I was wrong! (Perhaps)

[JulianEdgar]

When I was writing my book, there was a disagreement between the aerodynamicist I was working with (Dick Barnard) and another aerodynamicist giving me feedback on the draft (Adrian Gaylard).

The disagreement was basically this: are car vortices created by lift/downforce (as in an aircraft wing), or are the vortices themselves responsible for the lift/downforce (and of course drag)? Dick said vortices come from lift/downforce; Adrian said on cars they were not directly related to lift/downforce. So I did a panel in the book ('Chicken or the egg?' on Page 19).

Based on what Dick told me, I've been advising people that getting the car neutral in lift/downforce will therefore result in lowest trailing vortices, and so drag.

But I've now got feedback from two more professional car aerodynamacists that I think perhaps changes that.

One aerodynamcist has said that there will be vortex development associated with both up and down forces, and that therefore a car (as opposed to a wing) with zero lift may still have strong vortex drag. Furthermore, developing low pressures under the car to counteract lift may in fact cause more vortex drag than if this were not done.

The other aerodynamcist had a bet a little both ways, as I did in my panel in the book. But he put it even more simply, saying that he doesn't think so much in vortices as in separation - and it is separation that leads to vortex development. (But then he adds that my statement in the book that 'any shape that creates lift or downforce develops drag' is definitely correct!)

Both aerodynamicists I am quoting have worked for major car companies well known for their aero excellence. One is the current head of aero of his company.

So I still am not quite sure, but I think the statement that I have made here that lowest drag will come from having no lift / no downforce should at minimum be qualified.

But there's another dimension too: on cars with lift, surely it depends on how that lift is reduced. As Dick said to me, if you place a fence across the roof, that will reduce lift - but also create lots of drag. So a smooth undertray that demonstrably reduces lift - does it also increase drag? I wouldn't have thought so - compared to having no undertray, anyway. Solar race cars aim for lowest lift/downforce for lowest drag...

Now on a personal level, I'd go for no lift (or even downforce) purely for the stability and improved driving dynamics. But anyway, reducing lift may not always reduce vortex strength and so drag, it appears.

[RedDevil]

Felipe Nasr demonstrates that vortices can be generated by downforce too:

https://www.youtube.com/watch?v=_vIVcAqeCT8

[JulianEdgar]

Originally Posted by RedDevil

Felipe Nasr demonstrates that vortices can be generated by downforce too:

Yes, that's not in debate. What is in debate is whether if the car was developing neither overall lift or downforce, would it still develop strong vortices (from the car as a whole, not a wing).

[RedDevil]

I would think vortices can (and usually will) be created by disturbances in a local part of the body, so you can have several distinct vortices from both downforce and lift generating surfaces, as well as from disturbances that do not generate net lift or downforce at all.

In the F1 car it is clear that the vortices are a local affair, rotating around a core that originates from the sides of the rear wing.

Two other important vortices (not visible in the photograph) are generated by the gap between the nose and the louvres on the front wing, then tunnel between the front wheels and the nose and hit the barge boards behind the front wheels, forcing them out and helping establish a low pressure area below the car's floor.
The vortex also defeats or at least limits the turbulence created by the front wheels that would otherwise mess up any aerodynamic trickery in that area.

Vortices do not necessarily have anything to do with lift or downforce, but can be pretty useful for altering the flow in a controlled way.

[freebeard]

Dick said vortices come from lift/downforce

It's hard for me to accept that. Lift/drag is a summary action on the center of pressure vs center of mass.

edit:
Well, I've only read the introduction to this, but he stakes out a big claim.

https://professionalawesome.com/diy-downforce/

DIY Downforce – How to Design Front Splitters, Diffusers, Flat Bottoms, Wings and more!
july 9, 2018 by dan, posted in aerodynamics, testing

