Landracing Forum

Tech Information => Aerodynamics => Topic started by: Jack Gifford on November 15, 2016, 01:42:49 AM

Title: Drag coefficient fundamentals
Post by: Jack Gifford on November 15, 2016, 01:42:49 AM
Things I wonder about-

> Frontal area: the use of this term seems to confuse aero discussions. Some people look at a modified roadster with a tapered nose in place of a vertical grille and refer to its "small frontal area". Other people take the term to mean the area bounded by a projection of the car's outline onto a vertical flat plane perpendicular to the car's longitudinal axis (which would not be influenced by the nose piece mentioned above). Is there a bonafide definition of this term?

> Sixty-some years ago, competent motorsports engineers (Don Francisco, etc.) talked of drag coefficients in the ballpark of 0.9 for drop tank lakesters. No major drag-reducing changes have been introduced to this style car of today. So why do I read of a .27 drag coefficient in another thread? [Reply#90 of 'lakester aero help' thread in the 'Bonneville' forum]
Title: Re: Drag coefficient fundamentals
Post by: fordboy628 on November 15, 2016, 05:20:57 AM

> Sixty-some years ago, competent motorsports engineers (Don Francisco, etc.) talked of drag coefficients in the ballpark of 0.9 for drop tank lakesters. No major drag-reducing changes have been introduced to this style car of today. So why do I read of a .27 drag coefficient in another thread? [Reply#90 of 'lakester aero help' thread in the 'Bonneville' forum]


Based on what numbers I have been exposed to about aero, there is no way something with the wheels exposed fully to the air stream, could have a cd of .27

Maybe Woody will chime in with some authoritative numbers.

 :cheers:

 
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on November 15, 2016, 08:48:39 AM
You have to remember that the formula for total drag is D=CdA. The total drag is something that can be reasonably measured today and that the drag coefficient (Cd) is calculated IF you know an accurate frontal area. That area may be difficult to measure accurately or at least is very time consuming. Jack you are correct as I understand it that usually the A is the projection of the area in the frontal plane. I have tried to measure this on my car by a photo taken at a distance and using a drawing program to measure the area. Didn't work. Often for a usual road car the HxW numbers by an average 0.85 factor is used. Clearly a guess. Figuring this for a lakester would be very hard. A streamliner, easy. The Goldenrod was supposed to have a Cd of about .15 and frontal area 9.8ft^2. (per an article written by the designer). I recently observed a car in a wind tunnel. Published figures for Cd and frontal area were known. Initial runs in the tunnel suggested that the Cd was way off the published numbers. A number of changes were made that lowered the Cd by .12. One of those changes effectively decreased the frontal area (the car was lowered) so the total drag was reduced. If the new frontal area was used to calculate the Cd it would have been higher. I think if the final Cd that was found for this car was published without other comment, many people would call BS on it.

Interestingly Bonneville Pro has a Lakester as one of it's sample cars. It uses a frontal area of 16ft^2 and a Cd of 0.58. Perhaps realistic, who knows. (and 2000hp predicts 290mph) But the 0.9 you quoted is nearly a flat plate and I suspect these cars are better than that.

Bottom line of my thoughts are that in this situation, calculations are great as a baseline but actual data for these sorts of cars is harder to come by (those who know may not be talking) and the best way to find out is to measure, get as close as you can to the area and the magical low Cd falls where it may. The total drag is what counts. That sloping nose on the roadster does not have a lower frontal area, but may have a better Cd.

Woody published his calculations of body, axles and tires as an example and I don't doubt his thinking. But put the car or an accurate model in a tunnel and see what is measured.

Above is my understanding and thoughts about this stuff. FWIW
Title: Re: Drag coefficient fundamentals
Post by: Stainless1 on November 15, 2016, 09:56:36 AM
I think it was our old (RIP) buddy Jack Dolan that said

"Theoretical records are set by theoretical vehicles"   :cheers: Jack, we still think of you  :cheers:

What should be and what is appears to generally be different.  There are 2 affordable wind tunnels and a shit-load of software out there that can measure and predict.  Results can be affected by actual things like traction and the actual power you make on the race course.
Title: Re: Drag coefficient fundamentals
Post by: Bob Wanner on November 15, 2016, 10:23:37 AM
Jack,
Was that Francisco # .9 or .09  ? I still feel more lurker than racer on this list in the talks about anything Aero. I do think a wind tunnel session the best investment, info  gained second only to actually racing on a long straight. And, as for frontal area POVs, a F104 and a full grown pick-up  truck might have similar frontal areas, but which is more slippery  ?
Title: Re: Drag coefficient fundamentals
Post by: Jack Gifford on November 16, 2016, 01:27:45 AM
Thanks- more stuff to ponder.

jacksoni- I have yet to do it, but I'm hoping to fairly accurately measure the cross sectional area of my lakester. I'll take straight-on photos as you did (at each side and at top and bottom), but I'll only use them to spot which parts of the car comprise its projected outline. Then I can individually measure those parts and get a total area number.
When you mentioned area being reduced by lowering a car, I assume not an open-wheel car? I don't see that lowering a lakester could change its area.

