How About Wake Modification?

[vonk]

Having just joined this forum, I apologize if the following is old hat here.

Even on aircraft, stall only increases drag, and it’s induced by increased angle of attack. On an F1 car the rear wing's angle of attack is constant, so it can only be stalled by “spoiling” the flow over it. This can be done, for instance, by placing a small span-wise slot in the low static pressure area near the airfoil’s leading edge and feeding small amounts of air into that region. The effect would be boundary layer separation at that point, producing the equivalent of stall.

But why ever reduce down force at the expense of drag? I suspect there is something else at play here. How about wake flow modification? Stalling the wing at high speed would certainly do that. It would convert the trailing edge of the upper foil into a vertical knife edge producing a different turbulence pattern behind. What if something like a Kamm effect could produce a wake pattern that nets less drag? To really know, one would have to observe this with smoke trails in a wind tunnel. Somebody might have stumbled across this by increasing the angle of attack of the upper foil excessively.

Those drafting an F1 car know that there is a lot of energy in its wake, and it all comes from the car that produces the wake. In a macro sense, reducing that wake energy benefits the car. Wake flow behind an F1 car is hard to treat analytically, given the multiple influence factors affecting it. Long live the wind tunnel, not withstanding CFD.

[autogyro]

Having just joined this forum, I apologize if the following is old hat here.

Even on aircraft, stall only increases drag, and it’s induced by increased angle of attack. On an F1 car the rear wing's angle of attack is constant, so it can only be stalled by “spoiling” the flow over it. This can be done, for instance, by placing a small span-wise slot in the low static pressure area near the airfoil’s leading edge and feeding small amounts of air into that region. The effect would be boundary layer separation at that point, producing the equivalent of stall.

But why ever reduce down force at the expense of drag? I suspect there is something else at play here. How about wake flow modification? Stalling the wing at high speed would certainly do that. It would convert the trailing edge of the upper foil into a vertical knife edge producing a different turbulence pattern behind. What if something like a Kamm effect could produce a wake pattern that nets less drag? To really know, one would have to observe this with smoke trails in a wind tunnel. Somebody might have stumbled across this by increasing the angle of attack of the upper foil excessively.

Those drafting an F1 car know that there is a lot of energy in its wake, and it all comes from the car that produces the wake. In a macro sense, reducing that wake energy benefits the car. Wake flow behind an F1 car is hard to treat analytically, given the multiple influence factors affecting it. Long live the wind tunnel, not withstanding CFD.

If somebody 'stumbled' over this in a wind tunnel, then there is only one thing to conclude with the huge budgets for aero and all the 'clever' people involved in it.
The aerodynamacists are not doing their jobs properly and they have not been collecting enough data over the last decades to justify their existance.

[Mystery Steve]

Yep, it is just that simple. Collect a bunch of data, and a couple decades later you have all the answers to everything. What are these morons thinking?

[autogyro]

The Wright Flyer was more than a couple of decades ago.
You would think they have had enough time by now.

[jon-mullen]

I can't help you much, except to put the discussion back on track.

But why ever reduce down force at the expense of drag?

It makes sense for these cars on the straights, where they don't require as much normal force (and therefore friction) to get the power down in a straight line.

I suspect there is something else at play here. How about wake flow modification? Stalling the wing at high speed would certainly do that. It would convert the trailing edge of the upper foil into a vertical knife edge producing a different turbulence pattern behind. What if something like a Kamm effect could produce a wake pattern that nets less drag?

It's not clear if you're saying that this is HOW they're doing it or WHY they're doing it, would you mind rephrasing please?


PS @auto

The aerodynamacists are not doing their jobs properly and they have not been collecting enough data over the last decades to justify their existance.

And you're not doing sh*t except dominating the other thread that's already about aero regs, can this guy get his fair discussion without it being your soapbox? Also, many browsers come with a spell checker now.

[autogyro]

My comment was a fair answer to the question and I am allowed to voice it as much as you are to worship DF.
If you have a counter argument use it if not you are at liberty to ignore my posts.
I concluded that if the aerodynamacists have not got an instant answer to a basic question such as this, then something is wrong.
Oh and retreating into word semantics and spelling is one of the standard actions of those without answers. Shows a lack of balance.

[Mystery Steve]

My apologies to vonk for the distraction; I didn't intend to steer the topic. A quick note: the reason people "worship" downforce is because it allows the car to corner better, and reduces lap times. A very logical reason I would say. If you think that it hurts "the show" and should be addressed in future regulations, then I say fair point to you. In the meantime, some of us enjoy the intellectual "what ifs" and we want to talk about them. So unless you have something constructive to add to that discussion, please keep the off topic rants in other more appropriate threads. Thank you in advance for your cooperation.

------------------------------------------------

Back to the original topic...

