which of the following configurations is assumed to be more effective? Pros & cons of each?


to explain my MS paint drawing, and my thoughts:

The top aero mod has an air dam, a splitter, and an underbody surface added. The underbody surface is above the bottom of the air dam/splitter, therefore in my wee little brain, I imagine a lower pressure center behind the dam, as the volume of air that gets under the dam is relatively small compared to the larger area that it will try to fill under the front. The (again, imagined...) downside is turbulence and drag, and reduced velocity under the car.

The bottom aero mod has a similar airdam, with a splitter, that continues uninterupted under the car. The underbody surface is the same height as the splitter, therefore closer to the ground. I think this would provide smoother and faster airflow, which, per Bernuli's principle, implies downforce also. The downside I imagine is simply that the upper one makes sense.

So which is more common, and why? Is there a 3rd option available? Perhaps an underbody surface that meets the bottom of the airdam, but is angled upward as it travels back (best of both worlds: smooth airflow, and an increasing volume for decreased pressure.

TIA

 

The top one is going to create more drag, as a result of the pocket created behind the step. The idea is to smooth airflow out, while decreasing pressure under the car. I think ideally, it should be a combinatio of the two. Flush off the dam, but sweeps up the height of the top one. This will increase the volume of airspace, thereby reducing pressure, and creating downforce with a slight ground effect, but without the turbulance/ drag of the step.

 

There is simply no way to say without lots of testing. The air there is doing some really funky stuff, influenced by design factors from the radiator intake all the way to the back of the car. I don't even think it's safe to assume that there's laminar airflow under the nose of the car behind that lip.

K

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Knestis &raquo;</TD></TR><TR><TD CLASS="quote">I don't even think it's safe to assume that there's laminar airflow under the nose of the car behind that lip.

K</TD></TR></TABLE>

hmmm... vortex generators to the rescue? (not exactly bottoming-out friendly...)

 

I would generally vote for number 2 with an effort to smooth out the airflow unless you are in a situation where you can actually test and prove that a vortex is what you want there. I think the number they often repeat on maintaining laminar flow is about a 7 degree agle compounding as it goes along. Kind of hard to do on the garage floor with inexpensive components that are sacrifical like most air dams generally should be considered. I have done flat bottom air dams as that is probably the safest best for a performance gain with limited tools and no real ability to test.

 

3rd option following a naca shape, but what do i know? wheres johnny mac when you need him?

(consider that a few race classes (ie le mans) require flat bottoms to LIMIT aero downforce)


y2000-spec nsx. i'd do it like them (no longer allowed the underside aero)
http://asia.vtec.net/beystock/....html





Modified by Tyson at 9:58 AM 9/3/2004

 

There's an SAE paper on the Aero of an IMSA GT car that's interesting and generally useful - Go look for it.

If I recall correctly a slightly raked flat splitter tray is quite effective - and not terribly inferior to a curved profile or one that kicks up at the rear.

And you can have an angle of divergence between two flat surfaces of around 5 to 7 degrees without airflow seperation - In General, at our speeds, in decently clean airflow, etc, etc, etc.

And, yes, Drawing-2 (Flush) is the correct choice.

Scott, who'll dig out that paper later...

 

with no testing def option 2, with the rasied rear your gonna get some good turbulance

 

It is important to note that staying with the 5-7 degree slope is to prevent airflow separation, a loss of both lift/downforce and lots of drag.

Maintaining laminar flow is another issue entirely and nearly impossible to do so near to the ground plane. Maintaining laminar flow also has little to do with lift, only drag.

Andy

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by CRX Lee &raquo;</TD></TR><TR><TD CLASS="quote">no real ability to test. </TD></TR></TABLE>

taped tuffs w/ a pinehole camera mounted underneith?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Greyout &raquo;</TD></TR><TR><TD CLASS="quote">
hmmm... vortex generators to the rescue? (not exactly bottoming-out friendly...)</TD></TR></TABLE>

This was kind of touched on but vortex generators can only maintain laminar flow - in a big picture sense - NOT make it start happening. I'll bet that those yarn tufts would be flapping like crazy and pointing all kind of directions under there...

K

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Knestis &raquo;</TD></TR><TR><TD CLASS="quote">

This was kind of touched on but vortex generators can only maintain laminar flow - in a big picture sense - NOT make it start happening. I'll bet that those yarn tufts would be flapping like crazy and pointing all kind of directions under there...

K</TD></TR></TABLE>

??? Vortex generators maintain attached flow by creating a turbulent boundary layer... they destroy laminar flow.

 

Flush one.

 

Whichever one that results in the airflow having the highest velocity.

Higher velocity = lower pressure, so my guess would be the 'flush' verison, #2.

 

[edit] dont listen to me




Modified by Tyson at 3:01 PM 9/3/2004

 

#2 with a slight upward rake. Any possibility of a rear diffuser?

