Viscosity of air
It seems natural to see the origin of viscosity in
terms of the attractive and repulsive forces between molecules. However,
gases have substantial viscosity even though their inter-molecular forces
are weak, suggesting some other mechanism. Viscosity in gases arises
principally from the molecular diffusion that transports momentum between
layers of flow. A lot of fluid dynamics is concerned with in-viscid flow,
but the role of viscosity is crucial to understanding some of the most
important fluid phenomena, such as lift produced by a wing. (Author:
Fred Senese senese@antoine.frostburg.edu). Ref.
Touch a lift force in water
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Inner surface
The water "reflects" from spoon surface. According
to the Newton's third law ("For every action, there is
an equal and opposite reaction") the spoon moves to the opposite
direction. The motion is shown by arrows.
|
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Outer surface
The water follows the surface of the spoon. According
to Newton's third law, the spoon moves to the opposite
direction.
Why does water stick to the surface of spoon?
You can replace the surface tension's attractive force
by a repulsive one. Just put oil or fat film on the spoon's surface.
You will be able to see that the static forces (as capillarity) make no changes to
water stream. It still attracts the spoon. The effect has
a dynamic origin; It is explained by the Coanda effect.
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Touch a lift force in air
Take a shield of paper (A4 format). Cut it into 2 pieces.
Take a hair dryer and one of the paper strips. Make an experiment.
Peculiarities of dynamic ant static pressure
Dynamic pressure is the component of fluid pressure that
represents fluid kinetic energy (i.e., motion), while static pressure
represents hydrostatic effects.
| Static
pressure is isotropic - the same in all x,y,x directions.
In air, it is equal to the atmospheric pressure and does not depend
on the wing speed. |
|
Dynamic pressure of a fluid stream with density and speed u is given by
Dynamic pressure represents the fluid stream motion at a certain
direction. Air speed u is evaluated as a relative speed of the
wing.
Note1. Pillow phenomenon may explain why air flow
speed near the curved upper surface is accelerated and is higher when compared
to the air flow near the lower surface.
Note2: Air stream "sticks" to surface and bends down
near the upper surface of the wing, which is not caused by the Pillow phenomenon
but the Coanda effect.
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Note3: Do not confuse Dynamic pressure with a pressure
near airfoil surfaces. Air stream interaction with both airfoil
surfaces has a complex dependence and gives some relative
pressure, which may be applied to the normal of surface.
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Ref: |
Static pressure in a free air stream.
Static pressure is the pressure inside the stream measured
by a manometer moving with the flow. At the same time, the static
pressure is the pressure which is exerted on a plane parallel to
the flow. Thus the static pressure within an air stream has to be
measured carefully using a special probe. A thin disk must cover
the probe except for the opening. The disk must be positioned parallel
to the streaming flow, so that the flow is not interfered with.
If the static pressure is measured in the way outlined
above within a free air stream generated by a fan or a hair dryer
it can be shown that the static pressure is the same as in the surrounding
atmosphere. Bernoulli's law cannot be applied to a free air stream
because friction plays an important role. It may be noted that the
situation is similar to the laminar flow of a liquid with viscosity
inside a tube. The different velocity of the stream layers is caused
by viscosity. The static pressure is the same throughout the whole
cross-section. A free air stream in the atmosphere is exclusively decelerated
by friction. If static pressure in a free air stream is equal to atmospheric
pressure, some of the striking lecture demonstrations are interpreted
incorrectly since the effects observed are not caused by Bernoulli's
law.
Measurement of static pressure within a free stream
A sufficiently sensitive manometer can be produced easily
if not available in the lab. A fine pipe of glass is bent at one side
to dip in a cup and to be fixed according to figure 7. The meniscus
must be positioned in the middle of the pipe. The suitable inclination
should be 1:15 - 1:30. A rubber tube connects the glass pipe with a
probe. As has been pointed out before a flat disk must be glued on top
of the probe leaving the opening free. The disk has to be held parallel
to the streaming. If the static pressure is measured in such a way it
can be shown that it is equal to the pressure in the environmental atmosphere.
Information courtesy of Saulius Pakalnis, Research Support Technologies (researchsupporttechnologies.com)
