Vortex generators in diffusers

[aerohead]

Originally Posted by csnyder

We did some testing of Air-tabs on a 35 foot travel trailer using an aircraft engine blowing air down the side of the trailer and both tufts and smoke. Without the airtabs the tufts or ribbons just snapped around everywhere, tying themselves in knots behind the trailer, and the smoke curled tightly around behind the trailer. With the airtabs about 6 inches from the back of the trailer the tufts/ribbons stood straight out behind the trailer with virtually no flapping, and the smoke went back about 15 feet before starting to swirl. Wind speed was in excess of 60MPH down the side of the trailer. (about 12 feet? from a wall) I'd say they do pretty much what the information from airtabs claims they do. They sure keep the back of the trailer clean.

I don't know what their claim is.And I don't understand any mechanism which would allow for a drag reduction.The thickness of turbulent boundary layer would exceed the 'height' of the VGs,they would just be embedded in turbulence.The pressure at the same separation line would virtually be the same.That pressure determines the base pressure of the entire wake and basically the entire pressure drag of the truck.
I can see that they would alter the appearance of the wake,but uncertain that they'd have any effect on pressure.
If these are the 'Wheeler' wishbone VGs,Wheeler's original claim,was that on his HONDA Accord notchback sedan,that the VGs would allow reattachment onto the boot/trunklid of the car,which WOULD provide a drag reduction mechanism,as they do on the underside of a Lockheed C-130,preceding the tail boom.
Without any tail structure behind the trailer,there would be no facility for flow reattachment,locked-vortices,and pressure recovery,such that at the 'new' separation line,the pressure regime WOULD be increased.
And again,Hucho tested these sorts of things at Volkswagen,and he said that none of these 2-D palliatives worked on 3-D bodies.I don't know what else to add.

[vortex]

Air Tabs work much like an open air NACA submerged duct inlet. They influence and mix (till if you will) the boundary layer by drawing faster high energy air toward the surface.

Sure there are the counter-rotating vortices but these are a byproduct of function. The vortex filaments are not what delay downstream flow separation but rather the higher energy lower static pressure flow pushed against the surface making it stick longer before the boundary layer pressure gradient grows adverse. This is useful in extending Coanda flow attachment. That said aft facing Coanda attachment surface suction is generally bad for overall vehicle drag unless that pressure investment can be profitably applied saving a greater drag downstream, as a fairing costs drag but saves a greater drag as its return on investment.

[freebeard]

So could you say that the vortex generator rolls up the boundary layer into tighter streamlines, that pull the top of the boundary layer inward?

[vortex]

A series of vortex generators do energize the boundary layer but by the energy imparted to it by the VG blade and less from the higher speed outer boundary layer. Inside the vortex filament is a very low pressure core which keeps it attracted to nearby surfaces. You can visualize the low pressure by the way wing tip vortices condense vapor on a humid day. VG's are not free though as it requires a continual supply of (propulsive) energy to sustain the pressure deficit residing in the cores. The benefit you buy with that cost is greater resistance to catastrophic flow separation and (wing) stall.

It can be argued that the gyroscopic effect of the twisting air mass might tend to stabilize the filaments thereby reducing sensitivity to turbulence as the stream-tube filament row is passed over surfaces or even an open area such as a tractor-trailer gap. This is harder to visualize in air without introducing smoke, but witness the stability of the linked toroidal vortex in water and apply that intuition to a short straight vortex in air.

https://www.youtube.com/watch?v=ks3aQhEohTE

[aerohead]

Originally Posted by vortex

Air Tabs work much like an open air NACA submerged duct inlet. They influence and mix (till if you will) the boundary layer by drawing faster high energy air toward the surface.
Attachment 26162
Sure there are the counter-rotating vortices but these are a byproduct of function. The vortex filaments are not what delay downstream flow separation but rather the higher energy lower static pressure flow pushed against the surface making it stick longer before the boundary layer pressure gradient grows adverse. This is useful in extending Coanda flow attachment. That said aft facing Coanda attachment surface suction is generally bad for overall vehicle drag unless that pressure investment can be profitably applied saving a greater drag downstream, as a fairing costs drag but saves a greater drag as its return on investment.
Attachment 26163

I thought the VG's delta-wing vortices were what produced the low pressure,which induced inviscid flow kinetic energy into the boundary layer to forestall the reverse flow rollup,which would trigger eddies,then full-blown separation and turbulence?
The Reynolds number is enough to guarantee attached flow all the way to the back of the trailer.With or without VGs,the separation line remains fixed.
Turbulence cannot ever be converted to static pressure.If the VG accelerates the flow leading up to the separation line,under the Bernoulli Theorem,the pressure at the modified separation point would by default be at an even lower pressure.If the wake pressure is governed by the separation line pressure,this would lead to an even lower base pressure behind the trailer,increasing the pressure drag.
When you use the Coanda language,it is typically associated with a selective geometry perimeter nozzle which injects a 'tuned' volume of air from an internally provided source.I don't see how that's germane to a discussion of VGs.
With respect to a fairing,I might say that it aggravates frontal area,however, the superior,separation-free flow more than compensates,ending in an overall lower drag reduction.

