This paper investigates the thrust and flow structures

produced by submerged synthetic jet actuators. Inspired

by the propulsion methods of many sea creatures, such as

jellyfish, squids, and salps; synthetic jets use vortex rings to

create a net thrust. To assess the potential usability of these

thrusters for propulsion and maneuvering of small underwater

vehicles, a range of synthetic jet thruster prototypes were

built, and both flow visualization and thrust measurement

experiments were executed. Based on the experimental results

obtained from these models, we discuss the feasibility of using

these thrusters on small, slow, but maneuverable vehicles

Burdick, Joel

Mohseni, Kamran

Polsenberg-Thomas, A. M.

Caltech Authors - Main

An Experimental Study of Voice-Coil Driven Synthetic Jet
Propulsion for Underwater Vehicles
AM Polsenberg-Thomas1, Joel Burdick1, Kamran Mohseni2
1Engineering and Applied Science, Caltech, Pasadena, CA 91125
2Dept. of Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309
ampolsen@caltech.edu
Abstract—This paper investigates the thrust and flow struc-
tures produced by submerged synthetic jet actuators. Inspired
by the propulsion methods of many sea creatures, such as
jellyfish, squids, and salps; synthetic jets use vortex rings to
create a net thrust. To assess the potential usability of these
thrusters for propulsion and maneuvering of small underwa-
ter vehicles, a range of synthetic jet thruster prototypes were
built, and both flow visualization and thrust measurement
experiments were executed. Based on the experimental results
obtained from these models, we discuss the feasibility of using
these thrusters on small, slow, but maneuverable vehicles.
Index Terms—synthetic jet, AUV, vortex ring, optimization,
propulsion
Fig. 1. Dye visualization of the wake produced by a voice coil driven
synthetic jet
I. INTRODUCTION
Currently, AUVs (Autonomous Underwater Vehicles)
are most commonly used for large scale surveying and
mapping projects. Virtually all AUVs are propelled by a
single stern propeller. Additionally, propeller design and ef-
ficiency has been studied and improved upon for hundreds
of years[1], [2]. This configuration, while excellent for
cruising situations, does not allow these vehicle to easily
hover, and thus they are not well suited for slow speed tasks
involving tight maneuvering and station keeping. As more
and more new uses are being found for unmanned under-
water vehicles, it becomes interesting to investigate how
alternative propulsion schemes can add to the capability of
AUVs. Slow speed maneuverability can be crucial for tasks
such as tracking slow moving animals such as jellyfish and
larvaceans, and investigating enclosed or treacherous areas
where excellent maneuverability is required. The goal of
this work is to look at synthetic jets as a potential slow
speed propulsor well suited to handle tasks like biological
tracking and exploration of hazardous environments.
One major weakness of using propellers (ducted or
unducted) is that they typically protrude from the body
of the vehicle. The thrusters, and any other protruding
parts, are at risk of breaking off. In particular, during
launching, recovery and manuevering in enclosed areas
such as caves or shipwrecks, these protrusions are a huge
liability. Additionally, when the vehicles are used to inspect
or track animals, creatures have been killed by propellers.
In addition to desiring a propulsion system which will al-
low for highly maneuverable, controllable low-speed travel,
we believe that the following are equally important criteria:
• Minimal moving parts to increase mechanical robust-
ness
• No parts protruding from the hull of the vehicle to
become entangled with surrounding environment, or
the biological creatures being tracked by the vehicle
• Minimal volume, to allow more payload, which is
especially crucial in small vehicles
• Off the shelf components, thereby enabling low cost
small vehicles
A. Biological Inspiration
Pulsatile jet propulsion, as seen in squids, salps, and
jellyfish, may offer a propulsion scheme that meets many
of these design goals [3], [4], [5], [6], [6], [7], [8]. These,
and other animals, propel themselves by pulsing water out
of an orifice in their body. As the fluid exits the orifice,
it rolls into a vortex ring. The pulsatile jet created by
these rings has been shown to produce a greater average
thrust than that of a steady jet with the same mass flux
[8]. As Lane points out, the advantages of this approach
were discovered by nature: ”Ages before man discovered
jet-propulsion, cephalopods were jetting through primeval
seas [9].”
Nature suggests that this style of propulsion may offer
advantages for smaller vehicles operating at modest speeds.
Jellyfish and salps are relatively small, and while there are
some very large species of squid, the majority of squid
species are small [10]. (O’Dor proposes that the possible
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geometric and energetic constraints may play a role in
explaining the prevalence of smaller squid [10]) Whether or
not these constraints will apply to a synthetic jet propelled
vehicle remains to be seen, but this size distribution shown
in squids should not be ignored and has led us to begin
our investigation into small vehicles.
B. Pulsatile Jet Propulsion via Synthetic Jets
A B
C D
Fig. 2. The stages of synthetic jet operation: (A) initial inflow, (B) initial
outflow, (C) subsequent inflow, (D) subsequent outflow
Zero net mass flux jets, also known as synthetic jets, are
one means to generate pulsatile jet fluid flows. Synthetic
jets have long been used in the aerospace industry for flow
control [11], [12], [13], [14]. A synthetic jet consists of
a chamber with a vibrating membrane at one end and an
orifice on the other. During the membrane’s in-stroke, fluid
is sucked in through the orifice. During the out-stroke, fluid
is ejected through the orifice, creating a vortex ring. The
process repeats. A series of rotating rings entrain fluid,
creating a jet. This is illustrated in Figures 2 and 3 . Krueger
has shown the potential thrust augmentation benefits of
pulsatile jets [15], [16], [17].
The use of synthetic jets for underwater vehicle propul-
sion was first proposed by Mohseni [18], [19], [20]. In these
works, Mohseni and collaborators investigated the use of
solenoids and cams to actuate the synthetic jet stroke. In
Fig. 3. Vortex Rings Forming a Jet
orifice plate
membrane
chamber
actuator
Fig. 4. Schematic diagram of a synthetic jet prototype
this paper, we investigate a synthetic jet design based on
a voice coil actuator. This paper updates our prior work
on this type of actuator design [21]. We note that this
earlier paper [21] did not properly recognize prior work by
Kamran Mohseni on the use of synthetic jets for underwater
vehicle propulsion.
l
p
.50.0 1.0VP
Ins
tro
ke
O
utstroke
M
em
br
an
e 
D
is
pl
ac
em
en
t
Fig. 5. Membrane velocity profile (VP). VP represents the instroke
fraction of a single membrane cycle. (For example, a VP of .6 means that
the instroke accounts for 60 % of the cycle time.
