Parallel airfoils - Negative drag coefficient
 

Hello everybody!

I'm doing some simulations in FLUENT to study aerodynamic interference between two parallel airfoils (as the attached image), varying AoA, separation of the airfoils and the Reynolds number. Also, I'm using Spalart-Allmaras model for turbulence (in fact, I'm more interested in viscous effects, since I wasn't getting the laminar model to converge) and incompressible flow.


After a series of simulations, I checked the force report in FLUENT and it shows the drag coefficient due pressure and viscosity for both airfoils, and all of them are positive, except for the coefficient due pressure in the upper airfoil. I also checked the lift coefficients and they're OK.

The mesh is fine enough and the direction of the drag vector is correct (I think so). I tried to simulate the airfoils very spaced form each other and the results were similar to just one airfoil alone.

Any idea?