Diffuser / under body questions

[slimjim8201]

Back on topic...

I am a firm believer in aerodynamics...just not for the FSAE application. The speeds just don't get high enough to generate any appreciable amount of downforce. Most teams get more "downforce" from the weight of the wings and undertray than the actual aerodynamics. Many teams will quote 200 pounds at 65 mph or 100 pounds at 50 mph. Let me be the first to say BS.

The maximum speed that you encounter on an FSAE endurace course is right around 60 mph. None of the turns can be taken at that speed. The max turning speed is much closer to 30-35 mph. Look at all of the best and fastest teams...Cornell, Western Australia, Penn State, Virginia Tech...no aero devices.

[BreezyRacer]

I would have to say that you haven't seen the cars from from UTA (University of Texas Austin) or others such as Univ of Missouri. They pretty convincingly show the advantages of aero in FSAE. I've seen them run at the SCCA Solo 2 nats for the last few years. And keep in mind that FSAE is very much a power limited and compact car. With more HP you can be aggressive with wings, and with a larger car you can get better aero results and more underbody area to work with.

I would be willing to admit that the teams I've been impressed with realy have their acts together all across the board. However they've also been doing aero for years now and have learned that there is something to it.

[West]

Back on topic...

I am a firm believer in aerodynamics...just not for the FSAE application. The speeds just don't get high enough to generate any appreciable amount of downforce. Most teams get more "downforce" from the weight of the wings and undertray than the actual aerodynamics. Many teams will quote 200 pounds at 65 mph or 100 pounds at 50 mph. Let me be the first to say BS.

The maximum speed that you encounter on an FSAE endurace course is right around 60 mph. None of the turns can be taken at that speed. The max turning speed is much closer to 30-35 mph. Look at all of the best and fastest teams...Cornell, Western Australia, Penn State, Virginia Tech...no aero devices.

Huh, I thought I was the only one who thought that - it's probably the weight.

Just for reference,

http://www.mulsannescorner.com/aerodata ... agt04.html

http://www.mulsannescorner.com/aerodata ... ena99.html

Not even a Porsche Carrera GT can make those numbers. Of course, you can say they don't have the wings as some of those FSAE cars, but they've probably spent a lot more time in the wind tunnel.

[slimjim8201]

I would have to say that you haven't seen the cars from from UTA (University of Texas Austin) or others such as Univ of Missouri. They pretty convincingly show the advantages of aero in FSAE. I've seen them run at the SCCA Solo 2 nats for the last few years. And keep in mind that FSAE is very much a power limited and compact car. With more HP you can be aggressive with wings, and with a larger car you can get better aero results and more underbody area to work with.

I would be willing to admit that the teams I've been impressed with realy have their acts together all across the board. However they've also been doing aero for years now and have learned that there is something to it.

I know these cars/teams very well. UTA's car is very well thought-out and frankly...gorgeous. I am sure that it can place VERY well at a Solo 2 event. But Solo 2 and FSAE are COMPLETELY different.

The maximum speeds that an FSAE car can achieve are much higher at a Solo 2 event, but most importantly, the corners exhibit MUCH higher radii. As stated in the FSAE rules, the corners in the FSAE auto-x and the endurance course are designed for 25 and 35 mph, respectively. Downforce?? I think not.

Missoury-Rolla and UTA have some pretty legit cars but neither team is a perenial top performer. In 2005, Texas A&M was over 3 seconds faster than ALL other cars in the field on the endurance course with not a single aero device. Cornell consistently wins the whole event with...yep...nary an aero device.

The biggest benefits to having a full aero package like UTA and UMR cannot be realized at the FSAE competition. I'm not even sure that the judges give extra design points for them anymore as they do NOTHING but add weight at the event. The judges seem to be very focused on weight reduction at the moment.

[Reca]

reca you mentnion oil do you have a recipe for areo oil just disel or somehting else

Nothing special, we used the usual things, mostly kerosene with dyes or kaolin. Then the exact percentage varies a bit depending by the particular experiment. Anyway there’s enough literature on it to find precise indications.

[Nathan Treat]

I dont think its been mentioned before but you guys are missing one very large point with a properly designed underbody. The vortex generators. Most of them are not obvious but there are about 10 of them on a properly designed car. Nobody in FSAE has gotten close yet for wing profiles or underbody. To do it right takes alot of time in surfacing, CFD, and 50% tunnel. My advise would be to look at the lola or dp01 champ car underbody for ideas.

http://www.mulsannescorner.com/LolaUnderbody.jpg

the flow lines are quite wrong but it is hard to find publically avaliable pictures of underbodies.

[AeroGT3]

Back on topic...

