1. Symposium on Applied Aerodynamics and Design of Aerospace Vehicle (SAROD 2009)
December 10-12, 2009, Bangalore, India
Development and Implementation of High Frequency Pulsed Microactuators
for Active Control of Supersonic Impinging Jet
John T Solomon$, Alex Wiley*, Rajan Kumar@, Farrukh S Alvi#
Florida Center for Advanced Aero-Propulsion (FCAAP)
Advanced Aero Propulsion Lab, Florida State University, Tallahassee, FL 32310
johnts77@gmail.com, alvi@eng.fsu.edu
ABSTRACT
Actuators with high amplitude unsteadiness and tunable frequency are necessary for the effective and efficient control
of many high speed aerodynamic flow systems. In this paper, the design, development, characterization and
implementation of a novel high bandwidth micro fluidic actuator is presented. The first generation micro-actuators are
designed and their performance is tested in controlling the highly unsteady impinging jet flow field of a supersonic jet.
The remarkable feature of this micro-actuator is its high momentum mean flow along with high amplitude and a high
bandwidth unsteady component. Results show that the impinging tones are completely eliminated with the actuation of
these micro-actuators, whereas, new peaks at a frequency different from the actuation frequency and its harmonics are
observed in the spectra, the occurrence of which need to be further explored.
Key Words: Pulsed microjets, flow control, supersonic flow, micro actuators
1. INTRODUCTION better and efficient operation of many practical
aerodynamic systems. Aero-acoustic flow field
Active and adaptive control of shear and boundary generated by the impinging supersonic jets of a
layer supersonic flows have driven considerable STOVL (Short Take Off and Vertical Landing)
research initiatives in the recent years[1-6]. Use of aircraft, during the hovering mode, is an example of
efficient and effective actuators essentially leads to such a flow domain that necessitates novel active
control methods [1, 2]. Figure.1 shows a schematic of
such a flow that produces highly unsteady aero-
Nozzle
acoustic fields, governed by a well known feedback-
Lift plate
resonance phenomenon. As seen in the figure, the
Shear layer instability waves in the jet that originate at the nozzle
exit grow as they propagate downstream towards the
impingement surface forming large scale vortical
Upward traveling structures, and the acoustic waves that are produced
Instability
Acoustic wave upon impingement travel upstream and excite the
wave
nascent shear layer near the nozzle exit. Highly
unsteady flow field experienced by the weapon/cargo
bay of a military aircraft is another example that
Large vortex requires active flow control approaches. These cavity
Impinging
structures shaped storage spaces of high speed air vehicles
surface
produce high amplitude pressure fluctuations that
greatly affect the weapon trajectory dynamics and the
stability of the vehicle [3-6]
Fig. 1:An impinging flow field of a supersonic jet that Actuator technologies that are proved relatively
requires active flow control schemes successful in subsonic flows may not be ideal for the
effective control of high speed flows. Although
__________________________________________ various types of actuators have been and are being
$
Graduate Research Assistant explored, most designs have shown limitations either
*
Graduate Research Assistant in terms of performance and range of operation in the
@
Research Scientist, AAPL lab or the ‘cost’ of performance (including added
# weight and complexity) for eventual full-scale
Professor, Florida State University
implementation. There is a clear need for actuators
© F. S. Alvi, Florida State University
that produce high-amplitude disturbances, over a
SAROD 2009
broad range of frequencies. Furthermore, the output
Published in 2009 by MacMillan India Ltd.
of an ideal actuator should be ‘tunable’, both in terms
1

2. John T Solomon, Alex Wiley, Rajan Kumar, Farrukh S Alvi 2
of amplitude and frequency over a large dynamic source jet. Figure 2b and 2c represents the actuator
range. This allows their use in subsonic and model and the flow field respectively. The model is
supersonic flow control applications where their fabricated in plexi glass material.
properties can be adapted according to the specific
applications and flight/operational regimes. In this Source jet
paper we describe the development of a high
bandwidth micro-actuator and its implementation in
controlling resonance dominated supersonic
impinging flows.
2. MICROACTUATOR- INITIAL DESGIN Cavity
An actuator with high amplitude excitation, whose
frequency can be easily tuned over a large bandwidth,
is essential for the effective manipulation of high
energy structures of the shear or boundary layer of
high speed flows. To realize this goal, we have
Unsteady microjets
Nozzle Fig. 2b Actuator model Fig 2c Flow field
dm
hm
Source jet The main parameters that govern the properties of
the microjet array issuing from the actuator assembly
are: a) the distance of cavity from the source jet hm,
H=L+hm b) the length of the cylindrical cavity, L and c) the
L
Cavity source jet pressure `ratio, (NPR)m. The two geometric
parameters are indicated in Figure 2a. In the
preliminary study, we examined the effect of these
parameters on the flow issuing from the microjet
actuator to identify the optimal range and
combination of these parameters that produce the
Unsteady micro desired micro-actuator flow. This has helped us to
400µ nozzles jet array develop a preliminary design approach and scaling
laws for such actuators.
