Barton L. Smith

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...

Formation Criterion for Synthetic Jets

Af ormation criterion for synthetic jets is proposed and validated. A synthetic jet actuator is a zero-net mass-flux device that imparts momentum to its surroundings. Je tf ormation is defined as the appearance of a time-averaged outward velocity along the jet axis and corresponds to the generation and subsequent convection or escape of a vortex ring. It is shown that over a wide range of operating conditions synthetic jet formation is governed by the jet Strouhal number Sr (or Reynolds number Re and Stokes number S). Both numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric synthetic jet formation data in the literature. The data support the jet formation criterion 1/Sr = Re/S 2 > K, where the constant K is approximately 1 and 0.16 for two-dimensional and axisymmetric synthetic jets, respectively. In addition, the dependence of the constant K on the normalized radius of curvature of a rounded orifice or slot is addressed. The criterion is expected to serve as a useful design guide for synthetic jet formation in flow control, heat transfer, and acoustic liner applications, in which a stronger jet is synonymous with increased momentum transfer, vorticity generation, and acoustic nonlinearities.

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.

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)

Assessment of pressure field calculations from particle image velocimetry measurements

This paper explores the challenges associated with the determination of in-field pressure from DPIV (digital particle image velocimetry)-measured planar velocity fields for time-dependent incompressible flows. Several methods that have been previously explored in the literature are compared, including direct integration of the pressure gradients and solution of different forms of the pressure Poisson equations. Their dependence on grid resolution, sampling rate, velocity measurement error levels and off-axis recording was quantified using artificial data of two ideal sample flow fields—a decaying vortex flow and pulsatile flow between two parallel plates, and real DPIV and pressure data from oscillating flow through a diffuser. The need for special attention to mitigate the velocity error propagation in the pressure estimation is also addressed using a physics-preserving approach based on proper orthogonal decomposition (POD). The results demonstrate that there is no unique or optimum method for estimating the pressure field and the resulting error will depend highly on the type of the flow. However, the virtual boundary, omni-directional pressure integration scheme first proposed by Liu and Katz (2006 Exp. Fluids 41 227–40) performed consistently well in both synthetic and experimental flows. Estimated errors can vary from less than 1% to over 100% with respect to the expected value, though in contrast to more traditional smoothing algorithms, the newly proposed POD-based filtering approach can reduce errors for a given set of conditions by an order of magnitude or more. This analysis offers valuable insight that allows optimizing the choice of methods and parameters based on the flow under consideration.

Collaborative framework for PIV uncertainty quantification: comparative assessment of methods

A posteriori uncertainty quantification of particle image velocimetry (PIV) data is essential to obtain accurate estimates of the uncertainty associated with a given experiment. This is particularly relevant when measurements are used to validate computational models or in design and decision processes. In spite of the importance of the subject, the first PIV uncertainty quantification (PIV-UQ) methods have been developed only in the last three years. The present work is a comparative assessment of four approaches recently proposed in the literature: the uncertainty surface method (Timmins et al 2012), the particle disparity approach (Sciacchitano et al 2013), the peak ratio criterion (Charonko and Vlachos 2013) and the correlation statistics method (Wieneke 2015). The analysis is based upon experiments conducted for this specific purpose, where several measurement techniques are employed simultaneously. The performances of the above approaches are surveyed across different measurement conditions and flow regimes.

Uncertainty on PIV mean and fluctuating velocity due to bias and random errors

Particle image velocimetry is a powerful and flexible fluid velocity measurement tool. In spite of its widespread use, the uncertainty of PIV measurements has not been sufficiently addressed to date. The calculation and propagation of local, instantaneous uncertainties on PIV results into the measured mean and Reynolds stresses are demonstrated for four PIV error sources that impact uncertainty through the vector computation: particle image density, diameter, displacement and velocity gradients. For the purpose of this demonstration, velocity data are acquired in a rectangular jet. Hot-wire measurements are compared to PIV measurements with velocity fields computed using two PIV algorithms. Local uncertainty on the velocity mean and Reynolds stress for these algorithms are automatically estimated using a previously published method. Previous work has shown that PIV measurements can become ‘noisy’ in regions of high shear as well as regions of small displacement. This paper also demonstrates the impact of these effects by comparing PIV data to data acquired using hot-wire anemometry, which does not suffer from the same issues. It is confirmed that flow gradients, large particle images and insufficient particle image displacements can result in elevated measurements of turbulence levels. The uncertainty surface method accurately estimates the difference between hot-wire and PIV measurements for most cases. The uncertainty based on each algorithm is found to be unique, motivating the use of algorithm-specific uncertainty estimates.

Synthetic jets at large Reynolds number and comparison to continuous jets

Experimental measurements and flow visualization of synthetic jets and similar continuous jets are described. The dimensionless stroke length necessary to form a 2-D synthetic jet is between 5 and 10, with wider-nozzle jets consistently requiring a smaller value. Synthetic jets are wider, slower and have more momentum than similar continuous jets. Synthetic jets are generated using four nozzle widths that vary by a factor of four, and the driving frequency is varied over an order of magnitude. The resultant jets are in the range 13.5 < L{sub o}/h < 80.8 and 695 < Re{sub Uo} < 14700. In spite of the large range of stroke lengths, the near-field behavior of the synthetic jets scales with L{sub o}/h.

Vectoring of Adjacent Synthetic Jets

The formation, evolution, and interactions of a closely spaced pair of coflowing rectangular synthetic jets are investigated using particle image velocimetry. The dynamics of the combined jet is determined by the interactions of the counter-rotating vortex pairs that form each jet. These vortex pairs are formed along the long edges of the orifice of each jet by the time-periodic motion of a diaphragm that is mounted in a cavity underneath the orifice plate and is driven at resonance. It is shown that the phase between the actuation waveforms of the adjacent jets affects the relative timing between their blowing and suction strokes and consequently the formation of the vortex pairs that synthesize each of the jets. These phase variations also lead to controlled vectoring or bending of the combined jet toward the orifice of the jet that is leading in phase and can be varied dynamically on time scales that correspond to the actuation period.