Synthetic Jet Actuators for Flow Control

Among the various active flow control techniques, Synthetic Jet (SJ) actuators represent a very promising technology due to their short response time, high jet velocity and absence of traditional piping, that matches the requirements of reduced size and low weight. Therefore, understanding in depth the basic physical aspects driving the operation of these actuators is a key point. A practical tool, employed for design and manufacturing purposes, consists in the definition of a low-order model, lumped element model (LEM), which is able to predict the dynamic response of the actuator in a relatively quick way and with reasonable fidelity and accuracy. The research activity focused by the present author has tried to tackle different aspects to achieve various goals. A major task has concerned the development of LEMs to predict the behavior of two types of actuators: piezo-driven SJ and Plasma Synthetic Jet (PSJ) actuators. These models share the same philosophy: they represent valuable tools useful not only for design purposed, but also to obtain useful insights on the devices performances. A second crucial task has consisted in the design, manufacturing and characterization of various prototypes, which have been used to validate LEMs results. As an additional task of the research activity, applications both in automotive and aerospace fields have been considered. As regards the piezo-driven synthetic jets, the main activity was concerned with the extension of an already available lumped-element model in order to: derive a non-dimensional form of the governing equations; identify the main design quantities; estimate the device performances; shed light on the actuator energy efficiency with reference to the different stages involved in the operation process. Several issues have been faced including the technology required for bonding the piezo-element over the metallic shim (so as to realize the so-called diaphragm), the design and manufacturing of the experimental mock-up, the production of the different parts of the device, the post-processing analysis. A very interesting application of the piezo-driven SJ technology, which can have outcomes in the automotive sector, has regarded the manipulation of a continuous water spray. Experimental data, taken within a chamber test rig at two injection pressures, for different SJ positions, have been acquired. Starting from the flow field velocity distributions, detected with Particle Image Velocimetry (PIV), the effective influence region of the jet on the spray has been computed through a T-Test algorithm and corroborated by a vorticity analysis. Another innovative LEM, able to predict the temporal evolution of the main fluid-dynamic variables involved, has been developed for PSJ actuators. It is fully based on the gasdynamics equations, it includes viscous losses as well as radiative and convective heat transfer mechanisms at walls, and it considers real gas effect for air. OpenFOAM numerical computations have been carried out to perform a first calibration of the lumped model through the determination of key fitting parameters. Results for both single pulse mode and repetitive working regimes have been analyzed, providing insights on the major actuation characteristics. To validate the LEM model, a home-made PSJ actuator has been designed and manufactured. The overall experimental mock-up has been also designed, together with the control electrical system. Experimental measurements of the jet velocity, obtained with Hot-wire anemometer and Pitot tube, have completed the actuator characterization and have allowed the model validation

Design and Performance Evaluation of Piezo-Driven Synthetic Jet Devices

In 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

A Calibrated Lumped Element Model for the Prediction of PSJ Actuator Efficiency Performance

Among the various active flow control techniques, Plasma Synthetic Jet (PSJ) actuators, or Sparkjets, represent a very promising technology, especially because of their high velocities and short response times. A practical tool, employed for design and manufacturing purposes, consists of the definition of a low-order model, lumped element model (LEM), which is able to predict the dynamic response of the actuator in a relatively quick way and with reasonable fidelity and accuracy. After a brief description of an innovative lumped model, this work faces the experimental investigation of a home-designed and manufactured PSJ actuator, for different frequencies and energy discharges. Particular attention has been taken in the power supply system design. A specific home-made Pitot tube has allowed the detection of velocity profiles along the jet radial direction, for various energy discharges, as well as the tuning of the lumped model with experimental data, where the total device efficiency has been assumed as a fitting parameter. The best fitting value not only contains information on the actual device efficiency, but includes some modeling and experimental uncertainties, related also to the used measurement techniqu

Absolute instability in plasma jet

Stability features of a plasma jet are investigated by means of Linear StabilityTheory. The convective/absolute nature of the instabilities is determined by local spatio-temporal analyses of the impulse response of the flow for different stream-wise positionsand different operative conditions. Frequencies, shapes and growth rate of the leadingstability mode are compared to available experimental high speed camera recordings of thejet unsteadiness. The frequency range and mode shapes retrieved theoretically are in goodagreement with the experimental results. However, the growth-rates of these modes indicatea fast transition to the turbulent regime which is not observed in the facility, which couldbe explained by non-parallel base flow or non-linear modal growth of the mode effects

water spray flow characteristics under synthetic jet driven by a piezoelectric actuator

Particle 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

Free response of a gravitational liquid sheet by means of three-dimensional Volume-of-Fluid simulations

The volume-of-fluid (VOF) method is employed to simulate the dynamics of gravitational liquid sheets (curtains) issuing into an initially quiescent gaseous environment

Modal analysis of a 3D gravitational liquid sheet

Modal analysis of three-dimensional gravitational thin liquid sheet flows, interacting with unconfined gaseous environments located on both sides of the liquid phase, is performed in the present work. Numerical data of this relevant two-phase flow configuration are obtained through the single-phase formulation and the Volume-of-Fluid (VOF) technique implemented in the flow solver Basilisk. This class of flows exhibits a variety of spatial and temporal relevant structures, both in free and forced configurations, that are investigated through the Spectral Proper Orthogonal Decomposition (SPOD). By means of such methodology, we explore the effect of two main governing parameters on the flow dynamics, namely the liquid sheet aspect ratio, AR = W/H, where H and W are the sheet inlet thickness and width, and the Weber number, We = lU2H/(2), in which U is the inlet liquid velocity, lthe liquid density, and the surface tension coefficient. Finally, for the highest aspect ratio value considered (AR = 40), we investigate the forced dynamics of the system excited by a harmonic perturbation in transverse velocity component applied at the inlet section, comparing results with ones arising from a purely two-dimensional analysis of the flow. The obtained results highlight the low rank behavior exhibited by the flow, suggesting that reduced order modeling could be particularly appealing to reduce complexity and computational effort in numerical simulation of this class of flows

Facciamo il punto: miocardio non compatto

Il miocardio non compatto \ue8 una rara patologia del muscolo cardiaco caratterizzata da una marcata ipertrabecolatura parietale dovuta all\u2019arresto del processo di maturazione miocardica durante lo sviluppo fetale. Non sono stati ancora definiti dei criteri \u201cgold standard\u201d per la diagnosi; \ue8 inoltre necessaria la differenziazione da quadri parafisiologici e da altre cardiomiopatie. Rafforzano il sospetto clinico la familiarit\ue0 per la malattia, la coesistente presenza di malattia neuromuscolare, fenomeni tromboembolici, anomalie elettrocardiografiche, aritmie ventricolari, dilatazione e disfunzione ventricolare sinistra, fibrosi alla risonanza magnetica, mutazioni patogene. Molti aspetti rimangono tuttora controversi e dovranno essere chiariti con studi multicentrici, registri e studi osservazionali

Investigation of a Plasma Synthetic Jet Actuator for Flow Control

Among 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

Development of a numerical model for a PSJ actuator

Among 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