ANSYS Fluent NACA 4412 at Re = 50,000, AoA = 10° – Large CL Differences Between Transition Models and Boundary Conditions
- Post By Shadi Bentamy
- 20 hours ago
- Post Type Public
Hi all,
I am working on a final-year CFD project and have been stuck on this for 2 months with no luck investigating a NACA 4412 airfoil at Re = 50,000 and AoA = 10° in ANSYS Fluent 2025 R2. I am trying to reproduce low-Re experimental data before moving on to vortex-generator simulations, but I am getting very different lift predictions depending on the transition model and boundary conditions.
My current setup is:
NACA 4412, chord = 0.1 m
3D structured C-type mesh
Span = 0.025 m
Spanwise faces are now translational periodic
Airfoil remains horizontal; AoA is imposed through the inlet velocity components
Turbulence intensity = 0.35%
Transient simulations
Meshes tested: approximately 2.5M and 3.5M cells
The main problem is the lift coefficient.
Using Transition SST, I generally obtain roughly:
The mean suction-side wall shear indicates separation around 0.3 and 0.35, with the flow remaining separated over much of the aft chord. Increasing the mesh from 2.5M to 3.5M has not produced a major change so far.
I also tested the Transition k-kl-w model. It initially reached 0.6, but later became strongly unsteady, with lift oscillating significantly rather than remaining at that value.
I am now testing SST k-w with the gamma transport-equation transition model. This is producing much higher lift, around 1.06, with cd = 0.9 - 1, which is much closer to the experimental values I am trying to reproduce. However, I am concerned that part of the improvement may be coming from my outer-boundary treatment rather than the transition model itself.
One issue I am particularly unsure about is the correct far-field boundary setup when the airfoil is horizontal and the 10 degree AoA is imposed using velocity components. I have experimented with wall, symmetry, and inlet/outlet conditions on the upper and lower outer boundaries, and the aerodynamic coefficients appear quite sensitive to this.
I would appreciate advice on:
What is the most appropriate outer-boundary setup for this low-speed oblique freestream?
Is the Gamma transport model preferable to Transition SST for a laminar separation bubble at re=50000?
Is a cl difference this large between Transition SST, k-kl-w, and Gamma expected at such a low Reynolds number?
What quantities would you use to determine which model is physically correct cp, skin friction/separation and reattachment locations, intermittency, etc.?
Is a 25 mm periodic span sufficient for this baseline case, or should I first validate the problem in 2D?
I am trying not to simply choose the model that gives the experimental cl. I want to understand why the models are predicting such different separation/transition behaviour and establish a defensible baseline before adding vortex generators.
Any advice from anyone who has modelled low-Re airfoils or laminar separation bubbles in Fluent would be greatly appreciated.