 

In design #2, I am assuming that you are able to continue the flat bottom all the way to the rear of the car. If so, the airflow will be significantly smoother and faster than for #1. Typically, #1 is used by racers because the rules in NASA and SCCA for Honda Challenge/Improved touring would not allow #2.

Design #1 works well for flat front fascias since a large amount of stagnant (slow moving-high-pressure air is available to act on the top surface of the splitter. Splitters are not as effective on curved fronts due to the ability of air to move around the sides of the car - therefore the air above the splitter is lower pressure than for a flat front.


The height of the splitter above the ground should be determined by testing - coast down studies are effective in determining drag coefficients if your dad isn't a Bill Gates relative, and hence you probably can't afford wind tunnel time. A lower airdam increases effective frontal area and generates a disproportionally higher drag coefficient due to the high pressure in front and lower pressure in back of the dam.

Also, a deeper airdam - thus smaller window for underbody flow can also create pitch sensitivity problems since as you brake and the car pitches forward, the underbody window can completely close. This cuts off the air under the car, which increases the low pressure under the car at the front. This can lead to porpoising due to large changes in downforce and the resulting braking inconsistencies that cause the driver to lose confidence in the handling of the car. This is why you see some of the touring cars (BTCC, DTM, ect.) with mustache splitters. Of course you can increase your front spring rates and this problem can be somewhat mitigated.

On the other hand, a lower splitter height can improve the the high velocity air under the car and thus help generate higher downforce levels. But because most cars have dirty underbodies - littered with exhaust systems, unit body undulations, shift linkage, and on and on - this high velocity air can quickly be reduced to lower velocity air after it bounces off all of the stuff under the car. You will have to test to see the optimum height will be. And, to make matters worse, this optimum height will vary track to track. It's all about aero efficiency. If you want more downforce, then you will likely have to pay the price in increased drag.

In short, Design #2 is much better than #1 and only testing can optimize either one of these designs.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Knestis &raquo;</TD></TR><TR><TD CLASS="quote">There is simply no way to say without lots of testing. The air there is doing some really funky stuff, influenced by design factors from the radiator intake all the way to the back of the car. I don't even think it's safe to assume that there's laminar airflow under the nose of the car behind that lip.

K</TD></TR></TABLE>

And if the splitter was designed incorrectly, it's possible that you'll increase the drag substantially. The splitter should be aligned with the incoming airflow so that flow separation can be avoided. Run yarn tufts and a pinhole/lipstick camera at the opening on back to see what the flow is doing. If the tufts show reversed flow ( i.e forward facing flow), the flow is separating and the splitter is mounted at the wrong incidence angle.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by maxQ &raquo;</TD></TR><TR><TD CLASS="quote">It is important to note that staying with the 5-7 degree slope is to prevent airflow separation, a loss of both lift/downforce and lots of drag.

Maintaining laminar flow is another issue entirely and nearly impossible to do so near to the ground plane. Maintaining laminar flow also has little to do with lift, only drag.

Andy

</TD></TR></TABLE>


I had heard that it was higher, more towards 15 degrees before airflow begins to seperate from a surface?

Anywho #2 seems to my basic understanding to be better not at creating downforce but in smoothing out airflow under the car reducing drag. While there is certainly more advanced methods without creating a mold of some sort experiencing road contact or contact with other cars might prove laboriuos to replicate or fix in certain situations like a minor offcourse excursion

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Johnny Mac &raquo;</TD></TR><TR><TD CLASS="quote">

Run yarn tufts and a pinhole/lipstick camera at the opening on back to see what the flow is doing. If the tufts show reversed flow ( i.e forward facing flow), the flow is separating and the splitter is mounted at the wrong incidence angle.

</TD></TR></TABLE>

Coast down tests I understand to measure drag. Very easy to do with a data logger, and I have done it many times. But I want to see a picture of someone actually doing the tufts of yarn method that isn't from a 20 year old book.

By the way, here is a picture I took of the Champion Audi WCGT carbon fibre front splitter undertray brake duct combopiece while it was off the car. Here is how a very big bucks winning racing team does things.



And as installed on the car

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by FF lotus &raquo;</TD></TR><TR><TD CLASS="quote">


I had heard that it was higher, more towards 15 degrees before airflow begins to seperate from a surface?

Anywho #2 seems to my basic understanding to be better not at creating downforce but in smoothing out airflow under the car reducing drag. While there is certainly more advanced methods without creating a mold of some sort experiencing road contact or contact with other cars might prove laboriuos to replicate or fix in certain situations like a minor offcourse excursion

</TD></TR></TABLE>

Actually, you can possibly run a diffuser at 15 degrees. However, you need strong vorticity in the flow so the boundary layer stays attached - this is why the bottom edge of the diffusers are sharp corners.

The bottom picture also developes more downforce because the flow does not slow down as much, which means that it maintains high velocity and low pressure relative to the top design.