Blown Diffuser: How does it work actually and why it's there

[MIKEY_!]

@MIKEY_!: Of course, creating vortexes means drag (no free energy in our universe), but they are there anyway (no vortex-free airfoils in our universe either).
You can just throw them away (as seen clearly today on ML's rear wings), or you can try to gain some of this energy back.

Surely endplates are supposed to break up the vortex on an aerofoil. If the rear wing endplate was extended to the diffuser (as several teams do) the high pressure air above the diffuser couldn't create a vortex with the low pressure air under the diffuser therefore drag problem solved and more exhaust could be made to flow under the diffuser more effectively. Please correct me if i'm wrong as i probably am.

[speedsense]

Is the exhaust is going to reduce the angle (stretch) the helix shape of the vortex?

In the case of most of the cars, the exhaust driven diffuser vortex only has the about length of the tire to function. How may helix revolutions might be formed in that distance? Could there only be enough distance to form the initial rev/twist?

What might I research under to gain more info on the this type of vortex formation? Almost everything I have found is aviation specific.

Brian

I could be wrong, but I seem to remember some articles on heated vortices exiting jet engines. Though I would think the properties of combustion exhaust would be different, but I regress that I know not much about jet motors and their production of power and the temps following the motor....
In the years spent with ground effect tunnels, ambient air vortex flow generation were common studies in the windtunnel, mainly for their sealing of gaps outboard and sealing properties inboard, mostly forward of the mouth of the tunnel, ( meaning inboard and outboard of the tunnels). When the rules allowed, we had put strakes inside the tunnels for vortex generator towards the exit, but again these were ambient vortex production.
Of the highest interest was the rear tires and sealing them away from the tunnels, the gains are massive in prevention of not only the wheel "air pumps" but also the tires turbulence bridging the gaps below the tunnels. The gains were in the CP movement and how sensitive it was to this air flow at the rear of the tunnel.

[marekk]

@MIKEY_!: Of course, creating vortexes means drag (no free energy in our universe), but they are there anyway (no vortex-free airfoils in our universe either).
You can just throw them away (as seen clearly today on ML's rear wings), or you can try to gain some of this energy back.

Surely endplates are supposed to break up the vortex on an aerofoil. If the rear wing endplate was extended to the diffuser (as several teams do) the high pressure air above the diffuser couldn't create a vortex with the low pressure air under the diffuser therefore drag problem solved and more exhaust could be made to flow under the diffuser more effectively. Please correct me if i'm wrong as i probably am.

Endplates increase efficiency on low aspect wings and decrease vortex intensity, but we can talk of 5-10% improvements. Vorticies and drag are still there - as clearly seen on ML's rear wings in last few races.

[marekk]

Is the exhaust is going to reduce the angle (stretch) the helix shape of the vortex?

In the case of most of the cars, the exhaust driven diffuser vortex only has the about length of the tire to function. How may helix revolutions might be formed in that distance? Could there only be enough distance to form the initial rev/twist?

What might I research under to gain more info on the this type of vortex formation? Almost everything I have found is aviation specific.

Brian

I've found this one to be very informative: http://espace.library.uq.edu.au/eserv/U ... _16_07.pdf

[MIKEY_!]

Endplates increase efficiency on low aspect wings and decrease vortex intensity, but we can talk of 5-10% improvements. Vorticies and drag are still there - as clearly seen on ML's rear wings in last few races.

But not on RBR's wing... please explain... I think it might be more to do with the aggressive slots on the upper part of ML's endplates?

[horse]

But not on RBR's wing... please explain... I think it might be more to do with the aggressive slots on the upper part of ML's endplates?

The RB has a much smaller flap and with is therefore generating less powerful vortices. The main element will be generating vortices too, but they will not be as easy to see. I assume the "tip" vortices of the main element are broken down more as the end plate extends beyond its trailing edge.

[marekk]

But not on RBR's wing... please explain... I think it might be more to do with the aggressive slots on the upper part of ML's endplates?

The RB has a much smaller flap and with is therefore generating less powerful vortices. The main element will be generating vortices too, but they will not be as easy to see. I assume the "tip" vortices of the main element are broken down more as the end plate extends beyond its trailing edge.

For the first approximation vortex strength is proportional to the current load of the wing, so we can assume MacLaren has one of biggest loads on RW on the current grid.

Due to big endplates vorticies are shed directly form the working surfaces of the wing (we should see two - one very strong on the suction side, and one much weaker on pressure side), usually starting from lowest pressure regions.

