We start our aero modifications with the front of the body with analysis of basic splitters.
In stock form, the C5 has relatively low drag, but the "overbite" created by the bottom-breather design creates a bit of a front lift issue. The simplistic model used here for analysis is relatively correct with respect to body shape, but likely does produce optimistic underbody flows due to the absence of features such as exhaust pipes and suspension components. Fortunately, the C5 is pretty flat from the factory, so the simplified underbody shouldn't render the results useless.
As a refresher from the previous post on body aero, some exhibits showing pressure and flow for the stock body are presented below. The flow under the nose, high degree of lift over the cabin, and the turbulence and low pressure area at the rear bumper are of particular interest.
With base aero, the C5 hardtop produces 331 lbs of lift and 494 lbs of drag. All simulations were analysed for total drag and lift, as well as impacts to the front and rear of the car. The split between front and rear is defined as the middle of the body. Also, all tests were performed at 100 MPH, unless otherwise noted.
General Splitter Theory
As a very simplified background, first some thoughts on how splitters work. Basically, the idea behind a splitter is to take advantage of a high pressure area created by airflow over the nose of the car. As air hits the nose, it is forced either above or below the body; the introduction of a flat plane protruding forward helps separate the low pressure region under the car from the high pressure areas created by over-body flow.
Splitter Height
The first splitter attribute tested was the height. Three heights were tested. The base splitter was set at the height of the rocker panels, 4.5" above ground level. The second iteration moved the splitter lower by 40mm (toughly 1.5"), and the third lower by another 40mm. The third iteration is impractically low; with a height of 1.5" it would hit on pretty much any curbing and would scrape under cornering and braking. But it was analysed to show the effects of closing off as much front airflow as possible under the nose of the car.
The two exhibits below illustrate the splitter at work. Note how the flow in the base design lacks a hard division between above and below-body flow, resulting in a high pressure area under the nose. The splitter, in contrast, keeps the high pressure area from entering below the body. The leading edge of the splitter produces a fairly potent low pressure area.
The data showed the effectiveness of this change in flow: total lift was reduced by 124 lbs, with a very minor increase in drag. Initially, the increase in drag seemed counter-intuitive. However, reviewing flow traces covering the entirety of the body showed altering the front flow can have a meaningful influence on air behavior behind the car. So, one has to be mindful of how a change of flow on one end of the body can change behavior at the other.
To further study these dynamics, two more tests were performed, lowering the splitter by 40mm and 80mm, respectively. Overall, the results show reducing the amount of air entering the underside of the car has a large impact on aerodynamic efficiency.
As evidenced by the data below, splitter height has a critical impact on front downforce and balance. Effectively, the lower, the better. Because the -80mm design is not practical for actual use, the -40mm design will be used as the baseline for future tests.
Splitter Length
The second attribute tested was the length of the splitter, measured as the distance from the vertical body line to the edge of the splitter. Because the intent of the splitter is to help create a defined pressure differential between the high pressure at the nose of the car and the low pressure area below, it seems reasonable to expect the longer the splitter length, the more effective it would be. To test this, three different lengths were tested: the baseline 3", a 5", and a 7".
While the longer splitters did indeed increase front downforce, the impact was nowhere near as large as with changes in splitter height: the 7" splitter produced only 31 lbs more front downforce than the 3". In addition, the larger splitters also appear to result in a minor increase in drag, both front and rear.
The underbody flow images below show the improvements introduced by the addition of the splitter. The last two images, the lowered splitter and the 7", show the minimal changes to flow due to the increased length.
The side pressure plots show a similar story of no substantial differences between the 3", 5", and 7" lengths. One point to note, however, is the very defined pressure differential at the leading edge of the splitter relative to the high pressure area on top. It would make sense that the size of the splitter should be tailored to the individual use case and essentially match size and location of that high pressure area. If changes are made to the fascia or air dam that change the pressure in that area, changes to a larger splitter may be warranted. But, the larger length doesn't appear to be of use here, with this base configuration.
Conclusions
The analysis above shows how critical a device a splitter is to creation of front downforce. Key takeaways here are that controlling the airflow entering the underbody is critical, and that bigger is not always better with respect to splitter size.
Because of the impracticalities of using the more extreme designs tested above (the low spliiter and 7" length will both likely scrape constantly under braking and cornering load), we will settle on using the 3" length at a 3" height, since this still produces very good results while not suffering from the elevated danger of scraping. Overall, addition of this splitter should reduce front lift by over 250 lbs, while also reducing total drag. Next step will be improving rear downforce to balance out this front splitter.