# Reynolds stress model (RSM)

### From CFD-Wiki

## Introduction

The Reynold's Stress Models (RSM), also known as the Reynold's Stress Transport (RST) models, are higher level, elaborate turbulence models. The method of closure employedis 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, , 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, , may be written as follows:

or

Local Time Derivate + = + + + + - + + User-Defined Source Term

where is the Convection-Term, equals the Turbulent Diffusion, stands for the Molecular Diffusion, is the term for Stress Production, equals Buoyancy Production, is for the Pressure Strain, stands for the Dissipation and is the Production by System Rotation.

Of these terms, , , , and do not require modeling. After all, , , , and have to be modeled for closing the equations.

### Modeling Turbulent Diffusive Transport

### Modeling the Pressure-Strain Term

### Effects of Buoyancy on Turbulence

### Modeling the Turbulence Kinetic Energy

### Modeling the Dissipation Rate

### Modeling the Turbulent Viscosity

### Boundary Conditions for the Reynolds Stresses

### Convective Heat and Mass Transfer Modeling

### Return-to-isotropy models

For an anisotropic turbulence, the Reynolds stress tensor, , is usually anisotropic. The second and third invariances of the Reynolds-stress anisotropic tensor are nontrivial, where and . It is naturally to suppose that the anisotropy of the Reynolds-stress tensor results from the anisotropy of turbulent production, dissipation, transport, pressure-stain-rate, and the viscous diffusive tensors. The Reynolds-stress tensor returns to isotropy when the anisotropy of these turbulent components return to isotropy. Such a correlation is described by the Reynolds stress transport equation. Based on these consideration, a number of turbulent models, such as Rotta's model and Lumley's return-to-isotropy model, have been established.

Rotta's model describes the linear return-to-isotropy behavior of a low Reynolds number homogenous turbulence in which the turbulent production, transport, and rapid pressure-strain-rate are negligible. The turbulence dissipation and slow pressure-strain-rate

are preponderant. Under these cirsumstance, Rotta suggested## Model constants

The constants suggested for use in this model are as follows:

## Model variants

### LRR, Launder-Reece-Rodi

**Launder, B. E., Reece, G. J. and Rodi, W. (1975)**, "Progress in the Development of a Reynolds-Stress Turbulent Closure.", Journal of Fluid Mechanics, Vol. 68(3), pp. 537-566.

### SSG, Speziale-Sarkar-Gatski

**Speziale, C.G., Sarkar, S., Gatski, T.B. (1991)**, "Modeling the Pressure-Strain Correlation of Turbulence: an Invariant Dynamical Systems Approach", Journal of Fluid Mechanics, Vol. 227, pp. 245-272.

## Performance, applicability and limitations

## Implementation issues

## References

**Launder, B. E., Reece, G. J. and Rodi, W. (1975)**, "Progress in the Development of a Reynolds-Stress Turbulent Closure.", Journal of Fluid Mechanics, Vol. 68(3), pp. 537-566.