Transition

Three primary modes of transition are typically involved [376]:

1. Natural transition, in which a laminar boundary layer subjected to weak disturbances becomes linearly unstable beyond a critical Reynolds number at which point so-called Tollmien-Schlichting waves start to grow.
2. Bypass transition, the process of transition in response to large disturbances outside the boundary layer, typically free-stream turbulence levels in excess of 1%.
3. Separation-induced transition, in which separation of the laminar boundary layer gives rise to transition. The laminar boundary layer often reattaches in response to the enhanced mixing caused by the turbulent flow, forming a laminar separation bubble upstream of the transition location.

Examples of other important transition mechanism are: roughness-induced transition, which is often used in experiments to "trip" boundary layers towards a fully turbulent state, or cross-flow induced transition.

Transition models are necessary because turbulence models are theoretically derived to predict fully turbulent flows. The only exception are low-Reynolds number K-Epsilon models, which include damping functions for the turbulent eddy viscosity and the turbulent dissipation rate near the wall (such as the Standard K-Epsilon Low-Re model, the EB K-Epsilon model, or the Lag EB K-Epsilon model). These models show some transition behavior under certain flow conditions [383]. However, this transition prediction is more a result of a mathematical bifurcation between stable solutions than a desired feature.

In Simcenter STAR-CCM+, two approaches are currently available to account for transition:

1. Turbulence Suppression model—this model mimics the effect of transition simply by suppressing the turbulence in a certain pre-defined region and can be combined with any turbulence model.
2. Gamma ReTheta Transition model and Gamma Transition model—these models are based on correlations and solve additional transport equations that are coupled with the SST K-Omega turbulence model. For the Standard Spalart-Allmaras turbulence model, Simcenter STAR-CCM+ provides the SA Gamma Transition model.

The Turbulence Suppression model is a zero-equation model and is thus the fastest and least expensive one, but it requires that you already know the location of transition.

The Gamma ReTheta Transition model and the Gamma Transition model are more expensive, but provide a true predictive capability. The Gamma ReTheta Transition model solves for two additional transport equations in addition to the two-equation SST K-Omega model. The Gamma Transition model only solves for one equation—it is therefore faster and less computationally expensive than the Gamma ReTheta Transition model.