THE FU­TURE OF AERO­DY­NAM­ICS

Cars are start­ing to look the same and its be­cause of physics

Popular Mechanics (South Africa) - - Contents - BY EZRA DYER

There is a ba­sic ten­sion be­tween the aero­dy­nam­ics en­gi­neers and the car de­sign­ers. Each side will say that their mis­sions are in har­mony, that good de­sign should not pre­clude good aero­dy­nam­ics, and that aero­dy­nam­ics should not limit de­sign. But that’s not en­tirely true. The wind doesn’t care whether your grille is dis­tinc­tive. The wind wants your car to look like a rain­drop. Any­thing else is a com­pro­mise. Yes, ev­ery sliver of kilo­me­tre per litre or range mat­ters, but visual dis­tinc­tion sells cars. ‘I al­ways ask: “Have we per­fected the bot­tom of the car?”’ says An­drew Smith, global head of de­sign for Cadil­lac. ‘Let’s work on the part that no­body can see.’ The aero­dy­nam­i­cist, then, must please both the de­sign­ers and the laws of physics, cre­at­ing shapes that are vis­ually dis­tinc­tive, but are also aero­dy­nam­i­cally anony­mous, help­ing the car slip through the air with­out any drama. Maybe you’ve seen it: three lanes of mid­size crossovers that could trade badges and no­body would no­tice. All those shapes are dic­tated by in­te­rior space, power-train pack­ag­ing, gov­ern­ment reg­u­la­tions, and pro­duc­tion fea­si­bil­ity. But most of all, they’re dic­tated by aero­dy­namic ef­fi­ciency. When we started ask­ing auto man­u­fac­tur­ers how that works, we re­alised that it’s not in­cred­i­ble that so many cars look so sim­i­lar: It’s in­cred­i­ble that cars look dif­fer­ent at all.

And this can lead to some novel so­lu­tions. Con­sider the grille found on the elec­tric Jaguar I-pace. Why does an elec­tric car even have a grille, a relic of the in­ter­nal-com­bus­tion en­gine? Stand­ing next to a red I-pace in the park­ing lot of Jaguar Land Rover’s new North Amer­i­can head­quar­ters in Mah­wah, New Jer­sey, I ask. ‘ We’re still build­ing our brand,’ says Wayne Burgess, the pro­duc­tion stu­dio di­rec­tor at Jaguar Land Rover. ‘All the other mod­els have a grille, so we didn’t want to aban­don that with the I-pace. But we made it func­tional even though there’s no ra­di­a­tor be­hind it.’ He’s re­fer­ring to how the lower half of the grille feeds air to heat ex­chang­ers for the bat­tery’s cool­ing sys­tem, and the top half opens out to the bon­net, cre­at­ing a path through which air can ac­cel­er­ate over the wind­shield and roof and down over the rear win­dow to the up­right tail. ‘ You want to keep the air at­tached to the car, and then sud­denly de­tach it, to min­imise tur­bu­lence,’ he says. ‘ That’s why, in a plan view, look­ing down, the sides of the car are flat, but in pro­file you see curves at the tops of the fend­ers.’ It makes sense. When it’s mov­ing, this shape is ef­fi­cient and dis­tinct, a shark fin mov­ing through wa­ter. But Burgess does seem just a lit­tle bit wist­ful when he com­pares the I-pace’s flat flanks to the Jaguar F-type’s more cur­va­ceous fend­ers. ‘ The F-type has that beau­ti­ful Coke-bot­tle shape,’ he says, ‘ but it cre­ates tur­bu­lence. The air gets de­tached.’ You can’t have sexy, flared fend­ers and low drag.

AERO R&D A few weeks later, I’m at GM’S Tech­ni­cal Cen­ter up in War­ren, Michi­gan, stand­ing in­side GM’S new wind tun­nel. The tun­nel’s su­per­vi­sor, Nina Tor­tosa, fires up the 820 kw fan and spins the rolling road un­der a 40 per cent-scale Chevy Sil­ver­ado. These tread­mill sys­tems are now stan­dard in wind tun­nels be­cause, in the real world, your car moves, but the road and air don’t. The wind tun­nel must re­verse both pa­ram­e­ters, or else the re­sults will all be skewed.

