Diffuser stall problem

[hardingfv32]

What would be some of the general reasons for the diffuser to stall?

Brian

[jjn9128]

I assume you're referring to the issues McLaren and Williams are reporting. The main issue will be ride height - low ground clearances reduce pressure under the car to the point the attached flow cannot cope and separates.

I think the main issue, certainly for Williams, is getting the air to reattach after stalling. At high speed you don't need downforce, so you'd rather shed drag, so stalling the diffuser is desirable. It's the same reason for the bendy front wings ~7 years back. The problem comes when you brake and the rear ride height increases, you need that airflow to immediately snap back to the surface rather than taking half a tenth to get there - so you can get a hysteresis loop where when the ride height is reducing there is more downforce than when the ride height is increasing. This is the reason for dynamic ride height maps being created in the wind tunnel to try and identify those issues in the design phase - problem with wind tunnels is they're simulations so there are compromises and inaccuracies and sometimes these issues get missed.

Both the Williams spins today happened after the DRS flap was shut - so I think it's likely to be a rear wing issue where it struggles to reattach, especially with a bit of yaw across the wing. Both drivers committed to a hard turn right after the braking, so I wonder if they needed to either shut the flap a bit before hitting the brakes or turn slightly less aggressively in the initial phase of the corner to allow the wing to reattach with a straighter onset flow.

[Just_a_fan]

There was talk that the Williams issue was caused by airflow not going where intended as the front wheels are turned in to the corner. This is leading to diffuser instability and, in the case today, the rear end just snapping out of line. The car does look to be a handful to drive.

I guess the issue is caused by the front wing not directing the tyre wake as expected/designed.

[Big Tea]

I noticed, and remarked yesterday that the Williams rode like a brick. It seemed to have no suspension other than the tyres, which seemed to flop about like a bent wheel when the opposite wheel was on the curb.

It could be completely disrupting the air beneath the car and spilling it mid corner.

[Zynerji]

Are we expecting similar yaw instability with the 2019 in wash front wings?

[PhillipM]

They won't be inwash, they'll still be outwash, but a lot more upwash.

[hardingfv32]

Re-attachment issues:

'Assuming' that you get the diffuser back to the correct ride height/clearance, what are the general causes of poor or delayed re-attachment? Things like excess turbulence while stalled/detached?

Brian

[Zynerji]

Re-attachment issues:

'Assuming' that you get the diffuser back to the correct ride height/clearance, what are the general causes of poor or delayed re-attachment? Things like excess turbulence while stalled/detached?

Brian

I would assume it to be getting the clean vortexes generated by the barge boards stretched back to the diffuser.

[godlameroso]

Vortecies travel to the diffuser regardless, but by boosting them through the mid wing, the core of the vortex stretches, and it's integrity is less affected by any anomalies in it's path.



Notice at the leading edge of the floor the pressure is very low and the integrity of the vortex is well defined, and how it sort of breaks up a little before it reaches the rear diffuser. Well with the right device behind the bargeboard and the upwash created by the mid wing you can boost the strength of the vortex travelling under the floor so that it's blue(and well defined) all the way through because of the suction peak caused by the mid wing before the leading edge of the floor.

It's not easy to get it working because you have to feed the mid wing clean air, however it's hard when you have a tire in front of it, so you use vortex generators on the inboard foot plate of the bargeboard, which is receiving clean high energy airflow off the nose. There's a finite amount of airflow so getting the most out of it with the least effort is part of the challenge.

[Vanja #66]

I don't think we should discuss barge boards from more than 10 years ago and use them as an example for airflow when we talk about these cars. It's a completely different aero philosophy now from what it was then.

[jjn9128]

As if to prove my point about an issue being the rear wing, both Williams reverted to a 2017 wing for Silverstone.

[roon]

The image illustrates the point being made. FIA unable to alter laws of physics, yet. Current cars' VGs at the floor leading edge and bargeboard footplate to me suggest that they are employing multiple smaller vortices stacked side-by-side beneath the floor. Can't cram a large vortex into a small gap with out losing its rotation.

[Zynerji]

The image illustrates the point being made. FIA unable to alter laws of physics, yet. Current cars' VGs at the floor leading edge and bargeboard footplate to me suggest that they are employing multiple smaller vortices stacked side-by-side beneath the floor. Can't cram a large vortex into a small gap with out losing its rotation.

I thought that was the point of "stretching" them? Elongation causing a smaller radius. I think Toet wrote something about it a few years ago...

Edit: actually, I think it was a Ferrari engineer talking after the ban on the stair- stepping of the leading edge of the floor for 2009...

[roon]

Could be, I would think there would be limits to how much one could expect to contort the initial vortex.

[godlameroso]

As if to prove my point about an issue being the rear wing, both Williams reverted to a 2017 wing for Silverstone.

They still got lapped halfway through the race.