Aero Design Senior Project Community Ideas

[Aeroguy]

I'm not really sure yet. My best guess is that I'm shooting for the best downforce to drag efficiency. A boat tail sounds interesting I will look into it. I am somewhat familiar with the aero civic. And yeah, I have heard of the reynolds number. Since the project is mainly concerned with the difference between various setups, the number would be constant test to test. Please correct me if I am wrong.

[kach22i]

I put a little time into this attempting to backtrack on my thoughts.

Background to the concept I presented is needed - and right now.

Biomimicry
Here's 7 of the Best Examples of Biomimicry and Nature-Inspired Design | Digital Trends

Many of our modern aerodynamic designs rely on rather basic principles. To obtain optimal lift and minimal drag, sleek edges and clean lines are key. However, throughout the animal kingdom, many species, capable of exceptional lift. The Humpback whale, for example, uses bumpy, tubercle fins for propulsion — which seems rather counterintuitive.

A Harvard led research team determined that these nodules, enable the whales to choose a steeper “angle of attack.” The angle of attack is the angle between the flow of water and the face of the flipper. With Humpback whales, this attack angle can be up to 40 percent steeper than a smooth flipper. Due to these small ridges, sectional stalls occur at different points along the fin. This makes a full on stall much easier to avoid.

Tests conducted by the U.S. Naval Academy, using model flippers, determined these biomimetic fins reduced drag by nearly a third and improved lift by eight percent overall. Whale Power, a company based in Toronto, Canada has already capitalized on this latest tubercle tech. According to MIT, Whale Power’s biomimetic blades help generate the “same amount of power at 10 miles per hour that conventional turbines generate at 17 miles per hour.”

Aerodynamics by George Kachadoorian | Photobucket


And back in 2007............
New propulsion sytems for ships - Page 23 - Boat Design Forums


The fan/propeller from 2007 never came into being unfortunately.

The general idea here is to test a straight edge rear spoiler against a tubercle fin edge type.

The goals in mind would be the various angle of attack (vehicle in turns, going up and down hills etc.........) and measure the differences.

These straight edged spoilers we are so familiar with are perhaps just a reflection of man's linear thinking and surrender to manufacturing ease. Nature has some different things in mind apparently, and with 3D printing we can now give it a try.

2008
Bumpy whale fins set to spark a revolution in aerodynamics
http://newatlas.com/bumpy-whale-fins...dynamics/9020/

March 21, 2008 It seems despite man's endless ingenuity and the incredible modeling power available to inventors through CAD systems, we keep looking to nature to find ever more effective ways of doing things. Millions of years of evolution's trial and error approach have resulted in some incredibly effective designs that are ready to be incorporated into human constructions if we can only identify, understand and replicate them. The random-looking bumps on the humpback whale's flippers have just inspired a breakthrough in aerodynamic design that seems likely to dramatically increase the efficiency and performance of wind turbines, fans, flippers and even wings and airfoils. WhalePower's tubercle technology seems like nothing less than a revolution in fluid dynamics.

And the boat tread the article above was posted in including some discussion:

Whale of an idea for marine applications?
http://www.boatdesign.net/forums/boa...tml#post192994

[kach22i]

Originally Posted by Aeroguy

The actual stated goal (or really the goal that got approved) is to find the optimal wing/spoiler/rear trunk attachment.

1. You will need a baseline testing of the car model as stock, no attachments at all.

2. I suggest measuring a baseline for a standard aftermarket wing.

3. I suggest measuring a baseline for a standard rear spoiler similar to the one on your Z-car.

4. Once you have this baseline data you will have something to compare your new fangled designs to.

5. A definition of victory must be defined, a definition of "optimal" must be established. Is it to be low drag, high down-force, or some balance of the two set by some sort of complex math theorem or simple ratio?

[Grant-53]

If we are talking street cars versus racecars then look for minimum drag at zero lift. See materials written by Dr. Joseph Katz.

[freebeard]

I asked about the power just so you'd think about it as a parameter. 40mph is viable.

8x8 is 64sq in. The closest model I have to hand is a diecast 1/24th VW Type III, close enough to a Civic. It is 2.75x2.25in or 6.1875sq in. That a blockage ratio of about 10%.

I will defer to another member, aerohead, on what the maximum blockage ratio should be.

(funny coincidence: the guy in this video is one of my mentors).

He does good work, you could ask him.

[Aeroguy]

Thanks for the info on Biomimicry. I'm not trying to complain or anything but the CAD software I use (Inventor) really does not enjoy making waves.

Originally Posted by kach22i

1. You will need a baseline testing of the car model as stock, no attachments at all.

2. I suggest measuring a baseline for a standard aftermarket wing.

3. I suggest measuring a baseline for a standard rear spoiler similar to the one on your Z-car.

4. Once you have this baseline data you will have something to compare your new fangled designs to.

5. A definition of victory must be defined, a definition of "optimal" must be established. Is it to be low drag, high down-force, or some balance of the two set by some sort of complex math theorem or simple ratio?

1. That was one of the very first things I did.

2. I sort of have that, there isn't really a "standard" rear spoiler but I could certainly test the more popular ones like for instance this one is the same type of spoiler our racecar had before we slapped a huge superbird style wing to it (story for a different time).

3. I don't have a 240z, I wish. There was I think a spoiler that come with the civic model but it involved cutting into the trunk so that's out.

5. For the time being, the ultimate goal question is sort of one of those bridges I will cross when I get there. I think the downforce to drag ratio is an ok target.

I am very new to the world of wind tunnel testing and thus am relatively unsure of how the percent of blockage would effect results. Personally I think that as long as all the tests are down under mostly the same variables the numbers should be comparable.

