Dielectric-barrier discharge (DBD) for drag reduction

[freebeard]

I do hope the mental picture that immediately popped into my mind, has now done so in yours?

Sorry to disappoint.

Intellectually, I can appreciate that DBD at the point of separation can act as a Coanda effect. But I always struggle with reducing a compound curve surface to a single angle.

What is the orientation of these 'wings' to the actual boat tail?

[Logic]

Originally Posted by freebeard

Sorry to disappoint.

Intellectually, I can appreciate that DBD at the point of separation can act as a Coanda effect. But I always struggle with reducing a compound curve surface to a single angle.

What is the orientation of these 'wings' to the actual boat tail?

I think you actually got it on a deeper level than that simple explanation.




Ignoring surface friction and boundary layer, for simplicity:

Up to the point of maximum thickness, air has no choice but to get-TH out of the way of the wing.
ie; It acquires much momentum 'away' from that wing leading edge/s.

Where the surface starts to converge to a sharp tail:
That air has to;
Decelerate (away direction) and them accelerate back toward the surface.
Here it's relying on air pressure alone for that desired change in momentum.
vs
No choice but to get-TH out the way when displaced by the leading edge; A much greater force.
ie: Due to the continued 'away' momentum of all those molecules, a vacuum forms in this area.
Because air pressure just cant change the direction of momentum fast enough.
So it's here that turbulence starts.

Give the molecules (DBD) momentum parallel to the surface and said molecule is now WAY more 'back' along the surface for the same amount of 'away'.

Now the surface is converging toward the trailing edge.
Air pressure no longer has to overcome 'opposite' momentum, It just needs to have the oomph to 'press' molecules back toward the surface fast/well enough to avoid a vacuum large enough for turbulence to occur.

In HVAC the rule of thumb is 7 deg, but that # is way more fluid in open air, where the surface does not consist on sharp, cheap to make, changes in angle. (vs oncoming direction)


Add surface friction and boundary layer:
The fact that air molecules touching the surface doe not move makes the above even worse, dragging air in the direction opposite to that desired!
Molecule movement in the direction you do want is basically stopped, while (divergent) momentum away from it is unaffected.

With DBD electrons move along the skin at a significant fraction of the speed of light.
Sure; there are immediate collisions with air molecules, but that imparts momentum to air, in the desired direction
AND
being oppositely charged; said electrons and negatively charged air are heading for the downstream surface, in the desired direction,no matter what, NOT heading off into 'turbulent chaos space'.

One can almost imagine the electrons as a new surface that is not standing still but moving in the desired direction, along with it's surface molecules attached to it, like a conveyor belt.


Point being:
Both effects work in concert, in a fortuitous, desired way, with std air pressure getting a much needed hand at closing the wake.
+
that conveyor belt in the right direction for a 'surface' thing!

So much of a hand in fact that one can keep flow attached/laminar on a much shorter, much more practical boat tail.
ie: One with a much bigger converging angle vs oncoming airflow.
A lighter one!


(I really wish I had a clue at modern CAD and animation.
This thousand words for one picture thing suX! And the 'Cognitive Dissonance' finger is pointed directly at me! Time to learn!!
We are also virtually guaranteed that NO ONE besides you read all that!? )

[freebeard]

We are also virtually guaranteed that NO ONE besides you read all that!? )

Yeah, I read all that but I still don't grok the wings part

[Logic]

Originally Posted by freebeard

Yeah, I read all that but I still don't grok the wings part

No wings!
Just the wing profile:

So from Above; picture that, mirrored on the Chord line, with a car sticking out the front, going left, instead of the leading edge/s.
See the similarity between wing and boat tail now??

Now the angle of the boat-tail to wind direction increased (by ~17+ more deg on average), so that it's length is dramatically decreased.

ie: DBD would/could make for much shorter, more convenient, lighter boat tails, without the flow separation that would normally happen.

One might even incorporate a 'Weathercock Switch' to manipulate the DBD, to minimize buffeting And improve cornering. As just 'invented' be me!


(Similarly that 'Fake Feathers' thing I posted about.
But they work by stopping the reverse flow one normally gets on a wing's top trailing edge, turning what's left of it into a 'push'.
No power reqd)

[freebeard]

When I did these in 2014, I used a trailing edge angle of 45° (22½° on each side).



The rear wheel overskirts could be reduced, I didn't have the modeling skills.

[Logic]

Originally Posted by freebeard

When I did these in 2014, I used a trailing edge angle of 45° (22½° on each side).

The rear wheel overskirts could be reduced, I didn't have the modeling skills.

It seems you have the picture at last!?

Fact is;
thoughtfully placed DBD can reduce wake without any boat tail..!
Just some Kapton Tape, Conductive Tape and $5 PSUs from Amazon.

Better modeling skills than my non existent skillz.

[freebeard]

That was a $25 3D model, with the (light-colored) low poly shapes done in Wings 3D brought into a rendering program that added the cartoon outline. A long toolchain with no ability to merge the pieces and parts.

Today, it can all be done in Blender. If you want to dabble, there're tutorials. I've tried to do the same thing in Blender, but I get hung up with that model I bought a decade ago. Maybe I should find a low-poly model to start with.

[freebeard]

Here is the Citroen Tubik 21

Would it make sense to put DBD before, in, or after the venturi? Inner and outer faces?

This would create an air curtain for the back of the vehicle.

[Piotrsko]

Traditionally, anything after increases turbulence/drag in the turbulent flow but that is just nit picking ( sometimes you cannot measure the increase it is so small. Or your equipment cant measure that change)