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Scaling Studies of Cyclotronic Plasma Actuators for Active Flow Control Applications

Authors:
American Institute of Aeronautics and Astronautics
1
Scaling Studies of Cyclotronic Plasma Actuators for Active
Flow Control Applications
Joseph W. Zimmerman1, and David L. Carroll2
CUA, Champaign, Illinois 61822
and
Georgi K. Hristov3, Moiz Vahora4, Martin K. Motz5, David Reese Richardson6, and Phillip J. Ansell7
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
An innovative plasma-based flow control actuator has been developed in recent work.
The technique produces a high-voltage plasma arc across a coaxial pair of electrodes
positioned within the field of a strong rare-earth ring magnet. Plasma arc generation within
a magnetic field perpendicular to the current path results in a Lorentz force on the charged
particles, causing the arc to sweep about the center of the coax, forming an apparent plasma
disc. Being similar in concept to microwave-generating cyclotron elements, the resulting
actuator concept has been designated as a “Cyclotronic Arc-Plasma Actuator”. The
innovative aspect of this concept is the coupling of the thermal actuation of the plasma arc
filament along with the induced swirl component produced by the angular velocity of the
Lorentz forcing. Arrays of this actuator configuration can be applied in boundary layer
flow control by embedding the devices spanwise in an aerodynamic surface. The purpose of
the device is to alleviate turbulent flow separation, serving as a controllable vortex generator
that can be enabled or disabled on-demand (e.g., during takeoff and landing), avoiding a
parasitic drag penalty during high speed cruise. Demonstration of this technology in the
current research effort pioneers a class of plasma actuators aimed at addressing a notorious
problem in active flow control. The goal of the ongoing research effort is to demonstrate the
impact of these actuators in realistic flows, applying the devices in wind tunnel models and
on UAV platforms. Recent bench testing has been focused on optimizing and scaling the
actuators, and developing the capability of operating the actuators as modular arrays for
demonstration experiments.
Nomenclature
AC = alternating current
B = magnetic field strength
CAPA = cyclotronic arc-plasma actuator
CRT = cathode ray tube
DBD = dielectric barrier discharge
DC = direct current
F = force, Lorentz force
fps = frames per second
HV = high voltage
1 Senior Scientist at Champaign-Urbana Aerospace (CUA), Member AIAA.
2 President of Champaign-Urbana Aerospace (CUA), Fellow AIAA.
3 Graduate Research Assistant, Department of Aerospace Engineering, Student Member, AIAA
4 Graduate Research Assistant, Department of Aerospace Engineering, Student Member, AIAA
5 Undergraduate Research Assistant, Department of Aerospace Engineering, Student Member, AIAA
6 Undergraduate Research Assistant, Department of Agricultural and Biological Engineering
7 Assistant Professor, Department of Aerospace Engineering, Senior Member AIAA.
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AIAA Scitech 2019 Forum
7-11 January 2019, San Diego, California 10.2514/6.2019-0047
Copyright © 2019 by Joseph W.
Zimmerman, Georgi K. Hristov, Moiz Vahora, Martin Motz, David Reese Richardson, Phillip J. Ansell, David L. Carroll. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
AIAA SciTech Forum
American Institute of Aeronautics and Astronautics
2
I = current
MHD = magnetohydrodynamics
PIV = particle image velocimetry
RPM = revolutions per minute
UV = ultraviolet
V = flow velocity or voltage
V
= free stream flow velocity
VG = vortex generator
ZVS = zero voltage switching
I. Introduction
se of modern flow control techniques has been hindered by high power requirements, integration challenges,
and insufficient control authority of existing methods. Champaign-Urbana Aerospace (CUA) and the
University of Illinois at Urbana-Champaign (UIUC) have teamed to develop an innovative plasma flow
control actuator, to be applied for alleviation of turbulent boundary-layer separation. By positioning a pair of coaxial
actuator electrodes with high-voltage within a magnetic field, a sweeping plasma arc is produced which acts to
enhance mixing of the flow, similar to a traditional, passive vortex generator (VG). This novel flow control
technique, referred to as a cyclotronic arc plasma actuator (CAPA), combines the efficiency of vortex-based mixing
for inhibiting boundary-layer separation with on-demand capabilities. The present work has the objective of
demonstrating the plasma actuators as flow control devices in wind tunnel testing and on a UAV platform.