Bob- Don Francisco published a table of representative drag coefficients of various shapes, including at the low end the Auto-Union long-time record holding streamliner @ 0.08, and at the high end a flat plate @1.1x. A figure of 0.9 was shown for a typical drop tank lakester of 1953 (I don't know whether that would have been with wheel discs or with old Kelsey-Hayes wire wheels).

1953 was not the stone-age of aero knowledge. Especially in southern California, where wind tunnel data was not all that difficult to obtain. And engineers of the time were just as well grounded in Newtonian physics (including fluid dynamics) as any are today.

Title: Re: Drag coefficient fundamentals
Post by: jacksoni on November 16, 2016, 08:58:27 AM
Jack- yes, it was not an open wheel car. More or less stock production body. From a wind tunnel standpoint the space under the car is included in the frontal area, at least as I understand. A very low front air dam nearly kissing the ground certainly makes it so.   

There are several online programs that allow tracing around an irregular object and from that it calculates the area for you. I tried this with two different programs and got same problem with both. I had an accurate reference length in the photo and both the car with some known measurements and in this case the garage door, in which the car was sitting. The programs would not consistently show the known measurements as accurate when scaled to the reference length so the area calculations were bogus. I never figured what I was doing wrong and so we were left with the HxWx.85 deal. Bottom line the drag of the car in the wind tunnel using the calculated area produced a Cd number. Changing the car improved ( or not) that number as the drag changed. That is what we wanted. Don't care if the area or the Cd is "accurate", the drag improved and that was the goal.

Your plan to accurately measure the components of the car should work well to give a baseline. But to really come up with a Cd so you can guess at performance potential after you get your engine dyno'd is going to be harder. I am sure there are folks out there who have had their lakesters in a wind tunnel. Have not seen any of them pipe up with numbers.  :-(  So, we will continue to follow your build and look forward to the Great White Dyno giving the real answers.  :-D :cheers:
Title: Re: Drag coefficient fundamentals
Post by: Sumner on November 16, 2016, 10:28:11 AM
The following is from a page on my site where you can download various spreadsheets.  The spreadsheet it refrences is one where you can enter one set of data for frontal area/CD for the body and another set for the wheels/tires.......


Quote
Horse Power Needed For A Lakester or a Car to Run a Certain MPH -- Need to know how much HP you need to run a certain speed? This spreadsheet uses the formula from the spreadsheet above this one to figure total HP needed based on the frontal area/Cd for the body independent of the tires used and another frontal area/Cd for the tires/wheels. It combines the HP needed to drive both to the given speed.

I've found Cd's for tires that have the width/diameter ratio similar to Goodyear Land Speed tires with Cd's of between .18 to .58. So take your pick. I'd use numbers between .45 and .60 myself as that way you will be erring on the conservative side.

The Spreadsheet will also give you figures for the weight needed on the drive wheels for different salt conditions in order to get the HP/Thrust to the ground since you don't have the same traction as pavement.

"One point-- the "weight" required on the rear wheels doesn't have to be just ballast weight, rather it is the total of the weight plus the aerodynamic downforce. This can be really significant at high speed." Regards, Neil

NOTE: For a car or streamliner don't fill in the tire inputs.
( HP Needed For A Lakester- Excel Spreadsheet Format ) Version: 10-13-13

Here is a link to the page where you can download the spreadsheet if you so desire.....

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#HP needed for A Lakester

Sumner
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on November 16, 2016, 01:27:41 PM
Sumner- thanks for putting this up, I am sure it will be very helpful. The area of all the various pieces surely can be measured pretty well. Do you think that the wide variation of Cd of the tire samples is due to testing variations/difficulties or in fact different (though the dimensions seemed similar to the LSR tires as you mentioned) tires with similar sizes?  Also, what are your thoughts about the various components affecting the drag of adjacent parts? As in the "whole is greater (or less than as the case may be) than the sum of its parts?
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on November 16, 2016, 01:48:11 PM
 I was digging around in an old text on fluid dynamics that has a section on drag of motor vehicles. There is a bit on the 1934 Auto Union race car (google it for info) photo attached. This is at least similar to a lakester with respect to general lay out/shape and exposed wheels and wheel and tire size. There is more suspension in the breeze than a current lakester and it is bigger with a frontal area of about 12.4ft^2. A significant difference is radiator air flow which we know has a big effect. Anyway a wind tunnel developed Cd was .65 with all this. With wheel and axle fairings, Cd was .50. I am sure it would be much less with radiator blocked.

