Turbulence Chapter 4: Laminar-turbulent Transition Modeling
This chapter focuses on turbulence transition modeling in RANS simulations. The transition region affects boundary layer separation, drag, and heat transfer, and requires additional modeling beyond what fully turbulent solvers provide. The content includes the main transition mechanisms, typical modeling approaches, and Fluent-specific implementations.
Background: Why Transition Needs Modeling
In many engineering applications, the boundary layer does not remain either fully laminar or fully turbulent across the entire surface. Instead, transition occurs partway along the flow path. This change in regime significantly affects wall shear stress, boundary layer thickness, and overall pressure loss. Fully turbulent models often overpredict skin friction and fail to capture laminar separation regions unless transition is explicitly modeled.
Main Transition Mechanisms
Three types of transition mechanisms are typically considered:
Natural transition
Appears in low-disturbance environments. The laminar boundary layer first becomes unstable to Tollmien–Schlichting (TS) waves, which grow and eventually break down into turbulence. The process is gradual and sensitive to freestream conditions.Bypass transition
Dominates in flows with higher turbulence intensities (typically above 1–2%). Instead of TS wave growth, disturbances from the freestream directly trigger streaks and turbulent spots inside the boundary layer.Separation-induced transition
Occurs when a laminar boundary layer separates due to an adverse pressure gradient. The free shear layer that forms becomes unstable and transitions to turbulence before possibly reattaching. Common in turbomachinery and flows with curved geometries.
The type of transition mechanism depends on geometry, pressure gradient, turbulence intensity, surface roughness, and Reynolds number.
Transition Models in RANS
Standard RANS turbulence models assume fully turbulent flow throughout the domain and are not able to model transitional regions. Additional modeling is required to handle the onset and development of turbulence. Most modern approaches introduce an intermittency variable (ranging from 0 to 1) to control the local activation of turbulence production.
Three common transition model types:
Intermittency-based models
Use transport equations for intermittency and onset criteria (often in the form of Reynolds numbers or correlations). Turbulence production is only activated where the intermittency function predicts transition.
Examples: Transition SST, Intermittency modelLaminar kinetic energy models
Introduce a separate transport equation for laminar fluctuations. Transition is modeled as a gradual growth of pre-turbulent kinetic energy.
Example: k-kl-omegaEmpirical correlation-based models
Use experimental data to define critical Reynolds numbers or other criteria for transition onset. Often used in 2D or structured meshes.
Each model type has different strengths depending on flow regime and complexity.
Fluent-Specific Models
ANSYS Fluent includes several built-in transition-capable RANS models:
Transition SST (4-equation)
Extends the SST k–omega model by adding intermittency and momentum-thickness Reynolds number transport equations. Can handle natural, bypass, and separation-induced transition.Intermittency model (3-equation)
A reduced form of the Transition SST model, using a local formulation. Lower computational cost. Includes options for crossflow transition.k-kl-omega model
Tracks laminar kinetic energy separately. Useful for bypass and natural transition but does not predict separation-induced transition.
All transition models require:
Fine near-wall mesh (typically y+ < 1)
Accurate turbulence intensity at inlets
Second-order spatial discretization
Careful setup of solver controls and initialization
Practical Notes
For most cases involving bypass or mixed transition modes, the Transition SST model is preferred.
The Intermittency model is faster and often used for rotating machinery or swept surfaces.
k-kl-omega can be used in cases where turbulence intensity is moderate and separation is not critical.
Transition prediction is sensitive to inlet boundary conditions, especially turbulence intensity (Tu), which should be specified explicitly.
Summary
Transition modeling is required when the boundary layer changes from laminar to turbulent in a way that significantly affects the solution. This is often the case in aerodynamics, turbomachinery, and heat transfer.
Modern RANS models include additional equations or correlations to simulate transition onset and development. Fluent provides several options, and correct application requires mesh resolution near the wall and proper inflow conditions. Choice of model depends on the type of transition and the level of fidelity needed.