I al­ways thought that wind tun­nels blew smoke over cars so ev­ery­one could see how the air be­haved. It turns out no one re­ally does that. The process is messy, so it’s mainly a stunt for pho­tos or vis­i­tors.

Nat­u­rally, I want to see it. Tor­tosa points a wand over the top of the over­grown toy truck and the smoke glides smoothly across, trac­ing the shape of the bon­net and roof be­fore go­ing all chaotic over the bed and tail­gate. She moves the wand un­der the truck and re­veals that, yes, the ex­haust sys­tem, sus­pen­sion, and axles make for tur­bu­lence. It’s an em­pir­i­cal demon­stra­tion of why man­u­fac­tur­ers use front air dams – and even heigh­tad­justable sus­pen­sion – to min­imise air go­ing un­der the truck. You want as much air as pos­si­ble go­ing over the smooth parts up top rather than across the lumpy un­der­body.

In ad­di­tion, GM uses su­per­com­put­ers to run elab­o­rate fluid-dy­nam­ics sim­u­la­tions, the re­sults of which guide en­gi­neers to­ward prob­lem ar­eas. ‘It showed a

cir­cu­la­tion bub­ble on the quar­ter­panel of the Volt,’ Tor­tosa says. ‘ We couldn’t see that in the tun­nel.’

But a wind tun­nel is still es­sen­tial. ‘Once I build a com­puter model, if we change some­thing, it takes a day to run the new cal­cu­la­tions,’ says Ken Kar­bon, global head of sim­u­la­tion for aero­dy­nam­ics. ‘ Whereas, how many dif­fer­ent it­er­a­tions could you test in the tun­nel?’ He di­rects the ques­tion to Tor­tosa. ‘ Thirty,’ she says. ‘ We could try 30 dif­fer­ent things in a day.’ It’s a lot of work that re­sults in small but vi­tal mar­gins. ‘ We can af­fect aero by per­haps 10 per cent one way or the other – so if the co­ef­fi­cient of drag is .30, maybe we can get it down to .27,’ Kar­bon ex­plains. ‘And that might rep­re­sent a tenth of a kilo­me­tre per litre in fuel econ­omy, de­pend­ing on the ve­hi­cle.’

We go next door to the cen­tre’s orig­i­nal wind tun­nel, which is sim­ply un­fath­omably huge. The fan looks like a pro­pel­ler from the Queen Mary, fit­ted with six beau­ti­ful lam­i­nated spruce blades – wood is bet­ter than even car­bon fi­bre, be­cause if a piece of car flies off and dam­ages a blade, tech­ni­cians can eas­ily re­pair it. The space is shaped roughly like an oval track, so the air cir­cu­lates through the test sec­tion with the ve­hi­cle and back to the fan, re­quir­ing less power. You could host Wim­ble­don in­side the cham­ber be­hind the fan. See­ing this sta­dium-size fa­cil­ity in per­son now makes it pretty ob­vi­ous why not ev­ery­one has one. It is a very ex­pen­sive tool, and a fan­tas­tic and com­pli­cated ma­chine, so much so that no two tun­nels pro­duce the ex­act same re­sults.

‘ The three US com­pa­nies all have wind tun­nels, and we all test each other’s cars,’ says Tor­tosa. ‘ We share the re­sults, be­cause each wind tun­nel is dif­fer­ent. So we like to know what our truck got in Ford’s tun­nel, and vice versa.’ This pro­fes­sional aero cour­tesy does not ex­tend to for­eign com­pa­nies.

