Aero Design Progress 2014

Earlier this summer, I did a lap time simulation study earlier this summer to understand the sensitivity of lap time on a number of parameters (including mass, power, downforce, drag, and mechanical grip).  The results of the lap simulation showed mechanical grip to be the most important parameter, followed by downforce and then mass.  Since there is a limit as to what can be achieved with mechanical grip, past those limits aero is still the biggest enabler for decreasing lap times.

Setting goals can be a difficult with a brand new car since there is limited background.  Starting from DWAIT/OD-13, which was around 380-400lbf of downforce at 60mph, I wanted to try and get to ~550lbf at the same speed.  Unrealistic?  Looking back, probably since we're shorter and narrower.

The first set of CFD runs was somewhere around the 360lbf range, with ~35% front.  The first few changes were to get the front wing to produce more downforce for better balance, by making the span of each of the front wing halves larger.  This pushes the inside endplate closer to the body, reducing bypass flow that feeds the diffuser and rear wing.  However, CFD has showed this to be relatively insignificant as long as a reasonable gap is maintained, with a net gain in both downforce and balance.

That got us to around 380lbf, ~40% front.  Most of the work since then has been on the diffuser.  I'll talk about each part of the diffuser separately, though they all do interact in reality.

• Side tunnels profile: The maximum angle before flow separation starts to occur is ~15 degrees, much like what literature suggests.  However, using a smooth transition into the diffuser and a more airfoil-like profile, we can go to an overall angle of 21 degrees.  However, this is heavily dependent on how well the rear wing is driving the diffuser.
• Side tunnel exit location: Originally for tunnel volume I wanted the tunnel to exit as far rearward as possible.  I found however that having the diffuser under the "shadow" of the wing in planview was inefficient - the low pressure region from the rear wing was creating a significant amount of lift on that top diffuser surface.  Furthermore, with the airfoil type design on the side tunnel, I can get similar volume at a larger angle and have it exit near the leading edge of the rear wing.  This still resides in the low pressure region from the rear wing, which has the added benefit of allowing suspension points of moving forward, which enables us to get rid of the rear box completely
• Center tunnel: I thought that having the side walls flare out would create more downforce by creating a larger diffuser volume, but it turns out that extra planview area under the rear wing pretty much negates that gain.  Having a longer center tunnel creates the same issue, and thus the center tunnel is short and steep (15 degrees).  This is a good thing for packaging the jacking bar and its supporting structure.
• Secondary tunnels: due to the packaging of the frame and engine, secondary tunnels are a way to get a little bit more downforce.  I tried exiting them out at the rear with the center tunnel, at the same point as the side tunnel, and in between.  I think the forward exit is overall the best option.
• Top side flow: Because of the height of the diffuser and the height of the suspension in front of it, there's not a good way to transition from front closeout panel to diffuser.  I noticed that originally the nozzle on the front side of the diffuser was hurting downforce, as well as the low pressure region on the top side of the diffuser.  By opening the top side to ambient flow, we eliminate both these sources of lift.

Flow above diffuser

Diffuser under surface plot

Pressure plot at diffuser side tunnel plane

Diffuser air flow

With these changes, I've been able to double the amount of downforce the diffuser produces, from 60 to ~130lbf @ 60mph.  This is actually only a little more than last year's diffuser, though I'm not sure how good last year's number actually is - we have a lot more fidelity in CFD now than we used to.

With these few changes, we were up to ~450lbf downforce, ~43% front now.  Close, but not quite there yet.

From this diffuser optimization, we needed to do two things: increase downforce and shift the balance forward.  What is the actual balance required?  I'd think anywhere between 45-48% front is acceptable, considering the low speeds the car encounters since yaw stability will not be much of an issue unless we push the bias to ridiculous levels.  In an autocross car, I tend to think the extra front bias is a good thing.

The next steps were to increase the width of the aero.  We had originally used 52", using a 46" track and 6" tread width.  However, the section with of the tire is 8.5" when mounted on a 7" rim, allowing for an extra couple of inches of aero width.  Finally, to push the balance forward (and increase downforce), we lowered the front wing half an inch so that the endplate sits 2" above the ground now.  This change still requires a little bit of vetting to make sure that the endplate won't hit the ground under normal circumstances (particularly pitch change).

These changes have us just over 500lbf of downforce, 48% front, with an overall L/D of ~2.6.  I think this is about all we can extract from a car this size - and with our targeted 70whp, this puts top speed somewhere around 78mph. This is about 20% more drag than last year, but also 25-30% more downforce.  At these levels of downforce, any additional downforce tends to be very inefficient.

Rear wing airflow 

Front wing airflow - note the interaction with the rear wing

Tire airflow is something we may look at in the future as a drag reduction opportunity.  Though tires are never easy, there might be possible improvement in tire drag by injecting momentum into the flow behind the tires.