David CL Francis wrote:
On Tue, 28 Feb 2006 at 07:42:32 in message
. com,
wrote:
For a fan in open air the momentum of the air moving through the
fan is equal and opposite to the momentum of the air moving around
the fan to replace the air removed from the front of the fan. There is
no net momentum change in the air. For ducted flow that returns the
air to the front fo the fan, the net momentum is also zero. Net flow
and
net momentum through any closed loop is zero--else the 'loop'
is not 'closed'.
Followjng a wing in level flight, the downward momentum of the
air in the downwash is equal and opposite to the upward momentum
of the air to either side that moves up to replace the air that washes
down. There is no net momentum change in the air.
This whole discussion is becoming weird. The power required to drive a
fan goes somewhere. There is and must be a net increase in the air
velocity across the fan so there is a change of momentum from the air
entering to the air leaving. The power input results in a momentum
change. This principle applies to fans, helicopters, wings and other
things - even rowing! The air that leaves the driven fan, jet engine,
ducted fan, wing is the result of the thrust or vice versa, how ever you
like to think of it.
The momentum of an airplane in level flight at
constant speed is constant. Conservation of
momentum REQUIRES that there is no net
change in themomentum of the air. There is
momentum exchanged between the airplane
and the air. But there is no NET momentum
change in the air.
However, that speeded up air dissipates itself in the atmosphere
gradually giving up energy to the surrounding air as it all eventually
steadies down again. What does that do? Well in the ultimate I guess it
raises the temperature of the atmosphere slightly!
Yes. In closed loop ducted flow it can raise the temperature
of the air a lot.
An airscrew does much the same. It captures air from a tube somewhat
larger that its diameter, speeds it up and it goes out the back faster
then it came in. if you had a closed circuit like a wind tunnel it still
requires power to accelerate the air to the required speed. Less than an
open system though a carefully shaped return duct will slow the air down
again ready to be accelerated again. In that cases the losses have to be
made up.
What is a wing but a kind of linear fan?
There is a net transfer of energy from the airplane to the air.
There is not a net transfer of momentum from the airplane to the air.
Momentum is a vector, energy is a scaler.
--
FF