A synthetic jet can be built by taking a hollow cylindrical
chamber and placing a vibrating membrane at one end and
a lid with an orifice in it at the other end. We designed
modular prototypes so that elements such as orifice shape
and diameter, chamber length, and actuation device can
easily be changed to investigate the effects of these design
parameters on performance.
There are many possible ways to oscillate the membrane.
We have built two different voice coil actuated prototypes.
One prototype is actuated by using a Bruel and Kjael 4810
minishaker (designed to vibrate small objects). The second
prototype is built using a voice coil from a consumer-
grade audio speaker. For these prototypes there are two
variable parameters: the frequency of membrane forcing,
and the velocity profile of the membrane’s displacement.
In the experimental data presented below, the membrane’s
velocity profile was defined to be a periodic sawtooth
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function with a fixed amplitude. The profile parameter V P ,
ranging from 0 to 1, is the point at which the membrane
switches direction (see Figure 5). (Values of V P smaller
than 0.5 refer to profiles where the membrane pushes out
slower than it is pulled in.)
Fig. 6. Force measurement setup (pictured with speaker coil actuated
synthetic jet)
For the speaker coil driven synthetic jet, which will be
discussed in detail in Section II, the following are the
values for the variables shown in Figure 7.
delta = 3mm
ddisk = 25.4mm
dchamber = 50.8mm
dorifice = 15.875mm
II. FORCE MEASUREMENT
To measure the average net thrust produced by the
synthetic jet prototypes, the jets were attached to a strain
gauge force sensor (Interface Advanced Force Measure-
ment Part WMC-5) as is shown in Figure 6. The prototype
to be tested screws onto one end of the sensor while
the other end is rigidly attached to a stand. The setup
is submerged and forces are then measured through a
Measurement Computing Corporation PC-CARD-DAS16
analog data acquisition card.
Using the Bruel and Kjael minishaker prototype, average
thrusts of up to 0.25 N were measured. However, due to the
high price of the minishaker, we also developed a protoype
dorifice
dchamber
ddisk
delta
Fig. 7. Speaker coil driven synthetic jet
Fig. 8. Recorded force (for prototype actuated by off-the-shelf speaker
coil) as a function of forcing frequency and outstroke velocity profile
that employs an off-the-shelf voice coil actuator (as one of
the goals of this project is to design a low cost thruster).
As can be seen in the results for the consumer-grade
voice coil (Figure 8), frequency and velocity profile have a
significant effect on the produced thrust. In an actuation
region of 1–10 Hertz, increased frequency leads to an
increase in force. Additionally, in this region, a smaller
velocity profile (which corresponds to membrane forcing
profiles with a slow outstroke and a fast instroke) produced
a greater thrust.
In Figure 9, the same data is shown normalized by
outstroke velocity. This normalized data is particularly
useful, as it accounts for the fact that higher frequency
cases have higher average velocity, which skews the force
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Fig. 9. Recorded average force normalized by outstroke velocity
Fig. 10. Cross-Sectional image of vortex ring jet (pulsed at 10 Hertz)
obtained by DPIV. The jet flows from left to right.
measurement in their favor. While high frequencies may
produce a higher thrust, it is at a higher energy cost. After
normalization, our preliminary data suggets that lower
frequencies (approximately 2–5 Hertz) may provide the
most efficient propulsion.
III. FLOW VISUALIZATION
As most of the literature on synthetic jets is concerned
with jets in air, there is much to be learned about the flow
structures created by the jets in water. Early stages of this
project have been focused on gathering flow visualization
data, both qualitative and quantitative. The qualitative data
was gathered by using a high speed camera to capture the
wake created by the jet using food coloring. It is evident
from these early efforts that well defined vortex rings and
a coherent jet structure is formed. (See Figure 1.)
For quantitative data on the fluid flow induced by a
synthetic jet, Digital Particle Image Velocimetry (DPIV)
[22] was performed for trials using different jet parameters.
A pulsed 150 mW New Wave Gemini Nd:YAG laser was
used, with a time step of 10 ms between frames. The water
was seeded with 14 micron reflective particles. The data
was processed by using a window size of 32x32 pixels
with an overlap of 16 pixels. This processing allows for
the estimation of overall velocity and vorticity fields. For
this paper, DPIV was performed by using laser sheets
perpendicular to the jet. This allows us to see cross-
Sections of the vortex rings, as seen in Figure 10. Vortex
rings, seen as vortex pairs in this cross-Sectional image, are
clearly visible. Notice that the jet assumes a more complex
structure, resulting in blooming of the jet, in the far field.
Fig. 11. Illustration of proposed vehicle shape.
IV. VEHICLE DESIGN CONSIDERATIONS
This work seeks to demonstrate that low cost synthetic
jets can provide adequate propulsion and maneuvering
forces for small underwater vehicles moving at modest
speeds. Since one motivation for this project is to design
a vehicle capable of tracking a very slow moving sea
creature (such as a larvacean), our initial goal is to create
a vehicle which can reach speeds of approximately 5 cms .
(A potentially interesting hull shape would be an oblate
spehoid, as shown in Figure 11. This will allow for more
vertical stability, which would be beneficial for tasks which
require the vehicle to hover. The drag for this ”flying
saucer,” shape will be less than the drag of a sphere.
Thus the following calculations are a worst case scenario.)
Given the dimensions of our current thruster prototypes, a
reasonable diameter for the vehicle (in the horizontal plane)
is 0.25 meters.
Our initial concern was whether a single synthetic jet
would be sufficient to overcome the drag of such a vehicle.
The following calculations are done for a ”worst case
scenario” spherical vehicle, assuming steady flow:
Drag = CD
1
2
U2ρS
where U is the vehicle speed, ρ the density of water, and
CD is the drag coefficient. The drag coefficient can be
found as a function of the ambient flow’s Reynolds number,
Re = UD/ν, where D is the vehicle diameter and ν is the
kinematic viscosity of water.
As can be seen in Figure 12, the minishaker actuated
synthetic jet is sufficient for propelling the spherical vehicle
at speeds of well over 10 cms , while the consumer grade
speaker coil can propel the vehicle at speeds up to 8 cms
. Note that for the thruster configurations shown in Figure
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11, additional forces can be generated by simultaneous
actuation of two thrusters, thus increasing the speed of the
vehicle.
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0. 1
0
0.05
0. 1
0.15
0. 2
0.25
 
