Diffuser design factors

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Michiba
Michiba
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Joined: 28 Apr 2008, 08:58

Re: Diffuser design factors

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Mystery Steve wrote:
Michiba wrote:The fact that those rims are only on the front wheel and vastly different to the rear wheel rims makes me think they have more of a role than just diverting air away.
The front brakes generally create more heat than the rear due to dissipating more energy, so cooling requirements wouldn't necessarily be the same.
Yes, I understand that. However, if you were to just take the cooling requirements into account, then given that the front brakes work harder to dissipate heat, you would assume that the openings on those fairings would be larger on the front wheels compared to the rear wheels. Yet this is not the case.

What I was alluding to was the aero requirements of the front wheels vs the rear wheels. My point being that there is an effort to direct the air coming out of the front wheels, but not the rear wheels. This has lead to the discussion at hand.

xpensive
xpensive
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Joined: 22 Nov 2008, 18:06
Location: Somewhere in Scandinavia

Re: Diffuser design factors

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I'm not so sure Steve, the effect of the DDD's tells me that the cross-section area of the diffuser has a direct influence over the air-speed under the car and as a consequence amount of downforce generated.
"I spent most of my money on wine and women...I wasted the rest"

Mystery Steve
Mystery Steve
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Joined: 25 Sep 2009, 07:04
Location: Cincinnati, OH, USA

Re: Diffuser design factors

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Michiba:

I didn't intend to belittle you. Was just pointing out that the difference may not be aero-driven. Regarding the different sizes, I wouldn't think that the front would necessarily have to be bigger. The heat transfer is driven largely by mass flow through the duct. If you have a higher air velocity, which you would have because the airflow has not been disturbed as much as it would at the rear wheels, then you wouldn't necessarily need a larger inlet to achieve the necessary mass flow. The general uniformity of the airflow can influence the inlet size as well, with more uniform flow would require a smaller duct. It depends on the overall design.

As for directing the air coming out of the wheel, I really doubt that it would have an effect on underbody flow considering their is a slight upwash effect caused by the rotation of the tire. While there is a lower pressure region under the car, I'm not entirely sure it would be strong enough to pull that air underneath. I would have to think about it more. It would help if there was a copy of the video in English so that I could hear them describing what they're showing.

xpensive:

From Fundamentals of Aerodynamics by John Anderson:
A diffuser is a divergent duct whose role is to slow higher-velocity air down to a lower velocity at the diffuser exit.
It's an application of the continuity of mass. Assuming constant mass flow through the diffuser:



Assuming incompressible flow by the low Mach numbers, it simplifies to:



As the area increases, the velocity decreases. As you stated previously, as the velocity decreases the pressure will increase. This is the role of a conventional single diffuser.

Concerning the DDD, I confess I don't completely understand the reasoning that went into the different designs. My initial thought is that some of the lower pressure airflow is released before it is slowed down to the freestream, and this "bleed air" acts on the lower side of the "upper deck" of the diffuser, generating downforce. That could explain the two openings below the rear light on the Williams and the single opening in the same location on the Brawn:

Image

Image

If my reasoning is correct, the upper deck would technically be a wing and not a diffuser. So yes, in this case, the airspeed from under the car would affect the downforce because the airflow acts on the extra wing. However, a diffuser (by strict definition) does not specifically generate downforce, it conditions the exit flow. However, if you claim that because the flow from the diffuser acts on the upper wing then it could be considered part of the diffuser. This is what I was alluding to when I asked if we were saying the same thing different ways.

What was the loophole that the initial DDD users were using for their legality argument? Were they claiming that it worked based on airflow from the diffuser and was therefore a part of the diffuser rather than another rear wing, which would be a method of getting around the lost width of the rear wing in the regulations?

xpensive
xpensive
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Joined: 22 Nov 2008, 18:06
Location: Somewhere in Scandinavia

Re: Diffuser design factors

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At the end of the day, it's all about speeding up the air under the car in order to create downforce, all according to Bernoulli:

Static plus dynamic pressure is constant: ps1 + pd1 = ps2 + pd2, where pd is speed squared times density over two.

