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Author Topic: Positive and Negative Pressure  (Read 2193 times)
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« on: December 06, 2013, 02:30:23 PM »

This has been talked about before in various comments.  Usually the discussion has something to do with rear mud flaps.

However my question is under the bus, front bay.  If I cut a hole in my bay floor, am I going to deal with air being sucked out of my bay (negative pressure), or air being forced into my bay (positive pressure)?

Obviously I'm talking about while driving   Smiley


1989 MCI-102 A3
DD 6V92 Turbo, Alison
Tons of stuff to learn!
Started in Cheboygan, Michigan (near the Mackinaw Bridge).  Now home is anywhere we park
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« Reply #1 on: December 06, 2013, 02:42:48 PM »

That is just about impossible to answer because of all the things affecting air flow under a bus, it even varies at points not far apart.

It also depends on if there is another opening in the bay, especially at a distance from the bottom hole. If not there may be no pressure effect at all.

If there is negative pressure you will have a bay full of dirt!

In general, if you place a flap ahead of an opening it causes lower press at the opening, behind causes higher - but this isn't always true.

Ash Flat, AR
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1980 MCI MC-5C

« Reply #2 on: December 06, 2013, 04:54:38 PM »

Bernoulli's Principal states that when you take a fluid flow and force it into a smaller space it speeds up.  The increased speed lowers the pressure.  The space under the bus is turned into a high velocity low pressure area, along the entire floor, with the highest low pressure (if that makes sense) at the forward end, and the lowest low pressure at the rear of the bus as air spills back in along the sides of the bus.  If you want to further increase the pressure differential you can put a flap down, the area in front of the flap has higher pressure and the area behind the flap has lower pressure.  The air pressure along the side of the bus is like a teardrop - air flows out at the front and has low pressure at the bus surface, attaches again along the side of the bus, and has high pressure at the rear of the bus, hence side mounted radiators.  Generally speaking, if you have a hole in the floor of a bay it will tend to suck air out of the bay and into the space under the bus.  You can increase the effect by putting a flap in front of the hole, and reduce the effect by putting a flap behind the hole.


1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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