Bojan Vukasinovic

Vibration-induced drop atomization and bursting

A liquid drop placed on a vibrating diaphragm will burst into a fine spray of smaller secondary droplets if it is driven at the proper frequency and amplitude. The process begins when capillary waves appear on the free surface of the drop and then grow in amplitude and complexity as the acceleration amplitude of the diaphragm is slowly increased from zero. When the acceleration of the diaphragm rises above a well-defined critical value, small secondary droplets begin to be ejected from the free-surface wave crests. Then, quite suddenly, the entire volume of the drop is ejected from the vibrating diaphragm in the form of a spray. This event is the result of an interaction between the fluid dynamical process of droplet ejection and the vibrational dynamics of the diaphragm. During droplet ejection, the effective mass of the drop–diaphragm system decreases and the resonance frequency of the system increases. If the initial forcing frequency is above the resonance frequency of the system, droplet ejection causes the system to move closer to resonance, which in turn causes more vigorous vibration and faster droplet ejection. This ultimately leads to drop bursting. In this paper, the basic phenomenon of vibration-induced drop atomization and drop bursting will be introduced, demonstrated, and characterized. Experimental results and a simple mathematical model of the process will be presented and used to explain the basic physics of the system.

Dynamics of a sessile drop in forced vibration

The interfacial dynamics of a sessile water drop was investigated experimentally. The low-viscosity drop was forced by an underlying diaphragm driven vertically by a piezoelectric actuator. This high-frequency forcing produced very low diaphragm displacements, even at high acceleration amplitudes. As the driving amplitude was increased from zero, the drop exhibited several transitions to states of increasing spatio-temporal complexity. The first state of the forced drop consisted of harmonic axisymmetric standing waves that were present for even the smallest diaphragm motion. Wave modes up to 14 were observed and compared to theoretical results. As the forcing amplitude increased above a critical value, a parametrically driven instability occurred that resulted in the appearance of subharmonic azimuthal waves along the contact line. The critical accelerations and the resulting wavenumbers of the azimuthal waves were documented. For larger values of the forcing amplitude, the subharmonic azimuthal waves coupled with the harmonic axisymmetric waves to produce a striking new lattice-like wave pattern. With a further increase in the forcing amplitude, the lattice mode disappeared and the interface evolved into a highly disordered state, dominated by subharmonic wave motion. The characteristics of the lattice and pre-ejection modes were documented with phase-locked measurements and spectral analysis. Finally, as the forcing amplitude increased above another critical value, the interface broke up via droplet ejection from individual wave crests.

Active Control and Optical Diagnostics of the Flow over a Hemispherical Turret

The effects of flow control actuation on the aerodynamic characteristics and aero optical distortions in the near wake of a hemispherical turret model were investigated in a series of wind tunnel experiments at M = 0.3 – 0.64. Flow control was applied using a spanwise array of high-frequency synthetic jet actuators oriented such that the long sides of their rectangular orifices were aligned in the streamwise direction. The effects of actuation on the flow dynamics and aero-optical distortions were characterized using Malley probe measurements, along with distributions of static surface pressure and hot-wire anemometry. One of the main findings of the present work was that the dissipative, small-scale motions that are induced by the actuation (StD > 1) resulted in suppression of turbulent fluctuations within the separated flow over the hemisphere, and concomitantly in significant reduction in the levels of optical distortion (in excess of 40% at M = 0.4). The effect of the momentum limited actuation diminishes at M > 0.45.

Mechanisms of free-surface breakup in vibration-induced liquid atomization

The mechanisms of droplet formation that take place during vibration-induced drop atomization are investigated experimentally. Droplet ejection results from the breakup of transient liquid spikes that form following the localized collapse of free-surface waves. Breakup typically begins with capillary pinch-off of a droplet from the tip of the spike and can be followed by additional pinch-offs of satellite droplets if the corresponding capillary number is sufficiently small (e.g., in low-viscosity liquids). If the capillary number is increased (e.g., in viscous liquids), breakup first occurs near the base of the spike, with or without subsequent breakup of the detached, thread-like spike. The formation of these detached threads is governed by a breakup mechanism that is separated from the tip-dominated capillary pinch-off mechanism by an order of magnitude in terms of dimensionless driving frequency f*. The dependence of breakup time and unbroken spike length on fluid and driving parameters is established ...

Control of a Separating Flow over a Turret

Control of flow separation over a turret comprised of hemisphere on top of a cylindrical base is demonstrated in wind tunnel experiments at Reynolds number ReD ≈ 800,000. Highfrequency actuation (StD ≈ 15) is effected using spanwise arrays of individually-addressable synthetic jet actuators and the control effectiveness is characterized using high-resolution particle image velocimetry (PIV) and surface pressure measurements. The present work has demonstrated that high-frequency synthetic jet actuation can lead to substantial separation delay, and that extent of separation delay is directly proportional to both the jet momentum coefficient and the spanwise width of the actuator array. Detailed PIV measurements near the juncture between the hemisphere and the cylinder show that in the presence of nominal actuation there is virtually no recirculating flow within the measurement window down to the level of the juncture between the hemisphere and the cylinder, and suggest that the boundary layer on the surface of the hemisphere is actually attached. However, the pressure distributions show that the static pressure levels off at elevation angle greater than 145°, which may be attributed to 3-D effects. Estimates of the turbulent kinetic energy within the flow downstream of the hemispherical cap show a rather dramatic reduction in TKE in the controlled flow, as the recirculating flow domain is pushed below the hemisphere-cylinder juncture. It is also shown that the presence of a partition plate downstream of this juncture prevents vertical advection of the reduced recirculating flow domain and forces it towards the corner much like in the flow behind a hemispherical shell on a flat plate.

