Bumping this back up as I think this is still Merc’s problem.Hoffman900 wrote: ↑Thu Feb 24, 2022 4:29 pmNot 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.AR3-GP wrote: ↑Thu Feb 24, 2022 4:18 pmThe floor touching the asphalt is what causes the venturi to choke, which triggers the porpoising. You cannot porpoise without first contacting the asphalt.
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.
Vyssion wrote: ↑Fri Aug 10, 2018 9:40 amYou 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?e36jon wrote: ↑Thu Aug 09, 2018 8:45 pmMaybe 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?
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:
Your ride height now becomes a function of some sinusoidal frequency:
You also have a few other major parameters to consider:
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)
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.
- Added Mass Effect
(the displacing of a mass of air due to motion imparts a force on the aerofoil)
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.
jjn9128 wrote: ↑Mon Aug 13, 2018 6:27 pmI'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.e36jon wrote: ↑Mon Aug 13, 2018 5:04 pmThanks 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.
B.S. Mechanical Engineering, but math still scares me...
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.
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.TimW wrote: ↑Thu Mar 31, 2022 3:29 pmTheoretically 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.
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.
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.Hoffman900 wrote: ↑Thu Mar 31, 2022 9:06 pmNote: 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.