A quiz for people

[freebeard]

At Permalink #71 I posted links that explain that the air going over the top lags behind the air underneath.

The underbody is a [semi-]closed plenum while the upper body is in free air.

[aerohead]

Originally Posted by hat_man

I'm willing to learn something here but I'm going to need some guidance.

Is the lift of the Dream created by the attached airflow over the top of the profile or because the design of the underside creates more pressure on the underside than is created on the top (underside pressure >topside pressure. Lift created by a specific design.) or us the lift created by the basic shape resembling an airfoil and adds as such (Bernoulii ?) because it is positioned high enough off the road that it acts as a "wing" (not a specific underside design to create lift, but placement of a basic design in relation to the road to create lift)

1) there's a bunch of different Dream vehicles, so we'd want to discuss a particular version.
2) that said, lift will be caused by less pressure on top than on the bottom.
3) the contour determines the pressure, as it also determines the velocity, which are always dance partners.
4) all the cars are light for their dimensions, however, I believe the real issue is with crosswind and especially with gust.
5) If an Australian land train gust reaches the wheel fairings, they may act like a weathervane, turning the entire car sideways, where it takes on a much larger aspect ratio ( longer wingspan ) in relation to the car going straight forwards ( very little wingspan ). For the weight of car and driver, the coefficient of lift may be great enough, such that the lifting force exceeds the static load of the racer, and it must takeoff!
6) GM feared this in 1987, and removed the entire wheel fairing package as a palliative. Cd rose from 0.125, to Cd 0.147. In the documentary film, 'Who Killed The Electric Car,' there's a brief section filmed inside GM's wind tunnel, with Dr. Paul MacCready using a pitot-tube as a stethoscope to ascertain the limits of the boundary layer thickness. He's also responsible for calculating the 400-mpg Sunraycer would yield as an ICE vehicle.An inspiration for my projects.

[aerohead]

Originally Posted by freebeard

At Permalink #71 I posted links that explain that the air going over the top lags behind the air underneath.

The underbody is a [semi-]closed plenum while the upper body is in free air.

That's technically impossible. All the air reaches the rear of the car at the same time, regardless of path. It's a conservation of mass criteria, mandated by the Bernoulli theorem. Velocity and pressure can be all over the map, but it all arrives at the same time once to the rear end.

[JulianEdgar]

Originally Posted by hat_man

In my simple mind list can only happen if the pressure underneath is greater than the pressure over the top. (Under > over). Maybe it's simplistic but I'm seeing a wing "at rest" as under = over. Is my thinking so far correct?

It doesn't matter what the pressures are in absolute terms: it matters what they are compared to each other. That is, if the pressure on the top is lower than the pressure underneath, of course the pressure underneath is higher than the pressure on top.

Assuming a wing moving through the air at X mph, is the air moving under the wing equal to x and the air moving over the wing > x ? Or is it the other way around with the air over the wing equal to x and the air under the wing <x ? I'm assuming the former only because the air under would be relatively flat and the air over being the variable because of the "detour" it has to make over the top to reach the same point on the trailing edge?

Or is that the crux of the nit being picked?

You're back to discussing theories of lift, which as I said, is not at all straightforward and still causes debate among professional aerodynamicists. I see it a bit like debating why gravity occurs, rather than just working with the forces caused by gravity.

[freebeard]

All the air reaches the rear of the car at the same time, regardless of path.

Some sort of quantum entanglement theory? I follow sources that disagree.

[Fat Charlie]

The air doesn't get to the rear of the car, the rear of the car gets to it.

Wind tunnels aren't the real world- IRL, the air is already there and the car just moves through it. Air pushed upwards and having a hard time getting back down to where it was is called "lift".