learn to love turbulence?

[thentro]

Hi all, first post here. Thats the car I have here, a 2002 Subaru Impreza and I am wondering out-loud about improvements for aero. Some things are obvious like a partial grill block, block small cosmetic vents and so on but my question is about about the air as it travels over the hood, windshield, roof, and back.

Things I am thinking about, come from this article "efluids bicycle aerodynamics"

In section 11, they talk about making a sphere much more aerodynamic by forcing turbulence into the boundary layer of air with a wire on the leading edge of the sphere. Aka the "golf ball" principle. So why is it then, that we put things like vortex generators at the back of the car? Wouldn't it also help to create turbulence right above the windshield, or right below it? Especially if you have a poor aerodynamic shape to start with.

I am especially intrigued about how the glider community uses induced turbulence with "Zig-Zag Turbulator tape".

Any thoughts on this? Do I have it completely backwards? Also I love the oil test they to on the glider wings, it is like the fuzz test but you don't have to drive behind the car to see the final results!

[cfg83]

thentro -

Welcome to EM! Is it possible for you to remove the roof racks or are they frequently "in use"? If you need them, maybe find a way to attach side panels to close the gap under them.

I really like the idea of turbulator tape but I am not sure how to apply them. Also, I think ground driving cars are in a more "hostile" environment than planes. I am thinking that the tape would need to be replaced every X number of months.

CarloSW2

[Gone4]

As with everything in fluid dynamics, it depends. If you introduce turbulence on a golf ball it stalls flow separation. Conversely it induces additional skin friction or parasitic drag effects. Introducing turbulence early into the flow will promote skin friction but also encourage flow separation later as well, potentially. It can also expand the boundary layer substantially more...

This would need to be checked with some good CFD or real testing.

[LostCause]

If you like to read...

- LostCause

[thentro]

Thanks all.
cfg83 - I removed the cross bars but the lengthwise ones stay I am afraid. Just too useful. I will still close them up with rubber in the near future however.

LostCause - that was the post that started all of this I am afraid! What have you done?! Next thing you know my car is covered in olive oil and yarn...

[aerohead]

You don't have anything backwards,however the distinction between what is happening with the golfball and the Imprezza is unfortunately like the old "apples and oranges" metaphor.With the small golfball,the manufacturer is forcibly creating a turbulent boundary layer,by virtue of including the dimples in it's dsign.As your article states,the wire trips the turbulent boundary layer and it immediately transitions into a turbulent boundary layer.And as mentioned,while posessing greater skin-friction drag,the smaller wake made available by virtue of the turbulent boundary layer,reduces the "profile" drag,which is the major drag component for blunt bodies,and overall drag is reduced.With cars,and above 20-mph,drag coefficients are constant and the car demonstrates turbulent boundary layer which cannot be removed by additional smoothing of the body.And thats a good thing! So for your Subaru,or any car,the flow close to the skin would not benefit from additional roughening in the forebody.If you could see your car in a wind-tunnel,you'd see that,for the exception of a very small area in front of the windshield,the airflow is attached to the body,and remains so until it reaches the rear edges,where it breaks away into the turbulent wake( very different than the turbulent boundary layer).You are paying for turbulence through the engine bay,around the wheel houses,under the car,and finally,the enormous wake behind the car,which because it is a wagon,is basically the size of the entire frontal area of the car.Since your cars front divides the airstream such that more air is channeled over the roof,you'd find that the local air velocity over the roof is significantly greater than what your speedometer is indicating.The recommendation to remove your roof rack is based on this fact that air up there is colliding with the rack at velocities greater than those some airplanes fly at.Your biggest problem is behind the car and I recommend you check out what MetroMPG,Basjoos,and others are doing.

[aerohead]

Sorry! I meant to say the wire trips the laminar boundary.Where's that coffee cup?

[trebuchet03]

So why is it then, that we put things like vortex generators at the back of the car?

Whomever is doing that is placing them blindly To function properly, they need to be placed just before the transition zone... Where's that? Well, it varies from car to car and varies by speed etc. etc. - it's the million dollar question (with no money to be won! ).

The better question - why do we place them anywhere at all? It's a bandaid...

but, aerohead is right.... Golfballs and airplanes are good for explaining concepts... But gold balls are way smaller than cars and move much faster.... Planes are much larger than cars and move much faster.... Planes are also, generally, considered streamlined while cars are, generally, bluff bodies... Comparisons are a slippery slope in a best case scenario :/

[Otto]

What we need is one of you guys with an undertray to apply some dirty oil to the bottom of it, run her up to ~80, then take and post some pics of the oil streaks.

This would show us the true direction of airflow under the car, and therefore the best orientation for fairings or deflectors for the wheels.

[tasdrouille]

Originally Posted by Otto

What we need is one of you guys with an undertray to apply some dirty oil to the bottom of it, run her up to ~80, then take and post some pics of the oil streaks.

This would show us the true direction of airflow under the car, and therefore the best orientation for fairings or deflectors for the wheels.

That would be an awesome experiment to do. Here are 2 slides from an Hucho's presentation that might offer some insights.