Just for fun comparison.
Title: Re: Drag coefficient fundamentals
Post by: WOODY@DDLLC on November 16, 2016, 02:38:54 PM
I debated whether to add the Cd to that chart but since I did and you "caught" me I'll elaborate a bit.

One of the issues [not really a problem] with computer models is they are perfect!

None of the lumps, bumps, rivets, waviness, seams, warts, wrinkles or variations of real world parts. So that model says if those parts were exactly those shapes and they were placed exactly the same in a perfect wind tunnel this could be the potential Cd. Cd is a dimensionless number! WTF does that mean? We can measure density, frontal area and velocity but there is no number to measure shape - hence a dimensionless number is derived that enables us to compare different shapes. If the same methods and measurements are used to compare shapes then the "value" of that number has no meaning other than one shape is better than another shape. As has been stated - don't get too hung up on that dimensionless number!

Think of a pressure gage with no increments - if the needle goes CCW that's good but if it goes CW that's bad or vercey-visey! It still gets me the improvements I want just not quantified to any standard.

So the new chart show what happens if when we don't have perfect parts. So if we fudge some of the drag values to guesstimate what might happen if the tank is not as slippery or the tires are bigger or a different shape or you have a roll cage hanging out in the breeze you can see that the Cd goes south in a hurry. In all cases the tires and axles still have the most drag but the tank is getting pretty bad at the end. Wonder what some F1 tires would do? Double or triple the tire values and you will have an idea. If you can't change the shape you have to make it smaller.

Me thinks that just confirms many of the real world comments already stated in this and other threads.

As my byline says: All models are wrong but SOME are useful! That's my story and I'm sticking to it!  :x
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on November 16, 2016, 03:25:13 PM
In at least one of my earlier posts I said that Drag equaled area times the Cd. This is incorrect. It is proportional to the A x Cd. The velocity of the fluid (air in this case) squared and the density of same also are involved. My apologies.

Woody: Thanks for the reality check. I love computer horsepower. So much easier than building an engine. If only we could just send the results to the proper sanctioning body and clear a space on the trophy shelf.....
Title: Re: Drag coefficient fundamentals
Post by: Sumner on November 16, 2016, 04:39:00 PM
....Do you think that the wide variation of Cd of the tire samples is due to testing variations/difficulties or in fact different (though the dimensions seemed similar to the LSR tires as you mentioned) tires with similar sizes?

Good question and one I don't have an answer for as the CD's are all over the place.  Maybe it has something to do with how they hold the tire in the tunnel.  How about 10 of us put in $100 each and get some real data from a wind tunnel on various tires/wheels, with and without moon disks?  We could share the data between us.

....... Also, what are your thoughts about the various components affecting the drag of adjacent parts? As in the "whole is greater (or less than as the case may be) than the sum of its parts?

A good question for Woody.  I would recommend getting THE LEADING EDGE ....

https://www.amazon.com/Leading-Edge-Engineering-Performance/dp/0837608600

... not cheap but if one gained one little improvement it would be then considered a cheap source.  Most of the book is over my head :cry: (Rex understands it  :-)  ), but they summarize and that proves useful.  Especially parts about where things attach to the body.  In my case I have side pods, similar to Hammon's old car, and similar to how wings attach to a fuselage and they have some good info on that,