Fluidic Control of Aerodynamic Forces on a Bluff Body of Revolution

The aerodynamic forces and moments on a wind tunnel model of an axisymmetric bluff body are altered by induced local attachment of the separated base flow. Control is effected by an array of four integrated aft-facing synthetic jet actuators that emanate from narrow, azimuthally-segmented slots, equally distributed around the perimeter of the circular tail end within a small backward facing step that extends into a Coanda surface. The model is suspended in the wind tunnel by eight thin wires for minimal support interference with the wake. Fluidic actuation results in a localized, segmented vectoring of the separated base flow along the rear Coanda surface and induces asymmetric aerodynamic forces and moments to effect steering during flight. The aerodynamic effects associated with quasisteady and transitory differential, asymmetric activation of the Coanda effect are characterized using direct force and PIV measurements.

Controlled Streamwise Vorticity in Diffuser Boundary Layer using Hybrid Synthetic Jet Actuation

The formation of streamwise vorticity concentration by exploiting the interaction of surface-mounted passive and active flow control elements with the cross flow is investigated experimentally in a small-scale wind tunnel at high subsonic speeds (up to M = 0.5). Controlled formation of streamwise vortices can be a key element in the mitigation of the adverse effects of secondary flows in embedded propulsion system with complex inlet geometries that can affect pressure recovery and distortion at the engine inlet face. The evolution of these vortices is investigated on a converging-diverging insert along one of the test section walls that is designed to provide an adverse pressure gradient that mimics the pressure gradient within a typical offset diffuser. Counterrotating vortex pairs and single-sense vortices are formed and characterized using conventional passive micro-ramps and micro-vanes, respectively. It is demonstrated that similar streamwise vortices can also be realized using synthetic jet actuators having rectangular orifices that are slanted or skewed to produce single-sense vortices, or streamwise aligned to produce vortex pairs. Hybrid actuation is demonstrated by combining the passive and active actuation approaches to yield a “fail-safe” device with significant degree of controllability.

Hybrid Control of a Turret Wake

Effects of hybrid flow control and its active and passive components on the aerodynamic characteristics of flow over a 0.254-m-diam conformal optical aperture embedded in the hemispherical cap of a cylinder turret model (D = 0.61 m) are investigated at M = 0.3-0.5 and Re D = 4.4-7.4 x 10 6 . Resulting mean flows are characterized by surface static pressure distributions and oil-flow visualizations, while the separated-flow dynamics are assessed by hot-film measurements. Active flow control is effected by arrays of piezoelectrically driven synthetic jet modules distributed in multiple arrays upstream from the aperture. Active flow control is further assisted by global flow alterations induced by a passive forward partition plate, and, when combined, constitute hybrid flow control. It is shown that the hybrid flow control combines the positive effects of its component control elements to yield superior results in any cumulative aerodynamic aspect of the separated flow. This cumulative effect of the actuation is manifested by concomitant delay of flow separation and active, dissipative suppression of turbulent motions downstream of separation. It is also demonstrated by means of direct two-dimensional wave-front measurements that the overall aerodynamic improvements correlate with substantial suppression of optical aberrations through the separated flow. Furthermore, estimated Strehl ratios for the laser beam indicate that nearly invariant Strehl ratio is established within the range of tested aperture elevation angles, yielding improvement of about 50 % for the highest elevation angle.

Spray characterization during vibration-induced drop atomization

Vibration-induced drop atomization is a process of rapid droplet ejection from a larger liquid drop. This occurs when a liquid drop resting on a thin diaphragm is vibrated under the appropriate forcing conditions using an attached piezoelectric actuator. The resulting spray of small droplets is characterized in this work using high-speed imaging and particle-tracking techniques. The results show that the average spatial and velocity distributions of the spray droplets are fairly axisymmetric during all stages of the atomization. The mean diameter of the droplets depends on the forcing frequency to the −2/3 power. The ejection velocity of the spray droplets depends on both the magnitude and the rate of change of the forcing amplitude. Thus, controlling the characteristics of the forcing signal may lead to strategies for controlling the spray process in specific applications.

Fluidic Control of a Turret Wake, Part I: Aerodynamic Effects

The effects of direct small-scale actuation on the aerodynamic and aero-optical characteristics of the flow over a round 0.254 m diameter conformal optical window built into the hemispherical cap of a cylinder turret model (D = 0.61 m) are investigated at M = 0.3 and ReD = 4.46·10 6 (with additional measurements at M = 0.4 and 0.5). Flow control is effected by arrays of piezoelectrically-driven synthetic jet modules. The cumulative effect of the actuation is manifested by concomitant delay of flow separation and active, dissipative suppression of turbulent motions downstream of separation. The effects of actuation on aero-optical distortions are assessed from the flow dynamics using surface oil visualization, static pressure distributions and hot-film measurements within the separated flow domain. In addition, the suppression of optical distortions across the separated flow is estimated from Malley probe measurements over a range of elevation angles. These measurements show that for a fixed actuation level the suppression of spectral components of the optical distortion within the Malley probes’s resolvable frequency band 0.5 < f < 25 kHz at M = 0.3 is about 30%.