2022 cars 'porpoising' at high speed

[godlameroso]

It's a simple problem with complex solutions.

https://www.youtube.com/clip/Ugkxp0Se1R ... 4fviVd3kKI

As the clip shows, air is loud when it travels really fast. You can easily see this with your ears on a windy day. The aero instability is partly caused by sound. Sound = pressure waves, pressure waves cause buffeting.



What you see here is sound waves propagating upstream, the wing is in a transonic state. The top is Schlieren imaging, and the bottom is CFD.

Notice the pressure waves that propagate downstream effectively "pin" the flow to the wing, that is when the sound waves have a higher frequency, there is little if any separation or buffeting. The strongest vortex shedding happens at low frequencies like 1-2hz, at 6-10hz very little vortex shedding happens.

If it were as simple as simply running the suspension at a teeth shattering 6hz, the teams would do that. However the body is unsprung and the suspension is sprung, so 6hz at the axle won't translate to 6hz at the body.

The other way to deal with sound is to reduce the back pressure after the throat, or area of max velocity airflow. We can stop pretending that the airflow is not transonic in the diffuser, once we can accept that, we can begin to solve the problem.

The way to reduce back pressure in the diffuser is to jet the air through it. The edge wing air can be jetted into the diffuser, the geometry at the bottom can be cambered inward to promote more upwash. The rear wing endplates, the beamwing, and rear wing can all contribute to not just upwash but outwash as well. Red Bull's beam wing creates strong outwash which is why it helps so well. Ferrari's beam wing also creates a fair bit of outwash, as does Mercedes. Red Bull has the best ducted air, everyone else is just copying Newey.

Remember kids, these cars are a series of tubes.

[godlameroso]

[Hoffman900]

If the floor is not touching the asphalt, bouncing like this is decent

The floor touching the asphalt is what causes the venturi to choke, which triggers the porpoising. You cannot porpoise without first contacting the asphalt.

Not true. Vortex shedding can cause it as well, especially if the car is resonating at the same frequency of the now required mechanical springs or causing wing supports to flutter on some small level.

In my little world, we were joking about running a shaker rig in the wind tunnel, but it's not going to tell you much with the rules required 60% model.

Bumping this back up as I think this is still Merc’s problem.

Scarbs is saying suspension, but I suspect it’s both. James Allison didn’t say it wasn’t aero induced, he just said the stalling at low ride height / rebounding / stalling theory was incorrect. Scarbs, who I am a fan of, was one of the original sources of that theory that I can find.

Suspension alone, I believe this would have shown up on a shaker rig / shaker rig - pull down combo rig. I personally know of a GT car this has happened to and almost launched itself off the machine.

Wind tunnel doesn’t have suspension and if it does, it doesn’t scale.

Cfd are assumed infinitely stiff plus CFD doesn’t deal with Karman Vortex street very well.

Jin and Vyssion have talked about in an old thread from 2018 acting on the front wing: viewtopic.php?t=27525&start=15

Maybe on-topic? When they show the ultra-slow motion video of the front wings bouncing all over the place I have often wondered what that does to the wings performance. It would seem like there would be a significant tendency for detached flow when the wing is moving upward and therefore reducing the pressure on the bottom / back of all of the wing elements. And likewise an advantageous situation when the wing moves down causing the opposite effect...

Seeing it in writing I guess this same potential effect would apply to all of the downforce generating aero surfaces. And the upsetting force could be any vertical movement.

No one is talking about it, so it's probably not an issue, but to my mind the mechanics seem sound. Any thoughts?

You are entering into the world of "aero-elasto-dynamics" with that question. Incredibly complicated subject, so I'll only explain the top level stuff and you can see whether that informs your question?

So, to start there are two main types of wings that form a type of "oscillating wing" will exhibit: A heaving wing, or a flapping wing. To start, if I just simplify the case to a heaving wing in close proximity to the ground, which has a sinusoidal motion:

https://i.imgur.com/L714jJ7.png

Your ride height now becomes a function of some sinusoidal frequency:



You also have a few other major parameters to consider:

Reduced Frequency:


Non-dimensional "Plunge" Amplitude:


And your new "effective" Angle of Attack due to the motion:


So with a heaving aerofoil, the incidence is "positive" when the aerofoil is heaving "down" (i.e. the leading edge is lower than the trailing edge).

