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Design and Performance Evaluation of Piezo-Driven Synthetic Jet Devices
Full text linkIn the last two decades synthetic jet actuators have gained much interest among flow control techniques due to their short response time, high jet velocity and absence of traditional piping, that matches the requirements of reduced size and low weight. A synthetic jet is generated by the diaphragm oscillation (generally driven by a piezo-electric element) in a relatively small cavity, producing periodic cavity pressure variations associated to cavity volume changes. The high pressure air exhausts through an orifice, converting membrane elastic energy in jet kinetic energy. This review paper faces the development of various lumped-element models (LEM) as practical tools to design and manufacturing actuators. LEM can predict quickly device performances such as frequency response in terms of membrane displacement, cavity pressure and jet velocity, as well as efficiency of energy conversion of input Joule power into useful kinetic power of air jet. Actuator performance is analyzed also by varying typical geometric parameters such as cavity height and orifice diameter and length, through a proper dimensionless form of the governing equations
Development of a numerical model for a PSJ actuator
No full textAmong various devices used in active flow control, plasma synthetic jet actuators seem to be a promising technology to improve aircraft performances. This paper presents a new physical model able to predict the temporal evolution of the main thermodynamic variables of the device. Results for both single pulse mode and repetitive working regime are reported, providing insights of their characteristics. The work is completed by an analysis of the actuator frequency response, followed by a comparison with literature results
Numerical and experimental characterization of a double-orifice synthetic jet actuator
No full textA complete investigation of a doubleorifice
synthetic jet actuator, focused on the device
frequency response in terms of jet velocity, has been
carried out. Numerical simulations have shown that, in
many operation conditions, the flow within the
actuator cavity can be considered as divided in two
sub-volumes, each characterized by its own flow field.
An analytical approach, based on the previous consideration,
has allowed to obtain simple relationships
for the three resonance frequencies and to provide
further insights on the jets formation. The model has
been validated through experimental tests carried out
on two actuators manufactured in-house, having
different geometrical and mechanical characteristics.
Comparisons with the behavior of the twin singleorifice
device have been discussed and useful considerations
on the prediction of the actual formation of
the synthetic jet are included
Development of a physical model for plasma synthetic jet actuators
No full textAmong various devices used in active flow control, plasma synthetic jet actuators seem
to be a promising technology to improve aircraft performances. This paper presents a new physical
model able to predict the temporal evolution of the main thermodynamic variables of the device.
Results for both single pulse mode and repetitive working regime are reported, providing insights
of their characteristics. The work is completed by an analysis of the actuator frequency response,
followed by a comparison with literature results
Design approach to predict synthetic jet formation and resonance amplifications
No full textA predictive approach for the Synthetic Jet (SJ) formation, which couples formation criteria already available
from the literature, with outcome to Lumped Element Modeling (LEM), which is able to predict the resonance
velocity amplifications, is presented. In this respect, the formation criteria yield the minimum jet velocity to
obtain an experimentally observable jet, whereas the LEM approach predicts the jet velocity, to be compared
with the minimum one of the formation criterion. A deep experimental investigation is carried out to detect jet
formation and to measure resonance velocity amplifications on various types of actuators manufactured in
house, including, among others, the influence of single and double orifices. Jet velocity is recorded by means of
hot-wire anemometry, at both the device orifice exit and one orifice diameter downstream, i.e. downstream of
the expected stagnation (saddle) point. Jet formation and resonance amplification thresholds are identified in
terms of the dimensionless Stokes and Strouhal parameters. Based on the experimental finding, a description of
the relevant modeling is reported, devoted to the evaluation of the velocity magnification factor with respect to
the so-called incompressible (static) solution. It takes advantage from the physical consideration that the actuator
behaves as a driven system of two-coupled mechanical oscillators, the Helmholtz’s one and the structural
one, for which resonance frequencies are predicted by means of simple relationships, together with appropriate
damping factors. Jet formation thresholds closely agree with the present experimental findings. The predicted
overdamped conditions are compared with classic analytic boundary correlations of the literature and previous
experimental results as well. Practical and simple relationships, that can help the designer to manufacture a
device having desired performances, are presented
Investigation of a Plasma Synthetic Jet Actuator for Flow Control
No full textAmong active flow control techniques, plasma synthetic jet (PSJ) actuators seem to be a promising technology to improve aircraft performances due to their short response time, high jet velocities and absence of moving parts. An electrical discharge is produced within a cavity, increasing pressure and temperature, causing the exhaust of the gas through the orifice. After few cycles a periodic behaviour is reached generating a plasma synthetic jet.
A numerical and experimental investigation was conducted to characterize the performance of the actuator and its potential as an active flow control method. An original lumped-element physical model (LEM) able to predict the temporal evolution of the major thermo-fluid-dynamic quantities of the device was developed. The governing equations are fully gasdynamics based and include viscous losses; the air is modelled as a real gas and both radiative and convective heat transfer mechanisms are considered at walls. The correct simulation of the refill regime is guaranteed by the inertial term included in the unsteady Bernoulli's equation. Axisymmetric numerical computations, carried out with OpenFOAM computer code, has allowed one to calibrate the lumped model through the determination of some fitting parameters. Finally, experimental measurements have allowed the completion of device investigation, producing valuable information about pressure, jet velocity and duration of the discharge.