Why would you sacrifice downforce in the name of drag reduction? I'll flip the question on you: why would you increase downforce if it means increased drag? The basic answer is that vehicle dynamics is a control system. The process of systems optimization uses a cost function (lap time in this case) that is to be minimized, and the variables involved in the cost function can either be restricted or unrestricted in their respective domains. In the case of an F1 car, you can develop the cost function by dividing a circuit into a series of straights and turns with known lengths, radii, and banking angles. You can then calculate each "straight-time" and "turn-time" given known restraints on cornering and braking ability, and straight acceleration and top speed. These restraints are a function of downforce, tire selection, vehicle speed, ambient temperature, etc. Depending on the track, a lot of downforce may be beneficial, but at other tracks it is a detriment. This is why we see different aerodynamic setups at tracks on either extreme, such as Monaco and Monza.

What the McLaren "F-duct" is supposedly doing is introducing a non-linearity to the cost function restraints by reducing the amount of drag while traveling in a straight line relative to the drag experienced while in a turn. Near top speed, a car is not traction-limited, but actually drag-limited. So what McLaren may have seen is an ability to stretch the restrictions on the cost function in their favor. So a logical question would then be: so why are the Red Bulls consistently outperforming the McLarens in a lap time? (While the drivers certainly have different styles, and they are a major component of the control system, I will choose to ignore them in this discussion and assume they are all equally capable of pushing a car to its limit.) Straight-line aerodynamics is not the only variable restricting the cost function, and Red Bull may have a better suspension design which allows them to stretch the restrictions elsewhere, such as limit cornering. Perhaps the McLaren car has sacrificed too much cornering downforce by adding their vent/slit that they can't match corner exit velocity of other cars and their F-duct advantage is effectively negated. Who knows without looking at telemetry data?

I haven't thought too in-depth on the F-duct subject, so this is only educated speculation, but I think people have the F-duct philosophy backwards. As a wing's angle of attack increases, the drag force component increases, and the lift force component increases until the point of stall and then begins to decrease. While the rear wing angle of attack is fixed relative to some global frame of reference, the induced angle of attack continuously changes depending on the air flow around the wing due to the influence of other elements on the car. If you supply air to the underside of the rear wing while the car is at top speed, you can reduce the effective induced angle of attack and decrease the drag (and lift). Removal of that airflow switches the induced angle of attack back to a higher degree and produces higher downforce and higher drag. Maybe this has already been discussed in the other thread. I don't know since I've only read up to page 21 of what is currently a total of 69 pages. My apologies if it has been addressed already.

I don't have a direct answer for your wake question at the moment. I'll have to mull it over for a while. Maybe someone can add to the discussion in the meantime.

[autogyro]

No one in aero is going to give a strait answer to the question because reducing the effects of wake turbulance means reducing DF.
The Holy Grail.
Just like the 'Grail' it is an illusion.

[vonk]

[

But why ever reduce down force at the expense of drag?

It makes sense for these cars on the straights, where they don't require as much normal force (and therefore friction) to get the power down in a straight line.

I meant reducing down force and increasing drag. But I was wrong, because I overlooked that stalling the upper wing removes its reaward facing lift vector.

I suspect there is something else at play here. How about wake flow modification? Stalling the wing at high speed would certainly do that. It would convert the trailing edge of the upper foil into a vertical knife edge producing a different turbulence pattern behind. What if something like a Kamm effect could produce a wake pattern that nets less drag?

It's not clear if you're saying that this is HOW they're doing it or WHY they're doing it, would you mind rephrasing please?

The changed wake flow would be the natural consequens of the induced boundary layer separetion on the upper wing. The bit about the Kamm effect is pure wishful speculation.

[Jersey Tom]

The Wright Flyer was more than a couple of decades ago.
You would think they have had enough time by now.

The first pneumatic tire was developed sometime in the late 1800's (I believe). Yet as simple as they are, they are still an incredibly complicated, difficult item to both design and characterize.. with hundreds of millions of dollars spend annually on their research.

Think how far back medical research goes. Still vast unknowns with what must be billions upon billions of dollars of research.

Aerodynamics and fluid mechanics research in general is no different... otherwise places like the von Karman Institute wouldn't exist.

Any working engineer knows, within probably their first year in industry.. that cutting edge 'real world' engineering work is some immensely complicated, difficult ---. Good thing it is, as it keeps us employed. Decades of empirical data is nice, but data by itself is just data. Pile of numbers. It isn't necessarily 'knowledge.' Generating 'knowledge' from numbers is the talent of the best engineers and scientists. And yes, sometimes you just happen to stumble upon solutions.

[vonk]

Why would you sacrifice downforce in the name of drag reduction? I'll flip the question on you: why would you increase downforce if it means increased drag? The basic answer is that vehicle dynamics is a control system. The process of systems optimization uses a cost function (lap time in this case) that is to be minimized, and the variables involved in the cost function can either be restricted or unrestricted in their respective domains. In the case of an F1 car, you can develop the cost function by dividing a circuit into a series of straights and turns with known lengths, radii, and banking angles. You can then calculate each "straight-time" and "turn-time" given known restraints on cornering and braking ability, and straight acceleration and top speed. These restraints are a function of downforce, tire selection, vehicle speed, ambient temperature, etc. Depending on the track, a lot of downforce may be beneficial, but at other tracks it is a detriment. This is why we see different aerodynamic setups at tracks on either extreme, such as Monaco and Monza.

