What Is a Turbulence Model?

To obtain the Reynolds-Averaged Navier-Stokes (RANS) equations, the Navier-Stokes equations for the instantaneous velocity and pressure fields are decomposed into a mean value and a fluctuating component. The averaging process may be thought of as time averaging for steady-state situations and ensemble averaging for repeatable transient situations. The resulting equations for the mean quantities are essentially identical to the original equations, except that an additional term now appears in the momentum transport equation. This additional term is a tensor quantity, known as the Reynolds stress tensor, which has the following definition:

(140)

The challenge is thus to model the Reynolds stress tensor of Eqn. 140 in terms of the mean flow quantities, and hence provide closure of the governing equations. Two basic approaches are used in STAR-CCM+:

It is widely acknowledged that turbulence models are inexact representations of the physical phenomena being modeled, and no single turbulence model is the best for every flow simulation. Consequently, it is necessary to provide a suite of models that reflect the current state of the art.