Ari Glezer

The formation and evolution of synthetic jets

A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge of an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injection, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subjected to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transition to turbulence, slows down, loses its coherence and becomes indistinguishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near the jet exit plane, their celerity decreases monotonically with streamwise distance while the local mean velocity of the ensuing jet increases. In the far field, the synthetic jet i...

Aerodynamic Flow Control over an Unconventional Airfoil Using Synthetic Jet Actuators

Control of flow separation on an unconventional symmetric airfoil using synthetic (zero net mass flux) jet actuators is investigated in a series of wind tunnel tests. The symmetric airfoil comprises the aft portion of a NACA four-digit series airfoil and a leading edge section that is one-half of a round cylinder. The experiments are conducted over a range of Reynolds numbers between 3.1 × 10 5 and 7.25 × 10 5 . In this range, the flow separates near the leading edge at angles of attack exceeding 5 deg. When synthetic jet control is applied near the leading edge, upstream of the separation point, the separated flow reattaches completely for angles of attack up to 17.5 deg and partially for higher angles of attack. The effect of the actuation frequency, actuator location, and momentum coefficient is investigated for different angles of attack. The momentum coefficient required to reattach the separated flow decreases as the actuators are placed closer to the separation point. In some cases, reattachment is also achieved when the actuators are placed downstream of the stagnation point on the pressure side of the airfoil

Jet vectoring using synthetic jets

The interaction between a conventional rectangular (primary) air jet and a co-flowing synthetic jet is investigated experimentally. The nozzles of both jets have the same long dimension but the aspect ratio of the synthetic jet orifice is 25 times larger. Detailed particle image velocimetry (PIV) measurements of the flow in the midspan plane show that primary jet fluid is directed into the synthetic jet orifice and the interaction between the jets leads to the formation of a closed recirculating flow domain. The concomitant formation of a low-pressure region between the jets results in deflection of the primary jet toward the actuator jet despite the absence of an extended control surface (e.g. a diffuser or collar) and is balanced by a force on the primary jet conduit. For a given synthetic jet strength and primary jet speed, the vectoring force depends mainly on the volume flow rate of primary jet fluid that is diverted into the synthetic jet actuator. This flow rate is regulated by restricting the flow of entrained ambient fluid using a small streamwise extension of the synthetic jet orifice that scales with the orifice width. The response of the primary jet to the imposed vectoring is investigated using stepped modulation of the driving signal. The characteristic vectoring time and vectoring angle decrease monotonically with primary jet speed.

The formation of vortex rings

Vortex rings are usually formed by a brief discharge of fluid from an orifice. In previous investigations, the geometry of the vortex generator has varied greatly from one experiment to another, with important consequences for the ensuing flow. The present work categorizes the generating conditions for vortex rings and classifies the conditions under which a given vortex generator produces either an initially laminar ring, which may or may not undergo instability and transition to turbulence, or an initially turbulent ring. A particularly simple vortex generator was devised and measurements were carried out to provide systematic data over a range of the important dimensionless parameters. The results of this survey are used to construct a transition map that reveals a reasonably well defined boundary separating vortex rings that are turbulent upon formation from those that are not. High‐speed cinephotography of the formation and evolution of turbulent vortex rings suggests a possible connection between th...

Aerodynamic Flow Control Using Synthetic Jet Technology

The manipulation of global aerodynamic forces on bluff bodies using surface fluidic actuators based on synthetic jets technology is demonstrated in wind tunnel experiments using a 2-D cylinder model. Because synthetic jets are zero-mass-flux and are synthesized from the working fluid in the flow system in which they are embedded, their interaction with a cross flow results in formation of closed recirculation regions and in an apparent modification of the surface shape (and thus of surface pressure) with important consequences to flow separation. In the present experiments, the cylinder is instrumented with a pair of spanwise jet actuators and can be rotated about its centerline so that the angle between the jets and the direction of the free stream can be continuously varied. Azimuthal distributions of surface pressure measurements at Re D up to 131,000 over a range of jet angles demonstrate that the jets effect substantial increase in lift and reduction in drag. Velocity measurements in the near wake show that as a result of the actuation, the cross stream extent of the wake, its velocity deficit and all turbulent quantities are reduced. The response of the lift force and of the wake flow to a transient change in the control input are also investigated using pulsed amplitude modulation.