Sumner
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on November 16, 2016, 09:20:03 PM
Well, the guy at A2 told me they had been testing garbage cans so I guess a series with wheels only would be cool. 8-) The 10x100 will just cover the two hour minimum.. Best have ducks in a row to make changes. Hardest being how to hold the tire in place without the hold down interfering. They do use ratchet straps over the front wheels. Maybe a really thin wire strap deal could do it.
Title: Re: Drag coefficient fundamentals
Post by: WOODY@DDLLC on November 16, 2016, 10:01:24 PM
A2 uses small, compact straps on the front tires with a deflector to "simulate" the tire rolling. When you change tire sizes or wheel discs you find out how much HP you gain or lose real quick!  :cheers:
Regarding various components - it's a 3-dimensional interactive problem - leading to the two most common wind tunnel comments: "Wow, look at that!"  :-o and "I have no idea!"  :?
TEST - DON'T GUESS!  :cheers:
Title: Re: Drag coefficient fundamentals
Post by: Rex Schimmer on November 17, 2016, 05:39:21 PM
Jack Gifford said "When you mentioned area being reduced by lowering a car, I assume not an open-wheel car? I don't see that lowering a lakester could change its area." One of the things that you need to consider when lowering a tank (or any round bottom car) is that as the bottom of the tank gets close to the ground it begins to choke off air flow and can and will cause the attached air in the rear lower region of the tank to become attached which is the highest drag condition. Being low looks neat but it may not be fast. As I always reference: Look at the Contrivance modified roadster as a great example of providing sufficient ground clearance to allow the air to flow underneath it. (If you don't happen to know, the Contrivance modified roadster holds all of the gas records from E through AA plus some blown records also!)

Rex 
Title: Re: Drag coefficient fundamentals
Post by: Stan Back on November 17, 2016, 07:00:27 PM
I don't know aero from arrow.

But years ago, a friend of mine built a bike liner using what he believed were the aero theories used for the wing and belly tanks.  He'd found out that they were placed one radius below the effected body structure of the wings -- to be neutral.  So when they were jetisoned, the plane did not react to them.

The bike was underfunded and was designed for two Kawasaki triple 2-strokes and only ran with one Harley, so who knows what it ever proved.

(Disclaimer -- most or perhaps all of this is repeated with a failing memory.)

Stan
Title: Re: Drag coefficient fundamentals
Post by: Rex Schimmer on November 18, 2016, 02:05:48 PM
I said "tank to become attached which is the highest drag condition" I meant to say "unattached"! Attached flow is low drag unattached flow is high drag. As usual I will blame it on age! at my age everything has something to do with age!

Rex
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 22, 2017, 10:30:02 AM
A2 uses small, compact straps on the front tires with a deflector to "simulate" the tire rolling. When you change tire sizes or wheel discs you find out how much HP you gain or lose real quick!  :cheers:
Regarding various components - it's a 3-dimensional interactive problem - leading to the two most common wind tunnel comments: "Wow, look at that!"  :-o and "I have no idea!"  :?
TEST - DON'T GUESS!  :cheers:

Woody I suspect that tire "addition" is an attempt to get wheel lift to match rolling road conditions. I wouldn't put much stock into drag values off it.

It's amazing how many of the community have been steered towards using this style of tunnel. We need to have a serious discussion before someone gets hurt!
Title: Re: Drag coefficient fundamentals
Post by: Stainless1 on August 22, 2017, 11:55:14 PM
Well... Start having it right here if there is one to be had  :|
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 23, 2017, 11:21:20 AM
Well... Start having it right here if there is one to be had  :|

As with any tool, if used incorrectly you can take away the wrong answers.

My concern is a seeming lack of transparency on some key factors with this design of tunnel:

Blockage Ratio
Reynolds Number
Static Pressure Gradient
Boundary Layer Displacement Thickness

They may be doing everything they can to advise customers about the differences they might see between the tunnel and the track, I just don't know.

And that's what makes me nervous.
Title: Re: Drag coefficient fundamentals
Post by: Peter Jack on August 23, 2017, 11:46:12 AM
Well... Start having it right here if there is one to be had  :|

As with any tool, if used incorrectly you can take away the wrong answers.

My concern is a seeming lack of transparency on some key factors with this design of tunnel:

Blockage Ratio
Reynolds Number
Static Pressure Gradient
Boundary Layer Displacement Thickness

They may be doing everything they can to advise customers about the differences they might see between the tunnel and the track, I just don't know.

And that's what makes me nervous.

It might be more constructive if you based your insinuations on fact instead of pure speculation. Do a little research first if you're going to make spurious remarks.

Pete
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 24, 2017, 01:13:33 PM

It might be more constructive if you based your insinuations on fact instead of pure speculation. Do a little research first if you're going to make spurious remarks.

Pete

That's my exact point. What are the facts? Lets see a tunnel commissioning report, lets see the data reduction methods. Q control etc.

I have done 10 years of aerodynamic research in many different tunnels, scale and full size, rolling road and static ground. That's why I am concerned.

It could be I have no reason to be concerned, it could be that full disclosure of the testing limitations is fully explained to all customers. But I all I see is pictures of 2 and 300 mph (and some 400 mph) cars in a tunnel only capable of running 130mph with a blockage ratio of over 20% and the tail section of the car way into the collector of the tunnel in some cases, with what looks like a distributed suction system in the floor on a static ground. With comments like "some good downforce gains found".

