In the model, different modes of transition are modelled based on correlations obtained from experimental data, viz. natural and bypass transition, separation-induced transition and wake induced transition.

The natural and bypass mode of transition is modelled by a correlation based on the transition criterion by Abu-Ghannam and Shaw: The transition onset is modeled at the location where the Reynolds number based on momentum thickness, \(Re_\theta\), exceeds the value from the correlation.

The development of the intermittency is modelled on the same parameters as the transition onset.

Separation-induced transition is triggered when negative values of wall shear stress are detected. Generally, this transition mode depends on the same parameters as natural an bypass transition, such as free-stream turbulence intensity and pressure gradient. Furthermore, the effects of momentum thickness Reynolds number, \(Re_\theta\) and boundary layer shape factor \(H_{12}\) have been included in a correlation for the intermittency \(\gamma\). In this mode, the intermittency can reach values \(\gamma > 1\). This mechanism is used to circumvent weaknesses of two-equation models in correctly predicting reattachment of separated flows.

The effect of incoming wakes on the transition is modelled by setting the intermittency to \(\gamma = 1\) when high turbulence intensities ( \(Tu > 4\%\)) are detected at the boundary layer edge. Despite normally being run in unsteady mode, the model for wake-induced transition also possesses a "quasi unsteady" mode, in which the effects of impinging wakes on the transition onset are modelled in steady-state computations.

Because some of the underlying transition criteria are based on integral parameters of the boundary layer, the transition model can be characterised as a non-local model. The different modes of transition are overlapped by computing the intermittency for each mode separately, and then using the maximum intermittency found in the computation. The transition model is coupled to the source terms of the turbulence model, rather than directly reducing the modelled Reynolds stresses, so that transport of turbulent quantities is still possible even in non-turbulent regions. This feature is especially important for properly modelling wake-induced transition.

The model is described in [27] .