Figure 2c shows a representative schlieren image
Fig. 2a Schematic of the actuator of the flow field associated with the micro-actuator.
Large unsteadiness is seen in the source jet at certain
designed and developed an actuator system that can combinations of geometric and flow parameters that
produce pulsed supersonic microjets at any desired essentially force and excite the natural resonant
range of frequencies. This micro actuator produces modes of the actuator system at high amplitudes. The
high amplitude response by using a very simple flow field image given in Figure 2c corresponds to
geometric configuration that leverages the natural (h/d)m=1.3, L/dm=3 and (NPR)m=4.8. The secondary
resonance behavior of various components of this microjets are supersonic, as evident from the shock
micro-fluidic actuator system. cells present in the jet structure.
A schematic of the actuator is shown in Figure 2a.
2.1 Experimental details
As seen here, the micro-actuator consists of three
A 1 mm nozzle (dm), connected to a compressed
main components: a) an under expanded source jet,
nitrogen tank is used to generate source jet at various
which supplies the air into b) a cylindrical cavity
flow conditions. A Kulite probe is placed close to the
upon which the source jet impinges, and c) multiple
secondary orifices of the actuator to measure the
micro nozzles (i.e. microjet orifices) at the bottom of
unsteadiness associated with the secondary microjets.
the cylindrical cavity, from which the high-
The unsteady pressure signals were acquired through
momentum, unsteady microjets issue. In the present
high speed National Instruments digital data
design, the source jet was issued from a 1mm
acquisition cards using LabviewTM. The transducer
diameter (dm) converging nozzle and the micro
output was conditioned using a low-pass StanfordTM
nozzles array at the bottom of the cavity consists of
filter (cut-off frequency = 60 kHz) and sampled at
four 400 µm holes in the pattern shown in Figure 2a.
200 kHz. Standard FFT analysis was used to obtain
The cylindrical cavity has a diameter of 1.6 mm and
narrowband pressure spectra. A total of 100 FFT’s of
length L, and is located at a distance hm from the
4096 samples each were averaged in order to obtain

3. Development and Implementation of High Frequency Pulsed Microactuators for Active Control of Supersonic 3
Impinging Jet
statistically reliable narrowband spectra. Preliminary correlation that captures it; this is discussed in the
studies were conducted for different combinations of following section. More details of the actuator
geometric and flow parameters of microactuators characterization are available in reference [7, 8]
such as L/dm, (h/d)m and (NPR)m. In the present study,
L/dm is varied from 1-5, (NPR)m from 1.9 to 5.8 and
(h/d)m from 1 to 2. Figure 3a and 3b show the
representative spectra of secondary jets
corresponding to L/dm = 5. For this case, experiments
were carried out by varying (h/d)m for a fixed (NPR)m
= 4.8 and by varying (NPR)m keeping a fixed (h/d)m
=1.7.
3. ACTUATOR CHARACTERIZATION
3.1 Unsteady spectra- Effect of geometry and
source jet flow
The pressure spectra shown in Figure 3a and 3b
clearly show the presence of high amplitude peaks
indicating the presence of highly unsteady flow
issuing from the actuators. Here we see that for L/dm
Fig. 3b Actuator spectra, L/dm=5, h/d)m=1.7
= 5, the control knobs, (h/d)m or (NPR)m variation
produce high amplitude, unsteady microjets in the
Figure 4 summarizes the effect of (NPR)m and
range of 6-11 kHz. Equally noteworthy is the trend of
(h/d)m shown in Figure 3 but over a large range of
peak frequency variation, where a very small
parametric space.
variation of ∆(h/d)m, by ~600µm or a variation of
∆(NPR)m~1.5 , leads to a significant shift in the peak
frequency of ~5 kHz. Consequently, there is
significant potential for developing a compact,
robust, pulsed, tunable actuator with high mean and
unsteady properties. This design approach allows for
multiple ‘control knobs’ that can be used to modify
the actuator response in real time, as dictated by the
application.