The typical actuator configuration studied in the current work is shown in Figure 1. The Lorentz forcing on the
arc is described in the top view diagram, Figure 1a. The plasma arc is produced above a dielectric plate placed at the
end of a coax, and the magnetic field is provided by a rare-earth ring magnet placed within the coaxial gap, below
the dielectric, Figure 1b. High voltage (HV) is supplied to the center electrode to produce the plasma arc, while the
outer electrode is grounded.
High speed imaging was applied to show that the plasma is actually an arc moving around the circumference of
the coax at high speed, rather than a plasma disc (as it appears to the human eye), Figure 2a. Figure 2b shows a
Schlieren image of a coaxial actuator (side view) in quiescent air. Heating generated by the actuator produces
density gradients in the air which are visualized in the Schlieren images. Rising warm air, due to the localized
heating introduced by the arc filament, can be observed in combination with the swirling effect caused by the
rotating arc filament.
(a) top view diagram (b) side view diagram
Figure 1. Coaxial Actuator Configuration and Imaging Examples
U
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(a) high speed image capture of arc filament
(b) Schlieren image capture (quiescent)
Figure 2. CAPA Imaging Examples
By producing a plasma arc in the fashion described above, the cyclotronic plasma actuator is able to produce a
localized arc-induced perturbation, similar to existing SparkJet plasma-based flow control techniques, [1-4] but with
an additional swirl component. Most modern plasma-based flow control techniques, such as dielectric barrier
discharge (DBD) actuation, have been shown to provide an effective means to control aerodynamic flows with
minimal mechanical complexity [5-9]. However, traditional AC-DBD flow control devices have been limited to
very low-speed applications, due to the limited actuator authority provided by the actuation device. For the
cyclotronic plasma actuator, the additional swirl component is leveraged to produce three-dimensional, streamwise-
oriented vortical structures which are useful for preventing boundary-layer separation in the presence of high
adverse pressure gradients.
Furthermore, the use of plasma-based thermal manipulation for boundary-layer control, such as that used in
nanosecond DBD (ns-DBD) [10-12] has been shown to provide actuation amplitudes significantly greater than the
ionic wind actuation provided by traditional DBD actuators. Additionally, the use of pulsed-DC plasma actuation
has been an active area of investigation, which has recently shown to provide a plasma-induced thrust force that is
approximately six times larger than that of typical AC-DBD actuation, for a fixed voltage [13].
The fluidic mixing technique used by the cyclotronic plasma actuator is also similar to the approach used with
passive vortex generators, which are known to be a highly effective in for preventing boundary-layer separation for
fixed-wing aircraft [14-18] However, unlike vortex generators, the cyclotronic plasma flow control actuator can be
enabled on-demand, allowing the actuators to introduce vortices during critical phases of flight, like take-off, climb,
approach, and landing, as shown in Figure 3, while being disabled during cruise. This flexibility of the proposed
innovation allows the drag penalty associated with fixed, passive vortex generator devices to be avoided during
high-speed cruise. A wind tunnel model purpose-built for these studies is shown in Figure 4.
Figure 3. Illustration of low-complexity, on-demand
cyclotronic plasma actuator on an airfoil.
Figure 4. S8036 Airfoil Installed in the Wind Tunnel Test
Section with Actuators at x/c = 0.49
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Although the plasma flow-control technique described here is a new approach, the forcing of plasma discharges
in a magnetic field for fluid dynamics control is not a new idea. Magnetohydrodynamics (MHD) has been applied in
various areas of aerospace research such as space propulsion [19], plasma-assisted combustion (PAC) [20], heat-flux
mitigation for atmospheric reentry [21], high speed flow control [22], and power extraction from hypersonic flow
[23]. The review by Braun et al. summarizes a number of MHD flow control examples [24]. In a more recent
example, coaxial configurations with electromagnets placed around a micro-cathode discharge have been applied in
small-satellite propulsion techniques [25, 26]. MHD control of electric discharges has also been investigated,
measuring interactions of the electric discharge with direct and swirling flows, motivated by applications to flight
control and intensified mixing for combustion [27]. As quasi-DC discharges are now being researched for high
speed flow control [28] and combustion cases [29], there is the possibility that the arc-magnet actuator technology
explored for boundary-layer flow control in the current paper may also apply in these areas. The extension of the
coaxial arrangement to combustion burners [30] and chemical plasma reactors [31] is straightforward, having been
applied with coaxial microwave cavities; the introduction of a strong external magnetic field in reactors such as
these to exploit mixing and energy deposition effects offers a potential new research area.
