Diffuser vs. Rear Wing

[wesley123]

My idea is just simple, more underbody downforce, less downforce from wings.

I think the front wing has to be raised and the rear wing less angle of attack with a much smaller second plane.

[speedsense]

My idea is just simple, more underbody downforce, less downforce from wings.

I think the front wing has to be raised and the rear wing less angle of attack with a much smaller second plane.

IMHO, With the numbers I have seen as percentages of underbody forces vs. wing downforce numbers,say using, 20% front, 30% rear with the wings and 50% underbody as point of reference.
If we completely remove the wings, off the car... does the 50% number on the underbody remain? Or.. is the 50% underbody number there because of the wings?

Having actually done this in a windtunnel (with a diffuser)after wing removal, the percentage underbody was decreased by 30% (the ride height did change with removal of the wings).However when the ride height was moved to where it was with the wings, only 7% was regained.
Part of the problem is the aero balance is greatly disrupted when the wings get removed and do create a squat situation that allows more high pressure to enter the underbody and at the same time putting the diffuser closer to the ground making it more efficent.
So as a follow up we raked the car forward to keep the ride height (of the diffuser) as it was with wings.. we still only gained back a small percentage of the "supposed" downward force creation of the diffuser.
I question numbers that state percentages that under body devices create as "individual" pieces rather than "packaged pieces" of included wings. It is the under body devices that make wings more efficent than they would be on their own...IMHO

[F1_eng]

It is a complete system, things don't necesseraly work on their own.

Removing a wing to see what the actual diffuser forces are does not begin to analyse the diffuser forces independently. You are completely changing the flow conditions, both above and below the diffuser.
There are so many interactions, it is very difficult to quantify the effect of large global components individualy.

[speedsense]

It is a complete system, things don't necesseraly work on their own.

Removing a wing to see what the actual diffuser forces are does not begin to analyse the diffuser forces independently. You are completely changing the flow conditions, both above and below the diffuser.
There are so many interactions, it is very difficult to quantify the effect of large global components individualy.

I can name one that you can, the wing.....it is very safe to quantify that number, at least on the "normal" or standard positions on a race car.
So if a diffuser isn't effective without "other" devices acting on it, how we can we state, the device produces any forces at all by itself? I guess it's a matter of semantics or is it...
As far as the flow conditions changing, you are correct, however the changes to the chassis rake brought back the flow conditions very close to what they were with the wings, except the "exhausting" flow out of the diffuser was very different as was the center of pressure inside the diffuser, having moved closer to the "mouth" of the diffuser causing less effect.
When the rake was rearward, the Center of Press. was closer to the rear and the diffuser effect was greater (closer to the ground) but the floor contained higher amounts of high pressure (due to rake angle) and had an opposing force of lift.
Over all neither condition approached the expected downforce numbers of the wings + the diffuser vs just the diffuser.... and the diffuser should be regarded as a wing effectiveness enhancer, not a down force producer, in the purest sense of the word...

[F1_eng]

Please, I don't need you to tell me I'm correct.

You must not know much about how wings and other devices contriute to other downstream devices if you make the stated assumption. Front wing development has an enormous effect on conditioning the flow further along the car.
The front wing has a lot of understide vortex generators, compound flicks on the main planes, lots of geometry on the front wing end-plates, not to mention the whole upwash effect of the front wing itself.

I don't state that the diffuser produces x ammount of downforce in it's own. We don't test individual components in the wind tunnel for this exact reason, it would be 100x cheaper and easier than what we have to do.

"As far as the flow conditions changing, you are correct, however the changes to the chassis rake brought back the flow conditions very close to what they were with the wings, except the "exhausting" flow out of the diffuser was very different as was the center of pressure inside the diffuser, having moved closer to the "mouth" of the diffuser causing less effect. "

That is a contradicting statement, how can the flow be very close to winged conditions but then you go and say the "ehausting flow was very different", this "exhausting flow" is exactly what the diffuser is designed for.

I know exactly how much downforce the front and rear wings produce, downforce/lift of each wheel, overall car downforce. From this we can know the exact numbers for everything else on the car as a whole, safe to say the diffuser is a huge portion of this "everything else" number.

What car exactly were you evaluating?

[riff_raff]

As F1_eng points out, there are many considerations as to why both F/R wings and underbody flows are used to produce downforce on the car. Among the consideration for how each device is used are L/D characteristics and sensitivity to changes in AoA or pitch, yaw and roll, and how their trailing wake flows affect or interact with any other devices.