Perhaps it's not me who should be doing a little more research..... 
Title: Re: Drag coefficient fundamentals
Post by: Stainless1 on August 25, 2017, 12:30:52 AM
So go to either of the available tunnels and evaluate their methods, procedures and data, we will expect a full report from your expert evaluation...  :roll:
 :cheers:
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 25, 2017, 09:44:21 AM
So go to either of the available tunnels and evaluate their methods, procedures and data, we will expect a full report from your expert evaluation...  :roll:

Someone invite me to one of their tests as an observer. Or someone send me their data for review.

I won't publish any data in public.
Title: Re: Drag coefficient fundamentals
Post by: Rex Schimmer on August 26, 2017, 02:30:33 PM
I think you have brought up some very valid questions regarding many of the wind tunnels now being used by landracers. Standing in the tunnel with a smoke wand and the air speed probably around 50-60 mph really doesn't show much from a data point of view. I have also questioned the "blockage ratio" of many of the tunnels we see on various videos, only being a "side line kibitzer" and not a seasoned wind tunnel user, as Bratfink appears to be I have read enough to see that some of the present tunnels are questionable with full size cars. With the always improving CFD "world" I think that I would sooner give Woody some money to crank my little lakester through his computer if I need aero data.

Rex
Title: Re: Drag coefficient fundamentals
Post by: kiwi belly tank on August 26, 2017, 02:52:41 PM
That's about my line of thought too Rex! I do all my aero through experience & logic gained over the years & there's a few here that have taken my advice on their cars & picked up MPH.
  Sid.
Title: Re: Drag coefficient fundamentals
Post by: Ron Gibson on August 26, 2017, 04:59:18 PM
Wonder what it would cost for my street roadster?  Ha,Ha,Ha, Ha,Ha, :dhorse: :dhorse: :dhorse:
Kidding aside I see some of Bratfiks points, but I think smoke could tell of serious problems. Other than that I agree completely with Rex and let Woody do it.

Ron
Title: Re: Drag coefficient fundamentals
Post by: Slide on August 27, 2017, 11:25:56 AM
I'm Just here to learn.

But didn't redbull formula 1 team just experience this issue at the beginning of the season? The data collected from their wind tunnel did not manifest the same in real life? To the point that it was a significant detriment in races. And then they finally figured out how to adjust what they saw on the tunnel data or change something with the tunnel... and now their cars are finally picking up pace.

Different type of racing but it's still 4 wheels and a soap box punching through the air.
Title: Re: Drag coefficient fundamentals
Post by: Seldom Seen Slim on August 27, 2017, 11:42:20 AM
Ron, you've asked a good question - What does it cost to spend a session in a wind tunnel?  How many bucks do you need?

I dunno.  Back when the A2 tunnel (guy) was active on this Forum I seem to remember a cost in the $500/half day area.  That's only a hazy memory, though, and fresh information would be welcome.  I have no idea what Layne and Tom are charging at the Darko Tunnel - only that a lot of racers (and others) are using it - but they've got open times available.
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on August 27, 2017, 12:10:56 PM
Ron, you've asked a good question - What does it cost to spend a session in a wind tunnel?  How many bucks do you need?

I dunno.  Back when the A2 tunnel (guy) was active on this Forum I seem to remember a cost in the $500/half day area.  That's only a hazy memory, though, and fresh information would be welcome.  I have no idea what Layne and Tom are charging at the Darko Tunnel - only that a lot of racers (and others) are using it - but they've got open times available.
I think is (still) $495/hour with 2 hour minimum. In October spent 6+hours and made 24 runs there. Runs are pretty quick. Changes between not so much. Need to be prepared with lots of tape, foam, strips, aluminum, cardboard etc etc and have a good idea what you want to do before arriving to save time.
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 28, 2017, 10:32:31 AM
Thanks for opening up the debate. I was getting worried there that I was ostracizing myself by raising the concern.

A2/ Darko (basically the same tunnel design) are certainly an affordable tunnel for trying out ideas. And they both have some really smart guys working for them.

The racing industry in the US has moved towards using Windshear which is significantly more expensive. But does correct a number of the inherent issues with a small closed section fixed ground tunnel. It's not perfect, but it's closer.

I am a little out of touch with F1 these days, but it doesn't surprise me to hear of RedBull's issues. Rules limit full scale testing time, and scale model testing is under increased limitation. Most teams are doubling down on CFD, but mistakes are still made in basic validation between the 3. In an f1 car all it takes is a slight separation point miscalculation and you go from the front of the grid to the back.