Vectoring and small-scale motions effected in free shear flows using synthetic jet actuators

Novel fluidic actuators based on synthetic jet technology are used to effect thrust vectoring and manipulate small-scale motions in conventional air jets. Synthetic jets operate without net mass injection (and thus are comprised entirely of entrained fluid), have finite streamwise momentum, and are synthesized by the time-harmonic formation of a train of two-dimensional vortices at the edge of a sharp orifice. In the present experiments, millimeter-scale high aspect ratio actuator jets are placed along the long sides and near the exit plane of a primary rectangular jet scaling one to two orders of magnitude larger. The primary jet can be vectored either towards or away from the actuator jets at angles exceeding 30 deg (and 80 deg when pairs of actuator jets are operated in concert). The actuation frequency is at least an order of magnitude higher than the unstable frequency of the primary jet and thus results in direct excitation of small scale motions and enhanced turbulent dissipation. (Author)

Manipulation of free shear flows using piezoelectric actuators

An air jet emanating from a square conduit having an equivalent diameter of 4.34 cm and a centreline velocity of 4 m/s is forced using four resonantly driven piezoelectric actuators placed along the sides of the square exit. Excitation is effected via amplitude modulation of the resonant carrier waveform. The flow is normally receptive to time–harmonic excitation at the modulating frequency but not at the resonant frequency of the actuators. When the excitation amplitude is high enough, the excitation waveform is demodulated by a nonlinear process that is connected with the formation and coalescence of nominally spanwise vortices in the forced segments of the jet shear layer. As a result, the modulating and carrier wave trains undergo spatial amplification and attenuation, respectively, downstream of the exit plane. Strong instabilities of the jet column are excited when the jet is forced at phase relationships between actuators that correspond (to lowest order) to the azimuthal modes m = 0, ±1, ±2, and −1 of an axisymmetric flow. The streamwise velocity component is measured phase locked to the modulating signal in planes normal to the mean flow. Resonantly driving the actuators with different carrier amplitudes results in a distorted mean flow having a featureless spectrum that can be tailored to provide favourable conditions for the introduction and propagation of desirable low-frequency disturbances.

Role of Actuation Frequency in Controlled Flow Reattachment over a Stalled Airfoil

The effect of the actuation frequency on the manipulation of the global aerodynamic forces on lifting surfaces using surface-mounted fluidic actuators based on synthetic (zero mass flux) jet technology is demonstrated in wind-tunnel experiments. The effect of the actuation is investigated at two ranges of (dimensionless) jet formation frequencies of the order of, or well above, the natural shedding frequency. The vortical structures within the separated flow region vary substantially when the dimensionless actuation frequency F + is varied between O(1) and O(10). When F + is O(1), the reattachment is characterized by the formation of large vortical structures at the driving frequency that persist well beyond the trailing edge of the airfoil. The formation and shedding of these vortices leads to unsteady attachment and, consequently, to a time-periodic variation in vorticity flux and in circulation. Actuation at F + of O(10) leads to a complete flow reattachment that is marked by the absence of organized vortical structures along the flow surface

Aspects of Low- and High-Frequency Actuation for Aerodynamic Flow Control

Control approaches for separated flows over aerodynamic (or bluff) bodies in which the separated flow domain scales with the characteristic length of the body are distinguished by the frequency band of the actuation input. In an approach that relies on the narrowband receptivity of the separating shear layer that is coupled to the wake (shedding) instability and scales with the characteristic advection time over the separated domain, aerodynamic performance is partially restored by a Coanda-like deflection of the forced separating shear layer toward the surface. Because the instability of the unforced shear layer may already be driven by global vortex shedding, the advection of the vortices of the forced (or controlled) layer along the surface and their ultimate shedding into the near wake can couple to wake instabilities and, therefore, may result in unsteady aerodynamic forces in the controlled flow. A different control strategy that emphasizes full or partial suppression of separation by fluidic modification of the apparent aerodynamic shape of the surface relies on controlled interaction between the actuator and the crossflow on a scale that is at least an order of magnitude smaller than the relevant global length scales.

MODIFICATION OF LIFTING BODY AERODYNAMICS USING SYNTHETIC JET ACTUATORS

The control of separated flow on an unconventional airfoil using synthetic jet actuators was investigated experimentally. A symmetric airfoil based on the aft portion of a NACA four-digit series airfoil with a cylindrical leading edge was used in the experiment. The tests were conducted at Rec=3(10)5. For a>5°, the flow separated from the airfoil surface. Applying synthetic jet control near the leading edge, upstream of the separation point, reattached the separated flow fixangle of attack up to 18°. The effect of control location and amplitude was investigated for different angles of attack. Hot wire measurements in the nearwake of the airfoil revealed a transient passing of vortices associated with the transition from separated to reattached flow on the airfoil.