This paper summarizes the most recent experimental investigations performed for the development of the
cyclotronic plasma actuator. The experiments discussed here are a continuation of the recently reported work [32-
34] which evaluated the actuator configuration in low-speed wind tunnel tests using PIV in a zero-pressure gradient
boundary layer, as well as pressure recovery measurements with the actuator placed upstream of an expansion ramp.
It was observed that the actuator acts to introduce a local velocity defect immediately downstream of the actuator
location. However, a subsequent recovery in near-wall streamwise velocity was also observed, where actuation
produces a higher streamwise velocity in the region immediately adjacent to the wind tunnel wall. This velocity
defect and recovery is believed to be linked to the formation of streamwise vortex structures induced by the
actuation, with similarities to velocity profile development downstream of passive vortex generators [35]. From
investigation of mean vorticity and standard deviations in PIV data, actuation induces a shear layer above the wall,
which then develops into a region of high unsteadiness in the velocity scalar with increased streamwise distance.
This unsteadiness is linked to three-dimensional vortex structures, which enhance the mixing of the flow between
the high-momentum freestream and the low-momentum boundary layer regions. Such a mixing mechanism is
highly desirable for flow control actuation, as it can be used to prevent boundary-layer separation which may occur
in un-actuated applications.
The work described herein primarily encompasses characterization of scaled actuator variants in bench tests, and
also the operation of multiple actuators (CAPAs) from a single DC supply or battery. The objectives and
considerations for wind tunnel testing, and other future work are also overviewed.
II. Experimental Hardware and Setups
A. Diagnostics
For electrical characterization of the actuators and driver circuitry, a variety of tools were implemented: power
supply multi-meter readouts, high voltage probes, current probes, oscilloscopes. The voltage and current waveforms
of the actuator attached to the secondary side of the transformer were monitored using a Tektronix 6015 HV probe
and a Pearson model 411 current monitor. A Tektronix TDS 3034B scope was used to collect waveforms for V-I
measurements as well as tachometer frequency measurements.
An AlphaLab model GM-1-ST DC Gaussmeter was applied to characterize the magnetic field of various
actuators during benchtop tests. This meter uses a Hall-effect sensor to measure DC magnetic fields up to 20 kG
with a resolution of 0.1 G. The field strength of a variety of neodymium ring magnets were measured, both above
the ring along the center axis and above the midpoint between the inner and outer radii in order to verify the rated
performance. The dielectric material placed above the magnet must be made thick enough to avoid breakdown
between the center high voltage electrode and the magnet, but thin enough such that the arc gap is still within a
region of high B-field. In the actuator designs described below, 0.06”-thick (1.5 mm) alumina sheet were used. The
field around the ring magnets was also studied with FEMM software [36], to evaluate electrode placement to
promote a high uniform vertical field within the discharge gap. An example calculation is shown in Figure 5 for the
C2N design (described below).
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(a)
(b)
Figure 5. Comparison of Magnet Simulation in FEMM and Gaussmeter Measurements
In addition to the measurements of the actuator electrical and magnetic field characteristics, two tachometer
techniques have also been applied to characterize the rotation of the plasma arc. The first method (optical) applied a
United Detector Technologies (UDT) model UV-100 silicon sensor, which is responsive in the 250-1100 nm range
with peak response (amps per watt) around 830 nm. By placing this detector above the coaxial actuator, collimated
to view one segment of the arc path, and passing the output to an oscilloscope through an amplifier stage, a
frequency signal corresponding to the rotation rate of the arc was measured, as the detector responded to the
emission from the arc as it passed through the collimated region. This same sensor was used to measure low
frequency pulsing of the discharge in preliminary work by viewing the entire actuator [32]. An alternative method
applied a ferrite-core (215 H) inductor to sense the disturbance of the magnetic field near the actuator as the arc
rotated [33]. This was accomplished by placing the dielectric-coated inductor near the upper surface of the actuator
in its test stand, just outside its outer diameter, and passing the output signal of the inductor pickup to an
oscilloscope through an amplifier stage. The two methods were found to be in good agreement, and as will be
shown, agreed well with rotation rates determined with a high-speed imaging system (see [33-34] for details).
