In an air duct which is open at one end and having a small vent at
the other end. I want to know if the pressure build up or increases if
I make the duct longer. To clearify it further suppose that we have
two air ducts having a cross sectional area of 1 meter each with one
end facing the wind and the other is having a small vent. if one of
the ducts is 10 meters long and the other is 2 meters long, will the
pressure at the vented ends be the same.

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Request for Question Clarification by landog-ga on 11 Aug 2005 09:22 PDT

Without going into lengthy calculations. Take a look at the equation
on this page (scroll down to Determine Pressure Drop in circular
pipes):
http://www.engineersedge.com/fluid_flow/pressure_drop/pressure_drop.htm

Lets pretend your duct is circular and frictionless - absolutely smooth, then:

The delta P (Pressure drop) gets larger when we increase L in the equation. Thus
we can say pressure in a duct or pipe DROPS as we make it longer or
INCREASES if we make it shorter. Of course the Flow Velocity and
diameter play a factor in the pressure drop also.

All the above is in extreme simplified terms.

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Request for Question Clarification by landog-ga on 11 Aug 2005 09:27 PDT

Clarification:
Pressure drop increases if duct is longer. Longer duct = less pressure build-up.
Pressure drop decreases if duct s shorter. Shorter duct = more pressure build-up.

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Clarification of Question by daiba-ga on 12 Aug 2005 23:11 PDT

Thanks  landog for your comments

If you say the shorter the duct  the greater the pressure at the vented end,
and considering that  my aim is to maximize the pressure at the vented
end, then would using a cone shaped short duct  be the best choice?

What other parameters effect the pressure assuming that this cone
shaped duct is facing a wind at a speed of 20 miles/hour ?

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Request for Question Clarification by landog-ga on 15 Aug 2005 00:25 PDT

Hi,
In my mind it seems logical that a cone shaped duct would create more
pressure build-up at the vented end if compared to a duct with the
same diameter (all the length) as the largest diameter of the cone's.

Other parameters that will have effect:
- Air velocity (is the 20 mph constant? Is it a laminar flow?)
- Friction on the duct's walls.
- Vent hole/slit sizes or ratio.
- Air Turbulence inside the duct.
- Barometric pressure may also have minute effect.


regards
Landog-ga

Reminds me of the ventilation ducts on tramp steamers.  Apparently you
need to ?choke? down to the vent area as soon as possible.  Then it is
important that there is not a build up of pressure at the vented end,
i.e., if venting into an enclosed area, that there is adequate if not
active exhaust.

Actually, landog's prior analysis is incorrect.  The frictional
coefficient of the pipe depends on the Reynolds number which is
inversely proportional to the kinematics viscosity.  For a smooth(i.e.
frictionless) pipe the kinematics viscosity is zero, meaning the
Reynolds number tends to infinity.  Furthermore, the frictional
coefficient is inversely proportional to the Reynolds number. If the
duct is absolutely smooth as he suggests then the frictional
coefficient of the pipe goes to zero. Therefore the pressure
differential along the pipe is zero.  This is intuitively obvious in
the following way: In order to have a pressure differential along the
pipe work must be done on/by the fluid.  To mediate this energy
transfer we need some sort of loss mechanism, i.e. friction with the
walls of the pipe.  There exist no outside forces in landog's
simplification.  In the real world the fluid transfers energy (via
molecular collisions) to the walls of the pipe as it traverses the
distance L.  This progressive energy loss in the fluid results in a
pressure differential along the pipe.
Hope this helps!