# Reynolds stress model (RSM)

(Difference between revisions)
 Revision as of 10:15, 4 October 2006 (view source)← Older edit Revision as of 10:16, 4 October 2006 (view source)Newer edit → Line 11: Line 11: $[itex] - \underbrace{\frac{\partial}{\partial t}(\rho \overline{u'_iu'_j})} + \frac{\partial}{\partial x_{k}}(\rho u_{k} \overline{u'_iu'_j}) = - \frac{\partial}{\partial x_k}[\rho \overline{u'_iu'_ju'_k} + \overline{p(\delta_{kj}u'_i + \delta_{ik}u'_j)}]$ + \frac{\partial}{\partial t}(\rho \overline{u'_iu'_j}) + \frac{\partial}{\partial x_{k}}(\rho u_{k} \overline{u'_iu'_j}) = - \frac{\partial}{\partial x_k}[\rho \overline{u'_iu'_ju'_k} + \overline{p(\delta_{kj}u'_i + \delta_{ik}u'_j)}] [/itex]

## Introduction

The Reynold's stress model (RSM) is a higher level, elaborate turbulence model. It is usually called a Second Order Closure. This modelling approach originates from the work by [Launder (1975)]. In RSM, the eddy viscosity approach has been discarded and the Reynolds stresses are directly computed. The exact Reynolds stress transport equation accounts for the directional effects of the Reynolds stress fields.

## Equations

The Reynolds stress model involves calculation of the individual Reynolds stresses, $\overline{u'_iu'_j}$ , using differential transport equations. The individual Reynolds stresses are then used to obtain closure of the Reynolds-averaged momentum equation.

The exact transport equations for the transport of the Reynolds stresses, $\overline{u'_iu'_j}$ , may be written as follows: $\frac{\partial}{\partial t}(\rho \overline{u'_iu'_j}) + \frac{\partial}{\partial x_{k}}(\rho u_{k} \overline{u'_iu'_j}) = - \frac{\partial}{\partial x_k}[\rho \overline{u'_iu'_ju'_k} + \overline{p(\delta_{kj}u'_i + \delta_{ik}u'_j)}]$ $+ \frac{\partial}{\partial x_k}[{\mu \frac{\partial}{\partial x_k}(\overline{u'_iu'_j})}] - \rho(\overline{u'_iu'_k}\frac{\partial u_j}{\partial x_k}+\overline{u'_ju'_k}\frac{\partial u_i}{\partial x_k}) - \rho\beta(g_i\overline{u'_j\theta}+g_j\overline{u'_i\theta})$ $+ \overline{p(\frac{\partial u'_i}{\partial x_j} + \frac{\partial u'_j}{\partial x_i})} - 2\mu\overline{\frac{\partial u'_i}{\partial x_k} \frac{\partial u'_j}{\partial x_k}}$ $-2\rho\Omega_k(\overline{u'_ju'_m}\epsilon_{ikm} + \overline{u'_iu'_m}\epsilon_{jkm}) + S_{user}$

or

Local Time Derivate + $C_{ij}$ = $D_{T,ij}$ + $D_{L,ij}$ + $P_{ij}$ + $G_{ij}$ + $\phi_{ij}$ - $\epsilon_{ij}$ + $F_{ij}$ + User-Defined Source Term

where $C_{ij}$ is the Convection-Term, $D_{T,ij}$ equals the Turbulent Diffusion, $D_{L,ij}$ stands for the Molecular Diffusion, $P_{ij}$ is the term for Stress Production, $G_{ij}$ equals Buoyancy Production, $\phi_{ij}$ is for the Pressure Strain, $\epsilon_{ij}$ stands for the Dissipation and $F_{ij}$ is the Production by System Rotation.

Of these terms, $C_{ij}$, $D_{L,ij}$, $P_{ij}$, and $F_{ij}$ do not require modeling. After all, $D_{T,ij}$, $G_{ij}$, $\phi_{ij}$, and $\epsilon_{ij}$ have to be modeled for closing the equations.

## Model constants

The constants suggested for use in this model are as follows: $C_s \approx 0.25, C_l \approx 0.25, C_\gamma \approx 0.25$