B. Pulse Generation Circuits
Initial work applied a GBS Minpuls 2.2 system and also a neon sign transformer as a means to power the
actuators [32]. Recent work has focused on more compact circuits that can readily be configured to power arrays of
actuators. The majority of testing focused on applying a dual-MOSFET flyback-type tank circuit to excite the arc-
magnet actuator. This circuit operates on the concept of zero-voltage switching (ZVS), where the oscillation is
controlled by the resonant frequency of the tank circuit formed between the condenser capacitors and the primary
inductor {f ~ (LC)
-1/2
}. A detailed description of the ZVS circuit operation was given in a recent paper [33].
In addition to this fly-back type circuit, other options for driver circuit which will offer control of frequency and
duty cycle are being considered for future applications of the cyclotronic plasma actuator. One off-the-shelf option
under consideration is the GBS Minipuls 0.1, which is a lower power version of the GBS Minipuls 2.2 aimed at
UAV applications. Preliminary work was performed on clock-driven circuits where a timer circuit controls a bank of
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parallel MOSFETs to charge and discharge the primary-side coil from a battery supply. The configuration is similar
to a DC-DC step-up boost converter, where the inductive coil is replaced with the primary of a transformer used to
generate high voltage across the arc gap. Initial tests with a circuit using dual (parallel) IRFP250N power MOSFETs
have allowed powering of the coaxial actuators in the 3 to 15 kHz range (with variation in duty cycle). Preliminary
work has used the same CRT transformer applied in the ZVS circuit experiments, shorting the center-tap such that
the primary coil has an inductance of 23 H. A near-term goal is to optimize this type of circuit and rate its
performance compared to the ZVS flyback type driver, as both approaches are potential options for powering
actuators from a DC supply on a UAV. Preliminary results are described in this paper.
C. Scaled Actuator Configurations
The various types of actuators developed under this study have been summarized in previous papers [32-34].
The “Type 2” coaxial arrangement is focused on here, and was detailed in the most recent publications [33]. In this
design, the center electrode of the coax in placed within the inner diameter of the ring magnet, while the outer
electrode of the coax is placed around the outer circumference of the ring magnet. The electrodes protrude slightly
above the ring magnet, which is sheathed from the electrodes by dielectric material (e.g., alumina disks, mica
sheeting, PTFE, or Kapton film). The arc is formed in the air gap above the dielectric surface. The arc gap in this
design is therefore limited by the width of the magnetic ring (i.e., the difference between the outer radius of the
magnet and outer diameter of the center electrode), and the magnetic field strength varies with the size of the ring
magnet used, the dielectric thickness, and the magnet material grade. A recent design for an actuator module which
is being applied for integration in wind tunnel tests is shown in Figure 6.
Figure 6. Coaxial Arc-Magnet Actuator Type 2 (C2J) Design
In previous studies [34], the gap and B-field were adjusted, while the size of the magnet was restricted to ¾”
OD (19 mm). Therefore, sizing the gap relative to the fixed magnet size resulted in a significant change in the B-
field distribution shape and magnitude. In more recent work, scaled (bench test) actuators have been designed for
comparison of increasing gap and B-field while maintaining a similar uniform (vertical, non-divergent) distribution
of the B-field in the gap. Comparison of these three actuators serve as a size, weight, and power (SWaP) scaling
analysis for the Type 2 actuators. Sketches of the three variants are shown in Error! Reference source not found..
