Hey, I posted that right before I had to go live with the video! And during the video, I was able to show a clear effect. The box isn't fully sealed, and the better I seal it the more difference there is. But it's pretty neat to be able to just plug something into an inlet and see the efficiency increase. That's a 5 HP shop vac providing the vacuum.
take my money already damnit
Went & found the video. Here's a link for the rest of the class.
https://www.youtube.com/watch?v=BDdEURE0Sjo
While a bell can increase flow with your pressure difference in the box demonstration, if the pressure difference is much lower between the intake and brake rotor hat on the car, the restriction in intake diameter due to the thickness of the bell throat in the hose may negate any benefit. Our resident Mr. Stafford may have first hand experience. The hose diameter as well as thickness of the plastic throat for a particular diameter hose as a percentage of hose cross section area may also make a difference.
I always wanted to make a bell that sorta screwed onto the outside of spiral wound hose with a smooth"lip" inside the throat that the hose end butted up to. Then there wouldn't be a reduction in hose cross section and possibly less turbulence in the throat reducing resistance.
apexanimal said:take my money already damnit
let me ask the same question in a different way.
Keith, how can I/Scott facilitate ($$$) getting one or a pair of beta/development brake bellducts to Scott by July 24th so you can get some real-world data/testing before/during the July 31-Aug1 Grid.Life GLTC races? Scott's already got a Magnehelic gauge, and could give you a comparison of the "nominal market" ducts and these by replacing one side with yours and switching the magnehelic line between the two on the car.
2.5"? Do you have a relatively flat 5" diameter circle to mount it on? The real way to test these would be to put one on one side and a normal flush inlet on the other and measure rotor temp.
We have some internal procedures that limit our ability to get products in the catalog and on the shelf as quickly as I'd like. At least this product isn't subject to the complexities of an outside supplier, and our recent printer buying binge has finally left us with a bit of extra manufacturing capability. But I have two of these on my bench right now that were built with the express intent of being used for testing. So I think I could definitely get them to you.
In reply to Keith Tanner :
This is Scott's car (also, apexanimal on the forum, something else on MT):
those holes are already run with 2.5" brake tube behind. I'd say this is pretty close to an ideal 'market'/implementation test. imho
NOT A TA said:Went & found the video. Here's a link for the rest of the class.
https://www.youtube.com/watch?v=BDdEURE0Sjo
While a bell can increase flow with your pressure difference in the box demonstration, if the pressure difference is much lower between the intake and brake rotor hat on the car, the restriction in intake diameter due to the thickness of the bell throat in the hose may negate any benefit. Our resident Mr. Stafford may have first hand experience. The hose diameter as well as thickness of the plastic throat for a particular diameter hose as a percentage of hose cross section area may also make a difference.
I always wanted to make a bell that sorta screwed onto the outside of spiral wound hose with a smooth"lip" inside the throat that the hose end butted up to. Then there wouldn't be a reduction in hose cross section and possibly less turbulence in the throat reducing resistance.
The wall thickness on my full size test unit is 3mm. That is a surprisingly large loss in cross-section due to the fun of exponents, it's about a 20% loss. The wall could possibly be a bit thinner, I wanted something robust for my testing. Of course, any internal flange for mounting a hose is going to have a non-zero thickness so you'll never get the full 2.5" ID of the hose. If the wall was only 1.5mm (0.060"), there would still be a 10% loss in area. The bellmouth does accelerate the air which likely has an effect - I may do some more testing with my scaled down brake hose to see if I can get clean data. I need to anchor it to the box better.
The small test unit was scaled down equally in all dimensions, and the hole I was testing it in was laser cut to the exact ID of a quarter-scale 2.5" hose. So it suffered from the 20% loss in cross section and we saw a clear difference in performance even so. And besides, those hoses are not very clean inside, aerodynamically speaking. My blue test hose IS smooth inside so it's a better situation than you'd find in an actual brake duct using flexible SCAT hose.