As noted, the diffuser itself does not produce downforce. It is simply an aero device that creates a dynamic airflow region with lower ambient pressure in the trailing wake of the undertray. Since fluids tend to flow from regions of high pressure to low, this creates an environment where the air underneath the car naturally wants to flow (or more correctly, expand) out the back edge of the undertray faster than it can flow in past the restrictive front edge, creating a zone of lower dynamic pressure between the undertray and road surface.

Rules control where the diffuser zone starts and ends on the car. The trick to making it effective is to have its duct cross section increase as rapidly as possible without producing boundary layer separations and turbulence that would adversely impact flows.

It is logical that the undertray produces most of the downforce, since it can do so with the least drag penalty. But the front and rear wings are still very necessary, since they are easily adjusted to achieve the aero CofP the car set-up requires for the track conditions.

To answer the original poster's question, wings on a race car do normally tend to produce more wake turbulence than the underbody. But it's not because wing airfoils inherently have worse trailing flow characteristics than a diffuser/undertray arrangement. It's due to the fact that the (rules-limited span) wing elements are run at an AofA that maximizes downforce, and not L/D coefficient. In aircraft terms, they are used like wing flaps.

Best regards,
Terry

ps. Please forgive any errors in my rambling discussion of compressible flows. I'm a mechanical engineer, not an aerodynamicist.

[speedsense]

Please, I don't need you to tell me I'm correct.

You must not know much about how wings and other devices contriute to other downstream devices if you make the stated assumption. Front wing development has an enormous effect on conditioning the flow further along the car.
The front wing has a lot of understide vortex generators, compound flicks on the main planes, lots of geometry on the front wing end-plates, not to mention the whole upwash effect of the front wing itself.

You might also add, wheel inboard and outboard influences (mostly front but rears also at least from the inboard side. Suspension wakes, ground irregularities, tire influences and wakes, above/infront/behind, body/floor wakes and vortices (whether intended or not).

I don't state that the diffuser produces x ammount of downforce in it's own. We don't test individual components in the wind tunnel for this exact reason, it would be 100x cheaper and easier than what we have to do.

Other posts have stated that, my post is only reflecting on what the original poster has asked and the windtunnel test that was experienced and the overall genre of this thread.


"As far as the flow conditions changing, you are correct, however the changes to the chassis rake brought back the flow conditions very close to what they were with the wings, except the "exhausting" flow out of the diffuser was very different as was the center of pressure inside the diffuser, having moved closer to the "mouth" of the diffuser causing less effect. "

That is a contradicting statement, how can the flow be very close to winged conditions but then you go and say the "ehausting flow was very different", this "exhausting flow" is exactly what the diffuser is designed for.

Not as it pertains to a race car, where it's a diffuser in name only, as it's use and reason of use does not reflect the theory of use of a "diffuser" but rather the use as it pertains the floor in front of it. Controlling the exhausting flow only increases it's effectiveness (in small (+ - 5%) amounts, I might add) and effects the CoP point inside. My "close" statement is vague, I should have used the percentage number which was 87% when the ride height and rake were the same. The difference in rearward wake (going by the CoP critical movement) at the rear of the diffuser was only a difference of 4.5% between the wing off and the wing on, leaving all things the same, except the target ride height and rake.

I know exactly how much downforce the front and rear wings produce, downforce/lift of each wheel, overall car downforce. From this we can know the exact numbers for everything else on the car as a whole, safe to say the diffuser is a huge portion of this "everything else" number.

Is it safe? The chaotic aspect of a "measured" (via data acquisition) rolling race car at speed, tends to skew those numbers from windtunnel tests and computer generated geometry calculations from a stationary car. The numbers are only safe after real time measurement to back them up. Unless you also have a simulation program such as Adams or PiSim to draw calculations/conclusions from.

What car exactly were you evaluating?[/quote]

Van Dieman FF2000

[F1_eng]

What?

The diffuser, diffuses and the theory behind them are conventional. The floor is a completely different part.

How do you comapre wakes, stating a difference of 4.5%. How is this calculated from "CoP critical movement"?

You can be assured than conclusions I come up with from our measured data are as safe as safe can get. It is a colaboration of wind-tunnel, track data and CFD.

