The
Coandă effect ( ) is the tendency of a
fluid jet to be
attracted to a nearby surface. The principle was named after
Romanian aerodynamics pioneer
Henri Coandă, who was the first to
recognize the practical application of the phenomenon in aircraft
development.
Discovery
An early description of this phenomenon was provided by
Thomas Young in a lecture given to
the
The Royal Society in
1800:
The lateral pressure which urges the flame of a candle
towards thestream of air from a blowpipe is probably exactly
similar to that pressure whicheases the inflexion of a current of
air near an obstacle. Mark the dimple which aslender stream of air
makes on the surface of water. Bring a convex body intocontact with
the side of the stream and the place of the dimple will
immediatelyshow the current is deflected towards the body; and if
the body be at liberty tomove in every direction it will be urged
towards the current.
A hundred years later, Henri Coandă identified an application of
the effect during experiments with his
Coandă-1910 aircraft, which is the first
aircraft to use a
motorjet (an early type
of
jet engine).
In 1934 he obtained a
patent in France
for a
"Method and apparatus for deviation of a fluid into another
fluid". The effect presently named after Coandă was
described as the "Deviation of a plan jet of a fluid that
penetrates another fluid in the vicinity of a convex wall."
Causes
The Coandă effect is a result of entrainment of ambient fluid
around the fluid jet. When a nearby wall does not allow the
surrounding fluid to be pulled inwards towards the jet (i.e. to be
entrained), the jet moves towards the wall instead. The fluid of
the jet and the surrounding fluid should be essentially the same
substance (a gas jet into a body of gas or a liquid jet into a body
of liquid). In one application, a jet of air is blown over the
upper surface of an airfoil, which can have a strong influence on
the overall lift, especially at high angles of attack when the flow
would otherwise separate (stall). See
Blown
flap.
Applications
The Coandă effect has important applications in various high-lift
devices on
aircraft, where air moving over
the wing can be "bent down" towards the ground using flaps and a
jet sheet blowing over the curved surface of the top of the wing.
The bending of the flow results in its acceleration and as a result
of
Bernoulli's principle
pressure is decreased;
aerodynamic
lift is increased. The flow from a high speed jet engine
mounted in a
pod over the wing
produces enhanced lift by dramatically increasing the velocity
gradient in the
shear
flow in the boundary layer. In this velocity gradient particles
are blown away from the surface, thus lowering the pressure there.
Closely following the work of Coandă on applications of his
research, and in particular the work on his "Aerodina
Lenticulară,"
John Frost of
Avro Canada also spent considerable time
researching the effect, leading to a series of "inside out"
hovercraft-like aircraft where the air
exited in a ring around the outside of the aircraft and was
directed by being "attached" to a flap-like ring.
The first Avrocar being readied at the
Avro factory in 1958
This is as opposed to a traditional hovercraft design, in which the
air is blown into a central area, the
plenum, and directed
down with the use of a fabric "skirt". Only one of Frost's designs
was ever built, the
Avrocar.
The
VZ-9 AV Avrocar (often listed as
VZ-9) was a Canadian
vertical takeoff and landing
aircraft developed by
Avro Aircraft Ltd.
as part of a secret U.S. military project carried out in the early
years of the
Cold War. The Avrocar intended
to exploit the Coandă effect to provide lift and thrust from a
single "turborotor" blowing exhaust out the rim of the disk-shaped
aircraft to provide anticipated
VTOL-like
performance. In the air, it would have resembled a
flying saucer. Two prototypes were built as
"proof-of-concept" test vehicles for a more advanced
USAF fighter and also for a
U.S. Army tactical combat
aircraft requirement.
The effect was also implemented during the
U.S. Air Force's
AMST project. Several
aircraft, notably the
Boeing YC-14 (the first modern type to exploit the
effect), have been built to take advantage of this effect, by
mounting
turbofans on the top of wing to
provide high-speed air even at low flying speeds, but to date only
one aircraft has gone into production using this system to a major
degree, the
Antonov An-72 'Coaler'.
The C-17 Globemaster III uses the
Coandă effect for a comfortable ride at low flying speeds
The
McDonnell Douglas YC-15
and its successor, the Boeing
C-17
Globemaster III, also employ the effect. The
NOTAR helicopter replaces the conventional
propeller tail rotor with a Coandă effect
tail.
An important practical use of the Coandă effect is for inclined
hydropower screens, which separate debris, fish, etc., otherwise in
the input flow to the turbines. Due to the slope, the debris falls
from the screens without mechanical clearing, and due to the wires
of the screen optimizing the Coandă effect, the water flows though
the screen to the penstocks leading the water to the
turbines.
The Coandă effect is also used to make automotive windshield
washers which function without moving parts and to create pneumatic
logic circuits.
The operation principle of oscillatory flowmeters also relies on
the Coanda phenomenon. The incoming liquid enters a chamber that
contains 2 "islands". Due to the Coanda effect the main stream
splits up and goes under one of the islands. This flow then feeds
itself back into the main stream making it split up again, but in
the direction of the second isle. This process repeats itself as
long as the liquid circulates the chamber, resulting in a self
induced oscillation that is directly proportional to the velocity
of the liquid and consequently the volume of substance flowing
through the meter. A sensor picks up the frequency of this
oscillations and transforms it into an analog signal yielding
volume passing through.