With a periodic heaving motion, the aerodynamic forces show a period response at the same frequency as the heave motion, but with a bit of a "lag" relative to the motion itself. This "lag" of how the aerodynamic coefficients are affected depends mostly on the "Reduced Frequency" term that I wrote above.

Essentially, there are three main flow regimes present when you have something like a front wing:

• Ground Effect
  (ventui effect we are all familiar with)

• Incidence Effect
  (change in AoA = change in coefficients)

• Added Mass Effect
  (the displacing of a mass of air due to motion imparts a force on the aerofoil)

There is also a vortex shedding phenomenon present in the flow as well, but it comes in two types: forced and natural. Forced shedding is an inviscid phenomenon, whilst natural shedding is caused by viscous effects.

The forced shedding is linked to the relatively weak "starting" vortex during the transient initial period when the aerofoil accelerates from zero to a certain velocity (kind of like the result of a car hitting a kerb). The total amount of circulation in the flow is pretty much constant, however, there is a change in the effective angle of attack which happens at the mean position - where angle of attack is at its maximum.

The natural shedding typically comes about from bluff bodies but can appear on an aerofoil with sharp edges when separation occurs upstream. This type doesn't need a vertical motion to occur and it basically causes the stagnation point to move around the trailing edge until the flow stalls at which point, the stagnation point then reverses its direction back the other way (sort of a "move until stall and then go back" type thing).

There is something called "Theodorsen Theory" which describes an aerofoil under sinusoidal motion in the freestream (with a small amplitude) in terms of a 2 of the 3 main effects I listed above which you can go check out if you want to.

Thanks for another detailed reply!

I'm still feeling like we may not be on the same page, so here's my last attempt to ask this:

Looking at the top picture, if that airfoil was suddenly moved up vertically, my brain says that there would be an increase in negative pressure on the underside. My question is, does that increase in negative pressure cause large scale / complete flow separation, similar to a stall condition? If it does, then it would seem like a primary concern for the designers, given how often the car is hitting curbs. Given our conversation thus far, it seems like this is more of a 'corner condition' and is something they would check after the fact...

Edit: I looked at the top photo again a realized that the flow isn't attached already! There's a recirculation pocket that I missed... So, the question still stands as 'detached turbulent flow, as in a stall condition'...

Thanks again for the fun conversation. I really appreciate your level of engagement.

Jon
B.S. Mechanical Engineering, but math still scares me...

I'm going to try... aerodynamics are not steady state, nice smooth contour plots in CFD and wind tunnel results are the result of long time averaging intervals. In reality the trailing edge flow will be as Vyssion showed 2 posts back - more akin to a Karman vortex street. Wing's are also not infinitely stiff so this may result in some oscillations which will affect pressure distribution - which will cause the wing to pitch and heave - but the oscillatory forces are rather small so it's unlikely to create very big deflections. However if you watched the Haas sharkfin and T-wing last year you'll see those unsteady effects can get quite big - even to the extent that parts fail because of it.

On your kerb example the deflections can get rather big, even causing the endplates to hit the the ground - as those are set 85mm high when the car is flat to the ground those are some big oscillations. So yes as the wing is slapping up and down it will change the pressure distribution - if the car was static you may read some quite big forces because the front wing is a big surface area. However, the cars are generally travelling above 100mi/hr where the air under the wing is travelling at 4 or 5x that - so these oscillatory effects will be quite small as a percentage of total downforce. So it's unlikely to change the state of flow significantly.

[SiLo]

How would they even simulate this in CFD? Just run CFD on the car at various ride heights and then see how the flow under the floor changes?

[TimW]

Theoretically an aeroelastic simulation should be able to capture this, shouldn't it?

It seems to me that, at least theoretically, you do not necessarily need stall or vortex shedding as an excitor. When there are a big shifts in CoP of the underfloor, the 'angle of attack' of the underfloor would change, and you could get an unstable system this way.

[Hoffman900]

Theoretically an aeroelastic simulation should be able to capture this, shouldn't it?

