View Single Post
Old 06-10-2020, 04:49 PM   #29 (permalink)
Master EcoModder
aerohead's Avatar
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 11,814
Thanks: 19,042
Thanked 6,101 Times in 3,743 Posts

Originally Posted by 4drviper View Post
Taken the idea from modern airliners' nacelle strakes which is basically a huge VG on the side of the engine parallel to the airflow in level flight. Let's say the angle between the airflow and lengthwise centerline of VG, θ is 0 here. At level flight, the θ is 0 so no vortex is generated and marginal amount of drag introduced.

Now imagine the aircraft still moving level, but pitched up ~20 degrees; the airflow remains the same but the plane is turned up, resulting in θ = 20.

NOTE : In case you didn't know, it took me years until I was able to finally grasp the reality that airplanes rarely fly in line with the centerline of fuselage outside cruise. They pitch up and still slide forward (and downward a bit when landing) to create more lift at less airspeed but less efficiently. Which is why the VG is "under" the bottom of the wing but the air through the VG flows over the wing.

Now how is this relevant to a car? I can't actively pitch my car up or roll it to help turning, but the wind changes and I have to yaw (steer) the car to counteract the sideways forces which is never constant. And VGs on the trailing edge of a body seem to help "pressure recovery" as seen in the airtab wind tunnel civic data.

I've played around with several dozens of airtabs for past 5 years and even had 6 airtabs along the top of the rear window nearly eliminating any gusts when the weather app said up to 30mph.
Here's a little side note. For every airtab placed anywhere on the top or the side of 2007 Acura TL, it decreased my highway MPG pretty linearly by approximately ~0.5%. I tested from 2 to 12 airtabs using OBD2 fuel consumption data on a roundtrip of 7 miles (2 rural exits on the interstate little over 3 miles away) averaging around 21MPG.

Anyway, I concluded that airtabs having 1" height is the thickness of laminar flow at the end of 80ft box, not on a 15ft car. I needed something with less height as these are going to be placed across the hood, along the transverse frontal edges of stagnation bubble in the transition from hood to windshield. Airtabs could work in a pinch, but they're not as efficient (ECOmodder right?) being too tall for the airflow.

So what fits my bill? DIY VGs for general aviation planes.
These are 3/8" high and rigid.
These look about the same but flexible.
Both are less than $1 / piece, less than $1.375(2.75/2) / piece for Airtab.

Since I don't want people tripping and falling on my car's hood and suing me for the "dangerous barbed plastic pieces" on my car, I decided to go with the soft one.

Some advantages
- Designed to work around 60mph (40-60kts in testimonies)
- Clear color (most won't bother to notice)
- Semi-flexible (no injury)
- Removable, semi-permanent, paint safe adhesive unlike airtabs' the "Ultimate Epoxy in Sponge form" which never comes off clean.
- (PROBABLY) not going to induce much drag even if I fail to install it at exactly θ = 0 since they're meant to be on an airplane without inducing too much drag at an angle at all times.

After finding out the direction of airflow with tuft test, they will be placed in a obtuse V-shaped arrangement with the pointy side forward, aligned θ = 0 in no crosswind airflow.
MS Paint drawing to help visualize...

Some visualization of airflow around a sedan in crosswind. From this link on page 8 by Rossitto et. al.

Whatcha think??
I've no knowledge of anyone actually optimizing VG type, number, and location placement outside a laboratory environment.
Also, I believe that if you read the report on the Mitsubishi Lancer Evo VG research, that the TBL was an inch thick at the rear of the EVOs roof.
If you're not running a university supercomputer and Dassault-Exa POWERFLOW CFD software, you can expect nothing from any lesser CFD.
The cheapest wind tunnel you could get into is A2, in Mooresville, North Carolina. Two hour minimum,at a beginning $480/ hour, then $ 500/hour afterwards. And you'll catch hell from a certain member for not spending $4,000/ hour at Gene Haas Racing' moving-floor wind tunnel.
I strongly recommend you look at the other end of the car for mods.
Photobucket album:
  Reply With Quote