P.S. A practice becoming more commonplace is to seal underbodies with vortices at the sides. Barge boards & front wing end plates are the two most common places (I speculate) that are used to create these vortices.
I recently tried a study on inducing vortices on the sides to avoid the high pressure air from forming on the sides and spilling into the diffuser. Frankly, I did not achieve the desired results, but mainly because I couldn't devote enough time and energy towards this to make sure that I do get the results. But, I thought this must have been used by the teams for long time, say, atleast since the bargeboards took the shape they are in now !
Having gone through the process, I did achieve lower pressure on the lower parts of the sides. But this again prompted spillage from the top. Next stage in the study would have been(yes..that never happened ) using that spillage to generate vortices as these vortices-some sort of edge vortices by putting some'end plate' sort of strucuters- could be produced at a point further behind than the point where the side vortices are produced right now : bargeboards.
It wasnt a full car simulation. I left an aerofoil shaped wall curved similarly to the sides of an F1 car. Used an extruded and fillet-ed cicle(simulating wheel) to create wake, simulating the front wheel. Did not model a genuine sidepod 'hole'(thus, skipping the hot internal flows and in turn leaving out the possibility of using any exhaust gases to lower the pressure anywhere else). The only thing that might come close to reality is BARGEBOARD, but even this was done with dimensions found by scaling pictures, and the edges of the bargeboards were created using splines. And, add the fact that first order discretization was used and y plus values nowwhere near unity, this was a simulation that hold no significance with regard to finding exact values. More like helped me find if my thoughts were in right direction, cause I hadn't seen any discussion or article on the side vortices. (if I had, I wudnt have done a simulation..lol)
To conclude (if at all I can use that word), side vortices did give lower pressure, better negative lift in diffusers, higher drag, and pretty much same efficiency(was a bit lower than orig, in fact) for what I did. But again, as I said,modelling sidepods and internal flows, using exhaust gases and creating vortices from the spillage from the top part might add to the lift without much extra drag.