Theory of Turbulent Flows
1. Notations and units
Notation | Quantity | Unit |
---|---|---|
ρ |
fluid density |
kg⋅m−3 |
\boldsymbol{u} |
fluid velocity |
m \cdot s^{-1} |
\boldsymbol{\sigma} |
fluid stress tensor |
N \cdot m^{-2} |
\boldsymbol{f}^t |
source term |
kg \cdot m^{-3} \cdot s^{-1} |
p |
pressure fields |
kg \cdot m^{-1} \cdot s^{-2} |
\mu |
dynamic viscosity |
kg \cdot m^{-1} \cdot s^{-1} |
\bar{U} |
characteristic inflow velocity |
m \cdot s^{-1} |
\nu |
kinematic viscosity |
m^2 \cdot s^{-1} |
L |
characteristic length |
m |
2. Reynolds-averaged Navier–Stokes equations
The Reynolds-averaged Navier–Stokes equations (or RANS equations) are time-averaged equations of motion for fluid flow. The Reynolds averaging consists to decompose the solution variables of Navier-Stokes equations (like velocity \boldsymbol{u}) into the mean component (\overline{\boldsymbol{u}}) and the fluctuating component (\boldsymbol{u}^{\prime}), which can be written in the following form
2.1. Incompressible case
The incompressible Navier-Stokes equations required to find the velocity \boldsymbol{u} and the pressure p which verify following equations
with the stress tensor \boldsymbol{\sigma}
and strain tensor D(\boldsymbol{u}) :
By substituting the Reynolds decomposition on the velocity and pressure, i.e
into the incompressible Navier-Stokes equations and in computing the Reynolds average of these equations, we can derived Reynolds-averaged Navier–Stokes in the incompressible case. The RANS probleme corresponds to find \overline{\boldsymbol{u}} and \overline{p} such that
These system is very close to the incompressible Navier-Stokes equations exept that we have an additional term \rho\ \overline{\left( \boldsymbol{u}^{\prime} \cdot \nabla_{\mathrm{x}} \right) \boldsymbol{u}^{\prime}}. Then, this extra term can be rewritten (by using the continuity equation of the fluctuation) as