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Discussion Starter · #21 ·
Okay kids, now I'm looking at something really interesting. CURVED diffusers! This is where the drive/passenger sides of the diffuser are either higher or lower than the center. There was a study done some time back where the testing results were super interesting in that there is HIGHER downforce and LOWER drag by using a curved diffuser.

I've got an email to Kyle about this idea so apparently the billing has begun! Which is a pretty exciting step because I'm seeing results for my hard work. Who the hell knows, you might see this on a F1 car next year.
 

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Discussion Starter · #23 ·
May I ask you where you are getting these from?
That would be Amazon. Sometimes they deliver them on the same day.

The 24 x 48 sheet I would have preferred as 1/8 inch but the only stock they had was 1/16. No big deal, just needs a metal frame behind it.
 

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Discussion Starter · #24 ·
So, I've heard back from the consultant and yes indeed, curving a diffuser is not only done but done with great results. See the next post for his reply.

Spoiler sum up: A curved diffuser will do three things: Reduce the aero drag and increase downforce and load distribution.

At this point in the design phase, I'm looking at and researching additional vortex generation in the diffuser. When a diffuser is mounted on a vehicle, you will see a couple of things. First many of them are curved with the outsides of them being curved down. The Ford GT goes in the other direction.

Second, especially on Formula 1 cars, you will see a series of notches or "shark teeth" at the bottom of the strakes (the bits that are up and down at a 90 degree angle from the diffuser itself). The strakes themselves, especially at the very outer edges on the driver/passenger edges, prevent the "tire squirt" from disrupting the air flow inside the diffuser itself AND they create a vortex along the bottom edge of the strake.

That said, my objective is to ADD additional vortex generation to the diffuser to gain even more downforce. The reason I'm doing that is because of our old friend, the banana wing. While I've made great strides with the vortex generation with regards to the airflow over the wing, it itself is physically far enough forward that it not only adds downforce to the rear of the Stealth but also to the front - more than I would like. So, any additional downforce to the rear of the car is more than welcome.
 

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Hey Max, you should just buy THIS....


Curved diffuser is already done :)

Bob.
 

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Discussion Starter · #26 ·
Here is the communication from the Formual 1 consultant guy:

So with respect to the curved diffuser question, there is no reason you cannot curve the diffuser in the manner you propose, it is in fact quite a common thing done on racecars. Often we have the leading edge as all flat, but then the trailing edge could be either of those examples. For example, F1 cars have a higher centre than outside

While some racecars (ford GT comes to mind) go the other way

It is of my opinion that the higher centre, lower outside is the superior loading distribution, for three reasons:

1. The loading distribution is naturally more elliptical, which will inherently be more efficient if the edge vortices are not providing significant suction.
2. The lowered outer portion will decrease diffuser expansion near the tyre, reducing tyre squirt ingress
3. In the event that we are getting significant edge vortex suction, the ground proximity in this portion is lower, and therefore we will get more induced load.

As far as I can tell with the diffuser on the Ford GT too, one of the big reasons they are doing it that way is to clear their engine, gearbox and structure. Again, as I mentioned earlier, the diffuser kick should be flat and level, connecting with the main floor cleanly.

In both cases though, we would still run the strakes as full height, we are merely trying to manipulate the spanwise (across car) loading distribution and roof profile, not the profile of the strakes. Does that all make sense?

With respect to your paper, it is referring to a curve along the length of the car, i.e.

There are definitely benefits to curving and profiling your diffuser, however it is a very exact and specific problem. I would take the results of this paper with caution, as I would with any study looking at aero devices on a body without proper wheels. It is extremely rare that studies of this nature have any accurate translation into the real world, and a number of the results in this study (such as reducing diffuser volume in favour of adding curvature) go directly against what I have seen in CFD, in the tunnel, and on track.

Cheers,
Kyle
 

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Discussion Starter · #27 ·
Hey Max, you should just buy THIS....


Curved diffuser is already done :)

Bob.
Did you happen to notice something in the description?? The question is whether or not this thing flashes it's tour/sport light? If not, not interested.

  • Chassis: SBM14FCA0LW004512
  • 1,400 Miles
  • 4.0-Liter Twin-Turbocharged V8
  • Seven-Speed Dual Clutch Transmission
  • MSO Burton Blue
  • Power Retractable Electrochromic Roof Panel
  • Black Leather/Alcantara Interior
  • Carbon Fiber Interior & Exterior Trim Packages
  • Stealth Pack
  • Sports Exhaust
  • Carbon-Ceramic Brakes
  • Dual-Zone Climate Control
  • Front Suspension Lift System
  • Clean Carfax Report
  • Owner's Manuals
  • Window Sticker
  • Private Party Or Dealer: Private Party
 

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Discussion Starter · #28 ·
Enough rantin' and ravin', I've got the complete frame mounted on the car! The videos have been shot, the champagne has been drunk (one of the cats apparently thought it was a good idea to spray the interior of the car with it) and the yarn has been hung.