Which gives:

Static pressure differential = ps1 - ps2 = Rho *(v2^2 - v1^2)/2

The xtra openings on top of the conventional diffuser helps evacuate air from underneath the car, thus increasing v2.
Compare with the "chimneys" used last year to evacuate air from the radiators.
"I spent most of my money on wine and women...I wasted the rest"

Michiba
Michiba
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Joined: 28 Apr 2008, 08:58

Re: Diffuser design factors

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Mystery Steve wrote:Michiba:

I didn't intend to belittle you. Was just pointing out that the difference may not be aero-driven. Regarding the different sizes, I wouldn't think that the front would necessarily have to be bigger. The heat transfer is driven largely by mass flow through the duct. If you have a higher air velocity, which you would have because the airflow has not been disturbed as much as it would at the rear wheels, then you wouldn't necessarily need a larger inlet to achieve the necessary mass flow. The general uniformity of the airflow can influence the inlet size as well, with more uniform flow would require a smaller duct. It depends on the overall design.
It's cool mate, I didn't take it that way. I value rational constructive discussion. What does irk me on this (and pretty much every other forum) are baseless opinions stated as facts (ie. fanboyism or whatever you call it).

Back to the point, I'd have to disagree, because I think the front wheel fairings are definitely aero driven, with the main point being that all the teams have their devices pointing the air downward. There are a couple of recent exceptions like redbull and maclaren. The gurney-like tabs on theirs seem to direct air away from the wheel in general. However this is still what I would consider aero driven.

Mystery Steve
Mystery Steve
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Re: Diffuser design factors

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Michiba wrote:Back to the point, I'd have to disagree, because I think the front wheel fairings are definitely aero driven, with the main point being that all the teams have their devices pointing the air downward. There are a couple of recent exceptions like redbull and maclaren. The gurney-like tabs on theirs seem to direct air away from the wheel in general. However this is still what I would consider aero driven.
I agree, they are definitely aero driven. After all, they are aero devices used to manipulate the airflow. I just find it doubtful that they are using them to modify the underbody flow to improve downforce. I would imagine their goal is to just reintroduce the "used" air back to the free stream as smoothly as possible to reduce drag. Ultimately, what you're after is optimizing L/D. Although, if they are using it modify the underbody flow, it would be interesting to see how they're doing it. I would have to see some calculations or CFD to be convinced though.

Mystery Steve
Mystery Steve
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Joined: 25 Sep 2009, 07:04
Location: Cincinnati, OH, USA

Re: Diffuser design factors

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xpensive,

After doing more reading, apparently the reason for our disagreement is due to terminology. I was not aware that the racing community had adopted the term 'diffuser' to essentially mean the entire downforce producing underbody where as in the aeronautical community (my background) the diffuser only refers to the section of expanding cross section located aft of the underbody. My apologies for any confusion...

Also, it appears my thinking on the DDD was in the right direction, but admittedly a bit backwards. The diffuser is not acting on the wing element; the wing element is acting on the diffuser. The lower side of the wing element produces a low pressure region which increases the flow out of the diffuser.

As a few have already stated, the underbody creates downforce by creating a pressure differential via air velocity differential. Also, the mass flow under the car must satisfy continuity and is therefore constant for a given vehicle speed. The density of the air is not altered and the minimum cross-sectional area under the car is limited by the rules. This means that the velocity under the car is a function of the mass flow, which the DDD increases using the extra wing element.

The diffuser (using the aeronautical term) does not actually generate the downforce, but slows the flow back to free stream velocities to reduce turbulence and drag. This way you can generate the downforce while minimizing the drag penalty.

I think this is enough essays out of me for one day, back to real world work now...

dp35
dp35
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Joined: 15 Nov 2006, 00:58

Re: Diffuser design factors

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Thanks to all the smart people contributing to this thread & educating me. This is why I like this forum.

Interesting news regarding front wheel covers:
http://en.f1-live.com/f1/en/headlines/n ... 4902.shtml
F1 teams have agreed to ban wheel fairings for next season, Auto Motor und Sport claims.

We reported earlier this year that the devices, having first emerged last year, were likely to be banned for 2010 because they make it even harder for following cars to overtake.

Williams' technical chief Sam Michael said in July that studies have shown that the covers have "quite an adverse effect on the following car".

However, the Australian warned that banning the covers will not be a silver bullet.

"It is the sort of thing where you need to find three or four little things like that and that will add up to a difference," Michael added, "so I think it is going in the right direction but it just needs more."

newbie
newbie
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Joined: 29 Sep 2009, 23:33

Re: Diffuser design factors

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the agreement to ban wheel covers was a long time coming, I didnt realise it hadnt been reported in the media! the upper duct in the double-diffuser is simply a way to expand the volume and the exit area of the main diffuser. This simply means you get more mass flow under the car and thus more downforce. Even though the diffuser ramp brings the air to a higher pressure, the low-pressure peak underneath the car occurs just where the ramp begins; in effect the diffuser speeds up the air in a 'final kick' before slowing it down.