Going back to the original post in this thread, these inlets are usually placed in a spot with quite high pressure. So I'm pretty sure they'll drive the bellmouth well.
Having the bellmouth screw into the OD of the hose is an interesting idea. You may have trouble with the lip of the hose being messy - you have to have a flap of fabric on the lip, otherwise your steel wire comes loose - and that would likely negate your expected gains from an ultra-smooth transition. But adding a big coarse thread to the OD of the bellmouth along with a taper to avoid the step at the end would be easy enough to do on a printed part. I might try that.
If I remember my theory correctly, it doesn't really matter how big the minimum diameter is (up to a point)... if you're keeping the initial maximum diameter the same. Effectively the amount of air that's ending up in the duct is determined by the initial area, and everything after that determines what the flow's velocity/pressure are. That is, assuming that the flow doesn't go supersonic at somepoint in the area reduction; and that the flow has somewhere to go/exit and doesn't 'fill the volume' and then end up with a higher pressure than that at the face (i.e. force the flow out).
I don't necessarily expect either of those scenarios to happen... certainly not the former, even if it's a V8 miata. And the later, seems relatively unlikely since the flow is dumping into the wheel well.
So, effectively, you have an inlet area of A1 with pressure1, and V1. The minimum will end up at A2 which is ~= 1/2A1, thus V2 = 2*V1. When we get to the brake tubing, we're at A3 = 1.2*A2, thus V3 = V2/1.2. It'll still be significantly more speed and more mass flow than any of the 2.5" ducts on the market.
There's probably something to said for the fact that you might be making the initial diameter too large? Another refinement to consider, is that you provide some form of ramp at the outlet of the duct so there's less of an 'area step' when the flow hits the tubing.
This thing is not just a smooth ramp, it's actually accelerating the air. The profile is important - I used ELL-23-2349-3 from this article that has been referenced several times. That's where the initial diameter came from.
Or are you thinking of an undersized bellmouth followed by a divergent profile? We were talking about that in the shop after the video, I need to spend some time looking into it. I don't know if it would be better than a full size bellmouth or just easier to package. I don't think we have to worry about supersonic flow in this case :)
On Scott's car, do you have a way to measure rotor temperature? Tempilac or temperature strips or even an IR thermometer?
If you're looking into using diffuser behind a bell here's an article by Willem Toet I think is worth the quick read. https://www.linkedin.com/pulse/air-ducts-down-earth-guide-motorsport-applications-willem-toet/
Anyone with an interest in race car aerodynamics would probably find many of his articles on Linkedin interesting.
Keith Tanner said:On Scott's car, do you have a way to measure rotor temperature? Tempilac or temperature strips or even an IR thermometer?
They've (Scott and Becky / Robertson Racing) been measuring temp on the calipers with strips, and temp on the rotors with paint.
Thanks for the article link - a quick scan shows that's going to be a very interesting read and very applicable. There are some interesting follow-up experiments to figure out!
Sleepyhead, let me confirm that I'm good to ship these somewhere. And email me (keith@flyinmiata.com) an address.
In reply to Keith Tanner :
email sent
I've had a few people in various comment threads about the video complain that I was sucking air through the bellmouth instead of pushing it through, which is TOTALLY different. Pressure differentials don't matter :)
So tonight I turned it around. Put the bellmouth inside the box, put in a baffle to prevent the inflow from directly impinging on the inlets, and hooked the hose up to the outlet of the shop vac.
Open hole, 2.7" of water. Bellmouth, 2.4". That's right about the same difference as I saw when I was pulling vacuum in the box. No surprise, but I couldn't just leave it alone!
Much more interesting is what sleepyhead and friends find on the track. Will the potentially increased inlet airflow manifest itself in cooler brake temps? The article about a diffuser is really interesting, I'm not sure we'll get a chance to measure that immediately but I do want to play with it. It would only take 2" of length to ramp a 2" duct up to 2.5", so my initial fears about packaging may be unfounded. But where should the divergence begin, at the point of highest velocity? I think so. I'll have to see if I can determine where that is.