Once I'm ready to hang the strakes, I'll have measurements and other data to build one yourself.

The next step is to do some aero tuft testing and mount the stakes. Weeeeeeeeee!

Photo of main frame that slides into the slot under the gas tank mounted to the diffuser. You will note that I've built 2 strakes right into the frame. This will be the main tire squirt defense.

291595



Here is the diffuser mounted (!) on the car. You can see where the frame fits into the slot under the gas tank as well as the side strake. It looks a lot lower to the ground than it actually is.
291596
 

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Discussion Starter · #29 ·
Here's the video of the diffuser tuft test. I'm waiting for the Formula 1 consultant to take a look at it. I'm pretty confused by what's going on. Only thing left to do is hang the strakes and it'll be done. Next project is Barge Boards.

But, I am seeing that when a car passes in the other direction, there's drop in the flow.

 

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Discussion Starter · #30 ·
I have the recipe for the flo-vis paint but I'm under an NDA from the consultant so I can't share it. But before I start doing that, I've cut all the yarn to half length to try and combat the static build up on the diffuser. No, grounding it won't do any good.

I spent the day installing the bargeboards which I'm nearly certain will work to eliminate most of the tire squirt. That project was a royal pain in the ass. Not the normal kind but the royal kind. One side went on fine but the other side, the car is too close to the wall so my elderly pretzel routine wasn't a walk in the park.

Bargeboards are pieces of bodywork that serve a purely aerodynamic (as opposed to structural) function. They are vertical planes situated front and rear wheels. The concept that I'm using is different than what you see on a open wheel car in that I have attached them under the body kit in front of the rear wheels in order to shape and move the air flow into the diffuser. The way they're designed might work and might not. I'm expecting to have to connect some sort of ductwork to them to make them as effective as possible. There's enough room under the car to do it IF I flatten the ductwork.

291708


291709


291710
 

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Discussion Starter · #31 · (Edited)
The final (I hope) road test of the diffuser is done! The Bargeboards work! And as an added special bonus it's the premier of the all new opening credits!

What I'm looking for at 60 miles an hour is how the tufts are moving - to be precise, the stability of the movement. What I did was mount the bargeboards yesterday that direct air from the side of the car into the diffuser. I had some serious doubts as to whether or not they'd work but they do. I did a second run with the camera on the bargeboards themselves as well but that needs some work still.

What you'll see is the tufts closest to the camera are still moving in an "undecided" way - we suspect that this is due to the exhaust that hangs down across in front of the gas tank. I may repeat may put a shield over it to smooth that flow at some point.

The next step is to remove the diffuser, add the metal frame, the strakes (that create vortexes) and the actual vortex generators themselves. After that, hanging the diffuser with the final screws (instead of the current Gorilla Tape).


Introducing our new opening credits!!

This is the final road test up to 60 mph WITH the bargeboards installed

Aero Consultant: Kyle Forster
Support Kyle on Patreon!

Music: Roman Senyk, Epic Cinematic Dramatic Adventure Trailer
Support Roman on Patreon


Creative Commons license means that you free to share, copy and redistribute the material in any medium or format; adapt, remix, transform, and build upon the material for any purpose, even commercially. But you must give appropriate credit (name of author and song) and provide a link to original track.

Aerodynamic Consultant: Kyle Forster
Helicopters: Bell Aerospace
Cat Food Supplier: Friskies
Beer Supplier: Anheuser Busch, Bell Brewing Company
Inaccurate Information Manager: Hugh Jass
Lubrication Director: Fidel Castrol
NASA Consultant: Roger Houston
 

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I haven't watched the video yet, cuz I think I just peed my pants....

I thought I had a pretty good (ie weird) sense of humor, but you win!

You're crazy!

Bob.
 

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Discussion Starter · #33 ·
I haven't watched the video yet, cuz I think I just peed my pants....

I thought I had a pretty good (ie weird) sense of humor, but you win!

You're crazy!

Bob.
Come on Bob, it's only 2 minutes long. What are you going to do when they're 30 minutes? So make some popcorn and be sure to have a mouthful of liquids to spray on the computer.
 

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This is what got me:

"Helicopters: Bell Aerospace
Cat Food Supplier: Friskies
Beer Supplier: Anheuser Busch, Bell Brewing Company
Inaccurate Information Manager: Hugh Jass
Lubrication Director: Fidel Castrol

NASA Consultant: Roger Houston"

Classic.