I am a firm believer in aerodynamics...just not for the FSAE application. The speeds just don't get high enough to generate any appreciable amount of downforce. Most teams get more "downforce" from the weight of the wings and undertray than the actual aerodynamics. Many teams will quote 200 pounds at 65 mph or 100 pounds at 50 mph. Let me be the first to say BS.

The maximum speed that you encounter on an FSAE endurace course is right around 60 mph. None of the turns can be taken at that speed. The max turning speed is much closer to 30-35 mph. Look at all of the best and fastest teams...Cornell, Western Australia, Penn State, Virginia Tech...no aero devices.

I think your evaluation is perhaps dated?

100 lbs at 50 MPH is certainly not BS if properly designed. A rear wing well within the rules could achieve most of that. Our FSAE car is barely over 300 lbs . . . the downforce we feel we can easily generate at even 35MPH will have a *significant* effect on the cornering capabilities, but perhaps more importantly, the balance of the car. If you were at Fontana for FSAE West last year I think you'd find that the cornering speeds in the Endurance runs are quite higher than 25 MPH.

Your argument that all the best teams do not run Aero has very little to do with Aero at all. It says almost nothing about Aero, rather that great teams can out do poor ones with good suspensions and high budgets. That Cornell wins without Aero doesn't mean that they wouldn't be faster if they incorporated Aero into the car.

There are significant design points awarded for Aero, and our team was reamed by the judges for having absolutely zero of it. There are benefits like high speed stability and tuneability aero provide outside of pure performance as well.

Most teams get more downforce from the weight of the compoents? Aero does "nothing" but add weight? Please. Our rear wing setup weighs under 12 pounds and we'll have results from on car testing once the fuel injection is up and running . . . You can call BS then

[BreezyRacer]

Any effort to use df in FSAE requires a whole package philosophy. They include minimal ride height. rising rate suspension control, clean packaging, and lots of aero and suspension tuning options to vary balance dependant on speed. You'll find the balance changes as df increases so you need lots of adjustability.

When considered in this mode, rather than just hanging a wing or two on the car you get the most out of the df you create.

Getting the loads straight into the uprights, as the UTA cars do, also is very nice. Not nessasary, and complex for sure, but an example of great engineering and implementation.

As far as VG's go, yes, use them. However, I'm told that getting the VG undertray to really work you need lots of time in the wind tunnel. Given that we're talking undertrays you'll need a rolloing road and don't bank on CFD results. It would perhaps be better to take out a chassis, build a series of plywood undertrays with foam inserts to play with shaping and doing *GASP* real world testing. Then build off the results. Just my opinion, and I guess that the SAE wouldn't rank the results as highly as if you had all kinds of theoretical documention. You could use a model size vehicle on a gym floor if you wanted to. That would be a very controled environment.

[slimjim8201]

Back on topic...

I am a firm believer in aerodynamics...just not for the FSAE application. The speeds just don't get high enough to generate any appreciable amount of downforce. Most teams get more "downforce" from the weight of the wings and undertray than the actual aerodynamics. Many teams will quote 200 pounds at 65 mph or 100 pounds at 50 mph. Let me be the first to say BS.

The maximum speed that you encounter on an FSAE endurace course is right around 60 mph. None of the turns can be taken at that speed. The max turning speed is much closer to 30-35 mph. Look at all of the best and fastest teams...Cornell, Western Australia, Penn State, Virginia Tech...no aero devices.

I think your evaluation is perhaps dated?

100 lbs at 50 MPH is certainly not BS if properly designed. A rear wing well within the rules could achieve most of that. Our FSAE car is barely over 300 lbs . . . the downforce we feel we can easily generate at even 35MPH will have a *significant* effect on the cornering capabilities, but perhaps more importantly, the balance of the car. If you were at Fontana for FSAE West last year I think you'd find that the cornering speeds in the Endurance runs are quite higher than 25 MPH.

Your argument that all the best teams do not run Aero has very little to do with Aero at all. It says almost nothing about Aero, rather that great teams can out do poor ones with good suspensions and high budgets. That Cornell wins without Aero doesn't mean that they wouldn't be faster if they incorporated Aero into the car.

There are significant design points awarded for Aero, and our team was reamed by the judges for having absolutely zero of it. There are benefits like high speed stability and tuneability aero provide outside of pure performance as well.

Most teams get more downforce from the weight of the compoents? Aero does "nothing" but add weight? Please. Our rear wing setup weighs under 12 pounds and we'll have results from on car testing once the fuel injection is up and running . . . You can call BS then

The rules state a 35 mph design for the endurance courses and 25 for the auto-x. I seriously doubt any claim that an FSAE car will produce 100 lbs of downforce at 50 mph. Granted, it wouldn't take much downforce to have a measureable effect on an FSAE car, especially given their 500-700 pound gross weight, so I should retract any statement about the aero devices being "useless". I will retain my point of view that efforts would be better spent elsewhere. Tires and proper tire testing, reaching the 100 hp barrier (reliably), weight reduction all come to mind.