CFD is great tool (Woody you can forward the commission check later ;-) ) However, without validation to real or tunnel data you might as well be P*&$ing in the wind. The entire premise of CFD is based around solving 2 unknowns. In order to get one unknown (Lift/Drag) you have to make up the answer to the other. This is refereed to as the turbulence model. And which model you choose wildly changes the answer of the Lift/Drag. Not to mention all the other variables, Steady State vs transient etc. It's a real minefield. On a vehicle program (in my day job) I might do 10-15 transient CFD runs before I am happy the model correlates with my tunnel, not many people can afford 15 runs to throw away for validation. 

Those who say they prefer to test on the track are not as insane as they sound. With some data logging and some other basic principles a lot of data can be gathered about the aerodynamic performance of the car in the real world.

In short, continue to test and use tunnel facilities and CFD. But don't do so assuming the answers are right. always be critical of what you are looking at.

I am not trying to set myself up here, but if people feel they need help to understand numbers or procedures I'll try to help. I don't have all the answers, but I'll do my best.

I just want racers to be safe. Fundamentally these cars are on the edge of stability, and decisions made in a tunnel or CFD without knowing all the facts could push you over that edge. You don't know what you don't know. And the first key to solving a problem is knowing what you don't know.
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 28, 2017, 02:41:31 PM
Ron, you've asked a good question - What does it cost to spend a session in a wind tunnel?  How many bucks do you need?

I dunno.  Back when the A2 tunnel (guy) was active on this Forum I seem to remember a cost in the $500/half day area.  That's only a hazy memory, though, and fresh information would be welcome.  I have no idea what Layne and Tom are charging at the Darko Tunnel - only that a lot of racers (and others) are using it - but they've got open times available.
I think is (still) $495/hour with 2 hour minimum. In October spent 6+hours and made 24 runs there. Runs are pretty quick. Changes between not so much. Need to be prepared with lots of tape, foam, strips, aluminum, cardboard etc etc and have a good idea what you want to do before arriving to save time.

On the subject of brokered test time:

So what is available?

I am deliberately not releasing any prices here but needless to say they are all well above $500/hour.

What could be done for these high cost facilities is that a bunch of guys running similar class cars could all club together buy a shift and all share the data. Extend that idea and for a scale model facility, a reconfigurable model could be developed to fit a number of different classes, the cost shared among all the teams interested. Then combined tests could be had for those on a a budget and those wanting private testing could buy their own shifts. I had approached this idea previously but the facility in question didn't want to commit to the cost of a model without a firm commitment of sales.   

But the option is there and it can be revisited if there turns out to be the demand.
Title: Re: Drag coefficient fundamentals
Post by: Stainless1 on August 28, 2017, 10:22:11 PM
BF... ever been to Bonneville?  We do not have cars like nascar or F1, everything identical so info can be shared.... every vehicle running in the same class is different... even roadsters....
There may be some well funded teams, but rarely factory teams run, and when they do they often lose to backyard guys.... ask GM
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 29, 2017, 09:02:00 AM
BF... ever been to Bonneville?  We do not have cars like nascar or F1, everything identical so info can be shared.... every vehicle running in the same class is different... even roadsters....
There may be some well funded teams, but rarely factory teams run, and when they do they often lose to backyard guys.... ask GM


I may not be a veteran like many here but I have been most years since 08. I have worked on 4 cars in various different capacities from mechanic to driver to crew chief. No records yet, but working on it.

I disagree with you previous statement, many of the classes are similar enough (particularly in the roadsters) that a modular model system could be developed to replicate a large proportion of the field. They don't have to be identical to be aerodynamically similar. The point is is that that many of the basic tuning tips used by guys on the salt have a certain aerodynamic effect, it'd be quite useful for teams to understand the aero implications of making those changes to a roadster style vehicle. And having worked in Indycar, Nascar and on the periphery of F1 I can tell you the cars are far more dissimilar than you might imagine.

By factory teams I am not really referring to JCB, GM or any of that, I am referring to teams that through network contacts or sponsorship have managed to wrangle test time at a private facility.   
Title: Re: Drag coefficient fundamentals
Post by: Rex Schimmer on August 29, 2017, 04:56:13 PM
BF,
What are your thought of testing, either wind tunnel of CFD, different parts of a car. My son and I have a lakester which means the wheels and supporting parts are in the wind, how about making say a front wheel assemble, wheel, spindle and axle, and then running that in a wind tunnel, then make changes to things like wheel covers, spindle axle fairings etc. and one of my real interest the distance the wheel if from the body? The wheels and axles of a lakester are the largest contributor aero drag so maybe just looking at them would (could) be effective.