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(a) C2K (b) C2M (c) C2N
Figure 7. Coaxial Type 2 Designs for SWaP Study
Table 1 shows a comparison of the SWaP study designs. To allow for a comparison of performance with similar
B-field distribution, the coaxial electrode gap was centered on the average radius of the ring magnet and sized
proportionally to the radial thickness of the ring magnet. This corresponds to gaps of 2.3, 3.5, and 6.7 mm for
designs C2K, C2M, and C2N respectively. The C2K actuator is sized similarly to the wind tunnel design (C2J), but
with a slightly different gap size.
Table 1. Parameters of Actuators for Type 2 SWaP Study
Design Magnet Size
[OD x thickness]
Peak B-field
[G]
Coaxial Gap
[mm]
Actuator Mass
[g]
C2K 0.75” 0.50” 2378 2.3 58.0
C2M 1.00” 1.00” 4300 3.5 154.7
C2N 1.75” 0.50” 3230 6.7 209.5
III. Experimental Results
Recent bench testing focused on measuring the rotation rate of the arc in quiescent flow, using zero-voltage-
switching (ZVS) circuits as the pulse driver. I-V characteristic were measured, and optical tachometer measurements
were applied to quantify the rotation rate for various actuators. Additional studies were conducted to demonstrate
operation of multiple actuators from a single battery supply.
A. Actuator Scaling Study
Results from optical tachometer method, using the UV-100 sensor, are overviewed here. Three scaled variants
of Type 2 CAPAs were studied to look at the impact of actuator size, weight and power on performance. The
measured metric was arc rotation rate (by optical tachometer method), and the parameters varied were coaxial gap
size, and magnet size. The variants compared here are described above in Table 1 and Figure 7. The gap was sized
in proportion to the diameter of the ring magnet used. Rotation rate and arc velocity for the three variants are shown
in Figure 8 and Figure 9 respectively. Figure 10 shows the arc velocity divided by the actuator mass. Based on this
metric, the C2K design (similar to C2J used in wind tunnel) provides the best performance. However, the C2M
design, with a thicker magnet and the highest surface B-field provides the highest rotation rates and arc speeds. This
suggests the use of stronger and thicker magnets to provide higher B-field in the coaxial gap, but the tradeoff in
actuator weight (and volume) will be significant. A larger diameter magnet with a proportionally larger gap in C2N
produces a faster arc than C2K, but the corresponding rotation rate is lower.
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Figure 8. Arc Rotation Rate versus DC Power Figure 9. Arc Velocity versus DC Power
Figure 10. Arc Velocity / Actuator Mass versus DC Power
B. Operation of Arrays from Batteries
Since a project objective is to integrate the CAPAs into arrays on UAVs, bench test studies have been
conducted to demonstrate the powering of actuator arrays from batteries. The system was evaluated with three
sources:
1. A wall-plug power supply capable of 15 A and 17-28 VDC
2. A 6-cell 1800 mAh 22.2 V high-discharge LiPO Battery (Turnigy brand)
3. A 6-cell 2650 mAh 22.2 V high-discharge LiPO Battery (Turnigy brand)
The 6-cell (6S) batteries were representative of systems that could be applied in UAV tests. The planned system
layout is shown in Figure 11.
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The initial energy analysis indicated that the 1.8 Ah and 2.65 Ah batteries would support 18 and 27 runs
respectively, with a single battery powering a bank of three actuators for 30 seconds during each run.
Figure 11. UAV Actuator System Layout
The bench test setup for evaluating batteries is shown in Figure 12. Each battery was bench tested to confirm
the capability of providing the performance determined by the energy analysis. The batteries were used to power
actuators for 30 second periods with 40 second off-periods between runs. The bench test demonstrated that the 1.8
Ah and 2.65 Ah batteries would support 17 and 28 runs respectively, while maintaining a (safe) battery voltage of 20
V or higher. This was in good agreement with the energy analysis. A battery management system to provide alarm
and shutdown when battery voltage reaches a minimum operational limit is needed; this component will be added to
the UAV integration work.
Figure 12. Bench Test Setup for Evaluating Battery Performance in Powering CAPA Arrays
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C. Pulse Circuit Development
Another near-term goal is to continue developing driver circuits for improved power input to the actuators. Two
areas in particular are discussed here, (1) modification of the ZVS circuits to operate at different pulse frequencies,
and (2) use of a clock-driven dual MOSFET circuit.
