Favre averaging

Favre averaging is carried out for all dynamical variables except the pressure. For the velocity components, ${\displaystyle u_{i}}$, the Favre averaging is defined as:

${\displaystyle {\widetilde {u_{i}}}={\frac {\overline {\rho u_{i}}}{\overline {\rho }}},}$

where the overbar indicates the typical Reynolds averaging, the tilde denotes the Favre averaging and ${\displaystyle \rho (\mathbf {x} ,t)}$ is the density field. The Favre decomposition of the velocity components is then written as:

${\displaystyle u_{i}={\widetilde {u_{i}}}+u_{i}'',}$

where ${\displaystyle u_{i}''}$ is the fluctuating part in the Favre averaging, which satisfies the condition ${\displaystyle {\widetilde {u_{i}''}}=0}$, that is to say, ${\displaystyle {\overline {\rho u_{i}''}}=0}$. The normal Reynolds decomposition is given by ${\displaystyle u_{i}={\overline {u_{i}}}+u_{i}'}$, where ${\displaystyle u_{i}'}$ is the fluctuating part in the Reynolds averaging, which satisfies the condition ${\displaystyle {\overline {u_{i}'}}=0}$.

The Favre-averaged variables are more difficult to measure experimentally than the Reynolds-averaged ones, however, the two variables can be related in an exact manner if correlations between density and the fluctuating quantity is known; this is so because, we can write:

${\displaystyle {\widetilde {u_{i}}}={\overline {u_{i}}}\left(1+{\frac {\overline {\rho 'u_{i}'}}{{\overline {\rho }}\,{\overline {u_{i}}}}}\right).}$

The advantage of Favre-averaged variables are clearly seen by taking the normal averaging of the term ${\displaystyle \rho u_{i}u_{j}}$ that appears in the convective term of the Navier-Stokes equations written in its conserved form. This is given by^[4][5]

${\displaystyle {\begin{aligned}{\overline {\rho u_{i}u_{j}}}&={\overline {\rho }}\,{\overline {u_{i}}}\,{\overline {u_{j}}}+{\overline {\rho }}{\overline {u_{i}'u_{j}'}}+{\overline {u_{i}}}{\overline {\rho 'u_{j}'}}+{\overline {u_{j}}}{\overline {\rho 'u_{i}'}}+{\overline {\rho 'u_{i}'u_{j}'}}\\&={\overline {\rho }}{\widetilde {u_{i}}}{\widetilde {u_{j}}}+{\overline {\rho u_{i}''u_{j}''}}.\end{aligned}}}$

As we can see, there are five terms in the averaging when expressed in terms of Reynolds-averaged variables, whereas we only have two terms when it is expressed in terms of Favre-averaged variables.