Flawless floors

The concepts on this page are results of spontaneous thoughts and ideas that I've had over the last few months. This post is mostly me just putting these ideas to (digital) paper. As such, they are mostly untested concepts for the time being. I might test various elements of them in the future though, which would warrant a "part 2" post.  

Ground-effect-enhanced ground effect

The basic idea of this concept comes from the Gordon Murray T.50, which uses a high power fan to create a low pressure area immediately aft and above an aggressive diffuser kick point. The resulting pressure gradient helps the air through this rapid change of direction without separating from the diffuser roof, leading to all the benefits of an aggressive and big expansion with no downsides (other than the power to drive the fan). 

I like lightweight and simplistic cars though, possibly a mindset brought about by FSAE, with 160 kg no-aero cars able to sustain 2+ g cornering. A fan is going to be pretty heavy, and also take up a fair bit of space (as is evident just by looking at the rear of the T.50. It's also loud, especially if your car doesn't have a high-revving V12 in it. Would the same effect that the fan achieved be possible purely by using the energy of the car moving through the air? It wouldn't be as strong or as efficient, but it would be lighter, simpler, and quieter. So, I started coming up with ideas for what is effectively "ground-effect-enhanced ground effect", using a Venturi contraction to provide the low pressure source (not really "ground" effect but it sounds cooler) instead of a fan. The image below shows the layout of the design, which at this point is identical to the implementation on the T.50, except the duct is routed to the rear deck area instead of the rear of the car. 

This does introduce complexities in optimisation of the Venturi area. Where a fan would be optimised for maximum static pressure and/or mass flow, the focus now shifts to effective flow entrainment. 

I see two main concepts for mixing the flow:

1. The duct enters the channel from the base. This is the simplest option and provides no physical flow blockage (excluding momentum effects), but may be less efficient depending on the state of the boundary layer and how it's managed. A turbulent boundary layer would help flow mixing, but the effect of different flow momentums may alter the effectiveness. 

2. The duct extends into the centre of the channel. This would likely improve mixing and thus mass flow, but introduces issues relating to shaping both the duct and the Venturi cross-sectional area profile to avoid significant flow blockage or separation/turbulence/swirl that would decrease the pressure recovery.

There is an obvious limitation of this fan-less method though, being the reliance on vehicle velocity for the amount of suction provided to drive the duct. This undoubtedly means performance potential is reduced at lower speeds. Given the required momentum change of under-floor flow is lower at low speeds though, having the design still achieve its main goal of allowing aggressive expansion with no persisting separation is plausible. Figuring out to what extent, and identifying the presence of any remaining speed sensitivity would need wind tunnel testing.

There is potential for an adjustable, flexible floor profile that could reduce the curvature of the main kick, but this introduces significant mechanical complexities (a sliding overlap section would be needed), and undesirable aero characteristics such as imperfect seals and balance shift. Overall, I can't see any reality where this would be worth it for this particular purpose. 

An alternative, if speed sensitivity is an issue and performance drops significantly at lower speeds, is to keep a more "normal", less aggressive roof profile, and use the Venturi suction for boundary layer removal rather than momentum control, by aligning the duct with the flow in a scoop-like configuration. Boundary layer removal would allow use of a slightly more aggressive diffuser without separation, or would otherwise improve the efficiency of an existing diffuser by increasing the effective cross section. 

It would be particularly useful for double step diffusers, where the concave curvature between the kick points introduces a more significant adverse pressure gradient, which at the very least will thicken any boundary layer. Removing this thickened layer would again increase the efficiency, and also allow the rear kick to be more aggressive. Whether extracting the boundary layer forward or aft of the first kick  (see image below) is most effective would be down to testing, but I expect a more rearwards position may have the additional benefit of creating lower static pressure aft of the main kick point, thus driving under-floor flow harder. Desirable aero balance would also be a factor here though. 

While I haven't conducted any proper testing on these concepts, I did run a quick and dirty 2D CFD analysis to see if it had any credit at all. 

Two geometries were compared, one with a gentle single-kick diffuser and no duct, and one with a more aggressive forward kick with convex aft curvature, tested with and without a duct (the diffuser is still relatively gentle as a result of the test being 2D). Both diffusers had the same overall expansion ratio. The tests were run at 150 kph with standard air properties, and some minor optimisation was done to ensure close to maximum performance was achieved with the single-kick design. 

Unsurprisingly, the more aggressive kick with no duct resulted in significant separation and much worse performance, both overall and in terms of lower peak suction magnitude. Some minor separation was present on the single-kick design close to the exit plane. Results for the aggressive diffuser with duct were promising: Total downforce and drag across the whole floor and diffuser was on par with the more gentle single-kick design (not a particularly good metric as this will differ significantly between 2D and 3D implementations), but peak suction increased by 20%. Further, there was no separation at any point right up to the exit plane, suggesting the profile could be pushed even harder. This suggests an optimised 3D implementation could provide a significant performance improvement.

Total downforce on the top surface didn't change much when both setups had the Venturi present (about 3% lower when the duct was included), although the presence of the duct shifted the balance in this area forwards. The presence of the Venturi compared to just a smooth top surface was to reduce local downforce by again around 3%, and increase drag by a similar amount. It's pretty obvious that the Venturi shape and positioning used below could be vastly improved to reduce drag and limit impact on the surrounding flow. 

These results were promising for the potential of a full 3D implementation. Further improvements would be possible in 3D by incorporating both vertical and lateral expansion, increasing the drag efficiency of the Venturi tunnel. Care would need to be taken not to over-expand the corners though. Positioning is also key, both with respect to the available clean airflow, and the local body curvature. For a road-style car, positioning at the sides of the rear of the cockpit would likely be ideal (where many high-performance cars have rear cooling and/or engine intake inlets. For a prototype body car, the flat area between the rear of the monocoque and the rear wheel housings is prime real estate. 

For aesthetics, the tunnels could be incorporated into the body itself rather than as an appendage on top, which would leave only an entry and exit vent on an otherwise regular-looking body surface. The available internal space may be less forgiving on a prototype car though. 

The image below shows an example location, as well as highlighting the options for the Venturi diffuser. A flat-topped diffuser would be harder to implement as it takes up more internal space, but would improve downstream flow quality by down-washing the expanded flow. 

The main consideration for downstream flow quality is the rear wing. The 2D simulation shows static pressure is decreased slightly downstream of the Venturi, but by the rear of the car, it recovers to within 5% of ambient/gage. Upwashing the Venturi outlet could impose two negative effects, being the flow alignment for the rear wing, and the lifting of losses from the Venturi expansion and the floor duct up to the height of the wing. If the outlet flow is managed well, the rear wing will be negligibly affected and in turn improve the Venturi performance by reducing downstream pressure, thus driving it harder. 

There are a couple other potential sources of improvement that could be implemented to enhance Venturi suction. First is the introduction of vortices closely upstream, providing local suction. Expansion of the Venturi would need to be reduced to avoid over-expanding and blowing up the vortices, but the overall result may be worth it. 

The other option, limited to ICE vehicles, is to use the exhaust flow (and wastegate, if present), which could improve performance particularly at lower velocities. Managing turbulence from exhaust pulsing and the shear layers between exhaust flow and external flow could be difficult though. The Venturi could be sectioned to keep the exhaust flow separate from the external airspeed-dependant flow, or perhaps if there is sufficient mass flow through the duct then the momentum of the flow may be sufficient to dampen any transient suction differences at/across the duct outlet.