Fig. 4 Summary of actuator data7
As seen here, for a given actuator design, i.e.
fixed L/dm, very small changes in the source jet
distance and operating pressure allows one to sweep
the output frequencies over a rather large range of
∆factuator = 5-20 kHz. However, this plot also shows a
wide range of actuator frequencies can be produced
for a given (h/d)m or (NPR)m, by varying L/dm. In
order to better collapse the performance, in terms
Fig. 3a Actuator spectra, L/dm=5, NPR=4.8
actuator dimensions, we define a new variable H
which is defined by H=hm+L, where hm is the
As seen in Figure 3, the data from the parametric
distance of nozzle exit to the cavity entrance and L is
study is classified into two sets, one is the data
the length of the cavity, as before. This parameter H
derived from the (h/d)m variation (Figure 3a) and the
other set reflect the effect of (NPR)m variation (Figure represents the length of the jet column from the
3b). This grouping can then be used for micro-nozzle end to the impinging end of the cavity.
The actuator frequency is non dimensionalized
understanding the overall behavior in terms of these
using ideally expanded jet velocity of the under
parameters and for deriving a more general

4. John T Solomon, Alex Wiley, Rajan Kumar, Farrukh S Alvi 4
expanded source jet. The non dimensional frequency
is given by
Fig. 5 A correlation for actuator design
Fig. 6a Unsteady amplitude variation with (h/d)m
Stideal = fd m / U ideal (1) As seen in Figure 7a, this entire variation occurs
within an (h/d)m range of ~1 to 1.8 and is seen for all
In equation (1) f is frequency of the actuator, dm is the cavity lengths examined. This suggests the
source jet diameter and Uideal is the ideally expanded existence of a region where the flow is particularly
jet velocity of the under expanded actuator source jet. unsteady and where the instabilities are amplified, i.e,
The new parameter H is plotted against the non a region of instability.
dimensional frequency Stideal as shown in Figure 5.
Interestingly these new variables collapsed into a
single trend curve as seen in the figure. The collapsed
curve is approximated as an empirical correlation,
represented by equation (2)
Stideal = 0.4( H / d m ) −1.45 (2)
Equation (2) can be used as a guide for designing
high bandwidth microactuators for various
applications that demands high bandwidth actuation.
3.2 Unsteady amplitude of the actuator
For an unsteady actuator system, the amplitude of
unsteadiness is equally important as its frequency
response. The total energy in the unsteady component
of the micro-actuator flow can be captured by the rms
of the total pressure measured, Prms, by the Kulite Fig. 6b Unsteady amplitude variation with
total pressure probe. In the following, we describe actuator nozzle pressure ratio
how the geometric and flow parameters affect the
unsteady amplitude of the micro-actuator system. While discussing the pressure spectra of the
Figure 6a shows the variation of Prms with (h/d)m micro-actuator flow (Figure 3a) we have seen the
for different cavity lengths. It is observed that Prms emergence of distinct frequency tones when (h/d)m is
increases over a range of smaller values of (h/d)m and in the range ~1.3-1.8. In the preceding discussion, we
it remains nearly constant and decreases at larger noted high Prms, levels in the same h/d range. It is
values of (h/d)m. For example, for L/dm = 1, the Prms is clear that the discrete peaks in the frequency
144dB at (h/d)m = 0.75 reaches nearly 168 dB at spectrum, which are indicative of significant
(h/d)m=1.1 and remains nearly constant up to unsteadiness, are responsible for the high Prms. The
(h/d)m=1.6 and falls down to 158 dB at higher (h/d)m conclusion is that for a fixed NPR, there exists a
values. region of instability within which the variations of

5. Development and Implementation of High Frequency Pulsed Microactuators for Active Control of Supersonic 5
Impinging Jet
(h/d)m or H/dm give rise to high amplitude secondary around the periphery of the nozzle as shown in the
jet fluctuations. Furthermore, the unstable Figure 7. These actuator modules are designed to
frequencies can be controlled by selecting the generate microjets pulsing at 4-6 kHz at various NPR
appropriate (h/d)m and cavity length. The variation of values of the actuator source jet. The micro actuators
Prms with nozzle pressure ratio (NPR) is shown in are designed for this frequency range so that they can
Figure 6b. In the present experiments, at a fixed value be tuned to match the baseline frequency of the flow
of L/dm and (h/d)m (corresponding to large field. In this case, the baseline spectra of the
unsteadiness), the NPR is varied from 4 to 5.5. It is impinging jet have a dominant frequency component
observed that at each L/dm, with increase in (NPR)m near 6 kHz. The design details of the actuator are
(> 4.2) there is a sharp increase in OASPL, however available in [9].
