This idea may contribute to increasing or decreasing downforce, depending on the profile of the blades.
You don't give a speed range, and I'm not sure how this would compare with the downforce generated by underbody fairings, but it's not allowed under FIA rules.
The Brabham BT46 fan car was banned (as it was too damn good), and if you like you could compare the angular velocity of the car's 4 wheels with that of the fan. I think this is a good argument for saying you might be on to something, but although it's easy to work out the wheel speed, I can't find anything on the angular velocity of the fan.
This is getting into EngineeringSE territory, but as I am an F1 fan (sorry) myself, I can't resist a picture to allow you to compare diameters.
The FIA ban might suggest that it is a potentially effective idea, but that it is also inherently dangerous as, at high speed, you don't want one corner of the car to unbalance an otherwise symmetrical distribution of downforce if the wheel blade profiles are not matched properly.