Mythbusters tests "golf ball" dimples

[aerohead]

Looks like a 6.5-square-inch reduction in frontal area.
How it would affect the actual Cd we couldn't say with certainty.
For the 1982 Pontiac Trans Am Firebird, changing between 205mm and 195mm meant Cd 0.284, a delta- Cd 0.005 ( 1.5% )
185mm = Cd 0.278
175mm = Cd 0.273
165mm = Cd 0.268
One might plot the knowns on graph paper, create a trend line, and then extrapolate down to narrower tire estimated Cds.
Exploding tires are over-rated for fun, so be mindful of your load ratings.

[freebeard]

How does the vehicle weight compare to the BMWi3?

ecomodder.com/forum/showthread.php/bridgestone-announces-large-diameter-narrow-tires-25208

[Xist]

A friend commented that I definitely try hard to see my clients.

However, I usually get behind on my paperwork.

I just finished my progress reports for my second agency and I am now starting the notes that were due last night.

Normally my supervisor reminds me about notes on Thursday and our boss sends out an e-mail Friday morning, but I didn't hear from either, and somehow I thought notes weren't due, so those are late, too.

However, when I saw that aerohead responded to see what he had to say.

Regarding Mythbusters testing the tailgate, tonneau cover, and cargo net, we have found problems with their methodology before.

This time they used an electronic flowmeter.

Would that be more accurate than switching to a separate fuel tank, which you weigh before and after?

This time, instead of driving 1 mile on a closed course in the same direction 5 times they drove 5 miles on the highway in the same direction twice, although they indicated that one lap was 25 MPH and the other was 55 MPH.

Adam said that he was tired of driving after Jamie made 6 5-mile laps, although presumably they needed to drive back, where they didn't collect data?

I am trying to find something great that Metro said, but I found this first:

Originally Posted by BabyDiesel

You might be making a mistake going with the spare wheel. Here is my thread where I tried out using my usual tires on a 14x4 wheel:

http://ecomodder.com/forum/showthrea...hts-32033.html

It wound up hurting my fuel economy by increased weight and rolling resistance. There is also another thread located somewhere on here in which Darin (MetroMPG) did ABA testing on coastdown times with different air pressures. He used spare wheels and tires in a run and they failed miserably. Donkey CRX runs the Insight spare wheels on 165 width tires and has had zero problems with them. He gained mpg over the stock VX wheel IIRC.

What is your stock tire size? If it is thin enough, you can run spare wheels and not have the issues I did.

Also, the Insight spare wheels are an alloy, so they are uber light. I'd say less than 5 kilos each. If the spares are steel, you may not gain any weight loss.

Btw, welcome to Ecomodder

Lighweight donut/compact spare Toyota Yaris

So, it sounds like Donkey CRX is the one who tried RE92s on spacesavers.

Now I need to search for that?!

If the methodology is questionable, the results are questionable. It's that simple.

https://ecomodder.com/forum/showthre...tml#post332295

[aerohead]

There are some scientific flowmeters which have been used in the past, however, that was in the day of the carburetor, with no fuel bypass, just a 'one-way' trip.
On EFI, with a common fuel rail, we'd have to measure the 'gross' volume delivered, minus the return-flow volume back to the tank, to calculate the net volume actually consumed.
I believe that modern manufacturing practices are producing fuel injectors of such uniform tolerances that, given sensor technology, and a constant pressure fuel supply, the ECU's pulse-width-modulation data provides for very acceptable mpg data on the fly.
I've compared tank mileage to the readout on the Prius, and I trust the electronics more than my feeble attempts at accounting. And we're talking 2009 technology.

[Xist]

I was going to write my notes. I swear! I just saw a response to the boat tail video.

I replied to it and was about to write my notes.

But wait!

https://www.youtube.com/watch?v=mXx81YyjfL0

Except, they aren't dimples, they are outies, and they aren't on the entire car, they are on a roof scoop, which feeds the 8-liter engine.

They aren't normal outies, either, they are active, because they are supposed to hurt aerodynamics below 50 MPH, so they automatically extend above that--on a race car, which is supposed to hit 60 MPH in a couple of seconds, so it would only matter for 2.

Also, none of this was tested, it was all simulated on computers.

The annoying guy in the video ridiculed it.