In
air conditioning the Coandă
effect is exploited to increase the
throw of a
ceiling mounted
diffuser.
Because the Coandă effect causes air discharged from the diffuser
to "stick" to the ceiling, it travels farther before dropping for
the same discharge velocity than it would if the diffuser was
mounted in free air, without the neighbouring ceiling. Lower
discharge velocity means lower noise levels and, in the case of
variable air volume (VAV) air
conditioning systems, permits greater
turn-down ratios.
Linear diffusers and
slot diffusers that present a greater length
of contact with the ceiling exhibit greater Coandă effect.
In
cardiovascular medicine,
the Coanda effect accounts for the separate streams of blood in the
fetal right
atrium. It also explains why eccentric
mitral regurgitation jets are
attracted and dispersed along adjacent left atrial wall surfaces
(so called "wall-hugging jets" as seen on echocardiographic
color-doppler interrogation). This is clinically relevant because
the visual area (and thus severity) of these eccentric wall-hugging
jets is often underestimated compared to the more readily apparent
central jets. In these cases, volumetric methods such as the
proximal isovelocity surface area (PISA) method are preferred to
quantify the severity of
mitral
regurgitation.
In
meteorology, the Coanda effect theory
has also been applied to some air streams flowing out of mountain
ranges such as the Carpathian Mountains
and Transylvanian
Alps, where effects on agriculture and vegetation have been
noted. It also appears to be an effect in the
Rhone Valley in France and near
Big
Delta
in Alaska.
Demonstration
The Coandă effect can be demonstrated by directing a small jet of
air upwards at an angle over a ping pong ball. The jet is drawn to
the upper surface and curves around, diverting the flow downwards
over the back. This change in the momentum of the air flow is
reacted out in the reduced pressure on the upper surface of the
ball, this suction being sufficient to overcome the weight of the
ball (when there is enough air flow). This demonstration can be
performed using a vacuum cleaner if the outlet can be attached to
the pipe and aimed upwards at an angle.
A common misconception is that Coandă effect is demonstrated when a
stream of tap water flows over the back of a spoon held lightly in
the stream and the spoon is pulled into the stream. While the flow
looks very similar to the air flow over the ping pong ball above
(if one could see the air flow), the cause is not really the Coandă
effect. Here, because it is a flow of water into air, there is no
entrainment of the surrounding fluid (the air) into the jet (the
stream of water). This particular demonstration is dominated by
surface tension.
Another demonstration is to direct the air flow from, e.g., a
vacuum cleaner operating in reverse, tangentially past a round
cylinder. A waste basket works well. The air flow seems to "wrap
around" the cylinder and can be detected at more than 180° from the
incoming flow. Under the right conditions, flow rate, weight of the
cylinder, smoothness of the surface it sits on, the cylinder will
actually move. Note that the cylinder will not move directly into
the flow as a misapplication of the Bernoulli effect would predict,
but at a diagonal.
The effect can also be seen by placing a can in front of a lit
candle. If one blows directly at the can, the air will bend around
it and extinguish the candle.
If two lit candles are placed side-by-side, the heated air from
each candle rises and entrains surrounding air. Since both "jets"
are trying to entrain common air from the space between the two
streams, they are drawn towards one another. This is more apparent
if the candles are making a little smoke. This is a demonstration
of the Coandă effect without the presence of any surface. In some
sense, the plane of symmetry between the two flows can be thought
of as the surface.
Problems caused
The Coandă effect has disadvantages as well as advantages.
In marine propulsion, the efficiency of a
propeller or
thruster can
be severely curtailed by the Coandă effect. The force on the vessel
generated by a propeller is a function of the speed, volume and
direction ofthe water jet leaving the propeller. Under certain
conditions (e.g. when a ship moves through water) theCoandă effect
changes the direction of a propeller jet, causing it to follow the
shape of the ship's
hull.The side
force from a
tunnel thruster at the bow
of a ship decreases rapidly with forwardspeed.
The side thrust may completely disappear at speeds above about 3
knots.
See also
References
- Tritton, D.J., Physical Fluid Dynamics, Van Nostrand Reinhold,
1977 (reprinted 1980), Section 22.7, The Coanda Effect.
- [1], FLUID DYNAMICS by Mihaela-Maria Tanasescu,
Texas Tech University
- Yenne 2003, pp. 281–283.
- Milberry 1979, p. 137.
- [2], Coandă effect used in debris screen
design.
- United States Patent 4210283. Dual pattern windshield washer
nozzle. Available at:
http://www.freepatentsonline.com/4210283.html
- Spitzer, David W. "Industrial Flow measurement". Instrument
Society of America, 1990.
- Ashrafian H. The Coanda effect and preferential right atrial
streaming. Chest. 2006 Jul;130(1):300.
- Giles, B.D. Fluidics, The Coanda Effect, and some orographic
winds. Arch.Met.Geoph.Biokl. Ser.A. 25, 1977, 273-279
External links
Wikis
Quiz
Map
Search