1. ZVS Circuit Analyses and Tuning
ZVS driver circuit have been the primary approach applied in the wind tunnel so far. In these studies, the
voltage across the tank circuit, and the current in the primary transformer coil were monitored, for varied primary
coil windings. Figure 13 shows typical oscilloscope output for transformer voltages when the baseline ZVS circuit is
applied. These voltage measurements, along with current measurements are monitored to analyze the energy in the
tank circuit.
Figure 13. Voltage Measurements in ZVS Circuit
Transformer
The ZVS driver circuit was supplied DC power at 16, 20, and 24 V, and the transformer windings were varied.
This results in (i) a change in the characteristic frequency of the circuit, (ii) a change in the DC power for a given
voltage setting, and (iii) slight change in the energy exchanged between the tank circuit elements (capacitor and
inductor). The tank circuit energies computed from the voltage and current measurements for various primary coil
windings are shown in Figure 14. These results are in good agreement, and discrepancies are likely due to errors in
the tank capacitance (assumed 0.66 F). The frequency of the high voltage terminals of the transformer is plotted in
Figure 15. Neglecting resistive losses and thermal effects, the frequency f of the oscillation is driven by the product
of the capacitor bank C and the primary inductance L.
2


The operating frequency can be altered by changing the number of turns on the primary coils, as the frequency
increases with the inverse of inductance (or # of turns). However, there is an increase of DC power when the
inductance is reduced, due in part to higher switching losses. The study here was performed without breakdown, in
the secondary, and therefore these can be used as an estimate of power losses in the ZVS circuit at different voltage
settings. Further work will directly compare the rotation rate of the actuator plasma for different primary windings
used here, to determine how frequency impacts performance. Also, studies with different transformer ratios are
planned.
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(a) Current-based
(b) Voltage-based
Figure 14. ZVS Tank Circuit Energy based on (a) Current Measurements, and (b) Voltage Measurements
Figure 15. ZVS High Voltage Frequency versus Power Input for Varied Primary Coil Windings
2. Dual MOSFET Pulser Design
Because the current ZVS drivers are proving to be limited in power, the dual MOSFET circuit is being
considered. Bench tests were conducted, investigating the pulse tuning of this circuit, using it to drive a Type 1
plasma actuator (C1A) with a 1.6 mm gap (see [33-34] for more details). Figure 16 shows pulse data recorded for
operation of the circuit over a variety of clock settings for varied pulse frequencies, operating the circuit from a 12 V
source. As pulse width (discharge on) increases, the range of frequencies over which a discharge can be achieved is
lowered, Figure 16a. As the operating frequency is increase the charge time decreases for a given pulse width
setting, and the charge period (and charge duty) decreases. This impacts the energy that can be stored on the
discharge coil between discharge pulses. As shown in Figure 17, as the pulse frequency increases, the peak primary
coil current is reduced. The current values in Figure 17 correspond to a range of 8 to 16 mJ, decreasing as the circuit
is driven at higher frequency.
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The next step in maturing this circuit is to increase the energy stored during the charging phase. The charge rate
is proportional to voltage, and the time constant is proportional to the inductance. A simple model for the coil
current with no initial charge is
 
1

The energy stored in the coil is

1
2
Therefore, to increase the energy stored between pulses, the battery voltage should be increased, while the coil
inductance (primary of transformer) is adjusted to control the charge time and peak energy.
(a)
(b)
Figure 16. Pulse Data for Dual IRFP250N Circuit producing rotating plasma arc in C1A actuator: (a) Pulse Duty, and (b)
Charge Period.