On this photo from Jerez 2011 found on excelent Gordon McCabe's blog (http://mccabism.blogspot.com) we can clearly see strong vortex cores originating from the main element of the wing:

[horse]

Wow, that's an amazing photo, thanks. Marekk, I don't quite understand why there should be two vortices and, particularly why there should be one emanating from the pressure side. My understanding was that the vortices are a product of the differences between the two sides of the wing.

I would perhaps expect to see vortices from both the main element and the flap.

[raymondu999]

Hey folks; slightly off-topic, but just thought I'd love to get some opinions here. I think we've seen that after Valencia, Red Bull never really regained anything close to their pre-Silverstone form. Do you folks think that the Renault engine can't stand a race-long stint using off-throttle blowing, and so they have stopped using that, at least at Red Bull? McL, for example, seems comfortable running it in quali + the race.

[MIKEY_!]

It is interesting what you say about RBR's recent lack of form. I can still hear the rough engine note so a doubt they have stopped using hot blowing. My theory is that the other teams have started to catch up in the diffuser area and RBR have not been able to continue their advance in this area.

PS I agree with horse can marekk please explain this further, diagram would be nice thanks.

[marekk]

Wow, that's an amazing photo, thanks. Marekk, I don't quite understand why there should be two vortices and, particularly why there should be one emanating from the pressure side. My understanding was that the vortices are a product of the differences between the two sides of the wing.

I would perhaps expect to see vortices from both the main element and the flap.

Endplates prevent forming of tip vorticies, but there are still 2 separate regions of pressure difference (in reference to ambient air) between both sides of endplate - one stronger on the sucking side of the wing, one much weaker on high pressure side. This induces 2 vortexes, and as they are co-rotating, they eventually curl together down the flow (as seen near letter b on "bwin".

There is of course pressure difference between both sides of the flap (even when not nearly as strong as on main profile), but due to gurney this pressure gradient produces 2 counter-rotating vortices behind and along the trailing edge of the flap (perpendicular to car's axis). Both eventually are shedded near the endplate and merge with main vortexes, but are to weak to be visible (they are not very stable to).

Good reading: http://espace.library.uq.edu.au/eserv/U ... _16_07.pdf

[Smokes]

The question is how much energy are those votices using?

Also with Exhaust blowing how similar is it to propwash on prop plane wing. I.E where aircraft ground speed is less than the airspeed/Massflow rate going over the wing.

[horse]

Good reading: http://espace.library.uq.edu.au/eserv/U ... _16_07.pdf

Do you think the effect here of having two vortices near the wing is simply just caused by having very small end plates? This study is more like a front wing than a rear wing. The paper:

Ground Effect Aerodynamics of Race Cars
Appl. Mech. Rev. -- January 2006 -- Volume 59, Issue 1, 33 (17 pages)

doesn't seem to resolve this second vortex in their experiment studies (done by BAR honda, strangely enough) and they have a larger end plate. These are both front wing studies as well, so there are no ground effect influences on the rear wing.

Here's a good link to a racecar engineering article.

Here's a capture from that:



The two vortices are quite clear here although I think the lower one is generated by the wing, which I think is detailed in the Appl. Mech. Rev. and the other is off the top of the endplate itself. Thus a larger endplate takes this vortex away from the wing, like it says in the racecar engineering article.

How do the pair of vortices end up being co-rotating?

(Oo dear. Well off topic.)

[marekk]

Good reading: http://espace.library.uq.edu.au/eserv/U ... _16_07.pdf

Do you think the effect here of having two vortices near the wing is simply just caused by having very small end plates?

Of course bigger endplates mean lower pressure difference, but every 3D flow with pressure gradients ends in a vortex

doesn't seem to resolve this second vortex in their experiment studies (done by BAR honda, strangely enough) and they have a larger end plate. These are both front wing studies as well, so there are no ground effect influences on the rear wing.

Upper (on the pressure side) vortex is weak and not that well defined - authors of my referenced study struggled to find the core. On F1 RW it's even weaker, due to vents.

[godlameroso]

One thing that we should note, and probably has been noted to death already, is that vortex formation is also heavily dependent on pressure gradients caused by fluctuations in relative temperature. In other words, Tornadoes in the real world are caused by difference in high pressure humid hot air, and relatively colder lower pressure air in the upper atmosphere. What a better place to get relatively lower pressure, and relatively colder air than from the underside of wings, or the floor of the car? And where can you get hot high pressure air? The air travelling over the body is of relative higher pressure, and combined with exhaust gasses and hot air from the side cooling exits, and you have an area that is ripe for vortex formation.

All that's left is to tune the body work to exploit the vortexes, this isn't going to change much for next year, it's just going to be trickier because the exhausts will lose a lot of energy to interact with the air-stream, compared to this year.