EXPERIMENTAL INVESTIGATION OF VISCOUS FLOW NORMAL TO NACA 0012 AIRFOIL AT LOW REYNOLDS NUMBERS

Abstract

The low Reynolds number aerodynamics at high angle of attack is crucial for the design of unmanned aerial vehicles and wind turbine blades. The current study aims to enhance the insight on the near wake of airfoils normal to free stream. The near wake structure on a NACA 0012 airfoil normal to free-stream is measured with particle image velocimetry in the range of Reynolds number 7000 to 20000. The velocity and vorticity fields of the wake structures are studied and further analysis with Proper Orthogonal Decomposition is employed. The flow-fields are found to be Reynolds number independent.

Full text

International Symposium on Sustainable Aviation 2018
9 – 11 July 2018
Rome, Italy
ISSA-2018-020
EXPERIMENTAL INVESTIGATION OF VISCOUS FLOW NORMAL TO NACA 0012 AIRFOIL AT
LOW REYNOLDS NUMBERS
Erkan Gunaydinoglu1,2 and D. Funda Kurtulus1
1 Middle East Technical University, Department of Aerospace Engineering, 06800, Ankara, Turkey
2 Turkish Aerospace Industries, 06520, Ankara, Turkey
gunaydin@ae.metu.edu.tr, dfunda@ae.metu.edu.tr
SUMMARY
The low Reynolds number aerodynamics at high angle of attack is crucial for the design of unmanned
aeri al vehicles and wind turbine blades. The curr ent study aims to enhance the insigh t on the near wake
of airfoils normal to free stre am. The n ear wake structure on a NACA 0012 airfoil nor mal to free-stream is
measured with particle image velocimetry in the range of Reynolds number 7000 to 20000. The velocity
and vort icity fields of the wake structures are studied and further analysis with Proper Orthogonal
Decomposition is employed. The flow -fields are found to be Reynolds number independent
.
Keywords: Vortex dynamics, particle image velocimetry, aerodynamics, wake flow.
INTRODUCTION
Unmanned Aerial Vehicles (UAV) has gained an
increasing importance for civilian and military
purposes and their designs demand detailed
investigation of airfoil characteristics operating at low
Reynolds numbers. Most of the studies on low
Reynolds number airfoils focus on the laminar
separation and the dynamic stall phenomenon.
Recent studies on the wake structure of airfoils
(Mahbub Alam et al., 2010, Wang et al., 2014)
emphasizes on the intrinsic features at low Reynolds
numbers and addresses the need for further detailed
investigation. The low Reynolds number
aerodynamics of blunt bodies does not only broaden
our understanding for engineering systems, but also
helps us to reveal the dynamics of biological
systems (Holden et al., 2014). The studies on two-
dimensional flows normal to blunt bodies mainly
focus on flat plates since their sharp edges form
distinctive wake vortex structures (Dennis 1993).
The high angle of attack aerodynamics of airfoils is
especially important for the design of wind turbine
blades whereas most of those studies focus on the
force and moment coefficients (Michos et al., 1983).
The current study aims to experimentally
investigate the near wake structure of a NACA 0012
airfoil normal to free stream at low Reynolds
numbers ranging from Re=7000 to Re=20000 with
particle image velocimetry (PIV) measurements.
EXPERIMENTAL METHODOLOGY
The experiments are performed in a low-
speed open type wind tunnel with maximum velocity
of 20 m/s and turbulent intensity around Tu =0.5%.
An end-to-end acrylic NACA 0012 profile with chord
length of 0.06 m is placed into the square test section
(0.34 m x 0.34 m). The images are acquired with
Phantom v640 camera with 4 megapixels resolution
at the mid-plane to ensure the two-dimensionality.
The flow is illuminated with New Wave Solo PIV 120
Nd:YAG laser with output energy of 120 mJ. For
seeding TSI oil droplet generator model 9307-6 is
used where six-jet Laskin nozzle pressurizes the
olive oil and generates seed particles with the size of
1𝜇𝑚. The image pairs are acquired at 20 Hz and
mean flow-fields are achieved by averaging 500
vector fields. The acquired images are processed
with Dantec DynamicStudio 5.1 software with
adaptive cross-correlations starting from 64x64 pixel
windows to 32x32 pixel windows with 50 %
overlapping. The resulting field-of-view with these
settings is 127x79 mm2 which results vector spacing
around 0.8 mm. The experimental arrangement of
the Particle Image Velocimetry measurements is
given in Figure 1.
Fig. 1. Particle Image Velocimetry arrangement for the
experiments
RESULTS AND DISCUSSION
PIV measurements around a NACA airfoil at four
different Reynolds numbers Re=7300, Re=9700,
Re=11200 and Re=19500 are studied. The non-
dimensional streamwise and lateral velocity
components at given Reynolds number range are
shown in Figures 2 and 3. The trailing edge of the
airfoil is located at point y = 0.2 in all cases and the
velocity components are normalized with the free-
stream velocity to achieve comparative results. The
flow is coming from right to left in all cases.