Figure 17. Peak Primary Coil Current versus Pulse Frequency
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IV. Discussion and Future Demonstration Work
While preliminary studies showed the impact of the cyclotronic plasma actuator on wind tunnel flows [32-34],
the recent work has focused on improving the performance of the actuator, with the goal of improved effectiveness
and control authority. Future experiments are being developed in order to characterize and validate the cyclotronic
plasma actuator in realistic flight regimes. The goal is to improve the technology readiness level of the actuator
approach by investigating various important aspects of integration on a flight platform. Some preparations for future
research are described here, including:
(i) Further actuator array circuit development,
(ii) Wind tunnel testing on a flapped airfoil, and the NASA hump model
(iii) Flight test demonstration
A. Circuit Array Development
The cyclotronic plasma actuator device is intended to replace conventional vane-type VGs, and it is anticipated
these devices will perform best when organized in arrays, similar to the configurations used for conventional VGs
on fixed-wing aircraft. As the technology matures, this will become a key integration problem to solve, and designs
must be found which are both efficient and practical for installation in airframes. For near-term testing in the wind
tunnel, actuators can be configured to operate independently from wall-plug DC supplies. However, for integration
into aircraft systems, especially UAVs, a more elegant solution must be devised.
The ongoing work described here is investigating the operations of multiple actuator drivers off a single DC bus.
Examples reported here demonstrated an array of three actuator operated from a single LiPo battery. The current
rating (C-rating) of the batteries is large enough for the ZVS circuits currently examined, and should be high enough
to double the number of actuators from a single battery. However, some factors should be considered in further
development of the battery powered arrays such as: (1) improving the power input to the actuator will require larger
batteries, (2) increasing the number of actuators run from a single battery (obviously) proportionally decreases total
runtime, and (3) alternative circuits, such as the clock-driven MOSFET circuit discussed above, may require higher
C-rating than the ZVS circuits, due to the mechanism of charging.
B. Wind-Tunnel Demonstration Tests
In recent work, a flapped airfoil model has been constructed to serve as a wind tunnel testbed for cyclotronic
plasma actuators. The wind tunnel model, pictured above in Figure 4, uses an S8036 airfoil. This model was
constructed with a 3D printed body, mounted to a steel internal structure. Removable modules can be used to secure
arrays of actuators at various positions along the chord. The internal structural design allows the wiring for actuators
to be routed through the wing spars to the actuator modules; the layout of the cross section is shown in Figure 18.
The model also has three chordwise rows installed to evaluate the influence of the actuation on the local pressure
distribution and lift characteristics across planes coincident with or directly between actuators. The modules are
designed to allow for a span-wise array of up to 6 actuators to be embedded into the airfoil model. Additionally, four
module mounting locations are built into the model at different chordwise positions, allowing the performance
produced by actuation at various streamwise positions to be compared.
Figure 18. Cutaway Side-view of Reconfigurable Cyclotronic Plasma Actuator Modules
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A version of the NASA hump model has also been created in the 15” 15” tunnel at UIUC. The hardware is
reconfigurable to install either CAPAs or conventional vane-type VGs to study separation over the hump. The
vortex generator number, spacing and spanwise location was the same as that for the plasma actuators in order to
make a one-to-one comparison. The vortex generators and the C2J plasma actuators are shown installed in the
NASA hump model in Figure 19.
(a)
(b)
Figure 19. a) Vortex generators installed in the hump model; b) The cyclotronic plasma actuators revision C2J installed
in the model.
C. UAV Flight Testing Configuration
Another important goal is demonstrating effectiveness of plasma arc-magnet actuators on a UAV platform. The
team has begun overviewing the parameters of potential UAV platforms in order to define constraints on size,
weight, and power (SWaP) for the actuator designs under development. Currently the constraints associated with the
SR22T UAV platform are under consideration. This is a modified version of the 96.8” wingspan, 21%-scale Cirrus
SR22T, with upgraded motor, servos, and electrical systems. The model has been modified for a straight-wing,
similar to the configuration being applied in tunnel testing. Photos from the initial flight test of the UAV testbed
(without actuators installed) are shown in Figure 20.
(a)
(b)
Figure 20. Modified Hangar 9 Cirrus SR22T (a) with modified wings installed, and (b) after takeoff during initial flight
testing
Similar to the system being applied in wind tunnel tests, the integration of a 6-actuator system in a UAV is being
studied. A general system schematic of the CAPA UAV system is shown above in Figure 11. Based on the sizing of
bench test components for ZVS circuits described here and in previous work [33], a packaged system layout is being
developed. A 3D diagram showing a cutaway of the fuselage and various components is shown in Figure 21.
