Unsteady Lift Enhancement Using Active Flow Control
Diego Soares Gonçalves1
Illinois Institute of Technology, Chicago, Illinois, 60616
The improvement on the performance of aircrafts for flight situations such as gusts and
high-G maneuvers can be achieved by understanding better the application of active flow
control. Actuators are installed on the leading edge of a wing to evaluate the lift response to
pulse actuation. Using particle image velocimetry (PIV) data, time-resolved velocity field and
vorticity measurements are obtained.
Nomenclature
𝐶𝐿
= average lift coefficient =
c
𝑉𝑐𝑟
g
t
tchar
=
=
=
=
=
tconv
= convective time =
𝐿
2𝑆
1/2𝜌𝑉𝑐𝑟
chord
flight speed at minimum drag
m/s
gravitational acceleration
m/s2
dimensional time
𝑉
characteristic time for minimum drag flight vehicle = 𝑐𝑟 s
m
s
𝑔
𝑉𝑐𝑟
s
𝑐
t+
= fluid dynamic dimensionless time =
T
= flight vehicle dimensionless time =
Tg
U
V
x
y
max
∆𝐶𝐿
=
=
=
=
=
=
=
𝑡
𝑡𝑐𝑜𝑛𝑣
𝑡
𝑡𝑐ℎ𝑎𝑟
dimensionless gust period
dimensionless velocity component in x-direction
velocity component in z-direction
axis parallel to the flow, positive in the downstream direction
axis perpendicular to flow direction and aligned with gravitational vector
angle of attack
lift coefficient increment = 𝐶𝐿 (𝑡) − 𝐶𝐿̅
I. Introduction
THE
dynamic stall, phenomenon where the flow over a wing at a fixed angle of attack separates, causes a drastic
decrease on the lift and increase in pitching moment. This phenomenon can also cause violent vibrations and high loads
which limits the performance of the aircraft. One option to reduce the impact of dynamic stall is the use of active flow
control (AFC), i.e., pulse-jet actuators installed on the wing in order to change the flow characteristics. These actuators
improve the aircraft performance by delaying the boundary layer separation and increasing the maximum lift coefficient,
al.1.
1
Undergraduate Research Assistant, Dept. Mechanical Materials and Aerospace Engineering, dsoaresg@hawk.iit.edu
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Figure 1. Dynamic lift response following actuator input. (a) Single pulse actuation. (b) Multi-pulse actuation.
The dynamic lift response following the single pulse and multi-pulse actuations are shown in Figure 1. The lift
increment has different responses according to the number and frequency of pulses and obtaining a solid understanding
of the physics associated to these responses is the main objective of this paper.
II. Experimental Setup
The experiments were done on the Andrew Fejer Unsteady Flow Wind Tunnel, which is low speed wind tunnel. It
has a 0.61m x 0.61m cross-sectional area and test section length of 3.1m. At the maximum velocity, 40m/s, the test
section has a turbulence level of 0.03% and is capable of producing an unsteady flow component. The wind tunnel is
equipped with an IDT ProVision PIV system capable of measuring 3 velocity components in a plane. Twin 200
mJ/pulse New Wave lasers are mounted on a sliding traverse on the top of the wind tunnel.
A 0009 NACA wing with a 0.6m span and 0.245m chord length was used to perform the experiments. The wing was
equipped with 8 piezoelectric actuators (40mm length each, spaced 74mm from each other) over the leading edge and
surface pressure sensors at six chord-wise positions at the mid-span. The actuators jet emerged from two-dimensional
slots, 40mm long and spaced 74mm from each other. The wing was adjusted with a 12° angle of attack and the PIV
system was calibrated before starting the data acquisition.
To capture images with good quality CARBOWAX TM PEG particles were used on the experiments. Studies in a
closed loop wind tunnel has shown that PEG particles provides a very good signal initially, but the seeding disappears
rapidly with time, et al.2, so a correction on the valve pressure was necessary to maintain the good quality of the
images during the experiments.
Figure 2. Experimental setup. The first image shows one of the piezoelectric actuators that were installed on the wing’s
leading edge. The second image shows the wing equipped with the actuators. The third image shows the laser trigged,
lighting the sending on the flow over the wing.
2
Control algorithms for the pulse-actuators were designed using SimulinkTM and implemented in DSP hardware using
a dSPACE/ControlDeskTM1104 system. The computational power of the dSPACE system was capable of running 100
kHz sample rate. The single-pulse case and multi-pulse case were tested for a time period of 50 t + and for different
laser trigger delay values. The pulse tested had a pulse interval of 1.75𝑡 + and a pulse width of of 0.12𝑡 + .
III. Results
Figure 3. Horizontal velocity field variation with time, showing the lift response to a single pulse actuation.
Figure 4. Horizontal velocity field variation with time, showing the lift response to a multi-pulse actuation.
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Figure 5. Vertical velocity field variation with time, showing the lift response to a single pulse actuation.
Figure 6. Vertical velocity field variation with time, showing the lift response to a multi-pulse actuation.
Figure 7. Vorticity field variation with time, showing the lift response to a single pulse actuation.
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Figure 8. Vorticity field variation with time, showing the lift response to a multi-pulse actuation.
IV. Discussion of Results
The Figures 3 through 8 shows that the actuator produces a vortex that convects over the wing surface. The horizontal
velocity flow field shows the flow detachment at 𝑡 + = 1.5 for both type of pulse actuation, although on the single pulse
case this effect was more evident. The vertical velocity flow field shows two vortices on suction surface at minimum lift
for both single-pulse and multi-pulse cases. The dynamic lift reversal occurs at 𝑡 + = 1.5 for both cases. No change was
observed on the vertical velocity flow structure for the maximum (𝑡 + = 3) and minimum lift (𝑡 + = 1.5). The analyses
of the vertical velocity flow field and vorticity flow field showed new vortex structures emerging at 𝑡 + = 7.1, 10.5 and
14 in the multi-pulse actuation, which are not observed with single pulse. This new vortex structure explains the dynamic
lift oscillations. For the single-pulse case no new vortex formation was observed.
V. Conclusions
The use of the particle image velocimetry (PIV) allowed to identify differences in flow structure between single-pulse
and multi-pulse actuation. Flow structures appeared to correlate with dynamic lift oscillations.
Acknowledgments
The airfoil experiments were performed in the Andrew Fejer Unsteady Flow Wind Tunnel in Illinois Institute
of Technology (IIT). This work was supported by the U.S. Air Force Office of Scientific Research and by the Brazil
Scientific Mobility Program (BSMP). The big support of Xuanhong An and Dr. David R. Williams are acknowledged.
References
1Seifert, A., Bachar, T., Koss, D., Shepshelovits, M., and Wygnanski I., “Oscillatory Blowing – a Tool to Delay Boundary Layer
Separation,” AIAA Journal, Vol. 31, No. 11, 1993, pp. 2052-2060.
2Stanislas, M., Kompenhans, J., Weterweel, J. (Eds.) (2000) Particle Image Velocimetry: Progress Toward Industrial Application.
Kluwer: Dordrecht.
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