Rex
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 29, 2017, 06:20:59 PM
BF,
What are your thought of testing, either wind tunnel of CFD, different parts of a car. My son and I have a lakester which means the wheels and supporting parts are in the wind, how about making say a front wheel assemble, wheel, spindle and axle, and then running that in a wind tunnel, then make changes to things like wheel covers, spindle axle fairings etc. and one of my real interest the distance the wheel if from the body? The wheels and axles of a lakester are the largest contributor aero drag so maybe just looking at them would (could) be effective.

Rex

Excellent question!

More than just position of the wheel to body, you also want to consider the effects of the wheel rotating.

Single component testing used to be done in CFD when computing power was minimal and model complexity and compute times were an issue. These days it has fallen out of fashion. First it expanded to half car models and now full car. In the aircraft industry wing section 2d component testing resulted in the NACA profiles we all know and rear wing element assembly testing has been done by Indy teams for many years.

Benefit:

Cost:

The type of test you are suggesting I would set up in CFD using a Design Of Experiments (DOE) method. I would use the whole car model and in my meshing software create a morph of the suspension system allowing me to increase and decrease the offsets. I would run a number of increments of this and use another program to mathematically interpret the sensitivities of the certain variables. That way I don't have to do 100 CFD runs to get the full picture of the results.

I would then pick a few of the best outputs from that analysis to validate in the wind tunnel.

I realize that is way outside the scope of what you are asking. Just trying to give you an impression of how the industry might approach it.

Since wheels and suspension are so important in Lakesters. It might well be worth considering building a half car wind tunnel model to tune just that. And this sort of thing can be done in the back yard. If you have a body, you might well have a body mold which could be re-purposed as a wind tunnel buck. A simple suspension geometry could be fabricated, it doesn't need to hold the loads of the real car, just have the aerodynamic shape. The most complex thing is deciding if you want to rotate the wheels. Universities often have small wind tunnel facilities and no end of students looking for projects to help with grades. Might be worth approaching your local school. Wind is easy to create, just need to figure out how to keep it straight and find a novel method for measuring the loads. The Wright brothers did it, and they started as Bicycle mechanics. I have built my own simple tunnel to show school kids about aerodynamics, I might throw some plans up here for others to play with and expand on.       
Title: Re: Drag coefficient fundamentals
Post by: Stan Back on August 29, 2017, 06:48:47 PM
This'll show you how sophisticated I am!

Whataya think about testing models in a captive hydraulic environment by measuring the overall pull of the flow?  (Huh?)
Title: Re: Drag coefficient fundamentals
Post by: Elmo Rodge on August 29, 2017, 06:58:27 PM
Stan, since I moved to Utah I don't have my swimming pool anymore. I sure thought about that, though.  :cheers:
Wayno
Title: Re: Drag coefficient fundamentals
Post by: manta22 on August 29, 2017, 07:13:59 PM
This'll show you how sophisticated I am!

Whataya think about testing models in a captive hydraulic environment by measuring the overall pull of the flow?  (Huh?)

Stan;

I thought about that, too. How about letting a model roll down an underwater ramp and timing how long it takes to travel between two marks on the ramp. It won't give you quantitive data but it should show up differences between model shapes. The bigger the model the better.

Regards, Neil  Tucson, AZ
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on August 29, 2017, 08:53:14 PM
This'll show you how sophisticated I am!

Whataya think about testing models in a captive hydraulic environment by measuring the overall pull of the flow?  (Huh?)
I have seen exactly this done with a model using some fancy apparatus nature of which I forget to make bubbles to show the flow. I don't remember if the guy measured forces or just the path and turbulence. Is on YouTube. I'll see if I can find it. Bratfink likely knows about it and may chime in about it.
Title: Re: Drag coefficient fundamentals
Post by: Jack Gifford on August 30, 2017, 12:34:19 AM
So that's what Breedlove was doing in the canal! :lol:
Title: Re: Drag coefficient fundamentals
Post by: kiwi belly tank on August 30, 2017, 01:25:01 AM
From my novice standpoint, that's the way I've always dealt with aerodynamics, using fluid dynamics. It allows me to get a better picture of whats going to happen & after all, air is a fluid.
  Sid. (Hillbilly with a hammer!)
Title: Re: Drag coefficient fundamentals
Post by: Seldom Seen Slim on August 30, 2017, 07:24:17 AM
I do remember Scott Guthrie telling me how the Charlie Toy bodywork had been designed using water (water tunnel).
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on August 30, 2017, 09:05:25 AM
This'll show you how sophisticated I am!

