therefore, we can identify a sort of chiasmus: LES is sufficiently detailed but too expensive, RANS is sufficiently cheap but too superficial. For these reasons, seems natural trying to combine the two approach, obtaining hybrid RANS/LES methods. Applying the hybid filter to the Navier-Stokes equations it is possible to obtain a new set of formally correct hybrid equations. A new approach for the Reynolds stress tensor has been proposed in which no additional equations are required. This results in a very simple and cheap hybrid method, in which the RANS contribution is used to integrate, and not to substitute, LES in the context of coarse grid. contact: nicola.parolini@polimi.it