5. ANKARA INTERNATIONAL AEROSPACE CONFERENCE
17-19 August, 2009 - METU, Ankara - TURKEY
AIAC-2009-000
FLOW AROUND A PLUNGING AIRFOIL IN A UNIFORM FLOW
Mustafa Percin1, Aydin Misirlioglu2 and M. Fevzi Unal3
Istanbul Technical University
Istanbul, Turkey
ABSTRACT
Flow around a NACA 0012 airfoil undergoing a sinusoidal plunging motion has been investigated
experimentally and numerically. Different wake formations i.e. drag producing wake (Karman Vortex Street),
thrust producing wake (reverse Karman Vortex Street) and neutral wake have been visualized via tin
precipitation method. Numerical studies have been conducted by using commercial CFD Package Fluent
with dynamic mesh feature. Preliminary results show that leading edge vortex separation contributes to the
generation of thrust producing wake. It is planned to use DPIV method in order to acquire time dependent
velocity fields around a plunging NACA 0012 airfoil for a wide range of reduced frequencies and plunging
amplitudes. In order to examine the effect of leading edge vortex, experimental and numerical studies will be
performed for an airfoil with a sharp leading edge.
INTRODUCTION
The nature has always been a great source of inspiration for human being for ages. People have looked on
with wonder at the various mechanisms which are used by the other creatures in the nature. One of the
mechanisms which people have tried to understand the underlying aerodynamics/hydrodynamics is the
flapping wing systems of birds, insects, fish and cetaceans, such as whales and dolphins. The flapping
system, which has improved with millions year of evolution, bring these swimming and flying creatures in the
ability of staying aloft, propelling themselves and maneuvering with ease via generation of thrust and lift.
Recently, flapping wing aerodynamics has been of interest to researchers mostly due to the increased
design efforts of Micro Aerial Vehicles (MAV). MAV’s are small unmanned aerial vehicles length scales of
which are similar to those of birds and insects. Flapping wing MAV’s are given a great attention since they
would serve as mobile and stealth sensing platforms capable of gathering intelligence in hazardous and
physically inaccessible locations. When scaled to these sizes, traditional means of lift and thrust generation
become inefficient and thus a flapping wing propulsion system becomes a necessity.
On the contrary to conventional means of lift and thrust generation, natural flapping wing mechanism is
primarily based on the vortices separated from leading edge and trailing edge which result in low pressure
regions that may be used in order to create relatively higher lift and thrust compared to the fixed wing
applications [2].
Due to the aroused interest in the field of MAV’s, there have been a number of studies about flapping airfoils
and the generation of thrust. The phenomenon of thrust and lift production of flapping wing system takes its
seeds from the pioneering studies of Knoller and Betz [1]. They pointed out that a flapping motion of the wing
results in the generation of effective angle of attack. Consequently, the wing generates a lift force which has
a thrust component in the direction of free-stream velocity which is known as Knoller-Betz effect. This effect
was verified by Katzmayr by using a stationary wing in a sinusoidally oscillating wind stream and measured
an average thrust [6]. In 1924, Birnbaum used linearized potential flow theory in order to analyze the flow
around flapping foils and introduced a similarity parameter which is reduced frequency (k=2πfc/U ∞ ; f,
oscillation frequency; c, chord length; U∞, free stream velocity) [1].
1
GRA in Aerospace Engineering, Email:percinm@itu.edu.tr
Prof. in Astronautical Engineering, Email: misirli@itu.edu.tr
3 Prof. in Aeronautical Engineering, Email: munal@itu.edu.tr
2
Ankara International Aerospace Conference
AIAC-2009-000
Percin, Misirlioglu & Unal
Taylor et. al. examined the flapping frequency and amplitude of 42 species of flying animals during the
forward flight and they showed that these animals operate within the range of Strouhal number (St=fA/ U ∞; f,
oscillation frequency; A, wake width; U∞, free stream velocity) between 0.2 and 0.4 [3].
Needless to say that, the natural flight is three-dimensional and combines pitch, plunge and sweeping
motions of the wing. However, the current state of the art requires investigations towards understanding the
underlying physics of isolated motions of wing in two dimensions before extending the research into the third
dimension and the combined motion. On the other hand, the phenomena of interest in flapping wing
aerodynamics such as thrust generation, leading edge and trailing edge separation can be observed as a
result of plunging motion (vertical motion of the wing perpendicular to the free stream direction) [4].
The wake evolution of the plunging airfoil was studied experimentally and numerically by Jones et. al. [5].
Different wake formations depending on the reduced frequency and amplitude, i.e. drag producing wake
(Karman Vortex Street), thrust producing wake (reverse Karman Vortex Street), neutral wake (indicative of
zero drag) and thrust-lift producing wake (deflected reverse Karman Vortex Street), were defined and
visualized.
In early computational studies, which were based on the inviscid analyses, leading edge separation was not
taken into account. Nevertheless, leading edge vortex shedding and interaction of leading edge vortex with
the airfoil was shown to be crucial in the generation of lift and thrust [2] but still there is limited number of
studies about the effect of leading edge on the thrust producing mechanism of plunging airfoil.
In the current study, the flow around a NACA 0012 airfoil undergoing sinusoidal plunging motion is
investigated experimentally and numerically in a wide range of reduced frequency and amplitude. Then, the
results will be compared with those of the case in which an airfoil with a sharp leading edge will be used.
