22-06-2010, 01:59 PM
Turbulence models in CFD
CFD stands for computational fluid dynamics. A vast field of numerical analysis in the field of fluidâ„¢s flow phenomena is represented by this topic. Primarily CFD deals with understanding and solving ordinary and partial differential equations and much more than that. turbulence when present in a flow dominates all other flow phenomena.The modeling of turbulence thus greatly improves the quality of numerical simulations.
The main steps in solving a CFD problem is
- geometry and grid generation
-setting-up a physical model
-solving it
-post-processing the computed data
Complexity of the turbulence model
The Complexity in most of the turbulence models is due to the nature of Navier-Stokes equation which is inherently nonlinear, time-dependent, three-dimensional PDE. turbulence is also a random process in time. vortex structures move along the flow with very much varying lifetimes.
Classification of turbulent models:
The models based on Reynolds-Averaged Navier-Stokes(RANS) are:
-Eddy-viscosity models (EVM):
Here it is assumed that turbulent stress is proportional to the mean rate of strain
-Differential stress models (DSM):
Reynolds-stress transport models (RSTM) or second-order closure models (SOC) are part of this.
- Non-linear eddy-viscosity models (NLEVM):
In these kind of models, Turbulent stress is modelled as a non-linear function of mean velocity gradients
The ones based on the Computation of fluctuating quantities are:
-Large-eddy simulation (LES):
The time-varying flow is computed but the sub-grid-scale motions is modeled.
-Direct numerical simulation (DNS):
No modelling what so ever is applied here.
For full details, refer this pdf:
[attachment=3819]
CFD stands for computational fluid dynamics. A vast field of numerical analysis in the field of fluidâ„¢s flow phenomena is represented by this topic. Primarily CFD deals with understanding and solving ordinary and partial differential equations and much more than that. turbulence when present in a flow dominates all other flow phenomena.The modeling of turbulence thus greatly improves the quality of numerical simulations.
The main steps in solving a CFD problem is
- geometry and grid generation
-setting-up a physical model
-solving it
-post-processing the computed data
Complexity of the turbulence model
The Complexity in most of the turbulence models is due to the nature of Navier-Stokes equation which is inherently nonlinear, time-dependent, three-dimensional PDE. turbulence is also a random process in time. vortex structures move along the flow with very much varying lifetimes.
Classification of turbulent models:
The models based on Reynolds-Averaged Navier-Stokes(RANS) are:
-Eddy-viscosity models (EVM):
Here it is assumed that turbulent stress is proportional to the mean rate of strain
-Differential stress models (DSM):
Reynolds-stress transport models (RSTM) or second-order closure models (SOC) are part of this.
- Non-linear eddy-viscosity models (NLEVM):
In these kind of models, Turbulent stress is modelled as a non-linear function of mean velocity gradients
The ones based on the Computation of fluctuating quantities are:
-Large-eddy simulation (LES):
The time-varying flow is computed but the sub-grid-scale motions is modeled.
-Direct numerical simulation (DNS):
No modelling what so ever is applied here.
For full details, refer this pdf:
[attachment=3819]
