Hi Terry,

Looking at your pan without any skirts, the air displacement speed of the sucking device will have to be higher than the speed of the streamliner to generate any vaccum...

By my quick calculations, a streamliner 10 m long x 1 m wide and sitting 5cm off the ground has 0.5 cubic meter of air underneath.

This streamliner needs to move 10 meters to replace completely that volume of air.

So say it travels at 10 meters per second, 22.37mph, so in 1mn it will displace 18.65 cubic meters of air.

WE can then derive a ratio of air to speed ratio: 0.83.

Speed x ratio tells you how much air you have to displace 1st before generating any vacuum:

600mph * 0.83 = 500 cubic meters per mn

I know just enough about entropy to know air will be also gushing from the sides to replace the air displaced, but don't know if max air speed will be enough to totally replace the air moved and keep ATM under the car.

A vacuum might be created just by the speed of the car and the inability of the surrounding air to refill the space underneath the car above an unknown "balance" speed.

Patrick

Hi Patrick, thanks for the detailed comments.

JimL over at the Pan Skirts thread commented that he added skirts all around but left the rear open. Above 175 mph the natural vacuum created under the car sucked the belly pan off the car down to the salt surface. They had to tie it back up and eventually mount it rigidly to stay in place. So that lends credence to the idea that speed alone does indeed cause a vacuum to form where pan skirts are set in place.

If that's true then mechanical suction to further evacuate the air space bounded by the skirts should increase negative pressure up to the efficiency level of the skirt sealing. So the total volume of air being displaced by the pump second by second of car movement should equal whatever volume of air is passed through the skirts beyond the balance speed further enhanced by the amount of total negative pressure mechanically and aerodynamically created.

This design is based on a 30' L x 24" W body shell. I'm also allowing roughly 1" clearance between the belly pan and salt surface. The skirted area ends approximately 18' from the nose so all tolled we're looking at a static enclosed area of 3 cu.ft. or .085 cu.m. That's 5.1 cm/mn which at 12.27 mph is a 0.41 ratio. So I believe the more relevant number would be...600 * .041 = 246 cm/mn. But that's assuming no negative pressure is aerodynamically occurring which JimL has shown there is above some balance speed dictated by design factors.

I tend to believe static negative pressure will exist above a certain speed such that vacuum pump efficiency isn't tied to a volumetric speed ratio, but rather is a volumetric constant depending on the level of aerodynamic negative pressure within the suction zone. All theory of course that will have to be physically tested with appropriate sensors.

BTW... the big F-3X-140 Procharger draws 4500 cfm through a 6" inlet producing 60 lbs of boost at 72,000 rpm. So here we have a 6" discharge port drawing through a roughly 9" opening. Without gearing the impeller is turning between 8 to 10K but there's no appreciable inline back pressure to constrict cfm flow. This system would probably only work as designed if my assumptions about cfm flow as noted above are true. Interesting stuff... Thanks, Terry.