Doing a CFD study on a wing design for my uncle's club racing car; and noticing something that I can't wrap my head around.
Don't know its naca designation because searching airfoiltools database came up empty. This is for a single element spoiler.
Aspect ratio is 5. The width of the car's roof is 1300mm.
If I use a single surface / profile, at the wing's peak AoA (in relation to the air off the roof, not to the ground plane), with a gurney (roughly 2% chord length) produces about 105kg of negative lift.
Why am I seeing a decrease in performance (in terms of neg lift) when I switch it over to a multi-surface / multi-profile but w/o a gurney? There is an approx 5 deg difference between the air that's coming over the roof vs the air that's coming past the side window. The outer profile's AoA difference is exactly that - 5 deg; everything is pretty much identical incl. endplates. Only difference is I didn't CAD up a gurney for this thing. Does the gurney play that big of a role in a wing's efficiency in that even in the areas where the air hitting it isn't at its optimal AoA, it can still make the thing outperform on a design that is letting it see the air at an optimal angle across the span but w/o the lip? The performance decrease is roughly 4.7%; so it's not exactly chump change considering there's very little to begin with.