I am not totally familiar with Van Dieman FF2000, before my time but the pictures I have seen would suggest that the under-floor geometry does not lend itself very well to diffuser/floor downforce effects. The car looks very open and "loose" in my oppinion. Not much constraining/guiding of the air flow.

I have lost the direction of this thread im afraid.

[kilcoo316]

The difference in rearward wake (going by the CoP critical movement) at the rear of the diffuser was only a difference of 4.5% between the wing off and the wing on, leaving all things the same, except the target ride height and rake.

Am I reading that right?


You have measured the downforce of a car, rear wing on, and rear wing off... the difference in C.P. location is ~4.5%?

You have then extrapolated that to mean the wake effects are not much different?

[speedsense]

The difference in rearward wake (going by the CoP critical movement) at the rear of the diffuser was only a difference of 4.5% between the wing off and the wing on, leaving all things the same, except the target ride height and rake.

Am I reading that right?


You have measured the downforce of a car, rear wing on, and rear wing off... the difference in C.P. location is ~4.5%?

You have then extrapolated that to mean the wake effects are not much different?

The difference between removing the rear wing and the change of the rear wake, as it effected the exhaust flow of the diffuser, caused a 4.5% movement ( percentage of the length of the diffuser) in the Center of pressure inside of the diffuser, to a forward position (closer to the mouth of the diffuser-no rear wing). This measurement was just removal of the wing and no other adjustments.

I should point out, the wing assembly included two elements, an upper and lower wing assembly, with the lower wing within 12" (in height) to the exit of the diffuser. And also the diffuser started just behind the front of the motor.

The changes in the car's ride height and rake, resulted in movement of the Center of pressure (inside diffuser), 2% of the distance was regained, the rest of the distance was reasoned to be the wake difference of the diffuser, as when the wing was put back, the CoP shifted to the original position.

The original concept of this exercise, was to determine how much of the under body is generating down force and the efficiency of the diffuser) and whether we could enhance the flow of the diffuser regardless of the wing. We were planning on some wing changes that would involve this removal.
The percentages of calculated DF of the underbody where thought to be higher than what was observed.
The wing influences, not only in diffuser wake but in it's leverage over the under body were considered to be higher than the previously calculated amount and the CoP position inside the diffuser was of great importance to the size of the area in lowering of the low pressure area directly in front of the diffuser. It had a direct effect. The closer the CoP got the exit of the diffuser, the larger the upstream (before and in the mouth of the diffuser) "lower" pressure area increased in size.
The rearward wake of this diffuser and controlling of the wake with manufactured vortices, and reducing the height/width of the wake that was exhausting out into space, resulted in very small movements of the CoP inside by comparison.

Rake, ride height (and of course diffuser design) had a greater effect of efficiency of this diffuser, along with prevention side intervention of turbulent air (rear wheels, etc.) in terms of controlling the CoP. Further down the list is the rear ward wake exiting and controlling the expansion of the wake.

One the findings, the increase of DF between wings on and wings off, was that the under body down force was not as high as was calculated. A future test of running the car without wings at a race track confirmed this with live aero readings, specially noting the CoP inside the diffuser...the rake, roll, heave, warp movements of the car (less wings) rendered the diffuser almost ineffective, even with drastic changes in spring rate and lowered ride height.
Even with changes to the design of the diffuser and vortex generation above/around it, the down force numbers only changed enough to make a "mild" difference. Putting the wings back and taking the drag penalty was the largest step in regaining downforce numbers.

So when I read about how much downforce is "presumed" to be created by a diffuser/flat floor combination, my belief is that it's more about "wing leverages" than over body creation of downforce and the diffuser is more about removal of lift generation than DF production. The two did work hand in hand, with the wings having the greater amount of DF production and greater,but not equal to the under body generation. Without wings, the generation of DF was not 50% as was presumed, it's was more like 20%-25% and was not due entirely to the rearward wake changes...

BTW, this car went on to win the Championship, with two of the races on very high average speed tracks (we dominated them) and the car ran with a radically changed rear wing, this test allowed us to run the configuration with the knowledge we gained from these tests.

[speedsense]

What?

The diffuser, diffuses and the theory behind them are conventional. The floor is a completely different part.

How do you comapre wakes, stating a difference of 4.5%. How is this calculated from "CoP critical movement"?

4.5% movement of the CoP within this diffuser, of the distance of the entire diffuser. The CoP location was relative to the generation of lower air pressure area proceeding (upstream) the mouth of the diffuser.

