Wind Pressure Question

Can you explain why, if there is no flow , the pressure is the same. I am thinking , shouldn't it be Ptube*A1 = P(atmospheric) * A2 ?

therefore Ptube = P(atmospheric) * A2/A1 ( I am not sure) .

Because if the pressure were different, there would be flow!

The pressure at the inlet of A and B is the same. The pressure at the blocked ends in A is B is the same. Therefore, as there is no flow, because the ends are both blocked, the pressure in the middle is the same too.

Case C is analogous to:

Sew the small end up, and it no longer behaves like a windsock; it behaves like a flag.

Make sense.

Now for case C, there should be an analytical way to derive the pressure. Hope someone can show it . Thank you.

And in applying Bernoulli, realize that there is low pressure outside the discharge end of the tube, sucking the air from the tube. I don't know of any formula for calculating the pressure in such a system, that's why we have wind tunnels.

Read up on stagnation pressure.

GA. The pressure on the stagnant streamline is half the density multiplied by the square of the velocity, which is the same at the inlet in case A and case B. In case C there is no stagnant streamline.

It's actually very hard to calculate the pressure at the given points you show. There are a number of unknown factors. These diagrams may help illustrate the problem. These show standing waves for a closed tube like in a musical instrument. Depending on a number of factors the wind can set up different possible resonances, so you don't know if the point you've illustrated is at a node, or somewhere else.

For more information, look at:

http://cnx.org/content/m12589/latest/

May the forum know whether the duct is circular in cross-section, or is it something else, please? If so, what?

you can assume the tube is circular.

This brings up an interesting question :What if you designed an oblong funnel shaped intake for the front of a vehicle for engine combustion intake air?As your speed increased, the air would be forced into the engine under pressure that would increase efficiency.Not as good as a turbo charger or super charger, but with no moving parts an no load on engine, it would be a "free" boost.?

Any reasons this would not work?

Most of those highly tuned dragster engines do this, as do F1 engines. What percentage extra it brings at what speed, no idea, but it only has to be 1% or less for the racer boys to jump on the idea I guess....

Guessing more than a little, I don't think a day to day car or motorcycle engine ever goes fast enough, long enough for it to be worthwhile.....or the manufacturers would be doing it already!

Of course many everyday cars have a Turbo already, using the wasted energy in the exhaust to "Pump-up-the-Volume!".

Many Race car formulas are forbidden Turbos.....

That's my 2 (European) Cents worth!!! (They are worth more than 2 US Cents by the way!)

Thanks for the feedback.But consider this:

An intake funnel, facing forward, with a surface area 3 times the area of the throttle body or carburetor intake.

Any positive pressure provided has to be better than no pressure at all and should result in a "rammed air effect"

I know from experience that 750 Cubic Feet per Minute is all that is required for a high performance engine of around 5.0 liters.With 3 feet of area on the front funnel,the speed would only have to be 250 feet per minute (around 21 MPH) to reach break-even speed(For the moment, disregard losses thru filters,elbows,line losses, etc.)(Most of these losses will be there for normal aspiration anyway.)

The mass of air will increase as the speed increases,and the pressure should increase likewise.(1/2MxV2),so at 60 MPH,there should be significant boost pressure,considering that the 750 CFM is based on Wide Open Throttle of the engine.

This should result in a reduction of pumping losses and a reduced throttle opening for same speed.

There have to be some simple physics rules that I do not understand or have mis applied that would prevent this from being a viable performance enhancer.

I am not offended by criticism, so fire away.

I have never learned anything from someone that always agreed with me.

A related question:

What would be the pressure on a circular target, 25mm in diameter moving through stationary air, ignoring humidity,temperature, etc. for the time being,but consider if you must that it is 0% absolute humidity, and 25degrees C.The target speed is 60 mph.Ignore possible turbulence at the edges of the target.

(You see where I am going with this..)

The pressure will be too small. Even at 200 Kmh, the pressure is < 0.3 psi, from the formula given by pwslack, I have also confirmed same formula. 0.5x density of air x v ^2

Thanks!Makes sense when I look at the numbers.Just never really dawned on me how much air those turbo chargers and superchargers are pumping.Some very high performance engines require over 1200-1400 SCFM ,and to provide that much volume at several psi is a lot of work.