Open ended questions about exhausts and downforce

[wesley123]

Indeed the airflow goes in between the diffuser and the wheel. Exhaust placement gives it away really well.

[Carbon Dev Racing]

Good day everyone,

As a small intro before my thoughts I am a 23 year old from South Africa, I have been racing nationaly since 6years of age in various karts and later in the National FVee championships with various tests in other formulas. In 2009 I founded CDR in the pursuit of race car development (mostly theory although a few developments have made it into practical apps in FVee SA.)

Excuse my ignorance on the topic of aerodynamics as I know there are far brighter people on this forum, I do however want to share a project I did in 2010. One of the CDR projects was the investigation of the blown type diffuser (will outline the experiment and will dig up pics over the weekend) and the benefits on an FVee in SA. ** The rules in the SA Fvee championships do not permit the idea as the exhaust cannot be lower than 100mm from the ground.

however regardless I built a 1:4 scale version of the floor board with an hole made for the exhaust exit to lie. I hooked up the outlet exhaust pipe to an industrial heat gun with the velocity of the heated air just under 1:4 of the speed of the exhaust gas. The guns temp was set to more or less 1:4 of the air temp in the Fvee exhaust.

I then had two dyno fans +- half a meter diameter each set in to run in the opposite direction blowing into the relatively small square tunnel with home made flow stabalisers (hole bunch of 50mm ali tube cut to about 20mm long). I mounted the board with to a corner weight scale the board roughly 4mm above the ground.

With the fans blowing air out a scale 150kmh the board gave just over 7.2kg's of down force (not much but scale out to 1:1 its just over 28kg's of force). ***with out exhaust heated air charging the under body.

I then for the second test ran the exhaust charged floor at the same scale speed of 150kmh, the result was 8.7kg's of force (which after scaling up is about 35Kg's of force) which was a notible difference but nothing huge.

for the third experiment was the same as the second but at a scaled air speed of 80kmh, the result was 6 odd Kg's of force which when scaled up was more or less 24kg's of force.

The last run was without the exhaust charged floor at the same 80kmh scaled speed, the result was jus over 3kg's of force which equates to about 12kgs of scaled up force which is half that of the blown type.

I am not saying the experiment was fool proof but it seemed damn near fool proof! the result at high speed was notible but at a lower speed the difference was massive. My thought on this (may be incorrect) is that the blown diffuser works well at high speed but the main focus is on the lower speed corners - to me this seems to indicate that the expanded hot air does act to improve the speed of air passing the diffuser.


Please give me your thoughts on the above - I realise there may have been errors on the scaling up etc but let me know in order for me to learn.

Cheers gents all the best.

[olefud]

Something else to think about. The density of air at 400°C is 41% of air at 20°C at atmospheric pressure. Within reason, Bernoulli’s principle is a function of velocity and largely independent of density (a carburetor venturi will meter the same amount of fuel for a given air velocity independent of atmospheric pressure). Thus less dense heated air will better accelerate linearly, curvilinearly as well by Coanda effect while producing the same ΔP according to Bernoulli. However, according to Newton, the reactive force will be decreased by the lesser mass and increased by greater acceleration –essentially a wash.

Ignoring my usual soapbox spiel about viscosity, hot air is a better aero medium than cool air.

[pocketmoon]

My assumption was always that the exhaust was being used , in effect , to seal the sides or the diffuser thus keeping more air beneath it and therefore generate more downforce as it expands into the space under the diffuser and perhaps also fed over the top of the diffuser to extend it.

Rob

[stefan_]

Here is a video in which Enrique Scalabroni (former designer at Williams, Ferrari and Lotus) explains the Coanda effect and how it is used in Formula 1 with the exhaust gases.

[youtube]http://www.youtube.com/watch?v=gryojy2cHnI[/youtube]

[cwb]

My assumption was always that the exhaust was being used , in effect , to seal the sides or the diffuser thus keeping more air beneath it and therefore generate more downforce as it expands into the space under the diffuser and perhaps also fed over the top of the diffuser to extend it.

Rob

Yes the exhaust is being used to seal the lateral edges of the diffuser, but it is to stop air leaking into the diffuser not to "keep more air beneath it" . Downforce comes from pressure differences. The diffuser creates a low pressure zone underneath the car, and the high pressure air outside the car, aided by tyre squirt, wants to join the party. Trouble is the high pressure air spoils the party by increasing the pressure in the diffuser (or making it less negative). To avoid unwanted guests ruining your party you need to employ bouncers, so the teams are bringing these in in the form of high energy exhaust gases, carefully trained by coanda inc.