[AeroGT3]

The rules state a 35 mph design for the endurance courses and 25 for the auto-x. I seriously doubt any claim that an FSAE car will produce 100 lbs of downforce at 50 mph. Granted, it wouldn't take much downforce to have a measureable effect on an FSAE car, especially given their 500-700 pound gross weight, so I should retract any statement about the aero devices being "useless". I will retain my point of view that efforts would be better spent elsewhere. Tires and proper tire testing, reaching the 100 hp barrier (reliably), weight reduction all come to mind.

How could you doubt 100 lbs at 50 MPH? That is incredibly easy to achieve! The problem is that most Aero teams don't use aero effectively. Cars with trays aren't placing the rear wing low enough to assist the undertray, and some of the angles of attack are monstrously huge. And UTA, with their FOUR element wings? Keep it simple and 50 lbs @ 100 is very easy.

We have tested the piss out of our tires as we've used the same make/model for 4 years. Our car is 315 lbs, so no weight reduction over a few lbs is going to happen. Our single Cyl engine is never going to make 100 BHP, but we do have a team of 6 working on the intake/exhaust/fuel injection to squeeze out whatever we've got.

We would have won last year's comp had our engine made more than 16 BHP . . . this year it will make over 40 so besides the engine, Aero is really our only opportunity to make the car faster.

[Nathan Treat]

Given that we're talking undertrays you'll need a rolloing road and don't bank on CFD results.

careful there. In most cases you cant see vortex structures in the wind tunnel. You need the help of PIV or another volume based flow visualization. Only problem is that those are rarely avaliable or setup because of the huge time and money issues. A good 3D CFD(10-50million cells) case is your best bet of understanding whats really going on given a good numeric correlation with the tunnel has been established.

[BreezyRacer]

My understanding is/was that CFD cannot replicate the moving ground and ground boundary layer. Therefore I haven't seen any CFDs that could accurately predict lift and vortex flows. I studied a lot of Zin Xhang's work on diffusers (University of Southampton, which works with BAR Honda and others). He has published some really good papaers on diffusers in ground effect. All of his work has been in rolling road wind tunnels because CFD couldn't produce the results. It's been a while though and maybe CFD can replicate his results now, given enough computing power. BTW a few studies were done at velocities of 3ms, around 67 mph. Those studies show consider ground effect and indicate to me that undertrays should work well in low speed situations. His work includes a range of methods including particle image velocimetry (PIV), oil flow visualisation and
forces.[/img]

[Carlos]

If the rules of FSAE have the " Flexibility " -- I might suggest tire treatments to improve grip -- another consideration is suspension wishbone design incorporating aero to accent underbody- difuser efficency and perhaps down force -- I have noticed that the structural elements supplement the many other profiles and volumes and body structures &
aero surfacing of the most efficent devices ever created -- the wings of birds -- my intuition suggests there is not only flex -- as has been discussed on the threads concerning F! rear wing design -- I have long suspected an interaction between wing- body-rear wing (tail) of these biological examples -- these speculations have been influenced partly by "waveform architecture and design-- blobitecture' -- but most of these ideas have evolved from observations -- while sitting on park benches -- feeding pigeons and seagulls there are some small benifits to being semi-retired. I'll leave the real discussion and work to all of you.

Regards Carlos

[Nathan Treat]

rolling roads(translational surfaces) and boudary layers(prism layer) have been done for years with many commercially avaliable codes. It takes many cells(full cars ranging 10-50million) and some tricks with the turbulence equations but CFD is the most time effective place to explore vortex generation with the full car interaction.

[AeroGT3]

My understanding is/was that CFD cannot replicate the moving ground and ground boundary layer. Therefore I haven't seen any CFDs that could accurately predict lift and vortex flows. I studied a lot of Zin Xhang's work on diffusers (University of Southampton, which works with BAR Honda and others). He has published some really good papaers on diffusers in ground effect. All of his work has been in rolling road wind tunnels because CFD couldn't produce the results. It's been a while though and maybe CFD can replicate his results now, given enough computing power. BTW a few studies were done at velocities of 3ms, around 67 mph. Those studies show consider ground effect and indicate to me that undertrays should work well in low speed situations. His work includes a range of methods including particle image velocimetry (PIV), oil flow visualisation and
forces.[/img]

Ground effect and vorticity are quite easy to do in CFD! Simply specify your ground (wall boundary condition) as moving, and the same with your wheels. Get enough grid density in between the ground and the car and you will resolve your boundary layer just fine. I would think CFD to be superior to the tunnel for underbody Aero as it would be an extremely complicated wind tunnel problem.