izzy wrote: ↑10 Mar 2024, 10:16
So, as far as I understand it, separation is when the airflow detaches from the surface it's supposed to be following, and this is a stall. But bouncing is when the gap to the ground gets so small the two boundary layers meet and as they're slow the pressure suddenly rises and the car rises on its springs. Is Gary saying these two are the same thing?
I'm gonna go into a bit more detail, so feel free to skip when you find what interests you.
Flow separation is detachment from surface, yes. Full description would be straining of boundary layer flow and local flow reversal leading to local detachment. Depending on the flow conditions, this detachment can either immediately spread or remain locally contained. It's usually contained to a certain area and is called a separation bubble, unless there are also other external disturbances (dirt on the surface, damaged surface, strong side wind, etc)
Also depending on flow and geometry of the surface, separation can happen and aerodynamic forces can keep growing a bit more. For wings, trailing edge separation allows for a bit more angle to be added before lift starts dropping. For floor and diffuser, doesn't matter if it's a Venturi or a flat floor, typically the ride height is the crucial parameter. At a certain low ride height, flat floor diffuser exhibits initial separation. In controlled conditions, you can then still lower the floor a bit more and keep gaining downforce before the separation bubble grows too big, bursts and separation suddenly spreads out rapidly.
Venturi floor can experience the same phenomena in low ride height, but there are some key differences. Typical choke of the floor is the most extreme case, when the expansion after the throat is too big and the flow can't stay attached to the Venturi tunnel roof. It is reported by some insiders that no team in 2022 had this happen alone and cause porpoising, it was typically multiple factors that caused this to occur.
Due to the movement of the car and floor, bumps and kerbs on track and overall flow condition changes at different parts of the track, what happend most often was having an initial external influence cause the initial separation somewhere behind the floor throat. Since these Venturi tunnels are so big and generate 100s of kg of load, this separation could cause enough of a trouble to make the car lose enough load to start bouncing while the car would still accelerate on straights or high-speed corners. More speed later on means more load squared and thus the amplitude of bouncing would only go up. Once this process starts, there is a hysteresis and so it would be impossible to keep accelerating or even keep the same velocity and expect the bouncing to stop. Vertical movement caused by porpoising also typically meant additional vertical velocity component, which would also mess with the flow and add more disturbances.
There are two simple solutions to fight bouncing - higher ride height and stiffer suspension. Higher ride height affects downforce generation and tyre performance, while stiffer suspension usually affects low-speed cornering and tyre performance. So it's best when you can avoid bouncing with floor design that allows the floor to work more robustly and in various adverse conditions.
Not sure what Gary A wrote, but there are no two boundary layers, there's no boundary layer on the ground since the ground (and air) are typically static and the car moves.