TECH­NOL­OGY, NOT COM­PRO­MISE

Aero­dy­namic prin­ci­ples are fixed, non-ne­go­tiable. So how will GM, Jaguar, Cadil­lac, and ev­ery other man­u­fac­turer con­tinue to make cars more ef­fi­cient while avoid­ing ho­mo­gene­ity? How do we get per­for­mance cars that have high top speed (low drag) and high down­force (bet­ter han­dling)? One an­swer kept com­ing up: ac­tive aero, cars that don’t just pas­sively zip through the air, but ma­nip­u­late it to their ben­e­fit. This is the way you re­solve all the com­pro­mises in per­for­mance and styling. You build a Trans­former. MercedesAMG’S up­com­ing 750 kw su­per­car, the Project One, is a case in point. Be­fore go­ing to New Jer­sey and Michi­gan, I’m at the Mi­ami In­ter­na­tional Boat Show with the chief de­sign of­fi­cer for Mercedes, Gor­den Wa­gener. Look­ing at a full­sized clay re­pro­duc­tion of the Project One, I re­mark on the clean­li­ness of its sil­hou­ette. ‘ Well, there is a lot go­ing on un­der the car that you can­not see,’ Wa­gener says. ‘ But also, there’s ac­tive aero. There’s a big wing there, but you don’t see it when it’s parked.’

Ac­tive aero sat­is­fies both de­sign­ers and aero­dy­nam­i­cists be­cause it’s at once ef­fec­tive and mostly in­vis­i­ble. Lam­borgh­ini’s ALA (Aero­d­i­nam­ica Lam­borgh­ini At­tiva) sys­tem, as gets de­ployed on the Hu­racán Per­for­mante, doesn’t look like much more than a wing on the rear deck. But that wing’s hol­low, with air­flow through each of its stan­chions con­trolled by an elec­tric mo­tor – and there’s a sim­i­lar sys­tem in the front spoiler to bal­ance front – rear lift. On a straight stretch, the mo­tors can chan­nel air un­der the wing for low drag. Close the air in­takes and the wing makes high down­force, for ex­tra grip in cor­ners. And the car can vary the sys­tem from side to side for ‘aero vec­tor­ing,’ which gives you even more con­trol when steer­ing into tight cor­ners. Now, I’ve driven the Hu­racán’s pre­de­ces­sor, the Gal­lardo, at 320 km/ h

on a supersonic run­way in Florida, and it was spooky, wan­der­ing and float­ing across the tar­mac. I didn’t go that fast in the Per­for­mante, though as fast as I did go, it was glued to the road, with none of the Gal­lardo’s aero­dy­namic im­bal­ance. Lam­borgh­ini has al­ways pri­ori­tised top speed over han­dling, but ac­tive aero al­lows for both.

The Per­for­mante set a lap record at the Nür­bur­gring, the tried-and-tested mea­sure of high-speed sta­bil­ity. But never for­get it will also eas­ily gal­lop to 325 km/ h.

Suzy Cody, GM’S head of ve­hi­cle per­for­mance for aero­dy­nam­ics, says this tech­nol­ogy is the bridge be­tween de­sign and engi­neer­ing. ‘Look,’ she says, ‘ it doesn’t mat­ter how great your aero­dy­nam­ics are if only ten peo­ple buy the car. De­sign mat­ters. And ac­tive aero helps en­able de­sign.’ But what if, I posit, there’s a mas­sive propul­sion break­through? Right now, aero­dy­nam­ics are tied to fuel con­sump­tion and elec­tric range. What if we had bat­ter­ies that were good for 965 km of range and could fully charge in ten min­utes? Could we stop wor­ry­ing about ev­ery crease in the body­work? Could we just give those de­sign­ers the flared fend­ers and not sweat the small stuff? In other words, would the aero cease to be such a big deal? Cody, un­sur­pris­ingly, ap­peared aghast that I would even sug­gest such a thing. ‘Even if you had a bat­tery like that, good aero­dy­nam­ics gives you other op­tions. You could have a small bat­tery or make the car cheaper, give it more space, make it qui­eter. Aero will al­ways be im­por­tant.’

GM uses su­per­com­put­ers to run elab­o­rate fluid-dy­nam­ics sim­u­la­tions – but the wind tun­nel will pro­duce quicker re­sults. It’s in use 24 hours a day, seven days a week.

A TEACH­ABLE MO­MENT LE MANS, 1999: Crest­ing a hill + low down­force = flight. Some­how, no one was hurt.

SEMI-IN­TER­EST­ING AERO­DY­NAM­ICS FACTOID NO. 1 The shape known in physics as the Sears-haack body pro­duces the low­est drag per vol­ume at a supersonic speed. Imag­ine a chubby tooth­pick.

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