Bob.
 

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Discussion Starter · #35 ·
This is what got me:

"Helicopters: Bell Aerospace
Cat Food Supplier: Friskies
Beer Supplier: Anheuser Busch, Bell Brewing Company
Inaccurate Information Manager: Hugh Jass
Lubrication Director: Fidel Castrol

NASA Consultant: Roger Houston"

Classic.

Bob.
I'm not seeing the humor in all that. There's actually more staff members.

Auto-Body Expert: James Bondo & Matt Fiberglas
Auto Painting: Rusty Steele
Tire Consultant: Yessir Itsaflat
And finally,
End Credit Producer: Gladys Overwith

This ain't no fly by night operation over here you know.
 

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UNCLE.
We REALLY need to meet...

Bob.
 

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Discussion Starter · #38 ·
Point one: In talking with Kyle about the flow in the diffuser during the tuft tests, he's pointed out that yes, it's normal for the flow to vary and not be constant BUT the averaged out flow is far higher than not having the diffuser - thus the downforce it creates is quite high when designed properly, which he assure me mine is.


The aero design has taken a new and frightening turn because I'm dragging out all of my old physics and fluid dynamics textbooks. Nothing good EVER comes from that.

Here's an update while I'm taking a break from taping the diffuser confuser videos. I'm trying to reconcile something.

Even though I'm understanding the low pressure flow within the diffuser, I'm still having issues with the venturi effect. - I'm willing to take it on faith that it's happening but I'd like to understand the physical forces involved. I think at this point I understand - the angle of the diffuser is lower at the throat and higher at the exit , therefore the venturi effect is created using the pavement as the fourth side of the channels and due to the reversed wing shape of the diffuser. I also read the following which really helped explain the entire theory:

The air enters the diffuser in a low-pressure, high-velocity state after accelerating under the car. By gradually increasing the cross-sectional area of the diffuser, the air gradually slows down and returns to its original free-stream speed and pressure. The diffuser's aim is to decelerate the air without it separating from the tunnel walls, which would cause a stall, reducing the downforce and inducing a large drag force. By installing an inverted wing close to the diffuser exit it is possible to create a low-pressure area, which essentially sucks the air from the diffuser.


The above might not be entirely accurate but for the sake of my understanding, it's close enough.

Now I'm also thinking making the diffuser extend further underneath the car might also be a good idea. Or at the very least extending it so that it meets up with the bargeboard exits.

I cracked open the textbooks this afternoon and read up on free channel venturi effect and freely deformable fluid flow. But I'm finding it hard to take it on faith that there's an actual imiscible fluid phase involved in a diffuser without a closed bottom channel, thus enclosing all 4 sides, regardless of whether or not there's a stratified flow - I just don't see that much of a density difference that's going on.

However, if there's an actual free channel venturi effect going on, it seems like enclosing some if not all of the diffuser would only increase the speed of the flow, possibly increasing (decreasing?) the low pressure flow which seems to be totally incorrect on my part simply because nobody makes diffusers that are enclosed - I'm not buying that nobody has ever tried it which leaves me with to choices to consider:

A. It works too well and it's banned in racing
B. It has the opposite effect on downforce.

During my trip down the rabbit hole, I read about Colin Chapman's work on ground effects without side skirts and realized that accidentally, with my bargeboards, I've created a similar aero device without using the entire underside of the vehicle but rather introducing it via the airflow along the sides which makes me think bigger and longer might be better in this circumstance so I've ordered more ABS panels to build them. I also may have hit the correct angle of the bargeboards of 35 degrees to accelerate the air to it's maximum.

In review, I know that the air dam I built for the front of the car is reducing the amount of drag induced under the car with an additional two vortices that are created by the space between the sides dams and the center flap spoiler. That reduced the flow under the car enough that the bargeboards are having an enhanced effect on the diffuser.
 

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Discussion Starter · #39 ·
Update on the project. I'm waiting for one last sheet of ABS to get here - I've decided I'd like additional strength in the frame.

I'm also thinking of presentation of the device. After looking at a lot of street and track car designs, I've had to develop another forming method of the ABS to introduce curves to some parts. The bad part is that I only have 1 piece of material for the parts I want to curve so it's got to go right the first time around.
 

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Discussion Starter · #40 ·
The diffuser will not be done as quickly as I had thought. The hangup is all the math and triple checking the angles and ratios needed to add both the vortex and venturi effects so they're maximized.

And of course the first time in 3 years I start a major engineering project, my spine says "remember me?"
 
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