Michael Amitay

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

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.

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

Electronic Cooling Using Synthetic Jet Impingement

The efficiency and mechanisms of cooling a constant heat flux surface by impinging synthetic jets were investigated experimentally and compared to cooling with continuous jets. Effects of jet formation frequency and Reynolds number at different nozzle-to-surface distances (H/d) were investigated. High formation frequency (f=1200 Hz) synthetic jets were found to remove heat better than low frequency (f=420 Hz) jets for small H/d, while low frequency jets are more effective at larger Hid. Moreover, synthetic jets are about three times more effective in cooling than continuous jets at the same Reynolds number. Using particle image velocimetry, it was shown that the higher formation frequency jets are associated with breakdown and merging of vortices before they impinge on the surface. For the lower frequency jets, the wavelength between coherent structures is larger such that vortex rings impinge on the surface separately.

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.

Controlled transients of flow reattachment over stalled airfoils

Abstract The flow transients associated with controlled reattachment and separation of the flow over a stalled airfoil are investigated in wind tunnel experiments. Control is effected using surface-mounted synthetic jet actuators that are typically operated at frequencies, which are at least an order of magnitude higher than the characteristic shedding frequency of the airfoil. While at these actuation frequencies the circulation (and hence the lift) of the attached flow is nominally time invariant, actuation at lower frequencies that are commensurate with the shedding frequency results in a Coanda-like attachment of the separated shear layer, organized vortex shedding and substantial oscillation of the circulation. The transients associated with flow reattachment and separation are investigated using amplitude modulation of the actuation waveform. Phase-locked measurements of the velocity field in the near wake of the airfoil and corresponding flow visualizations show that the transients that are associated with the onset of reattachment and separation are accompanied by the shedding of large-scale vortical structures and oscillations of the circulation. Pulsed modulated actuation of the actuation waveform is used to capture these transient effects and augment the increase in lift that is obtained by conventional time-harmonic actuation.

Separation control in duct flows

Active control of separation in a duct flow is achieved using an array of fluidic actuators based on synthetic-jet technology. A two-dimensional serpentine duct model is designed to produce controlled separated flow in two configurations, in which the flow is either completely separated or has a separation bubble. An array of synthetic-jet actuators is placed within the separated flow domain in the diffuser section downstream of the onset of separation. Actuation leads to complete flow attachment up to U in =75 m/s (M≃0.2) and to partial reattachment up to U in = 105 m/s (M≃0.3)

Evolution of finite span synthetic jets

The streamwise and spanwise evolutions of finite span synthetic (zero net mass flux) jets were investigated experimentally using particle image velocimetry. The synthetic jet was produced over a broad range of length and time scales at Reynolds numbers from 85 to 364, stroke lengths from 16 to 50 times the slit width, and two formation frequencies, f=300 and 917Hz. The velocity and vorticity fields were measured in two planes, across the slit (i.e., along the short axis of the orifice) and along the slit (i.e., along the long axis). The paper presents the effect of the slit aspect ratio on the development of the synthetic jet, and the spatial evolution of secondary three-dimensional vortical structures in the flow field. The measurements in the plane along the slit revealed a unique flow pattern, where near the orifice the flow is two-dimensional, while farther downstream the vortex pair lines develop secondary counter-rotating structures. The streamwise and spanwise spacing between these structures vary ...

Three-dimensional interactions between a finite-span synthetic jet and a crossflow

A complementary experimental and numerical investigation was performed to study the three-dimensional flow structures and interactions of a finite-span synthetic jet in a crossflow at a chord-based Reynolds number of 100,000 and a 0° angle of attack. Six blowing ratios in the range of 0.2–1.2 were considered. Experiments were conducted on a finite wing with a cross-sectional profile of NACA 4421, where particle-image velocimetry data were collected at the centre jet. To complement the experiments, three-dimensional numerical simulations were performed, where the numerical set-up matched not only the physical parameters (e.g. free stream) but also the physical dimensions (e.g. orientation and location of the jet. For the low blowing ratio cases, spatial non-uniformities developed, due to the finite span of the slit, which led to the formation of small and organized secondary structures or a streak-like pattern in the mean flow. On the other hand, for the high blowing ratio range, turbulent vortical structures were dominant, leading to larger spanwise structures, with a larger spanwise wavelength. Moreover, the phase-locked flow fields exhibited a train of counter-rotating coherent vortices that lifted off the surface as they advected downstream. In the mid-blowing ratio range, combined features of the low range (near the slit) and high range (in downstream locations) were found, where a pair of counter-rotating vortices issued in the same jet cycle collided with each other. In all cases, the spanwise extent of the secondary coherent structures reduced with downstream distance with a larger decrease at higher blowing ratios. Similar observations were made in earlier studies on finite-span synthetic jets in quiescent conditions.