Results for both single pulse mode and repetitive working regimes are obtained, providing insights on major actuation characteristics. High frequency oscillations in the time interval between two subsequent discharge pulses are observed and analytically justified resorting to the Helmholtz resonator model. A comparison between measurements and simulations is performed, showing a satisfactory matching of the data and demonstrating the validity of the LEM model in the prediction of a PSJ actuator behaviour
Plasma synthetic jet actuators for active flow control
No full textThe plasma synthetic jet actuator (PSJA), also named as sparkjet actuator, is a special type of zero-net mass flux actuator, driven thermodynamically by pulsed arc/spark discharge. Compared to widely investigated mechanical synthetic jet actuators driven by vibrating diaphragms or oscillating pistons, PSJAs exhibit the unique capability of producing high-velocity (>300 m/s) pulsed jets at high frequency (>5 kHz), thus tailored for high-Reynolds-number high-speed flow control in aerospace engineering. This paper reviews the development of PSJA in the last 15 years, covering the major achievements in the actuator working physics (i.e., characterization in quiescent air) as well as flow control applications (i.e., interaction with external crossflow). Based on the extensive non-dimensional laws obtained in characterization studies, it becomes feasible to design an actuator under several performance constraints, based on first-principles. The peak jet velocity produced by this type of actuator scales approximately with the cubic root of the non-dimensional energy deposition, and the scaling factor is determined by the electro-mechanical efficiency of the actuator (O(0.1%–1%)). To boost the electro-mechanical efficiency, the energy losses in the gas heating phase and thermodynamic cycle process should be minimized by careful design of the discharge circuitry as well as the actuator geometry. Moreover, the limit working frequency of the actuator is set by the Helmholtz natural resonance frequency of the actuator cavity, which can be tuned by the cavity volume, exit orifice area and exit nozzle length. In contrast to the fruitful characterization studies, the application studies of PSJAs have progressed relatively slower, not only due to the inherent difficulties of performing advanced numerical simulations/measurements in high-Reynolds-number high-speed flow, but also related to the complexity of designing a reliable discharge circuit that can feed multiple actuators at high repetition rate. Notwithstanding these limitations, results from existing investigations are already sufficient to demonstrate the authority of plasma synthetic jets in shock wave boundary layer interaction control, jet noise mitigation and airfoil trailing-edge flow separation.Aerodynamic
Scaling properties of resonant cavities driven by piezo-electric actuators
No full textAcoustic-structural properties of piezo-electric driven resonant cavities usually employed to generate the so-called synthetic jets are theoretically and numerically investigated in order to characterize the performances of such devices. It is shown that the actuator behaves as a two-coupled oscillators system and the dimensionless form of the governing equations allows one to identify various operating conditions, in particular those leading to their decoupling. The theoretical predictions are validated through analytical, numerical and experimental findings for devices having different mechanical and geometrical characteristics, designed to achieve an increasing coupling effect. Considerations about the strength of jet formation at the Helmholtz frequency are made as well
Modelling of efficiency of synthetic jet actuators
No full textA comprehensive and detailed modelling to evaluate the efficiency of energy conversion of
piezo-electric actuators driving synthetic jets is developed. The contribution is original because the
analysis is based on the energy equations of the two coupled oscillators, the membrane and the
acoustic one, which are directly derived from the corresponding motion equations. The modelling is
validated against numerical as well as experimental investigations carried out on a home-made
actuator having an aluminum shim on which the piezodisk is bonded. A major result is that for the
actuator under investigationthe global efficiency (representing the conversion of input Joule power to
kinetic power) decreases with increasing the applied voltage. Another finding is that the conversion
process of mechanical power transferred from the driving membrane to Helmholtz oscillator kinetic
power scales dramatically with
the coupling degree of the oscillators. The coupling degree infuences the efficiency of two cavities
actuators sharing the same piezo-diaphragm as
well. Considerations are reported to relate the theoretical orifice efficiency to the practical jetefficiency
issuing in the external field
Water spray flow characteristics under synthetic jet driven by a piezoelectric actuator
Full text linkParticle Image Velocimetry (PIV) and Phase Doppler Anemometry (PDA) have been applied to investigate the droplets size and velocity distribution of a water spray, under the control of a piezo-element driven synthetic jet (SJ). Tests were carried out under atmospheric conditions within a chamber test rig equipped with optical accesses at two injection pressures, namely 5 and 10 MPa, exploring the variation of the main spray parameters caused by the synthetic jet perturbations. The SJ orifice has been placed at 45° with respect to the water spray axis; the nozzle body has been moved on its own axis and three different nozzle quotes were tested. PIV measurements have been averaged on 300 trials whereas about 105 samples have been acquired for the PDA tests. For each operative condition, the influence region of the SJ device on the spray has been computed through a T-Test algorithm. The synthetic jet locally interacts with the spray, energizing the region downstream the impact. The effect of the actuator decreases at higher injection pressures and moving the impact region upwards. Droplets coalescence can be detected along the synthetic jet axis, while no significant variations are observed along a direction orthogonal to it
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