Question: Don't you actually want that bellmouth to be oblong in shape because the front of the air dam is curved and it cannot be mounted in line with the direction of airflow?
Edit: I guess it doesn't matter that much, but I would be curious what CFD says.
In the case of a straight hole like shown in the OP's pics it seems the full value of the tubing diameter is likely not being realized.
Trying to match the curve of a semi-randomly placed hose on a home-made air dam is a challenge :) I certainly can't do it from here. I think that's entering "perfect is the enemy of good" territory.
In reply to ProDarwin :
In addition to what Keith mentioned, the airflow might not be in the direction we might expect. In highway speed tuft testing I've done, I was surprised the direction of flow was more downward in the area where the openings on Scotts car pictured above are placed (think 5 o'clock). Every case is different due to shapes, ride heights, etc.
On of the benefits the bellmouth entry has, is it helps align mis-aligned flows with the direction of the duct, and the compression helps to smooth turbulence out at the same time, at the 'expense', it would seem, of lowering the pressure.
i think part of the reason we see these on engine intakes is, the intake stroke ensures that the exit of the tube is always at a lower pressure than even the reduced pressure of the exit of the bellmouth.
also, I've cracked open my textbooks and started up a spreadsheet to try and unwrap my head from the axle... or, in this case, brake duct. Thanks for the link to Toet's article, John!
Interesting! I didn't know about the alignment effects. So it makes the intake less sensitive to local airflow direction instead of more?
Keith Tanner said:Interesting! I didn't know about the alignment effects. So it makes the intake less sensitive to local airflow direction instead of more?
yeah, you can see that in the illustration of the intake manifold in Toet's article that Not A TA / John linked:
In other news, I've managed to ballpark some numbers I had from Scott's Magnehelic gauge from when we were testing the original "brake duct scooped from the side of the radiator duct", including losses.
edit:
one issue I have, though, is that a ~4.75" opening duct, that reduces to ~2.25", the math indicates there will be negative pressure at the exit of the bellmouth. Which, I think really means that the original duct Keith printed is oversized?
sleepyhead the buffalo said:On of the benefits the bellmouth entry has, is it helps align mis-aligned flows with the direction of the duct, and the compression helps to smooth turbulence out at the same time, at the 'expense', it would seem, of lowering the pressure.
Yeah. I'm just playing devil's advocate here.
If you know the approx. area they will be mounted (you do), and you know the curvature of the nose of a Miata (check), there is no reason it has to be misaligned at all :) It may also help with the hose routing behind the bumper as now there is less direction change the hose needs to accomodate.
Now if it were a universal part designed to work on the front of any car, it doesn't make sense to optimize that (much).
sleepyhead the buffalo said:edit:
one issue I have, though, is that a ~4.75" opening duct, that reduces to ~2.25", the math indicates there will be negative pressure at the exit of the bellmouth. Which, I think really means that the original duct Kieth printed is oversized?
That seems to depend on the profile of the bellmouth. I used the ELL-23-23-49-3 profile from the Professor Blair article, and on the second page you can see the difference in behavior between that and other profiles. Looks like it reaches terminal velocity maybe two diameters in? I'll try a pressure test using both a straight pipe and a divergent one to see what happens.
I have been on uncle duty for a 7 year old all day, I need to sit down and fully internalize all these articles.
well you guys are over my head, but if we can provide a platform for testing we will do our best and help out!
If one is having brake over-heating issues, why not go for a bigger hole? In regards to miatas, I know the 949 guys upgraded to 3" ducts instead of 2.5" and it remedied a majority of their issues.
I think it might be relevant to do your best to limit the numbers of bends after the inlet too. Maybe?
I've never had brakes issues on miatas before, but I've always had pleab power. Scott are you having issues with your car since the kswap?
3" doesn't package well on a Miata. I've gone from 2.5" to 3" on my car and it helped a lot, but there are lots of clearance problems and full lock is not possible.
I've got a couple of interesting variations to play with - coming off the printer today - then they're heading to the track for testing on Scott's car.
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