Modeling of Turbulence

This section covers the numerical modeling of turbulence by various turbulence models, near wall modeling and inlet turbulence parameters specified for turbulence models.

Although turbulence has been researched for centuries it continues to pose some of the most difficult and fundamental problems in physics because it is a complex, nonlinear phenomenon.

It has the following characteristics:
  • Three dimensional and time dependent
  • Irregular and chaotic in nature
  • Non repetitive
  • Contains a vast range of length and time scales
  • Has a length scale reduction with increasing Reynolds number
  • Is sensitive to boundary and initial conditions
Considering these turbulence characteristics, resolving the turbulent quantities directly by means of Direct Numerical Simulation (DNS) is impractical due to the limitations of currently available computational resources. For this reason, the preferred approach is not to resolve turbulence but to model it with consideration of the following observations:
  • Statistical averages of quantities in turbulent flows are reproducible.
  • Engineering problems are concerned with the mean flow motion, not the instantaneous motion.

    Given these justifications, turbulence models are developed to account for the effects of eddies on the mean flow field in order to make Computational Fluid Dynamics (CFD) plausible on industrial scale engineering problems.

    In this section representative turbulent flow cases are provided to explain why turbulent flow simulations could be challenging. Resource requirements for DNS are examined in order to justify the need of turbulence modeling. After a brief comparison of resource requirements for various turbulence models the Reynolds-averaged Navier-Stokes equations (RANS) are introduced and turbulence models are presented to close the RANS equations. The focus then shifts to Large Eddy Simulation (LES).

    Overall, this discussion is primarily aimed at motivating the need for turbulence models when investigating high Reynolds number turbulent flow applications and to introduce commonly available turbulence models to new users.