Anyone have 800 Lbs of clay handy? New mythbusters episode

This is all great - but can someone post a pic of the dimpled mod??? I've run through the threads, and still don't see the relevance... Not that I am relevant - just wondering why and how [ame=http://en.wikipedia.org/wiki/Laminar_flow]Laminar flow - Wikipedia, the free encyclopedia[/ame] is implemented -so I/we can test it...

 

I remember seeing an article in pop science a few months back about a company making a dimple skin for cars... MPG-Plus™, Drag Reducing Technology, Improve MPG, Vehicle Wraps, FastSkinz™

 

Ah yes, here we go Today's Mileage Tip: Turn Your Car Into a Shark - Green Car Reports

 

As I understand it, laminar flow is the holy grail of aerodynamics. It's relevant to just about everything that moves, and even some things that don't. The more easily air moves around objects - planes, trains, automobiles, and even buildings - the less fuel we waste.

 

This article may help some Laminar Flow Airfoil You don't need some type of suction to draw boundry air away from a surface to achieve laminar flow, but it is one way. Achieving perfect laminar flow is next to impossible, but as with anything getting close does have real advantages. In the wind tunnel both the B-24 and the P-51 did real well, but in reality manufacturing tolerences, dirty surfaces from operating off of grass strips etc. kept either from achieving laminar flow, but the P-51's real sucess came from it's great range, something it wouldn't have had without that wing.

 

The idea of a rough surface having less drag than a polished one is in theory the rough surface would "hold" the boundry layer of air stationary against the object and the drag would come from one layer of air "shearing" against another layer of air.
A smooth surface on the other hand would have airflow across the smooth surface. Which generates the most drag? I don't know.
I do know for sure that it is normal and easily demonstrated that a well waxed and polished airplane is faster than the exact same airplane is dirty.
What do the dimples on a golf ball do? Well, they are vortex generators Aerospaceweb.org | Ask Us - Golf Ball Dimples & Drag
Like those thing some of you called "roof fins"

 

If somebody really wanted to get serious with this, all you need to determine when airflow separation occurs is a ball of yarn and a roll of scotch tape. You would be surprised how accurate "tufting" a vehicle is.
I would theorize that the lower the CD of a vehicle you have, the less possible gain there would be from using vortex generators to increase the laminar flow.

 

What we need is thicker air. If we can replace air with a low Reynolds number fluid it will be easy to get laminar flow.

Tom

 

The Los Angeles basin. You can actually see the air on a summer day. :madgrin:

 

Hi All,

This has been discussed some on ecomodder. Which is probably the best collection of amateur drag reduction aerodynamics knowedge regarding cars on the internet. They accepted based on the show that an actual improvement in fuel economy occured.

The best reason I have read on there as to why, is that the Ford Taurus used, has the rear window at the worse possible drop angle for aerodynamics. It turns out that you need to keep that angle below about 20 degrees (like a Prius), or greater than 50 degrees for the best drag. This is a published professional aerodynacist result (Hucho?).

Consequently, the flow is seperating on the back glass. And conseqeuntly the dimples helped by delaying the seperation till its flow cross section was less than the cross section of the laminar flow boundary layer further forward on the car.

Some words have been put out. Such as "Bluff". There are very few cars that are not Bluff bodies . The Golf ball is a Bluff body. These are both the same type of animals aerodynamically. Cars can benefit from bluff-body aerodynamics tricks.

Even though the older Taurus looked aerodynamic, it apparently is not.

Fast skinz is a sham. Its been proven not to work. In the Mythbusters episode they were careful to scale the dimples to the same relative size of the car, as they are on a golfball. Shark skin works on ships, where the flow is hydrodynamic. It also works on high-subsonic jets - where apparently the Reynolds number is similar. Its been said that for aerodynamic ground vehicles (such a an Aptera) that the speed would have to be in excess of 250 mph to get into that same aerodynamic regime as a + 20 knot boat hull/fish and high sub-sonic jets. A typical car, even a Prius will always be a bluff body however.

 

Hi All (again),

So where does one place dimples on a Prius? Hard to say. Possibly nowhere. If the flow is laminar down the rear glass, then turbulence will increass drag.

I think some sorta difusher plate from the lowest reach of the rear axle, to the rear bumper skirt , with dimples would be good. That area is poor aerodynamically, I think.

I noticed the other day, (as I have not cleaned my car lately) that I am getting an accretion of stuff just forward of the rear gas cap about 9 inches high, in line with the door handles and between the rear door and the fuel filler door. This apparently is the speration point for the flow on my car. Remember, it might be the same on your car, as I have the flush hubcaps and turbulators on the rear-view mirrors and A-pillar. Now, this is where the car starts to sweep up to the meet the narrower tail. Could dimples behind the rear door opening, at that level help? Or maybe a turbulator behind the rear door handle? Or possibly filling the door handle dimples with foam rubber? Again all hard to say. Is this air coming out from under the car? Could side damns get rid of it ? Could filling in the rear wheel openings get rid of it? Its too complicated for me!

 

You need to get together with rrolf. He has a wind tunnel in his backyard. :madgrin:

 

If your serious, you don't need a wind tunnel, all you need is a ball of yarn and a roll of scotch tape. A video camera is helpful though.
The illustrated guide to aerodynamics - Google Books
Read the part on tufting. The easiest way to tuft is to pull a long piece of yarn the entire length, tape it every six inches or so and cut after taping. The yarn will show exactly where turbelent airflow begins

 

And I have plans to build one in my basement. But then, I have plans for many things that have yet to happen.

 

Me too. :madgrin:

 

I have to show this to my sister in law who bought a car with hail damage... While I am at it, I will ask her about fuel consumption and compare it to spritmonitor.de

I've been thinking of adding an inflatable tip at the back of an RV, like the 'tip' of a droplet of water. In the Netherlands that could even cover the bicycles on the back.
Something like this but inflatable (preferably even inflated by driving speed):


I sold my RV some years back, so not that much chance of testing myself...

I first spoke to mine the day after she turned 19. Not sure I would use that option...