Maximum force produced by synthetic thruster
in optimization experiment
Vehicle Velocity (m/s)
Maximum thrust produced 
using “minishaker” voice coil actuator
Maximum thrust produced 
using “off the shelf” voice coil actuator
Drag on a “vehicle” modeled as
a sphere with diameter .25 mForce
   (N)
Fig. 12. Comparison of forces produced by prototype synthetic jet
thrusters and the drag of a spherical ”vehicle.”
V. CONCLUSION
There are many potential advantages to using synthetic
jets as propulsors for AUVs. The compact size of the
thrusters, single moving part, single seal and absence of
protruding elements make them ideal for operation in
complex environments and in very close proximity to
small, slow moving creatures. While small, the forces
they produce are sufficient for the vehicle scale that we
are considering. Finally, the wake structures generated by
synthetic jets are likely to be less obtrusive for underwater
surveillance of biological organisms.
VI. ACKNOWLEDGMENT
This work could not have been accomplished without the
assistance of Professor John Dabiri, Dr. Michele Milano
and Professor Mory Gharib of Caltech. Caltech student
Kathleen Fischer is responsible for much of the early
DPIV done on these prototypes. We would also like to
thank Caltech students Joanna Cohen, Aimee Eddins, Jesse
Escobeda, Vicente Fernandez, Maxwell Grazier G’Sell,
and Amish Patel for their assistance in the productions
of the thruster prototypes. The first author was partially
supported by a Department of Defense fellowship and a
PEO Scholar Award. Additonal support was also provided
by the National Science Foundation, grants NSF9402726
and NSF0413078.
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An experimental study of voice-coil driven synthetic jet propulsion for underwater vehicles

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An experimental study of voice-coil driven synthetic jet propulsion for underwater vehicles

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