[aerohead]

Originally Posted by vortex

A series of vortex generators do energize the boundary layer but by the energy imparted to it by the VG blade and less from the higher speed outer boundary layer. Inside the vortex filament is a very low pressure core which keeps it attracted to nearby surfaces. You can visualize the low pressure by the way wing tip vortices condense vapor on a humid day. VG's are not free though as it requires a continual supply of (propulsive) energy to sustain the pressure deficit residing in the cores. The benefit you buy with that cost is greater resistance to catastrophic flow separation and (wing) stall.

It can be argued that the gyroscopic effect of the twisting air mass might tend to stabilize the filaments thereby reducing sensitivity to turbulence as the stream-tube filament row is passed over surfaces or even an open area such as a tractor-trailer gap. This is harder to visualize in air without introducing smoke, but witness the stability of the linked toroidal vortex in water and apply that intuition to a short straight vortex in air.

I'm okay with the 2-D wing,under flight conditions, at large angle-of-attack,as there is always a flap,aileron,trim-tab,elevator, engine nacelle,or tailcone section behind the VG, for which the flow can reattach.Ludwig Prandtl was demonstrating attached flow on wings at up 40-degrees AOA in the 1920s.
Without a reversed-step box cavity or 'steep' boat tail behind the trailer,there would be nothing for flow to reattach to.Without that,essentially,there cannot be any pressure recovery of attached,decelerated flow.So we're stuck with the same base pressure,and same pressure drag.
Can we dig even deeper?

[aerohead]

Originally Posted by freebeard

So could you say that the vortex generator rolls up the boundary layer into tighter streamlines, that pull the top of the boundary layer inward?

There aren't really streamlines in the boundary layer.Those are all outside the boundary layer,out in the free stream flow (inviscid flow).Turbulent boundary layer us allowing laminar flow outside itself.And the layer of air immediately adjacent to the skin is completely at rest.It has no velocity whatsoever.
Beyond the point of the body's maximum cross-section,the flow is in a very hostile pressure regime.There's very little reason for the flow to be attached.
By definition,if you try to decelerate the flow any more than a streamlined body would,you're asking the air to decelerate,when its already at rest.That's impossible! So at that point the flow will begin to reverse in direction,flow forwards towards the area of minimum pressure,and as it does,if lifts off the surface,rolling up into,first small eddies,then full-blown turbulence.Once you have turbulence you've completely lost the game.All that swirling kinetic energy can never be recovered into any useful pressure.It will eventually be worn down by viscous attrition,just adding heat to the atmosphere.
On an aircraft,the VG induces vorticity (delta-wing lift of the Concorde aircraft ) , kinetic energy, as if it were in a more favorable angle of attack,where energy of the inviscid flow could crash through into the boundary layer (think of a water jet from a garden hose nozzle,pushing water uphill on a driveway,which would otherwise be draining down to the gutter under the influence of gravity) energizing it such that it can adhere to a 'steeper' surface.
As Vortex says,there's a price to pay for the vorticity.If you get it right,the drag reduction from the smaller wake possible with the VGs will overshadow the drag penalty of these excrescences and their frontal area, interrupting to flow.

[Tahoe_Hybrid]

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?

I have seen some Vortex generators on side mirrors Ford Festa I though I share that they were tiny but it had them

[aerohead]

Originally Posted by Tahoe_Hybrid

I have seen some Vortex generators on side mirrors Ford Festa I though I share that they were tiny but it had them

Sometimes,noise will show up in the acoustic wind tunnel.VGs can be used to tune the resonant frequency of the mirror,kill any booming,etc.,to quiet them down,as perceived from the cabin.
Also,the EPA actually has certain noise criteria for passenger cars.An annoying whistle could be attenuated with cheap aero fixes.

[MeteorGray]

I installed Air Tab vortex generators on the rear of my 27-foot Jayco travel trailer around 10 years ago. I can average about 14 mpg when pulling the trailer with my diesel F250 at about 60mph, and whether the Air Tabs help contribute to that mileage is impossible to say since I got about the same mileage before I installed them. If they do benefit the mileage, it is less than 5% and thus hard to discern with all the variables involved in hauling a trailer here and there.

I do know that the trailer seems to benefit from additional stability while on the Interstates when being passed by traffic or in high winds. And I can see a much clearer picture behind my trailer when towing in wet weather. And the back of the trailer stays noticeably cleaner. So, they are doing something to reduce the back-draft back there.

I don't think the Air Tabs are miracle cures for whatever towing problems might ail you, but I do believe the operating principle is solid and, for me, was worth the couple of hundred bucks they cost to help with stability and with whatever small fuel savings they might offer.