International Symposium on Sustainable Aviation 2018
9 – 11 July 2018
Rome, Italy
ISSA-2018-020
Fig. 2. Normalized mean streamwise velocity contours around NACA 0012 airfoil at (a) Re=7300, (b) Re=9700,
(c) Re=11200 and (d) Re=19500
Fig. 3. Normalized mean lateral velocity contours around NACA 0012 airfoil at (a) Re=7300, (b) Re=9700,
(c) Re=11200 and (d) Re=19500

International Symposium on Sustainable Aviation 2018
9 – 11 July 2018
Rome, Italy
ISSA-2018-020
Fig. 4. First three POD modes in x-velocity (left column) and y-velocity (right column). Every second vector is shown.
The near wake structure follows classical bluff
body characteristics and the sharp trailing edge does
not show a distinctive effect on the wake structures.
The wake bubble is found to be symmetric and
nearly similar through whole Reynolds number
range. The width of the bubble is found to be around
0.8c which agrees with Alam et al. (2010). The area
of maximum streamwise velocity is found to be
largest at Re=7300 and decreasing with increasing
Reynolds number.
The velocity data is also analyzed with Proper
Orthogonal Decomposition (POD). The POD
provides useful information on identifying the
coherent structures. The analysis consists of
acquisition of snapshots of the flow and generating
a correlation matrix for fluctuations and solving an
eigenvalue problem. The detailed information on the
theory and application of POD for turbulent flows
could be found in Lumley (1967) and Sirovich
(1987). Figure 4 shows the first three energetic POD
modes for x- and y-velocity components. Figure 5
shows the energy content of the POD modes. The
first mode shows a universal vortex structure sitting
on the center of the wake bubble.

Gunaydinoglu and Kurtulus
ISSA-2018-020
4
Fig. 5. Modal energy distribution for Re=19500
Fig. 6. Normalized mean out-of-plane vorticity contours around NACA 0012 airfoil at (a) Re=7300, (b) Re=9700,
(c) Re=11200 and (d) Re=19500
The magnitude of velocities is equal to the
turbulence intensity of the wind tunnel flow, so first
mode could be related with the in-flow turbulence.
The third mode shows similarity with the mean flow
as forming a wake bubble but this time with a smaller
bubble width.
The non-dimensional mean vorticity contours for
the cases are given in Figure 6. At this relatively low
Reynolds numbers inertial forces show minor
contribution to the wake structures. The streamlines
-not shown here- and vorticity contours are also
similar for the whole range. Rather than the small
variations in flow-fields which is in the order of
measurement uncertainty the velocity fields are also
found to be similar. The minimum streamwise
velocity is found to be -0.48U∞ in the wake bubble at
lowest Reynolds number
The vortex shedding frequency is at the center of
the flow-of-interest is measured to be St= 0.15 at
Re=7300. Keeping in mind the low resolution of the
current methodology the Strouhal number is found
to be comparable with available Laser Doppler
Anemometry measurements of St=0.144. Due to the
reflections around the airfoil, leading and trailing
edge velocities could not be measured.
CONCLUSIONS
The viscous flow normal to a NACA 0012 airfoil
at Reynolds number range of 7000 to 20000 is

Gunaydinoglu and Kurtulus
ISSA-2018-020
5
investigated through PIV. The acquired data is
further analyzed with POD to characterize the most
energetic contents of the flow. The near wake flow
in the studied range is found to be Reynolds number
independent which agrees with the force and
moment measurements in available literature.
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Viscous flow normal to a flat plate at moderate Reynolds
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