Vortex generators in diffusers

[kach22i]

The last two posts ask some very good questions.

Aerospaceweb.org | Ask Us - Wing Vortex Devices


May 15th 2019
BAE Systems 3D printing enables first ever flight using supersonically blown air
https://3dprintingindustry.com/news/...wn-air-155452/

Drone development undertaken by BAE Systems in collaboration with The University of Manchester has reached a new landmark.

Earlier in May this year, the MAGMA unmanned aerial vehicle (UAV) successfully completed trials proving the capabilities of its “flap-free” flight technologies.

As a result, MAGMA is believed to have become the first vehicle in aviation history to be manoeuvred in flight using supersonically blown air, a technology which improves the complicated task of controlling an aircraft at low-speeds.......................

“Today BAE Systems is 3D printing our components out of titanium and we are flight testing them on the back of a jet engine in an aircraft designed and built by the project team. It doesn’t get much better than that.”...............

“We made our first fluidic thrust vectoring nozzle from glued together bits of plastic and tested it on a hair drier fan nearly 20 years ago.”

The thing to perhaps keep in frame of mind is the squeezed high velocity air trapped under the car once liberated from it's confines will slow and expand and increase in pressure very rapidly, hence causing trailing drag behind the car.

What ever one can do to delay or mitigate this disruptive transition should help lower aerodynamic drag.

I just don't see these tiny shark fin devices located where they are having much of a chance to do that but welcome viewing any CFD or wind tunnel images anyone can come up with.

[Vman455]

Originally Posted by freebeard

I don't think anything can act at right angles to the airflow. Any effect will propagate downstream.

Not a right angle, necessarily, but an acute one--there has to be low enough pressure on the trailing side of the VG, since that's what induces the vortex.

[aerohead]

Originally Posted by fastflyer

Hi, has anyone tried the Vortex generators or airtabs as if they were a diffuser?

In theory, they should push the air upwards, enhancing the effect of the diffuser, not?

If the diffuser is designed for low drag,it will not have any separated flow for a VG to 'solve.'
The 'long' diffuser will be 2.8-degrees
The 'short' diffuser will be 4-degrees
That would be the end of story if designing for low drag.
VGs would only be a drag-inducing excrescence.
For a 'trackable' car,you'd want to follow Ferrari,which has 'valves' to control the diffuser as a drag reducing device.I think Freebeard is touching on that.

[aerohead]

Originally Posted by fastflyer

Has anyone tried something like this?



Site: sigmaautomotive

I,as others,would question their placement.
Typically,a VG is positioned such that is enhances separated flow to reattach onto a trailing panel,downstream of the VG.
Since the VGs pictured are already at the trailing edge of the vehicle,I can't imagine what the expectation would be for their performance.If there was a box-cavity or something back there,it would be a different story.No testing that I know of ever created a reduced wake,'phantom' tail using any type of device such as this.

[aerohead]

Originally Posted by kach22i

The last two posts ask some very good questions.

Aerospaceweb.org | Ask Us - Wing Vortex Devices


May 15th 2019
BAE Systems 3D printing enables first ever flight using supersonically blown air
https://3dprintingindustry.com/news/...wn-air-155452/



The thing to perhaps keep in frame of mind is the squeezed high velocity air trapped under the car once liberated from it's confines will slow and expand and increase in pressure very rapidly, hence causing trailing drag behind the car.

What ever one can do to delay or mitigate this disruptive transition should help lower aerodynamic drag.

I just don't see these tiny shark fin devices located where they are having much of a chance to do that but welcome viewing any CFD or wind tunnel images anyone can come up with.

Hucho mentioned and emphasized that, never has a 2d flow device ever lowered the drag of a 3D body.
And history has shown that a simple low drag body is the best way to achieve low drag.They're not as 'sexy' as a race car,their just faster and more fuel efficient.

[fastflyer]

Originally Posted by aerohead

If the diffuser is designed for low drag,it will not have any separated flow for a VG to 'solve.'
The 'long' diffuser will be 2.8-degrees
The 'short' diffuser will be 4-degrees
That would be the end of story if designing for low drag.
VGs would only be a drag-inducing excrescence.
For a 'trackable' car,you'd want to follow Ferrari,which has 'valves' to control the diffuser as a drag reducing device.I think Freebeard is touching on that.

I do not question the operation of a diffuser. But in a car without diffuser, put the Vortex generators. For less drag and more downforce?.

He left a DYNA study on vgs and airtabs. And how they are placed in the back of the car.

Dyna

[aerohead]

Originally Posted by fastflyer

I do not question the operation of a diffuser. But in a car without diffuser, put the Vortex generators. For less drag and more downforce?.

He left a DYNA study on vgs and airtabs. And how they are placed in the back of the car.

Dyna

Thanks!
I would say that the results would be valid,only for the specific vehicles tested,exactly how they were configured, and any extrapolating of the performance equivalency to any other vehicle would be dubious.
If their numerical simulation satisfies the full Navier-Stokes equation,then I'm okay with that.
The base pressure of a vehicles wake is established by the local pressure at the 'first' separation line at the rear.The presence of VGs at the separation line would not alter that local pressure.There's no ramp behind the VGs on which the edge vortices induced by the VGs to act.I don't understand the drag reduction mechanism they purport to achieve.
It would have been more proper for MIRA to have conducted,rapid,back-to-back runs,with and without the VGs on the same vehicle,to isolate any variables between the two.I'm quite surprised that they conducted the testing in the manner in which they did.The SAE would not have allowed it.

[fastflyer]

I think that with the VG happens the same thing as with the publicity of the airtab

[aerohead]

Originally Posted by fastflyer

I think that with the VG happens the same thing as with the publicity of the airtab

I know that what they promote is implied in the animation.I'm not sure that what actually happens is in agreement with the facts.
Details such as this would require a 'full-scale',full Navier-Stokes equation, numerical analysis,which currently requires about 2-days run time on a supercomputer.
Again,Hucho ran the Volkswagen full-scale climatic wind tunnel for 10-years,and he tested these sorts of things.He never saw anything lower drag ,except for box-cavities and boat tails.
I'm not calling Air-Tabs a 'liar',but I'd want a more rigorous investigation.
They haven't altered the frontal area,nor pressure at the separation line.I'd like an explanation for how they achieve a higher base pressure without some sort of reversed step.

[fastflyer]

Other example:



Rear undertray of several Toyota. Those two fins has any explanation?