The position of the front wheel covers simply serve to allow positioning of the exit air so that the required effect on controlling the wheel wake can be obtained. If somebody had CFD images of the exit airflow they probably wouldn't post it here! ;)

Michiba
Michiba
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Joined: 28 Apr 2008, 08:58

Re: Diffuser design factors

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I would certainly like to see a CFD diagram of the flow. There must be more to the flow than has been discussed here in terms of the negative effects on a trailing car. How would it affect a following car anyway?

User avatar
ringo
230
Joined: 29 Mar 2009, 10:57

Re: Diffuser design factors

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Mystery Steve wrote:xpensive,

After doing more reading, apparently the reason for our disagreement is due to terminology. I was not aware that the racing community had adopted the term 'diffuser' to essentially mean the entire downforce producing underbody where as in the aeronautical community (my background) the diffuser only refers to the section of expanding cross section located aft of the underbody. My apologies for any confusion...

Also, it appears my thinking on the DDD was in the right direction, but admittedly a bit backwards. The diffuser is not acting on the wing element; the wing element is acting on the diffuser. The lower side of the wing element produces a low pressure region which increases the flow out of the diffuser.

As a few have already stated, the underbody creates downforce by creating a pressure differential via air velocity differential. Also, the mass flow under the car must satisfy continuity and is therefore constant for a given vehicle speed. The density of the air is not altered and the minimum cross-sectional area under the car is limited by the rules. This means that the velocity under the car is a function of the mass flow, which the DDD increases using the extra wing element.

The diffuser (using the aeronautical term) does not actually generate the downforce, but slows the flow back to free stream velocities to reduce turbulence and drag. This way you can generate the downforce while minimizing the drag penalty.

I think this is enough essays out of me for one day, back to real world work now...
Yes you hit the nail on the head. I always nod my head every time i hear the term free down force and diffusers reduce pressure. I know Nozzles reduce pressure.
I always thought of the diffuser as being a device that gives a smooth transition between 2 pressures. Diffusers only seem to create downforce because it scavenges of the low pressure at the back of the car. If for instance the car is a perfect streamline shape and the pressure behind it is atmospheric, then the diffuser would be useless.
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BreezyRacer
BreezyRacer
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Joined: 04 Nov 2006, 00:31

Re: Diffuser design factors

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Michiba wrote:
Conceptual wrote:Wouldn't the low pressure area under the floor be pulling in the air released from the brake ducts? When I watched the video, my forst thought was that it was just getting sucked under by the low pressure. Exactly what the sliding skirts were made to prevent!
yes, that's exactly what I thought, but breezyracer had other ideas. Would anyone have an CFD images to show how they could possibly work?

The fact that those rims are only on the front wheel and vastly different to the rear wheel rims makes me think they have more of a role than just diverting air away.
Here's a non incriminating image (I hope) that gives you an example about airflow curving out and then back in. This is not an F1 car so the diffuser strength isn't nearly as strong since there is no wing fitted to this example. But it does show the curved airflow I referred to. Don't be alarmed .. the front wheel is removed in this view.
http://picasaweb.google.com/lh/photo/bu ... directlink

BTW, this has nothing to do with brake ducts per se. It's simply about airflow, in general, in the undertray area. This basic curved flow is in every design I have seen in a variety of design variations, including efforts to tune it out. I don't get to see any of these with rear wings though, and these designs have to conform with class rules, etc.

BreezyRacer
BreezyRacer
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Joined: 04 Nov 2006, 00:31

Re: Diffuser design factors

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Here's a good shot of what is actually going on in F1 as regards to VGs for under trays. While this piece is off the car (McLaren, and this shot was taken in Germany) you can imagine that this combined with a barge board channels the airflow outwards towards the side of the car to create a vortex that helps produce a low pressure area as far forward in the under tray area as possible. One of the cool things about this shot is that it's a dirty part so you can see the flow lines along the bottom.

It's also interesting to note the amount of massaging that this area has undergone. You can see fillers and sanding marks. Maybe McLaren actually created a channel in this area for the VG flow and then were told it had to be flush. I don't know but we're getting into a fuzzy rules interpretation area here under the argument of where does the edge end and how much of a radius can the edge have, etc.

Image

Here's an interesting image of same at Monaco (an earlier race), showing it installed on the car along with barge board placement. So you can see that McLaren have been working on this idea for quite a while.
http://picasaweb.google.com/lh/photo/1z ... directlink

These VGs come in many fashions like the cow catcher on the Brawn or the curved up sides on the RB and Williams. But the idea is the same .. lower the air pressure in the under tray as far forward as possible to extend downforce as far forward in the chassis as possible.