Downloaded by Joseph Zimmerman on February 15, 2019 | http://arc.aiaa.org | DOI: 10.2514/6.2019-0047
American Institute of Aeronautics and Astronautics
15
Figure 21. Diagram of Key Components for CAPA Integration in UAV Testbed
V. Conclusions
Overall, the results of the effort described here are encouraging toward the advancement of this new innovative
plasma actuator concept. The scaling studies have shown the ability to improve the rotation rate and velocity of the
plasma, by sizing the gap and magnet thickness proportionally. The CAPA size being applied in current wind tunnel
and UAV demonstrations can be considered mass-efficient when the “arc-velocity to mass ratio” is evaluated for a
given power input to the circuit. The cyclotronic arc-plasma actuator is envisioned to be used as a method for
performing active control of turbulent boundary-layer separation on a variety of aerodynamic geometries. Benchtop
testing and design work showed that variants of the cyclotronic plasma actuator were straightforward to produce,
and that various driver circuits could be applied to achieve the desired effect of arc rotation in a magnetic field.
Tachometer measurements and high-speed visualization of updated testbeds have revealed that the rotation of the arc
could be controlled depending on the basic design parameters (arc gap, system voltage, and magnetic field strength).
The most recent designs are more compact than preliminary bench tests, and based on lessons learned, actuators can
be sized and integrated into a wind tunnel test bed which is now available for testing.
Further work is needed to produce power- and volume-efficient actuator array circuits operating from a
common battery supply, which can be packaged in UAVs for flight test. Also, the examination of the arc behavior in
flow conditions similar to wind tunnel model and UAV takeoff airspeeds, with high-speed imaging, is necessary.
Future CAPA research is planned to result in successful development and demonstration for realistic flight regimes,
with the following strategy:
Advanced wind tunnel tests of actuators embedded in wind tunnel airfoil models
Study of scaling effects of actuator arrays, both by analysis of flow interaction and by producing efficient
and compact driver circuits for arrays
Preliminary flight test demonstrations with actuator system integration on a UAV
Acknowledgments
This work was supported by NASA SBIR contract #NNX17CL08C “Cyclotronic Plasma Actuator with Arc-
Magnet for Active Flow Control.” The authors wish to thank the program technical monitors Stephen Wilkinson,
Luther Jenkins, and Latunia Melton of NASA Langley Research Center for helpful interactions and guidance.
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... This paper summarizes the most recent experimental investigations performed for the development of the cyclotronic plasma actuator, and is a continuation of the recently reported work [32][33][34][35]; the technology was recently patented [36]. Early work evaluated the actuator configuration in low-speed wind tunnel tests using PIV in a zeropressure gradient boundary layer, as well as pressure recovery measurements with the actuator placed upstream of an expansion ramp [32]. ...
... The configuration is similar to a DC-DC step-up boost converter, where the inductive coil is replaced with the primary of a transformer used to generate high voltage across the arc gap. Initial tests with a circuit using dual (parallel) IRFP250N power MOSFETs have allowed powering of the coaxial actuators in the 3 to 15 kHz range (with variation in duty cycle) [35]. Preliminary work has used the same CRT transformer applied in the ZVS circuit experiments, shorting the centertap such that the primary coil has an inductance of 23 H. ...
... The various actuators developed under this study have been summarized in previous papers [32][33][34]. The "Type 2" coaxial arrangement is focused on here, and was detailed in the most recent publications [33,35]. In this design, the center electrode of the coax is placed within the inner diameter of the ring magnet, while the outer electrode of the coax is placed around the outer circumference of the ring magnet. ...
... The electrical energy is transferred to the swirling kinetic energy by the imposed magnetic field. This field structure generally occurs in current physical devices that are designed to generate and utilize MHD swirling flow, such as advanced space thrusters [4,5], plasma vortex actuators [6], MHD stirrers [7], the rotating plasma reactors [8], and soft X-ray sources [9]. Also, in theoretical fluid mechanics research this MHD swirling flow is used to study the stability of rotating flow [10,11]. ...
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