I'm impressed! I'm fully aware of the multitude of tradeoffs, and I can see how this formal optimization is a good tool for planning and initial setup. But when it’s Friday morning, will everything be perfect? I trust you're not saying that teams respond to driver comments with formal system optimization.

What the McLaren "F-duct" is supposedly doing is introducing a non-linearity to the cost function restraints by reducing the amount of drag while traveling in a straight line relative to the drag experienced while in a turn. Near top speed, a car is not traction-limited, but actually drag-limited. So what McLaren may have seen is an ability to stretch the restrictions on the cost function in their favor. So a logical question would then be: so why are the Red Bulls consistently outperforming the McLarens in a lap time? (While the drivers certainly have different styles, and they are a major component of the control system, I will choose to ignore them in this discussion and assume they are all equally capable of pushing a car to its limit.) Straight-line aerodynamics is not the only variable restricting the cost function, and Red Bull may have a better suspension design which allows them to stretch the restrictions elsewhere, such as limit cornering. Perhaps the McLaren car has sacrificed too much cornering downforce by adding their vent/slit that they can't match corner exit velocity of other cars and their F-duct advantage is effectively negated. Who knows without looking at telemetry data?

I agree. I think the F-duct thing is intended only for high speed where lateral friction is no issue. Maybe Red Bull just have the better diffuser?

I haven't thought too in-depth on the F-duct subject, so this is only educated speculation, but I think people have the F-duct philosophy backwards. As a wing's angle of attack increases, the drag force component increases, and the lift force component increases until the point of stall and then begins to decrease. While the rear wing angle of attack is fixed relative to some global frame of reference, the induced angle of attack continuously changes depending on the air flow around the wing due to the influence of other elements on the car. If you supply air to the underside of the rear wing while the car is at top speed, you can reduce the effective induced angle of attack and decrease the drag (and lift). Removal of that airflow switches the induced angle of attack back to a higher degree and produces higher downforce and higher drag. Maybe this has already been discussed in the other thread. I don't know since I've only read up to page 21 of what is currently a total of 69 pages. My apologies if it has been addressed already.

I don't have a direct answer for your wake question at the moment. I'll have to mull it over for a while. Maybe someone can add to the discussion in the meantime.

Thanks for the detailed comments.

[Mystery Steve]

I'm impressed! I'm fully aware of the multitude of tradeoffs, and I can see how this formal optimization is a good tool for planning and initial setup. But when it’s Friday morning, will everything be perfect? I trust you're not saying that teams respond to driver comments with formal system optimization.

I'm not implying that they necessarily make adjustments to the car based on the results of the optimization. Rather, I would say they probably figure out a general middle ground setup that should be competitive and then make adjustments based on driver feedback and telemetry data. If you're clever, you might be able to improve the results by developing some metrics so that the driver is mathematically part of the system. Certainly the setup you bring on Friday won't necessarily be perfect since weather conditions can be unpredictable and the aerodynamic and tire curve fits are empirical and may not capture the full behavior. But it definitely gives you something to start with, and is a valuable tool in the early conceptual design phase. I'm sure McLaren probably did something similar to justify allocating resources for the development of the F-duct.

Maybe Red Bull just have the better diffuser?

Maybe Red Bull does have a better diffuser, but maybe not. The important thing to remember is they are not driving diffusers, wings, tires, or engines. They are driving a CAR, which is a collection of components working together. At the moment, Red Bull is doing the best job of putting these components together in a way that their drivers can utilize effectively (at least during qualifying).

[kilcoo316]

It would convert the trailing edge of the upper foil into a vertical knife edge producing a different turbulence pattern behind. What if something like a Kamm effect could produce a wake pattern that nets less drag?

Well... the Kamm design does remove, or at least vastly reduce, the 2 longitudinal vortices that are produced off a car's C pillars.

Obviously by stalling out the wing, you don't have the same low pressure area at the wing's suction surface, and thus the pressure differential does not exist to induce those longitudinal vortices.


However, simply due to the extreme AoA of the 2nd element of the rear wing, stalling it also reduces the pressure drag. Don't forget the "lift/downforce" component of the wing will be roughly normal to the wing camberline - which for the rear wing 2nd element is probably as little as 20-30 degrees off the horizontal!

To really know, one would have to observe this with smoke trails in a wind tunnel.

Smoke trails?!?!

Its all PIV or hot wire.

Somebody might have stumbled across this by increasing the angle of attack of the upper foil excessively.

Nah - they've known about it for ages - how long has it been since the first flexi wing controversies? 10 years or so?

[vonk]

To really know, one would have to observe this with smoke trails in a wind tunnel.

Smoke trails?!?!

Its all PIV or hot wire.

As we know, F1 technology is a highly creative and experimental art. New ideas are at a premium and brain storming produces a lot of good and bad concepts. To separate the wheat from the chaff, quick qualitative, experimental diagnosis can often save much time, money and pride.

It seems USF1 was going to do it all from the keyboard.

[vorticism]

If a powered fan were permitted on the car, this could be used to accelerate the radiator outflow up to freestream velocity, which should help reduce the turbulence in the wake. True or false? Also, it might allow downsizing or the radiators and bodywork, affecting the size of the wake furthermore.