# Turbulent Transition Models

All of the previously described models are incapable of predicting boundary layer transition. To include the effects of transition additional equations are necessary.

Transitional flows can be found in many industrial application cases including gas turbine blades, airplane wings and wind turbines. It is known that conventional turbulence models over predict the wall shear stress for transitional flows. Thus, transition models can be used to improve the accuracy of CFD solutions when flows encounter turbulent transition of the boundary layer.

## γ-Reθ Model

Langtry and Menter (2009) proposed one of the most commonly used transition models in industrial CFD applications. The $$\gamma -R{e}_{\theta}$$ model is a correlation based intermittency model that predicts natural, bypass and separation induced transition mechanisms.

The $$\gamma -R{e}_{\theta}$$ model is coupled with the Shear Stress Transport (SST) turbulence model and requires two additional transport equations for the turbulence intermittency ($$\gamma $$) and transition momentum thickness Reynolds number ($$R{e}_{\theta}$$). The turbulence intermittency is a measure of the flow regime and is defined as $\gamma =\frac{{t}_{\text{turbulent}}}{{t}_{\text{laminar}}+{t}_{\text{turbulent}}}$ where ${t}_{\text{turbulent}}$ represents the time that the flow is turbulent at a given location, while ${t}_{\text{laminar}}$ represents the time that the flow is laminar.

For example, while the intermittency value is zero, the flow is considered to be laminar. A value of one is for fully turbulent flow. The transition momentum thickness Reynolds number is responsible for capturing the non local effects of turbulence intensity and is defined as $R{e}_{\theta}=\frac{\rho \overline{u}\theta}{\mu}$ where the momentum thickness is $\theta =\underset{0}{\overset{\delta}{{\displaystyle \int}}}\frac{u}{\overline{u}}\left(1-\frac{u}{\overline{u}}\right)dy$.

To couple with the SST model, Langtry and Menter (2009) modified the production term and dissipation term of the turbulent kinetic energy to account for the changes in the flow intermittency.

### Transport Equations

### Production Modeling

- Production ${P}_{k}={\mu}_{t}{S}^{2}$
- Strain rate magnitude $S=\sqrt{2{\text{S}}_{ij}{\text{S}}_{ij}}$
- Strain rate tensor ${\text{S}}_{ij}=\frac{1}{2}\left(\frac{\partial \overline{{u}_{i}}}{\partial {x}_{j}}+\frac{\partial \overline{{u}_{j}}}{\partial {x}_{i}}\right)$
- Constant ${\beta}^{*}$

- Blending function for specifying constants $\alpha ={\alpha}_{1}{F}_{1}+{\alpha}_{2}\left(1-{F}_{1}\right)$
- ${\alpha}_{1}=\frac{5}{9}$ for the inner layer
- ${\alpha}_{2}$ = 0.44 for the outer layer.
Turbulent Intermittency $\gamma $

where ${F}_{onset}=max\left\{min\left(max\left[\frac{R{e}_{\nu}}{2.193R{e}_{\theta 0}},{\left(\frac{R{e}_{\nu}}{2.193R{e}_{\theta 0}}\right)}^{4}\right],2\right)-max\left(1-{\left(\frac{{R}_{T}}{2.5}\right)}^{3},0\right)\right\}$ $R{e}_{\nu}=\frac{S{d}^{2}}{\nu}$, ${R}_{T}=\frac{k\nu}{\omega}$

Transition Momentum Thickness Reynolds Number ${\text{Re}}_{\theta}$

where ${F}_{\theta}=\mathrm{min}\left(\mathrm{max}\left[{F}_{wake}{e}^{-{\left(\frac{d}{\delta}\right)}^{4}},1-{\left(\frac{\gamma -1/{C}_{e2}}{1-1/{C}_{e2}}\right)}^{2}\right],1\right)$, ${F}_{wake}={e}^{-{\left(\frac{R{e}_{\omega}}{1E+5}\right)}^{2}}$, $R{e}_{\omega}=\frac{\omega {d}^{2}}{\nu}$, $\delta =\frac{50\text{\Omega}d}{\overline{u}}{\delta}_{BL}$, ${\delta}_{BL}=7.5{\theta}_{BL}$, ${\theta}_{BL}=\frac{\overline{R{e}_{\theta}}\nu}{\overline{u}}$

### Dissipation Modeling

where ${F}_{\text{turbulent}}={e}^{{\left(-\frac{{R}_{T}}{4}\right)}^{4}}$

### Modeling of Turbulent Viscosity ${\mu}_{t}$

- The second blending function ${F}_{2}=tanh{\left(max\left(2\frac{\sqrt{k}}{{\beta}^{*}\omega d},\frac{500\nu}{{d}^{2}\omega}\right)\right)}^{2}$,
- Fluid kinematic viscosity $\nu $,
- Constant ${\beta}^{*}$.

### Correlations

where ${\lambda}_{\theta}=\frac{{\theta}^{2}}{\nu}\frac{d\overline{u}}{ds}$ is the pressure gradient.

### Model Coefficients

${\beta}^{*}$ = 0.09.

The following constants for SST are computed by a blending function $\varphi ={\varphi}_{1}{F}_{1}+{\varphi}_{2}\left(1-{F}_{1}\right)$: ${\sigma}_{\omega 1}$ = 0.5, ${\sigma}_{\omega 2}$ = 0.856, ${\sigma}_{k1}$ = 0.85, ${\sigma}_{k2}$ = 1.00, ${\beta}_{1}=\frac{3}{40}$, ${\beta}_{2}$ = 0.0828. ${C}_{e1}$ = 1.0, ${C}_{e2}$ = 50, ${C}_{a1}$ = 2.0, ${C}_{a2}$ = 0.06, ${C}_{\theta 1}$ = 1.0, ${C}_{\theta 2}$ = 2.0, ${\sigma}_{f}$ = 1.0.