Flapping wing aerodynamics
in micro-aerial vehicles
There is an ever-increasing interest in the field of MicroAerial Vehicles (MAV’s) all around the world with recent
developments in micro- and nano-technologies. MAV’s are
defined as small Unmanned Aerial Vehicles (UAV’s) with
length scales similar to birds and insects. In addition to
comparable physical dimensions, MAV’s also have similar
flight mechanisms with those in nature, such as flapping
wing propulsion.
A local example of a flapping wing MAV is the ‘The DelFly’,
which was born in 2005 as a student project at TU Delft.
The design was inspired by nature, particularly the
‘dragonfly’, and it was really promising. After two years of
development, it evolved into a lighter and smaller model,
the ‘DelFly II’. It has a wing-span of 28cm and it weighs
only seventeen grams with an onboard camera that is
employed for observation and vision-based control
purposes. However, the ultimate goal of the project is to
build a DelFly of even smaller proportions, with a wingspan of five cm. To reach this goal, the first step was taken
in the building of the ‘DelFly Micro’. It is still the smallest
member of the DelFly family and has a wing-span of ten
cm and weighs three grams, still being equipped with an
onboard camera.
The goal of the project is to understand the underlying
aerodynamic mechanisms of flapping wing mechanisms,
particularly ‘clap-and-peel’ motion of the DelFly wings, and
use this information for futher development. Special focus
is given on the effect of wing flexibility which is shown to
be a crucial parameter in flapping wing aerodynamics.
PhD Candidate: Mustafa Percin
Department: AWEP
Section: Aerodynamics
Supervisor: B. W. van Oudheusden
Promoter: F. Scarano
Start date: 01-10-2010
Funding: STW
Cooperations: NATO STO
Sketch of the top view of the experimental setup
As a result of these experiments, the general wake
topology is described in conjunction with the behavior of
distinctive flow structure, in particular, the occurrence and
development of tip vortex, trailing edge vortex, and root
vortex. Different vortex mechanisms are observed for
different flapping parameters.
Reconstructed 3D wing geometry
Fundamental Understanding of Flapping Wing
Aerodynamics
In addition to the experiments performed on the DelFly,
simplified experiments were performed in the context of
the project. In accordance with the activities of NATO AVT202 panel, Tomographic Particle Image Velocimetry (TomoPIV) measurements were carried out in a water during the
accelarating phase of rotating flat plate. The aim of this
project is to explore the evolution and stability of the
Leading Edge Vortex (LEV), which is regarded as the major
element in the generation of forces.
Aerospace Engineering
Iso-surfaces of vorticity magnitude z vorticity (wz) at
the end of upstroke (k=0.5 (left) and k=1 (right))
How much do the DelFly Wings Deform?
It is known that flexibility plays an important role in the
generation of forces in flapping wing mechanisms. Previous
studies on the DelFly wings also showed that force
generation and force production strongly depends on the
wing thickness, stiffener position, and orientation.
Therefore, assesing deformation of the wings during the
flapping motion is crucial and also valuable as an input for
the numerical simulations.
Experimental setup for Nato AVT-202 Basic Test Case
Measurements
The same setup was also used to investigate the effect of
wing flexibility on the vortical structures generated during
the clap-and-peel type flapping motion. It was shown that
the flexibility affects the magnitude and the behaviour of
the LEV.
DelFly II in flight
Wake Flow of the DelFly II
Understanding wake flow in the flapping wing systems can
reveal variety of characteristics related to the force
generation mechanisms. Especially, it is crucial particularly
for the DelFly, which has two wing pairs and a tail that is
presumably interacting with the wake structures of the
wings.
In order to gain better understanding of this phenomenon,
a detailed experimental study was performed in a wind
tunnel. Time-resolved velocity measurements were
performed in the wake of the flapping wings of the DelFly
II in forward flight configuration, using the Stereoscopic
Particle Image Velocimetry (Stereo-PIV) technique.
Images were recorded at several spanwise oriented planes
in the wake of the flapping wings with three high speed
cameras and a high speed laser. Then, three-dimensional
reconstruction of the wake was perfomed by use of Kriging
regression technique with local error esimates [2]. A simple
error model that is derived from the local PIV correlation
map was used to estimate local error for each velocity
vector.
Images taken by two high speed cameras at an instant
of the flapping motion
Two high speed cameras oriented at an angle with respect
to each other recorded the DelFly II wings flapping at high
recording rates. This time, the DelFly II wings had a
structured grid of 89 small markers on each of them. A
white background was generated by use of a strong light
source and a diffusive surface underneath the DelFly
wings.
Images of markers were captured at high recording rates.
The processing of the images involves image preprocessing, detection of lines (leading edge and stiffeners)
and points (grid points), and triangulation of detected
points to find their coordinates in three-dimensional space.
As a final step, the complete wing geometry is
reconstructed by use of Kriging regression technique with
local triangulation uncertainities. During the regression, a
special attention is given to the conservation of wing
surface area between the markers.
Iso-surfaces of vorticity magnitude at f=0.75 Hz
Future Works
Although the flow in the wake of the DelFly II wings are
visualized for a wide set of parameters, force
measurements are still missing. A new balance system has
been realized recently which will be used for force and
torque measurements simultaneously with the deformation
measurements.
A water submergible force sensor will also be integrated to
the water tank system for a better interpretation of the
visualization data.
Publications
- M. Percin, Y. Hu, B.W. van Oudheusden, B. Remes, and F. Scarano, “Wing Flexibility Effects in Clap-and-Fling”, International Journal of Mico Air Vehicles, 3, 4, 2011.
- J.H.S. de Baar, M. Percin, R.P. Dwight, B.W. van Oudheusden, and H. Bijl, “Kriging regression of PIV data using a local error estimate,” submitted to Experiments in Fluids, 2012.