Saturday, February 15, 2014

Nigel Bennett, Gordon Murray, and vortex generators

Erstwhile Formula 1 and Penske designer Nigel Bennett has published a superb autobiography, Inspired to Design, which provides a reminder that several important aerodynamic concepts, prevalent in Formula 1 to this day, were actually invented in Indycar.

One of the recollections in the book even suggests that the use of vortex generators to enhance underbody downforce, was co-conceived by Bennett and Tony Purnell:

"Tony Purnell and I discussed some research he was doing at Cambridge University regarding laser viewing of vortex sheets, an element of which was trying to measure the low pressure generated at the centre of a vortex. Tony explained that if the vortex was trained to run between two plain surfaces, the low pressure would act on those surfaces.

"So, in our wind-tunnel tests, we set out to see if we could use this phenomenon to create more downforce from the car, and sure enough, it worked in that by creating a vortex at the front of the underbody such that it directed air at the underwing and chassis intersection, we were able to gain some 30-40lb [13-18kg] of downforce (full-size at 150mph) without an increase in drag. We developed a series of triangular sharks' teeth, fitted at an angle to the normal air stream just in front of the lower edge of the radiator intake duct, and the air would spill off these and form the swirling vortex. Later work using flow visualisation techniques showed where this vortex ran, and indeed, other vortices from the outer shelf edge did much the same thing in the outer rear corners," (p97).

It seems, then, that Bennett and Purnell were the first to systematically investigate and apply vortex generators. This work appears to have been undertaken as part of the design for the 1988 Penske PC 17. However, it should be recalled that Gordon Murray (featured in this month's Motorsport magazine podcast) introduced inch-deep vortex generators on the underside of the 1975 Brabham BT44, also with the intention of creating downforce. (Murray explains this in diagrammatic form when interviewed by Steve Rider for Sky Sports' F1 Legends Series).

For those seeking a rigorous insight into vortex generators, Lara Schembri Puglisevich has recently submitted a PhD thesis at the University of Loughborough, reporting the results of Large-Eddy Simulations of vortical flows in ground effect. This work includes a comparison (pictured below) of a vortex generator above: (i) a smooth, stationary ground plane; (ii) a smooth, moving ground plane; and (iii) a rough, stationary ground plane. The images show vorticity isosurfaces, colour-contoured by streamwise velocity. The flow is from left-to-right, with the vortex generator suspended from the floor above.

This is the first attempt to understand the potential interaction between a vortex and the roughness of the ground plane. Unfortunately, it wasn't possible to make the rough ground plane a moving plane, hence the stationary ground plane builds up its own boundary layer, which interacts with the vorticity shed by the vortex generator.

Nevertheless, these LES images vividly demonstrate just how 'messy' real vortices are.


Unknown said...

Hello Professor McCabe,

Does sending vortices under the floor entrain more mass flow under the car ? If so, how ?

I read in an old issue of Racecar Engineering from 1992 that they would send vortices into the venturi tunnels which would increase the flow rate ? Presumably this works
simply by mixing the momentum of high speed core into the boundary layer to reduce it and increase the area ratio and thus the efficiency of the venturi tunnel ?

Can sending a vortex through the air simply suck/entrain more air along with it than otherwise possible with out the vortex ?

Happy Easter,

Peter from Canada

Gordon McCabe said...

Hi Peter.

A vortex will entrain non-vortical air between its spirals during the rollup process. And if a vortex has a jet-like axial-flow profile, then that will help to drag more air along. But I think the vortices increase mass-flow under the floor simply by virtue of creating low pressure, thereby accentuating the pressure gradient which drives air under the floor.

The vortices which are pulled off the fences in the diffuser are each side-edge vortices, much like the one detailed in the academic literature on racecar diffusers. Each one creates an upwash in its respective compartment, which helps to keep the flow attached. Keeping the flow attached in the face of an adverse pressure gradient towards the trailing edge of the diffuser will increase mass-flow through the diffuser.