EXPERIMENTAL SETUP
Experiments will be performed in a large-scale water channel in Trisonic Laboratories at the Faculty of
Aeronautics and Astronautics of the Istanbul Technical University. The cross-sectional dimensions of the
main test section are 1010mm  790mm.
The airfoil made of plexiglass and manufactured in the CNC milling machine has a chord length of 10 cm
and span of 30 cm. It is transparent and allows laser light to illuminate both suction and pressure sides. The
airfoil is mounted vertically in the water tunnel about its quarter chord and at the top end it is connected to a
servo-motor. Two end plates made of plexiglass are positioned about 3 mm above and below the airfoil. The
top end plate has a slit to ensure the maximum plunging amplitude. Plunging motion of the airfoil is
accomplished with a Kollmorgen/Danaher Motion servo motor. The sinusoidal motion profile for the plunging
motion is generated by a signal generator Labview VI (Virtual Instrument).
The motion of the airfoil can be described with the following equation.
y(t) = y0+acos(2πft+φ)
In above equation, “y0” stands for the initial vertical position of the wing and “a” for the plunge amplitude. ”f”
is the plunge amplitude and “φ” is the phase angle.
Preliminary experimental studies have been conducted via tin precipitation method. Three strips of tin foil
were wrapped around the airfoil at three different spanwise positions, at the mid-span location and two
locations 8 cm away from the mid-span point in both directions in order to examine three dimensionality of
the wake flow. The flow was illuminated by a dual cavity Nd:Yag laser (max. 120 mJ/pulse). A 10-bit camera
with 16001200 pixels resolution and 30 Hz frame rate is used for image acquisition. The schematic of the
experimental setup is shown in Figure 1.
Flow visualization of the flow around the plunging airfoil has been performed at the Reynolds number of
1000 and reduced frequencies of 4, 6, 8, 16 for a dimensionless plunge amplitude (h=a/c; a, plunge
amplitude; c, chord length) of 0.1.
Quantitative flow images will be captured and processed by a Digital Particle Image Velocimetry (DPIV)
system. Two 10-bit cameras with 16001200 pixels resolution and 30 Hz frame rate will used for image
acquisition. The flow will be illuminated by a dual cavity Nd:Yag laser (max. 120 mJ/pulse). The water will be
seeded with silver coated spheres of 10μm diameter.
Ankara International Aerospace Conference
AIAC-2009-000
Percin, Misirlioglu & Unal
Figure 1: Experimental Setup
NUMERICAL METHOD
The unsteady flow field around the plunging airfoil has been acquired by using the commercial CFD Package
Fluent 6.26 with the incompressible solver, second order upwind spatial discretizations, and pressurevelocity coupling via PISO algorithm. A “C” type 191590 grid points (first normal grid point at 10-3 chord
lengths from the surface), boundaries at 20 chords in the downstream and 15 chords in the upstream, upside
and downside directions, was used for preliminary runs (Figure 2). The motion of the airfoil has been
accomplished with the dynamic mesh feature of the Fluent by plunging the body and the grid sinusoidally
with respect to free stream.
Figure 2: Computational Grid
Ankara International Aerospace Conference
AIAC-2009-000
Percin, Misirlioglu & Unal
RESULTS
As indicated above, preliminary experiments have been conducted via tin precipitation method and three
different wake modifications, i.e. drag producing wake, neutral wake, and thrust producing wake, are shown
in Figure 3. It is evident in Figure 3c that flow separates near to the leading edge and contributes to the
generation of thrust producing wake.
Figure 3: Flow visualization results (Rec= 900) a) Drag producing wake – Karman street (k=4, h=0.1)
b) Neutral wake (k=6, h=0.1) c) Thrust producing wake – Reverse Karman street
(k=8, h=0.1)
In Figure 4, a vorticity map, which is acquired from a numerical run for a case of k=1.6 and h=0.4 at the
Reynolds number of 5000, is shown in order to emphasize the effect of leading edge seperation. The airfoil
herein has a thrust producing wake with a massive leading edge seperation.
.
Figure 4: Numerical result (k=1.6, h=0.4, Rec=5000)
Ankara International Aerospace Conference
AIAC-2009-000
Percin, Misirlioglu & Unal
References
[1] Platzer, M. F., Jones, K. D., Young, J., and Lai J. C. S., Flapping-Wing Aerodynamics: Progress
and Challenges, AIAA Journal, Vol. 46, No. 9, Sep 2008.
[2] Young, J., Numerical Simulation of the Unsteady Aerodynamics of Flapping Airfoils, The
University of New South Wales, Sydney, May 2005.
[3] Taylor, G. K., Nudds, R. L., and Thomas, A. L. R., Flying and Swimming Animals Cruise at a
Strouhal Number Tuned for High Power Efficiency, Nature (London), Vol. 425, pp. 707–711, Oct
2003.
[4] Lewin, G. C. and Haj-Hariri, H., Modelling Thrust Generation of a Two-Dimensional Heaving
Hydrofoil in a Viscous Flow, Journal of Fluid Mechanics, Vol. 492, pp. 339-362, May 2003.
[5] Jones, K. D., Dohring, C. M., Platzer, M. F., Wake Structures Behind Plunging Airfoils: A
Comparison of Numerical and Experimental Results, 34th Aerospace Sciences Meeting & Exhibit,
AIAA, Jan 1996.
[6] Katzmayr, R., Effect of Periodic Changes of Angle of Attack on Behavior of Airfoils, NACA TM
147, Oct. 1922.
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Ankara International Aerospace Conference