Someone in the comments led me to this video:

https://www.youtube.com/watch?v=snNL5GgOq_c

About a year later we got an email from someone who worked with one of the big three automakers that they were curious about this and what you might not know is that big car prototypes are actually made out of clay. Yeah that's one of the key ways in which they prototype cart forms. So they pulled this clay SUV frame they had out of storage and some poor guy carved a thousand detents in the outside of this clay car and the this automaker put it in their wind tunnel and then they wrote us to say that they did not come to the same results we did.
That didn't bother me at all one they were using a totally different kind of car so i don't consider that it's valid. It's apples and oranges not really a scientific methodological approach that i would approve, but secondly we wasted the time of a big three automaker for like a week. That is so cool! That's like that's one of those ways in which Mythbusters allowed all of us to feel kind of like peers with all the scientists.

He is holding the model car on which they tested dimples before they went full-scale.

"It's [...] not really a scientific methodological approach that i would approve."

The automaker tested a full-size model, not on a one-mile track, but in a wind tunnel.

I really feel that if we knew about this when it happened we could say there is nothing else to discuss.

I just wish that I knew what they did find.

[redpoint5]

Golf ball dimples are subtle, too. Just because a car is bigger than a golf ball, that doesn't mean the dimples need to be scaled up in size too. That model looks cratered, not dimpled.

[aerohead]

Originally Posted by Xist

I was going to write my notes. I swear! I just saw a response to the boat tail video.

I replied to it and was about to write my notes.

But wait!

Except, they aren't dimples, they are outies, and they aren't on the entire car, they are on a roof scoop, which feeds the 8-liter engine.

They aren't normal outies, either, they are active, because they are supposed to hurt aerodynamics below 50 MPH, so they automatically extend above that--on a race car, which is supposed to hit 60 MPH in a couple of seconds, so it would only matter for 2.

Also, none of this was tested, it was all simulated on computers.

The annoying guy in the video ridiculed it.

Someone in the comments led me to this video:

He is holding the model car on which they tested dimples before they went full-scale.

"It's [...] not really a scientific methodological approach that i would approve."

The automaker tested a full-size model, not on a one-mile track, but in a wind tunnel.

I really feel that if we knew about this when it happened we could say there is nothing else to discuss.

I just wish that I knew what they did find.

The only conclusion I could take way from the dimple experiment was that, on the Ford, above the backlight, and down the C-pillars, they functioned as vortex generators ( VGs ), and drew the flow down and in a bit, allowing reattachment, or enhanced reattachment onto the boot, before finally separating.
At the enhanced upper rear separation line, the static pressure would be increased, raising the base pressure of the wake, lowering pressure drag, and lowering overall drag.
The 'OEM' super-critical Reynolds number would have taken care of the rest of the car.
Had they only applied VGs to the roof, as with the Mitsubishi LANCER EVO, it's highly likely that they'd have seen similar results, without all the dimples.

[freebeard]

Agreed that localized application is sufficient. That model he's holding in the thumbnail -- they must have had an intern dimple the roof. Compare the hood and rear deck.

Can VG dimples form a vortex street?

Kármán vortex street
In fluid dynamics, a Kármán vortex street is a repeating pattern of swirling vortices, caused by a process known as vortex shedding, which is responsible for the unsteady separation of flow of a fluid around blunt bodies. Wikipedia

Interspacing might be a factor.

[aerohead]

Originally Posted by freebeard

Agreed that localized application is sufficient. That model he's holding in the thumbnail -- they must have had an intern dimple the roof. Compare the hood and rear deck.

Can VG dimples form a vortex street?


Interspacing might be a factor.

The Karman vortex-street is associated with 2D flow past a cylindrical section, in an 'infinite' un-bounded flow field, with two degrees of freedom.
Since a VG is attached at a 'wall', it's constrained, as far as it's access to the flow field, operating in 'finite', 3D, bounded flow, with no degree of freedom.
--------------------------------------------------------------------------------------
And since VGs can be 'nubs', 'sub-boundary layer thickness', 'boundary layer thickness', super-boundary layer thickness', 'co-rotating', 'counter-rotating', 'large eddy breakup devices', etc., we'd have to be specific.

[Piotrsko]

All these years I have believed the main purpose of VG's were to keep the airflow attached, typically at sub critical Reynolds numbers because when your flow becomes detached you fall out of the sky stalled, at any speed. I also understand that at some distance from the surface, the airflow returns to being laminar so I suspect dimples or divots are just keeping the surface from stalling (detached airflow) which for wings in flight is the highest drag point.