It seems to me that, at least theoretically, you do not necessarily need stall or vortex shedding as an excitor. When there are a big shifts in CoP of the underfloor, the 'angle of attack' of the underfloor would change, and you could get an unstable system this way.

It is VERY hard to simulate and that simulation is an approxmiation. Further more you would have to concurrently simulate the suspension / chassis, road surface, and tire at the same time, and good luck with running all of that together, and even better luck if it actual is close to what is happening in the real world.


Frank Dernie talks about how active suspension came about here, mostly do to instabilities related to the flat floor. Reading between the lines, though there problems were different, the active suspension came about due to these instabilities due to aero issues. Feeding back to what Scarbs is hinted at. Merc is looking to fix both, but the suspension is a big part of getting it under control, but at the end of the day, it is an aero problem.

~ 0:41:00 in.


Imo, a lot of this is down to the rules. When you look at PVRS vs. a metal valve spring, you can assume that the air PVRS won’t resonate and you are really restricted by keeping the follower in contact with the lobe via air pressure. Contrast this to a metal valve spring, and the springs (and pushrods it applicable) can go into resonance and that’s when things break. The hydraulic active suspension is providing the same benefit and a hydraulic heave spring like they had, would as well.

Also remember, a coil spring is just a wound torsion spring (what F1 cars use). If you think of a crankshaft in terms of a torsion spring, you can imagine how things go into resonance (and why harmonic dampers are installed).

[SiLo]

Active suspension should absolutely have been mandated, and the control unit could even have been a spec part. Would have saved the teams a lot of money.

[Polite]

can porposing be managed by reducing the entry of air into the venturi at high speeds?

if yes, there is a fast and easy way to manage it.

[Hoffman900]

can porposing be managed by reducing the entry of air into the venturi at high speeds?

if yes, there is a fast and easy way to manage it.

Probably only in the sense of weakening the downforce produced by the floor, thus it represents less of the total downforce load, as Jin and Vyssion hint at in regards to why the front wing in that case didn’t induce this. I’m an engine guy though, so I think either one of those two would have to comment.

Mt thoughts are this is why raising the car works and why Mclaren, if they lack df, why their car may not. I also could be wrong.

[hollus]

Going back to how all teams seem to have missed something and were surprised in the first testing session, and apologies if this has already been brought up:
Trouble seems to start only at about 300km/h and above. With the acceleration of flow in the diffuser throat, it will approach 500km/h very locally… mach 0.5 so compressibility effects might start to make things a bit murky. CFD does not account for compressibility, and in the wind tunnel, with the 180 km/h and at scale, it will not start to appear either. So that is a new effect, likely unmodeled, which would only appear for the first time at full scale and full speed: testing day 1.
Now for a mind twister: the 500 km/h transient speed is only between car and air, subtract 300km/h for relative air-road speed. Does this mean that something unmodeled happens only at the throat and onlt in the carbon fiber side of it?
And would this extend a bit under the plank?

[Hoffman900]

Great video and what I have been saying all along.

As I brought up before, only Jean-Claude Migeot (aerodynamcist of similar vintage) said as much as well back at the Barcelona test while everyone was about the other theory.

[Just_a_fan]

CFD does not account for compressibility,

CFD can account for compressibility. That it doesn't usually is a different issue.

[Hoffman900]

Note: one thing I didn’t consider is the overpowering of the sidewall at least causing it become an undampened spring.

Also suggests why Merc / Lewis was trying higher rear tire pressures, which ultimately results in a loose car when taken too far.

[vorticism]

Note: one thing I didn’t consider is the overpowering of the sidewall.

That tells me the sidewall is the ultimate limit and they may have to decrease df to make it work. That’s also an incredible thought because of how stiff those sidewalls are to deal with the loads they do.

Tires are the main undamped moveable component at work here. Suspension damping can only do so much to control what's going on downstream from them. That said, they are spec tires. The porpoising force delta would play into how much the tires are to blame. A RB or Ferrari may have relatively weak porpoising force delta compared to Merc.

Merc have used a cable stay on their floor, maybe part of their problem is floor flexibility. Either with fluttering causing detachment or simply deflection leading to lateral flow being reduced at the floor edge.

[godlameroso]

There might be a solution in this video.