Slipstream solves the incompressible Navier–Stokes equations on a grid, on your GPU, every frame. Air enters from the left, your shape is a no-slip wall, and the pressure field that forms around it is integrated over the surface to get drag and lift.
Cd is drag divided by ½·ρ·U²·A, where A is the frontal height of your shape, corrected for tunnel blockage with Maskell's method, the same correction real wind tunnels apply. A plain cylinder reads about 1.4 and a flat plate about 2, which is what two-dimensional experiments give at this Reynolds number; the familiar three-dimensional numbers (1.2 for a cylinder, 0.3 for a car) are lower because real air can also slip around the sides. Cl uses the streamwise length as its reference, the way airfoils are quoted. Readings are averaged over a couple of seconds, so give a new shape a moment to settle.
This is a 2-D, low-Reynolds-number (a few hundred), pressure-only simulation: no skin friction, no turbulence model, no 3-D effects. Bluff bodies (plates, boxes, cars) are represented well; slender bodies read lower than a real tunnel because their drag is mostly skin friction. The ordering between shapes is trustworthy, the third decimal is not.
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