You can be assured than conclusions I come up with from our measured data are as safe as safe can get. It is a colaboration of wind-tunnel, track data and CFD.

I am not totally familiar with Van Dieman FF2000, before my time but the pictures I have seen would suggest that the under-floor geometry does not lend itself very well to diffuser/floor downforce effects. The car looks very open and "loose" in my oppinion. Not much constraining/guiding of the air flow.

The sides are very open to intrusion of high pressure. Though ride height and side panel changes, splitters, were allowed at the time. The speeds of the car didn't exceed 140 mph at the time. Basically it's a FF with wings and higher horsepower...

I have lost the direction of this thread im afraid.

My apology, I'm a chassis engineer/data analysis not a writer...

[riff_raff]

I love the technical level of this discussion (or argument). Keep it up!

speedsense quotes a rearward CofP movement of "4.5%" on a Van Diemen FF2000 chassis after removing the rear wing entirely. I don't really know squat about the aero configuration of that particular vehicle. But based on that comment, I can assume one (or more) of the following is true:

A)That's a very stable underwing design.

B)That's a very ineffective underwing design.

C)That's a very ineffective rear wing design.

And finally, in regards to your comment, "The speeds of the car didn't exceed 140 mph at the time", I find it truly impressive that a FF2000 car can even achieve any speed close to 140 mph while making any appreciable amount of aero downforce. =D>

Best regards,
Terry

[xpensive]

As noted, the diffuser itself does not produce downforce. It is simply an aero device that creates a dynamic airflow region with lower ambient pressure in the trailing wake of the undertray. Since fluids tend to flow from regions of high pressure to low, this creates an environment where the air underneath the car naturally wants to flow (or more correctly, expand) out the back edge of the undertray faster than it can flow in past the restrictive front edge, creating a zone of lower dynamic pressure between the undertray and road surface.

Sorry Terry, but there are a few errors in there. The general idea of the diffuser is to help speed up the air under the car in order to create a higher dynamic pressure and as a consequence lower static such.
A restrictive front edge of the floor is not necessary, au contraire, the most efficient shape is a full "venturi", where the front edge curves up, which is banned and the reason for the flat bottom rule between wheel centerlines.

[speedsense]

I love the technical level of this discussion (or argument). Keep it up!

speedsense quotes a rearward CofP movement of "4.5%" on a Van Diemen FF2000 chassis after removing the rear wing entirely. I don't really know squat about the aero configuration of that particular vehicle. But based on that comment, I can assume one (or more) of the following is true:

A)That's a very stable underwing design.

B)That's a very ineffective underwing design.

C)That's a very ineffective rear wing design.

And finally, in regards to your comment, "The speeds of the car didn't exceed 140 mph at the time", I find it truly impressive that a FF2000 car can even achieve any speed close to 140 mph while making any appreciable amount of aero downforce. =D>

Best regards,
Terry

Typically at most tracks, the car saw 130 but at few long straight away tracks and very trimmed we did see 139.5 (I rounded up) with a bullet motor.
We did do some redesign work on the diffuser, that stabilized the CoP and were able to move the distance further towards the rear. What was "learned" was that the distance of the CoP from the mouth of the diffuser was vitally important to increasing the "lower pressure" area in front of the diffuser, thus increasing the size of the lower pressure area. The longer this distance was from the mouth of the diffuser, the greater in size and depth of the lower pressure area was. Small percentages netted reasonable size gains.
The removal of the rear wing caused enough change in the exhaust wake that the CoP moved aft and reduced the Lower pressure area in size and strength, by a small percentage. Created vortices above and on the sides of the trailing edge brought back most of the distance of the CoP.
The wings on this car weren't the greatest high efficiency wings, they could have been better, but rules did not allow us to change the profile.
Progressing to a high downforce, high speed car such as an atlantic car with full ground effects, the CoP location and distance from the tunnel's lowest point is just a critical, if not more.
While controlling the exhaust wake is an important "tuning" tool and has an effect on the CoP there are far more effective things that can be done that will make a greater difference and have a greater impact on lower pressure creation.

[sknguy]

Just a note of interest, but wouldn't relying on the diffuser/groundeffects be more dangerous than relying on the rear wing for downforce. I should think that the wing would provide some safety features which the diffuser could not. Better braking and stability for an unstable chassis perhaps? Would this not be a correct asumption?