[Tim.Wright]

Are all diffusers so anti-social? or just those in F1?

[Richard]

Good analogy cwb.

Reminds me of party tips from Veronica Smalls. Lay out some newspaper and shoot the high pressure gate crashers. Way off topic so I'll only link not embed this video ... http://www.youtube.com/watch?v=VkmIQDrqmqM

[Pup]

Interesting quote from Brawn on why the Renault teams opted for a different exhaust treatment from the McLaren style...

"RBR's and Lotus' solution is problematic off throttle, because in off throttle mode there is less blowing from the exhaust and therefore less downforce contribution by the exhaust, which means that one has to rely more on the cokebottle. However, the cokebottle on the RBR and Lotus is less pronounced because of their ramp behind the tail pipe. Therefore, their solution is inferior at corner entry compared to the solution that was pioneered by McLaren, and which all the teams except RBR and Lotus followed. It's possible that they wanted to compensate with clever engine mapping.

This quote was in the context of a clarification that the Renault teams asked for during the Barcelona tests, seeking more leeway with their throttle maps. And it jives with earlier observations that the McLarens are better on corner entry while the Red Bulls are better on exit.

[amouzouris]

IIRC that is the same reason why Webber looked better than Vettel before Red Bull managed to get their exhaust-sidepod configuration working properly

[shelly]

I have found this VERY interesting post by Gordon McCabe, on his very good blog McCabism:

http://mccabism.blogspot.it/2010/12/vor ... icity.html

Here he explains much better than me with picture, why vortices are so important in F1 aero and whyit is so important to accelerate them along their axis to get downforce.

I think this, along with sweeping away tyre squirt, is the main benefit of ebd/coanda: accelerate the diffuser footplate vortex.

[riff_raff]

Are all diffusers so anti-social? or just those in F1?

It's not that diffusers are themselves anti-social, instead it's a situation where diffusers create an environment that encourages air to engage in a behavior that it wouldn't normally without the diffuser's influence. Think of a diffuser as a bar and the air as being someone drinking beer in the bar. The exhaust flows are like whiskey shots that accelerate things late in the process, just before the air/drunk is finally tossed out the exit.

[Cam]

Here is a video in which Enrique Scalabroni (former designer at Williams, Ferrari and Lotus) explains the Coanda effect and how it is used in Formula 1 with the exhaust gases.

[youtube]http://www.youtube.com/watch?v=gryojy2cHnI[/youtube]

After his basic demonstration, I have no idea what he was saying - but 'knifey spoony' - that I got clearly. Great video in layman's terms that all can understand. Thank you. +1

[stefan_]

Here is a video in which Enrique Scalabroni (former designer at Williams, Ferrari and Lotus) explains the Coanda effect and how it is used in Formula 1 with the exhaust gases.

http://www.youtube.com/watch?v=gryojy2cHnI

After his basic demonstration, I have no idea what he was saying - but 'knifey spoony' - that I got clearly. Great video in layman's terms that all can understand. Thank you. +1

Yes, the talking part is a little bit difficult to understand (with the boundary layer, how it can go wrong and all that), but with my basic physics knowledges and imagination (the drawing helped a lot), I managed to get a little bit more than the basic idea of Coandă and how the teams are using it after watching the video for about 4 or 5 times.

[godlameroso]

I can verify Olefud's point because when studying hurricane formation one notices that it is primarily the sun and ambient temperature plus humidity that creates a large mass of steaming sea water. All this hot water wants to rise, and rise fast it does, and as it rises it carries heat with it, and this heat rushes upward in order to fulfill it's desire for equilibrium. Sometimes the mass is too much for it to equalize and balance out as mere rain because the constant high pressure weather system keeps feeding sunlight to this steaming pile of seawater. This mass is so large and requires so much negative delta of temperature and pressure to balance it out that it almost reaches the mesosphere. The pressure wake that results from this act of equilibrium is those spinny things of death we call hurricanes.

The point of all this is that heat is a very good way of creating increased flow, increased flow equals greater pressure differential on any exposed surface. Heck you just need a loosely organized weather system to keep a hurricane together(which is why hurricanes are steered so easily by low and high pressure systems), and F1 cars have a discrete surface attempting to organize flow (freaking bodywork). I wonder when teams will get to the point of trying different textures on different parts of the car to further influence flow and vorticity generation(Kind of like how textures(topography) on earth's surface affect hurricanes).