Negative Drag Coefficient of main airfoil in Multi-Element rear wing
 

I'm simulating a multi-element wing (so far in 2D) for race cars . Both main and flap are S1223 airfoil with 0.5% chord trailing edge gap. I've been using both SST-K-Omega & Realizable K-E with enhanced wall treatment in "Steady" mode.
1)Y+~1
2)Pressure-Velocity Coupling Scheme: SIMPLE & sometimes Coupled
Turbulent Intensity for both Velocity Inlet & Pressure Outlet boundary conditions= 1%
3)Turbulent Viscosity Ratio for both Velocity Inlet & Pressure Outlet boundary conditions=10%
4) Velocity Inlet=66.67 m/s
5) AoA of main airfoil= 0 -- AoA of flap= 30 deg
6) Overlapping= 0 mm
7) Gap between airfoils=8 mm
8) Boundaries: Left side, top & bottom are "Velocity Inlet" bc. & Right side is Pressure Outlet. & two airfoils are walls with no slip.
9) for Reference values I chose calculate from Inlet & for the Area & Length I put the projected length of the System Chord on X axis. (Unit depth in Z direction)
10) In Monitors (force Monitors) I didn't change the axis of Cl & Cd and left them as they are (Cl= (0,1) & Cd=(1,0))

Now the problem: As you can see in the Force Report (attached photo), the Pressure Drag & finally the total drag coefficient of main airfoil is negative. I get this result in all turbulent models and in all different arrangements of airfoils even when I simulate it in Transient mode.

QUESTION:
Do you think the problem arises from Parasitic Drag due to the flap airfoil or it's choosing wrong solution monitor coordinates for Cl & Cd of the airfoil system that results in negative drag of main airfoil.

Hello,

have you found a solution?

I have the exact same problem, for a similar configuration. (velocity 11m/s, main element angle 3°, second element angle 38° (respectively S1223, Eppler420).


As the angle of the second element is so high, maybe I should try LES instead of k-w SST low Re.

Hi Remi,

Unfortunately not yet, I just accept it as it is. I hope someone finds a reasonable answer for it!
About your suggestion for using SST-k-omega, I should say that I used SST-k-omega in majority of simulations however my free stream speed was 66.67 m/s for a 240mm main element airfoil in 0 deg & 84mm flap in 35 deg $ 40 deg. I also tried Spalart-Allmaras, realizable k-epsilon but the same thing occurred.

Thanks for your reply.

I will investigate with LES, and try to "see" what happens!
Maybe it's as you said, a parasitic drag occuring from the second element. In that case, it would be extremely beneficial for an aircraft, as it would push it forward. I highly doubt it, though (but that would be great :D).

I'll keep this post updated if I find anything.

I am also simulating a 2 element airfoil for a race car and have negative drag on the main element. However my overall drag is positive. My I don't know if this is right but my supervisor told me that it is due to a phenomenon called leading edge suction. You should be able to see that your stagnation point on the main element is underneath the airfoil (the high pressure side) and that the flow raps around the leading edge at very high speed . Apparently it is actually separating and reattaching very quickly and this high speed results in very low pressure on the leading edge which creates a suction force.

As long as your overall drag is positive I think you are ok.