Drag reduction - A new theory?

[donee]

Hi Trebuchet and Aerohead,


But the 1/2 PSI is exhausting through the very thin slot not the whole Coanda surface. So, based on available exhaust volume, one sizes the slot to get greater than 1/2 PSIG within the conduit. That way, the exit velocity in the slot is sufficient. The idea is to get the slot exit velocity greater than the boundary layer flow velocity. I think this is doable. Ever hold your hand over an open exhaust pipe while the car is reved?

The Slot and Coanda surface acts as an air-amplifier, causing up to 20 times the volume (of the exhaust flow) to be pulled around the surface. This is sorta an aerodynamic impedance matching. Think of Pressure/FlowRate to be similar to Voltage/Current. The boundary layer impedance is very very low (high flow rate) and the exhaust flow has a very high impedance.

Your very correct on the air-through the vehicle. One cannot just expect much air to flow through a small pipe. In dust collection, good flow does not really occur till at least 8 inch tubing. These small 4 inch tubing they sell for home dust collection are semi-worthless at keeping dust in suspension. The only way to do this with a frontal pressure passive system would be to use the passenger compartment as the conduit. Bad Hair Day , anybody ?

Actually, the 1970's Opel Manta (I had a '74 as my first car) actually did use the passenger compartment as the aero duct conduit, however. It had small ducts in the C pilar (B pilar actually, two door car), and the air would just howl through that car when the vents were opened up in the summer. Which was synergistic, in that the car did not have AC. But it was really not a problem, as long as one was doing at least 40 mph. To get it to really work, though, one would have to pressurize the cabin to at least 1/2 PSIG (probably with a blower), and have slots all the way round the outside of the rear of the car. A turbo blower would not be too safe for this, due to the possibility of a seal failure in the 200 K RPM shaft seals.

I think I read somewhere that the Aptera is doing this, but do to the small wake area, the amount of flow needed is very small.

[racerob]

These are very interesting ideas but I do not see the energy savings. For example: If you are going to put solar cells on a vehicle why not charge the battery rather than run the alternator? Wouldn't it create the same (or better) net gain? And if you are running the exhaust into a port which creats 1/2lb pressure what effect does that have on engine efficiency? I do not see the 'free energy' which operates the air pump.
My thoughts are that reducing frontal area is the most direct route to reducing the wake. Maybe a duct at the windshield base to feed air toward the sides at critical points would be more feasable.

[Christ]

Originally Posted by racerob

These are very interesting ideas but I do not see the energy savings. For example: If you are going to put solar cells on a vehicle why not charge the battery rather than run the alternator? Wouldn't it create the same (or better) net gain? And if you are running the exhaust into a port which creats 1/2lb pressure what effect does that have on engine efficiency? I do not see the 'free energy' which operates the air pump.
My thoughts are that reducing frontal area is the most direct route to reducing the wake. Maybe a duct at the windshield base to feed air toward the sides at critical points would be more feasable.

Except removing the high-pressure zone at the windshield will do more toward compromising aero, by creating an area where flow can attach, then will have to "ramp" the windshield-roofline bridge, which will result in more detached airflow along the top of the car, causing even more of a low-pressure area on the roof.