[freebeard]

http://ecomodder.com/forum/showthrea...low-34134.html

Originally Posted by aerohead

I don't have my work book with me,nor calculator but here are some considerations:
* a 1/10-scale passenger car tunnel would require a test section cross-sectional area of 4-meters-square minimum,to counter blockage effects.
*a minimum test section airspeed would be 200-mph to reach a critical Reynolds number/turbulent boundary layer.(dynamic similarity).
*at 200-mph,we have 17,600 feet/minute,and with 44-sq-ft of cross-sectional area,we get 774,400-Cubic Feet per Minute (CFM).
*high-speed tunnels have boundary layer thickness issues,and the ground borad must be elevated above the boundary layer,or suctioned off the flooor of the tunnel,as in the real world,the ground has no boundary layer.
*you'd need help from an air conditioning company to determine wall friction coefficients/losses,and total static pressure requirements to determine net horsepower for the air mover(s).
*Alan Pope published a great book on Low Speed Wind Tunnel Design.It's worth an inter-library loan if your school doesn't have it.
*small-scale model features cannot be successfully investigated except in near-full-scale CFD or wind tunnel.(Ferrari is spending 100-hours just to refine a single spoiler in their tunnel,at a cost of $4,000 (US)/hour.[$400,000]
*Texas Tech had a small tunnel and did 1/12th-scale work.With 50-hp (electric) they could get an 80-mph section velocity,not enough for a TBL Reynolds number,and they had to back things up in a water tow tank.

aerohead hasn't posted in a week, he's busy prepping for the A2 wind tunnel. The quote is from a similar thread.

I see elsewhere that blockage should not exceed 5%.

[Grant-53]

Check the local hobby stores or E-bay for 1/24th scale model kits.

[ChazInMT]

A wind tunnel is like using a chain saw to slice onions, you get an idea about what might be happening, but the slices will look dramatically different.

The 3 primary differences between a wind tunnel and real life are,

1st) In real life, the car is moving through air that isn't. (Assuming a calm day) the drag created by the car is due to the energy required to move the air out of the way, and drag it along behind you. So, calm air just sittin there.....along comes a car, the air needs to move sideways/up/down to allow the car to pass. As the air is being shoved out of the way, some of it gets snagged on the car, or pushed along by it, making it move in the direction the car is going. In order for the air to move, you have to add energy to it, that is your drag. In the worst case, air moving at different speeds to get out of the way, and/or be entrained, creates differing air pressures due to the Bernoulli principle. When these differing pressures create a secondary air movement (Vortex) that secondary movement is also felt as drag, because, the air was just sitting motionless and the passing of your vehicle is what made it move, so the only place to get the energy to be moving comes from the car.

In a wind tunnel, the air is moving, and the car tries to slow it down.....can you see the significance? The wind tunnel car stops the air, the rolling car is starting the air.

2nd) The air affected by the car outside extends out many yards in each direction in front of, to the sides, and up. And by affected, I mean made to move....and we remember from before that making air move means energy expended. Behind the car, the air is in motion for many hundreds of yards before it settles back to being still and just sitting there again. Believe it or not, those air molecules hundreds of yards behind are still sort of attached to your car and anything that affects them, will be felt as a change in drag on your car.

In a wind tunnel, the sides and top of the tunnel don't allow the air to move like it would outside, and the air behind is seriously affected too. The rear wake has no opportunity to develop as it would outside, it is either ejected to still air, or turned around to be recycled way before it would have normalized. Either way, you lose how the air reacts outside, and thus have a different drag number.

3rd) The ground plane outside has still air sticking to it, as you drive over it, the air stuck to the bottom of the car is moving in the direction of the car. So outside, there is a shear effect happening, the air is essentially moving in 2 directions.

In a wind tunnel, the air on the ground and the bottom of the car are moving in the same direction....front to back. This is a dramatic difference, the air is moving very differently, thus, a different drag number.

There are other nuanced effects as well I'm sure, but these 3 are fairly glaring.

This is not to say that wind tunnels are complete crap, just that their results may not represent what a cars Cd will be in the wild. However, if you just want to see how one design element variation changes the aerodynamic characteristics of the car, it should work well. If nothing else, you learn a lot in how to conduct experiments, record data, and report the results.

I hope this helps you to understand things a little better. There is way more to aerodynamics than meets the eye. Also, what our intuition tells us is true, may need to be thought out much more deeply to understand the reality of it.

Aeroguy, I'm very heartened by your measured responses in this thread, and you seem to be a wicked smart person as well. With a keen intellect and good people skills, I'm sure you are going to be a massive success in what ever you choose to do in life. People like you make this forum very rewarding to participate in. Thanks for being here.

[aerohead]

Full-size automobiles achieve a stable coefficient of drag at around 20-mph.
Testing mods in full-scale would require at least a 20-mph test velocity.
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At 1/24-scale,do to rules of dynamic similarity (verisimilitude),the air velocity in the test section of your wind tunnel would require a minimum 480-mph to achieve the necessary Reynolds number.This would introduce compressible flow/transonic flow effects which are not encountered in 'real' driving.
Typically,aero testing is conducted with a minimum 1/10-scale,and critical Reynolds number must be factored in.And this is 'rough' testing.
'Winners' from 1/10-scale tests would graduate to 3/8-scale,then winners from that would finally graduate to 1:1 scale testing,required to 'prove' a small structure as with a wing.
Exa Corporation's PowerFLOW CFD can analyze small features but require enormous digital CAD files,require 35-100 parallel computers running simultaneously to 'solve' a single iteration after many hours of run time.
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You can learn some pick and shovel kinds of results at 1/24 so it can be worth playing around with it.