| |
By Ralf Kroeger, Fluent Germany
View the pdf of this article
Particle tracks show the air movement caused by the flapping wings
The ability to fly has always been a dream of man. In the early 1800s,
Sir George Cayley made many observations of soaring birds and applied
his findings to the development of a series of gliders in England1. In
1853, his coachman served as the passenger on the first manned glider
flight. In 1889, Otto Lilienthal published Birdflight as the Basis of
Aviation, the first book on aviation2. In it, he demonstrated the general
concepts of aeronautics as well as the mechanical principals of bird flight,
and he reported on his experiments and measurements on curved gliding
wings. It served as an inspiration for the Wright Brothers during the
years prior to their first flight in 1903. When early attempts at human
flight were made, only the gliding technique of large birds was considered.
Recently, however, engineers have begun to examine the flapping-wing technique
of small birds and insects as a possible basis for small, highly-maneuverable
aircraft.
Early aircraft engineers realized that to make a gliding airplane fly,
two mechanical components were needed to separately produce thrust (the
engine) and lift (rigid wings). The quasi-steady aerodynamics of fixed-wing
aircraft are now well understood, and engineers have built airplanes that
are both fast and capable of carrying heavy loads. Airplanes are not very
flexible for maneuvering, however, and to gain this ability, helicopters
were conceived. Helicopters have a primary propeller that provides lift
and thrust, by pulling the aircraft in the direction of the orientation
of the propeller axis.
The angle of attack vs. time (top) and the wing tip path as seen from
the side of the fly (bottom)
Despite the broad range of capabilities offered by airplanes and helicopters,
birds and insects have far more maneuverability for two reasons. First,
their small size permits it, and second, the principle of flying used
by these creatures is completely different. Rather than separating the
thrust and lift functions, evolution took a different path and designed
one single body part that can produce these two effects at the same time.
Small, flexible, almost two dimensional wings evolved over millions of
years to move rapidly in a very complex way, and result in versatile flight.
Engineers have examined insect wings in wind tunnels and measured their
steady state lift and drag coefficients. Their early observations suggested
that some insects are not able to fly in a steady-state mode. To better
understand the flight mechanism used, unsteady wing motion and transient
lift and drag characteristics have been recorded from living insects.
To complement the wind tunnel measurements, which are difficult to perform,
enlarged robotic models of insects have been built to “fly”
in water tunnels. These experiments have helped engineers to better understand
the principle of flapping-wing flight. Insects have one or two pairs of
stiff but flexible wings, which flap between 20 and 600 times per second.
Each wing moves not only up and down, but forward and backward as well,
and during the flapping cycle, the angle of attack can be varied by rotating
the wing around its axis3. In certain sudden maneuvers, insects move each
wing of a pair differently.
Surface mesh on the fly
Scientists now understand that there are three basic principles that
contribute to flapping-wing aerodynamics4. While delayed stall (which
is undesirable in steady-state airplane flight, but effective for insect
wings that move quickly) occurs during the translational up and down stroke
of the wing at high angles of attack, rotational circulation (similar
to back spin on a tennis ball) and wake capture (to regain energy from
vortices that were shed during the last flapping cycle) occur during the
fast wing rotations at the top and bottom end of the stroke. The latter
two effects are most likely used by insects to maneuver. While the main
principles have been identified, the exact interactions between them are
still the subject of ongoing research.
As the understanding of insect flight aerodynamics has progressed in
recent years, along with that of micro-technology, engineers have turned
their attention toward developing unmanned micro-aircraft, which are essentially
flying micro-robots. Such devices, about the size of a thumb, could be
used for search and rescue missions or to detect harmful substances or
pollutants in areas that are not accessible by, or too dangerous for,
humans. Since experiments in wind tunnels are still very complicated to
perform, and scale-up to enlarged models can introduce added uncertainty,
CFD will be a key ingredient in the success of this effort. With the dynamic
mesh capability in FLUENT 6.1, the threedimensional, unsteady aerodynamics
of flapping wings can be examined in great detail, providing insight that
could be applied to the design of micro-aircraft in the future.
As an example, the airflow around the flapping wings of a fly has been
simulated in FLUENT. A realistic fly body (in stereolithography, or STL
format) was imported into GAMBIT and, with the help of TGrid, a mesh of
600,000 cells was created. The complicated wing motion of a real fly was
adopted from literature data5. The wing itself was idealized to be a rigid
body with one fixed point at the joint of the wing, around which the wing
was rotated. The rotation vector was varied in size and direction over
the duration of the flapping cycle, according to the wing kinematics.
The fly makes 125 wing flaps per second, so each cycle is 8 ms in duration.
Two complete cycles were simulated, using a time step of 5 ms. The insect
was assumed to move in a Surface mesh on the fly Pressure on wings from
below Delayed stall, occurring midway down the stroke; white surfaces
represent shedded vortices forward direction at a speed of 2.75 m/s, so
a laminar flow of air at 2.75 m/s was imposed at the inlet, upstream of
the fly. Only the mesh in the vicinity of the moving wing was included
in the dynamic mesh zone, where remeshing was performed at every time
step.
Delayed stall, occurring midway down the stroke; white
surfaces represent shedded vortices
Pressure on wings from below
The formation of vortices behind the wings was observed in the simulation,
and these vortices were found to shed during the quick rotation of the
wings at the top and bottom extremes of the stroke. Other unsteady effects
like delayed stall and wake capture were observed during the cycle as
well. Pressure distributions on the upper and lower sides of the wing
were integrated to obtain unsteady lift and drag coefficients.
The FLUENT simulation has shown that with the dynamic mesh model, it
is now possible to simulate the three-dimensional flow around flapping
wings to better understand the unsteady aerodynamics of insects, and to
contribute to the development of micro-aircraft. The model can also be
used to examine critical flight situations for fixed-wing aircraft, like
the undercarriage lowering at low air speed, or the movement of swept
wings on fighter jets at high air speed. Expanding beyond flight applications,
the dynamic mesh model can be used for simulations of moving heart valves
in the biomedical area (see p.25), or small flapping membrane valves in
micro-equipment.
References:
- www.flyingmachines.org/cayl.html
- O. Lilienthal, Birdflight as the Basis of Aviation, London/New York,
1911 / Hummelstown, 2001.
- C.P. Ellington, Phil. Trans. R. Soc. Lond. B305, p. 1-181, 1984.
- M.H. Dickinson, F.-O. Lehmann and S.P. Sane, Science 284, p. 1954-1960,
1999.
- W. Nachtigall, “Dipterenflug” In: Lokomotion in Fluiden,
Biona-Report 11, p.115-156, Fischer Verlag, 1997.
|
|
|