its value saturates beyond (NPR)m = 4.6 within the For the present study, a 100psia Kulite (Model
range tested. XCE-062-100A) was flush mounted in the ground
plane at the stagnation point of impingement (r/d=0)
4. IMPLIMENTATION OF ACTUATOR to measure unsteady loads on the ground. A second
5psid Kulite (Model XCS-062-5D) was flush
The impinging flow field to be tested for the mounted in the lift plate at r/d=2 to measure the
effectiveness of the high bandwidth actuator is unsteady loads experienced by the aircraft. For near
generated by an ideally expanded supersonic jet, field acoustic measurements, a microphone at r/d=10
issued vertically through a Mach 1.5 C-D nozzle as was mounted in plane of the nozzle exit (see Figure
shown in the Figure 7. 7). All three measurements were recorded
4 Actuator modules simultaneously.
integrated to the lift plate
10d CD Nozzle High bandwidth
Actuator integrated
Micro phone to the lift plate
close to the nozzle
Lift Plate exit
10d
h/d Fig. 8a Impinging spectra with actuator
operating at NPR=5.4
The results shown in Figure 8a and b correspond
Ground plane to a nozzle-to-ground distance of h/d=4.5 where a
dominant impinging tone is generated at 5.3 kHz in
Ground Kulite the baseline flow. The control effects of the actuator,
` operating at (NPR)m=5.4 and 6.5, on the baseline
Fig. 7 Schematic of test facility and actuator flow are shown in Figure 8a & b respectively. At
integration (NPR)m= 6.5, the microjets are pulsing at 5.3 kHz. It
Temperature is controlled using an inline heater is important to note that the impinging tones are
which maintained a temperature ratio, TR=1.0 (where completely eliminated in both the cases. Also note
TR=stagnation temperature/ ambient temperature) for that a new tone is generated at ~6.7kHz along with its
all experiments. To simulate the presence of an harmonics, which is neither present in the base flow
aircraft in hover, a circular plate (referred to as the lift nor with the actuator. This needs to be investigated
plate) of diameter 10d (d is C-D nozzle throat further.
diameter, =25.4 mm) is flush mounted with the Although these new tones are of similar
nozzle exit. Four actuator modules that can generate amplitude, they are narrower than the impinging
16 pulsed microjets were integrated in the lift plate tones resulting in lower energy content. This is

6. John T Solomon, Alex Wiley, Rajan Kumar, Farrukh S Alvi 6
reflected in overall sound pressure level (OASPL) REFERENCES
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and Krothapalli, A., “Control of supersonic
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Look at Supersonic Cavity Flows and Their
Design and development of a novel, simple and Control,” AIAA Paper 2005-2803, presented
robust micro actuator is described in this paper. The at 11th AIAA/CEAS Aeroacoustic
first generation actuator consists of a source microjet, Conference and Exhibit, Monterey, CA,
under expanded into a short, cylindrical cavity and June 2005.
multiple secondary microjets emanate out of the 6. Zhuang, N. Alvi, F. S., Alkilsar, M. and
cavity through multiple micro orifices. The Shih, C., “Aeroacoustic Properties of
remarkable feature of this micro-actuator is its high Supersonic Cavity Flows and Their
momentum mean flow along with high amplitude and Control,” AIAA Journal, vol. 44, No. 9,
a high bandwidth unsteady component. Based on a Sept. 2006, pp. 2118-2128.
detailed parametric study and characterization, a 7. Solomon, T. J., Kumar, R. and Alvi, F. S.
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performance was tested in controlling the highly Active flow control” AIAA paper 2008-
unsteady impinging jet flow field of a supersonic jet. 3042.
The results show that the impinging tones were 8. Solomon, T. J., Kumar, R. and Alvi, F. S.
completely eliminated with the activation of these “Development and characterization of high
micro-actuators, but, new peaks at a frequency bandwidth actuator” ASME paper, 2008-
different from the actuation frequency and its 3042.
harmonics were observed in the spectra. These need 9. Solomon, J.T., Hong, S., Wiley, A., Kumar,
to be further explored. The current design actuates R., Annaswami, A.M., and Alvi, F. S. “
only 30 % of the circumference of the main jet. Control of supersonic resonant flows using
Actuator modules that span a larger spatial extent, high bandwidth Micro actuators” AIAA-
around the entire periphery of the main jet may 2009-3742
further enhance control effectiveness. Also, we are in
the process of integrating this actuator for controlling
the flow field associated with other high speed
applications.
Acknowledgment
The development and testing of pulsed actuators
was primarily supported by the Florida Center for
Advanced Aero-Propulsion (FCAAP). We also want
to thank the Air-force Office of Scientific Research
(AFOSR) which has consistently supported our prior
work on the control of impinging jets.