2D multi-element airfoil negative drag
 

Hello,

I am doing a drag-lift 2D CFD analysis on a multi-element airfoil constituted by a "main-body" (named after "MB") and a deployed slat (see pictures). I am using Fluent.

I am new to CFD analysis and I am asking myself a lot of questions about the quality of my simulation results because:
-I obtain negative drag on the main-body
-I obtain negative lift on the slat

Can someone tell me if I missed something, such that I get a negative drag coefficient on the main body?

After convergence, I obtain:
Cd-MB: -0.085
Cl-MB: 1.96
Cd-SLAT:0.15
Cl-SLAT:-0.3

Aerodynamic conditions:
1)Vinf: 67m/s (Mach ~0.2)
2)AoA: 5.5deg
3)Pinf: 101325Pa
4)Tinf: 15°C
5)Chord: 2m

Fluent setup:
1)Y+<1
2)Spalart Allmaras viscous model (Turbulent Viscosity Ratio for both Velocity Inlet & Pressure Outlet boundary conditions=5).
3)Boundaries: Left side, top & bottom are "Velocity Inlet" bc. & Right side is Pressure Outlet. & main-body and slat are walls with no slip.
4)For Reference values I chose calculate from Inlet.
5)Monitors (force Monitors) I didn't change the axis of Cl & Cd and left them as they are. Direction vector for drag: (cos(5.5);sin(5.5). Direction vector for lift: (-sin(5.5);cos(5.5)).
6)Pressure-based solver
7)Incompressible flow assumption (as M<0.3)
8)Operating pressure: 101325Pa Gauge pressure: 0Pa

Mesh:
1)Farfield: 100*chord
2)Number of elements:280000 (Max skewness:0.82, Min orth quality: 0.12)
3)Main-body & slat wall mesh size: 10-3 m
4)Max edge size: 2m
5)Growth rate: 1.1
6)Triangle elements in the inner region (~4*chord away from the airfoil). Quad-dominant meshing method for the outer region.



Best regards,
Etienne

Apparently, nobody can help me. After some researches I think I found some explanation on the physics of the phenomenon observed.

Maybe it could help for others...


Negative drag on main body: In my point of view the presence of the slat induces a pressure decrease downstream of it. As a result, a pressure loss appears in a region where the faces normal vectors are oriented "on average" in the direction opposite to Uinf. The negative drag could then be generated by this pressure effect near the leading edge of the main body.
However on the entire structure (slat + main-body), the total drag is positive.
You can see on the Cp figure that the stagnation point on the main-body is located on the intrados. Normally for a "single-element airfoil", I think would observe a stagnation point location more closer to the leading edge.

Negative lift on slat: due to the slat deflection, the stagnation point on the slat is located sufficiently "high" on the extrados so that the pressure loss appears on the slat intrados. So generating negative lift on this component. However on the entire body (slat + main-body) the total lift is positive.

Does anyone agree?

Are you sure you are extracting forces parallel and normal to the freestream flow direction and not the chord line? It's a simple mistake, but one I have definitely been guilty of in the past. You can switch between normal/axial and lift/drag using an euler rotation that essentially breaks down to this:

Cl=Cn*cos(AoA)-Ca*sin(AoA)
Cd=Cn*sin(AoA)+Cd*cos(AoA)

Also, just glancing at your mesh... It doesn't look like you have done much of anything to resolve your wake. That could also be a source of error.

Thanks a lot for your advices. I checked once more but I extracted forces parallel and normal to the freestream flow direction.

Indeed maybe my mesh isn't refined sufficiently to resolve the wake... I will try a local refinement. Thanks a lot!