We see people saying that trying to improve downforce for lower power cars is a waste of time because the cars don’t have the horsepower to make up for the added drag. There may be instances where this may be true, but we have seen this isn’t often the case. We’ve worked with 90 hp Mazda Miatas and 1000 hp Mitsubishi Evolutions. The information applies for both. You can make downforce without adding much drag, and in some cases, even reducing drag!
....
We’re going to start at the front of the car and work our way to back. I know when I first started getting into upgrading aero, getting the front to perform was always my biggest fear. At the back of the car, you could slap on a wing and make it as big as your wallet allowed. But how do you get more performance out of the front? Well, after years of reading, testing and research, my mindset has flipped. Downforce at the front of a car is relatively easy! Why? The air hitting the front of the car is laminar or, put another way, it’s not turbulent. The rear of the car is dealing with all sorts of vortices, boundary layer effects and issues that you really need CFD or a wind tunnel to study. Without having those issues to deal with, designing for the front is much easier.

I'll see how it does, but first— food and hygiene.

[JulianEdgar]

Originally Posted by freebeard

It's hard for me to accept that. Lift/drag is a summary action on the center of pressure vs center of mass.

It's fairly easy to justify, actually. Let's imagine a typical car with a lot of lift. That lift comes from having a lower pressure on the upper surfaces compared with the underneath surfaces.

If there is a higher pressure underneath than on top (the other way of putting it) then you'd expect some of the airflow to be moving from underneath towards the top surfaces. As the car moves forward, that airflow then develops a corkscrew pattern, leading to the development of a trailing vortex. Now apply that idea to anywhere lift forces are occurring, eg as airflow wraps around rear angled pillars (eg C pillars in a sedan - airflow moving from side of car to lower pressure on trunk lid). You can see that as lift increases, so would trailing vortex intensity. That was Dick's basic point (I hope I have paraphrased him correctly) ie all lifting bodies develop trailing vortices. That is, overall lift (or downforce of course) causes the vortices. Therefore, one would seek to reduce lift to zero in order to minimise vortex development.

Adrian Gaylard (Jaguar Land Rover) and Rob Palin (ex Tesla) said to me that is much too simple. They say that with cars, there is no direct correlation between the amount of lift and the strength of vortices. They say that to suggest there is, is a carryover from aeronautical aerodynamics (eg as with wing tip vortices, that do develop roughly as outlined above). Rob goes on to say that any surface on the car developing lift or downforce sheds vortices. Therefore, one would seek to reduce any surfaces developing lift or downforce, (of course without in turn then doing anything that creates drag).

Thomas Wolf (Porsche) said to me that he thinks not in terms of vortices, but in terms of separation. Any separation produces vortices, so therefore, to minimise vortex development, attempt to minimise separation. You can see how Thomas's point would apply to things like A pillars, around which there is often a small amount of separation. His point would also apply to separation at the rear end of the undertray under the car, and at the trailing edge of the trunk lid on top of the car (etc).

[freebeard]

As the car moves forward, that airflow then develops a corkscrew pattern, leading to the development of a trailing vortex.

Tell me about it, I drive a VW Beetle.

The action of a vortex is around a central axis longitudinal to the flow. Perhaps they only can act in pairs?

[JulianEdgar]

Originally Posted by freebeard

Tell me about it, I drive a VW Beetle.

The action of a vortex is around a central axis longitudinal to the flow. Perhaps they only can act in pairs?

That would be supported by the airflow on each side of the car moving in the direction of low pressure. Furthermore, Dick's point is supported by the fact that the trailing vortices rotate in different directions if the car develops overall downforce or overall lift.

As I have said, vortex development was the only area where I simply couldn't get my advising experts to agree.

[freebeard]

As I have said, vortex development was the only area where I simply couldn't get my advising experts to agree.

Mama always told me not to look into the eye's of the sun
But mama, that's where the fun is.

Manfred Mann

I can grok involuting and evoluting pairs, but I can't fathom what a single unpaired vortex can do except exert a retarding force.
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It's off-topic, but I enjoyed this video: Cavitation - Easily explained!. Turns out when a cavitation bubble collapses, the weakest point collapses into it's antipode and creates a toroid that has a powerful poloidal jet. That's what wreaks havoc with propellers and impellers.

[RedDevil]

I'd think every windmill under load produces a single big vortex, and maybe one smaller vortex per wingtip.