Whataya think about testing models in a captive hydraulic environment by measuring the overall pull of the flow?  (Huh?)
I have seen exactly this done with a model using some fancy apparatus nature of which I forget to make bubbles to show the flow. I don't remember if the guy measured forces or just the path and turbulence. Is on YouTube. I'll see if I can find it. Bratfink likely knows about it and may chime in about it.

Here is the youtube link. This is the first of maybe 8 or more videos this fellow made using his water tunnel to illustrate flow around several cars. An accurate model is needed.  https://www.youtube.com/watch?v=quDLzxmJl5I
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 30, 2017, 09:45:17 AM
This'll show you how sophisticated I am!

Whataya think about testing models in a captive hydraulic environment by measuring the overall pull of the flow?  (Huh?)

Water tunnels (or tow tanks) are an excellent way to maintain the Reynolds number in small scale without having to flow at very high speed. Essentially increase density instead of velocity to balance the dimensional equation. To save power many of the older water tunnels were gravity fed, as in the x plane of the test section was vertical.

Here's the problem; friction coefficients (viscosity) for water are much higher than air. Thus wake distortion, separation points and some other things behave differently to how they would in air. Still a valid test form though.   
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 30, 2017, 10:08:32 AM

Here is the youtube link. This is the first of maybe 8 or more videos this fellow made using his water tunnel to illustrate flow around several cars. An accurate model is needed. 

Yup, this is like what I was talking about. However, I wouldn't describe that as an accurate model. No cooling pack, no open grilles, no open wheel arches with rotating wheels, no underbody detail, no cut line sealing. All of which have an impact on the aerodynamics. But this is a great tool for basic shape development.

We actually do flow visualization similar to this in the wind tunnel but it requires lasers and high speed cameras. And what can't be made better by using lasers  :evil:     
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 30, 2017, 10:11:18 AM
Stan;

I thought about that, too. How about letting a model roll down an underwater ramp and timing how long it takes to travel between two marks on the ramp. It won't give you quantitive data but it should show up differences between model shapes. The bigger the model the better.

Regards, Neil  Tucson, AZ

Why not do it in air? Simple Coastdown testing. Run it up to speed, shut it off and shift to neutral see how long it takes to stop. You have to be careful to keep all the variables as constant as possible, but people do use this method.
Title: Re: Drag coefficient fundamentals
Post by: manta22 on August 30, 2017, 11:18:49 AM
I proposed doing it with a model, not a full-size vehicle. It is far easier to evaluate different model shapes in a swimming pool. Yes, I know people use a "coast-down" test but that is using a car that is already built.

Regards, Neil  Tucson, AZ
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 30, 2017, 05:18:55 PM
I proposed doing it with a model, not a full-size vehicle. It is far easier to evaluate different model shapes in a swimming pool. Yes, I know people use a "coast-down" test but that is using a car that is already built.

Regards, Neil  Tucson, AZ

For shape development that is defiantly an option.
Title: Re: Drag coefficient fundamentals
Post by: Seldom Seen Slim on August 30, 2017, 05:24:56 PM
Yes, it IS!
Title: Re: Drag coefficient fundamentals
Post by: Stan Back on August 30, 2017, 05:35:51 PM
Is that defiantly or definitely?
Title: Re: Drag coefficient fundamentals
Post by: jacksoni on August 30, 2017, 07:58:34 PM
Is that defiantly or definitely?
Why yes! I believe it is!   :cheers:
Title: Re: Drag coefficient fundamentals
Post by: Bratfink on August 31, 2017, 08:25:37 AM
Is that defiantly or definitely?
ha ha "defiantly". I didn't notice auto correct changed that.
Title: Re: Drag coefficient fundamentals
Post by: Seldom Seen Slim on August 31, 2017, 08:44:52 AM
The "defiantly/definitely" error is a very common one I've seen countless times, so maybe it is an auto-correct problem.  This validates my feeling that auto-correct/auto-spell software isn't the end-all that'll make proofreading obsolete.

Thanks for letting us yank your chain.  Stan and I defiantly enjoy the exercise. :cheers: :cheers: :evil:
Title: Re: Drag coefficient fundamentals
Post by: floydjer on September 06, 2017, 09:19:23 AM
Gosh Jon, Ewe keep picking on the gnu  guise and their going two go aweigh................... :wink:
Title: Re: Drag coefficient fundamentals
Post by: tallguy on March 18, 2024, 12:55:44 AM
So that's what Breedlove was doing in the canal! :lol:

Some of the "fluid(s)" involved there may have even been considered "semi-solid".