Dipesh1995 wrote: ↑
Wed Jul 14, 2021 2:54 pm
Just_a_fan wrote: ↑
Wed Jul 14, 2021 2:07 pm
godlameroso wrote: ↑
Wed Jul 14, 2021 1:34 pm
I would suspect that some airflows are close to the .7 mach number that creates these compressibility effects. However the pressure analogy is correct. Pressure is pressure, whether it's you pumping iron or air lifting a plane, or pipes bursting. Good video though.
I'd be very surprised if compressibility is even close to being an issue in F1. The fastest speed an F1 car has done in a race is over 100km/h slower than the speeds where compressibility is even considered to be starting to be an issue.
If local airspeed started to get in to the compressibility speeds, we'd see shockwaves developing and things would go wrong pretty quickly. A shockwave under the car, for example, would choke the floor leading to rear instability and much crashing in to the scenery.
A free paper for y'all.
https://www.researchgate.net/publicatio ... racing_car
"A numerical investigation into the influence of compressibility effects around a simplified open-wheel
racing car was completed. Results demonstrated that for high-lift aerodynamic designs operating in
close ground proximity the effects of compressibility are significant at speeds well below the Mach 0·3
threshold normally applied. Incompressible simulations are generally unsuitable even at Mach 0·15
and below, with pockets of local velocities exceeding three times the freestream value and creating
exaggerated low-density, low-pressure flow.
Notably, changes to the interactions between components were the key contributors to some of the
most significant observed points of difference. Components operating in the most extreme ground effect
were observed to be most affected. For the front wing, incompressible simulations underestimated the
extent of flow acceleration and therefore maximum suction. The negative lift produced by the floor
and diffuser was found to be the most influenced component; the consideration of compressible flow
revealed a more detrimental interaction with the rear wheel and resulted in markedly less predicted
negative lift when compared to the incompressible prediction even at Mach 0·0882 (30ms–1), but
most notably at Mach 0·2646 where discrepancies of over 20% were observed. The effects towards
the behaviour of the prominent vortices off the front and rear wings were small relative to the overall
pressure distributions acting on components. Smaller vortices surrounding the rear wheel and diffuser,
and general wake velocity deficits were found to be more affected due to compressibility. The car
simulated here has a significantly less aerodynamic efficiency compared to modern vehicles, so the
estimates presented here are expected to be conservative – actual discrepancies may therefore be
greater than those evaluated in this study.
The incompressible assumption is clearly not suitable for racing car aerodynamics if absolute accuracy
is a key goal, despite the additional computational expense required to conduct compressible simulations. Compressible simulations become particularly important at Mach numbers of 0·15 and above,
and may be one of several factors in poor correlation between wind-tunnel results and incompressible
CFD simulations. Observed trends are non-linear and complex due to the close interaction between
components. It is clear that a simple compressible correction would not be sufficient."
That says it all pretty much. On a race car treating the airflow as if it's non compressible will not be totally accurate. As I estimate, the local flows and pressure gradients are such that there is some compressibility present in the bulk flow field.