OpenFOAM I think is the go to for open source CFD, although I’ve never tried it myself. There’s also XFOIL which, since you’re talking about a flying wing, might be enough for your use case.

Even for a finicky flying wing, I think if you keep an eye on the way the pitching moment shifts with pitch, I don’t see why picking an airfoil shape for the frame and hand calcs couldn’t get you to a design that flies reasonably.