Help verifying lift and drag coefficinets setup

Project Support
1

Hi, I am simulating a NACA 0012 finite wing in incompressible external aerodynamics and measuring lift/drag coefficients and forces across angles of attack.

Here is the link to my project: https://www.simscale.com/workbench/?pid=1752549643485203317&location=simulationRun:1143d89b-a9f5-4acf-aaa1-9f433b143af4

I vary AoA by changing the inlet velocity direction in the xy plane. For an AoA α, I set:

Lift direction = (sin(α), cos(α), o); Drag direction = (cos(α), sin(α), 0)

I use V = 10m/s; Chord = 0.3m; Reference area 0.6m^2

The problem is that both my lift force and lift coefficient keep increasing across all angles. I was expecting stall to occur between 12 and 16 degrees, meaning that both lift force and lift coefficient were supposed to reach their peak and start decreasing. Instead, the rate at which they increased was decreasing, but the value kept growing even at 30 degrees. Moreover, my drag coefficient was decreasing at the start and then happened to have negative values, which contradicts my positively growing drag force value across all AoA.

I changed the orientation of the drag coefficient from (cos(α), sin(α), 0) to (-cos(α), -sin(α), 0), and of the lift coefficint from (sin(α), cos(α),0) to (-sin(α), cos(α), 0); this helped to solve the problem with negative drag coefficients. At that moment, drag was growing.

The remaining problem is that my Lift force, along with the lift coefficient, keeps increasing instead of reaching its peak and going down. I know that something in my setup is probably wrong, but I cannot determine what exactly. I would really appreciate any advice or a possible explanation for my problem.

I am simulating drag and lift to be able to check my circulation Г values with Zhukovsky’s theorem and plot a “lift produced per unit of drag” graph.

3

Hi,

I’d suggest reviewing this simulation project. There’s also a description if you scroll down: https://www.simscale.com/projects/dlynch/naca0012_validation_3/

4

Hi Elena,

The reason your wing isn’t stalling is due to a conflict between how you angled the wind and how your simulation box is set up.

1. The Wind is Crashing into the Ceiling: Looking at your mesh, the outer box is straight and horizontal. By angling the inlet velocity to change your angle of attack, you are blowing the wind diagonally directly into the top and bottom walls. Because those walls are set to “Symmetry,” they act exactly like solid ceilings. The air can’t escape through them, so it gets unnaturally squeezed between the ceiling and the wing. This squeezing forces the air to stay glued to the wing surface, making it physically impossible for the wing to stall.

To let the air flow naturally and capture that stall, you have three options depending on how you want to set up the physics:

Option A: The Standard Approach (Pitch CAD + Symmetry)

Keep the wind blowing perfectly straight horizontally (x=10, y=0, z=0) and physically rotate the 3D wing model itself inside your CAD software (like SolidWorks) before importing it. Leave the top and bottom boundaries as Symmetry. Because the wind is straight, it will flow perfectly parallel to the boundaries without crashing into them.

Option B: The Freestream Approach (Flat CAD + Angled Wind) If you prefer to keep the CAD flat and angle the inlet wind, you must change the top and bottom boundaries from Symmetry to Freestream (or Custom Inlet/Outlet). This tells the solver to let the air pass through the ceiling and floor of the box freely, preventing the artificial squeezing effect.

Option C: The Wind Tunnel Approach (Pitch CAD + Walls)

Keep the wind straight, rotate the CAD model, but change the outer Symmetry boundaries to Walls. This simulates a physical closed-test-section wind tunnel. Just make sure to set them as Slip Walls (zero friction); if you leave them as standard no-slip walls, friction will create thick boundary layers on the ceiling and floor that will artificially choke the airflow around your wing.

Simplified Math for Options A and C: By keeping the wind straight and rotating the wing, you completely avoid the negative drag issue and the confusing sine/cosine math. Your force directions become extremely simple for every single run:

  • Drag Direction: (1, 0, 0)

  • Lift Direction: (0, 1, 0)

Any of these three setups will fix the blockage and finally show you that lift drop-off between 12 and 16 degrees.