Random Wind Tunnel and Smoke Pictures Thread

[aerohead]

Originally Posted by CFECO

At what speed was the beetle tested at? That could have been 20 mph to make it look aero.

They'd choose a speed which gave the critical Reynolds number,to ensure a turbulent boundary layer.And 20-mph would have been adequate.
They're not cheating.This is the way the air behaves.And they are exposing the rear separation,where all the drag is.They are NOT injecting the smoke into the wake,so,in this respect they are avoiding the most unflattering aspect of the Beetle's flow.Here's another view.Forebody flow is fine

These are the money-shots!

[freebeard]

So where is the wake-injected Beetle body? I want to see that. It would explicitly demonstrate the separation line.

[CFECO]

Speed has No effect on flow or attached flow?

[freebeard]

Yes, for certain values of 'speed'.

OTOH, the value may not change, but the magnitude does. For instance, at higher air-flow more heat is convected.

[CFECO]

Ok ???

[kach22i]

Originally Posted by CFECO

Speed has No effect on flow or attached flow?

Just going off my sometime faulty memory, but I recall Aerohead or somebody saying 30-250 mph will get you your standard Cd number in testing.

And that 250-600 is it's own range, then we get into near supersonic/supersonic, and above that hypersonic speeds.

I suspect some odd things can happen between 15-30 mph depending on the shape being tested, the complexity of the forms and so forth.

I would think that at very low speeds(15-30) there would not be enough pressure built up at the front/nose to ensure attachment.

Would the air flow just rise up as if being sucked up to the heavens?

Again, I'll await Aerohead to correct any misgivings which I may have misspoken.

[NeilBlanchard]

Yes, the same aero rules apply up to 250MPH. Cd is a drag ratio and has no dimension.

[aerohead]

Originally Posted by freebeard

So where is the wake-injected Beetle body? I want to see that. It would explicitly demonstrate the separation line.

I've been waiting patiently for 40-years now.Too bad they didn't do it at the A2 wind tunnel when they had the Beetle there.

[aerohead]

Originally Posted by CFECO

Speed has No effect on flow or attached flow?

*the air adjacent to the car's body surface is always at zero velocity because of air viscosity.
*there's no reason for the air to stick to the surface unless there's a favorable pressure gradient around it.
*if it can,the air will always flow towards the lowest pressure area.
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*for a smooth car,at low speed,as the air that was accelerated around the body gets to the windshield area,it's at it's fastest velocity and encountering the lowest pressure on the car,according to the Bernoulli Theorem,and wants to stay right there.

*As the flow slows onto the roof,it's pressure begins to build back up,which requires that it slow down.
*But the air against the body is already at rest and it can't 'slow' anymore,and will begin to move forwards towards the low pressure.As it does,it rolls into eddies and swirls,lifts off the car,and blooms into full-blown turbulence.
Here you can see this at the left with the bowling balls

If you get the car moving fast enough,the boundary layer flow will transition over to a turbulent boundary layer which can strafe kinetic energy into the stationary air and pin it down, where it cannot flow forwards;as long as the strafing angle isn't too steep,otherwise the kinetic energy just ricochets off the air as a flat stone over water.
*It's a Reynolds number effect,named for Osborne Reynolds who published on the effect in the 19th Century.
In the following table you'll see two Reynolds numbers along the right hand margin and you'll notice that the Cds are lower at the higher Reynolds number.

*And Reynolds number is a function of length and velocity.
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*for a typical automobile,it's length is sufficient such that at around 20-mph,it will have a full,turbulent boundary layer and it's drag coefficient will be constant up to transonic velocity,which for cars can be as low as 250-mph.
*the only exception is with cars which have a rear end with angles in the range of around 28-32 degrees downslope.These angles produce a 'bi-stable' flow phenomenon in which at one instant the car has a fastback wake,then in another instant a squareback wake.The oscillation creates a booming and could actually lead to structural failure if weakly constructed.
*a 'tearing edge' can fix the situation,a 1930s invention by Koenig-Fachsenfeld and used today on rear bumper fascias.

[freebeard]