Since the review of periodic flow phenomena by Berger & Wille (1972) in this journal, over twenty years ago, there has been a surge of activity regarding bluff body wakes. Many of the questions regarding wake vortex dynamics from the earlier review have now been answered in the literature, and perhaps an essential key to our new understandings (and indeed to new questions) has been the recent focus, over the past eight years, on the three-dimensional aspects of nominally two-dimensional wake flows. New techniques in experiment, using laser-induced fluorescence and PIV (Particle-Image-Velocimetry), are vigorously being applied to wakes, but interestingly, several of the new discoveries have come from careful use of classical methods. There is no question that strides forward in understanding of the wake problem are being made possible by ongoing three- dimensional direct numerical simulations, as well as by the surprisingly successful use of analytical modeling in these flows, and by secondary stability analyses. These new developments, and the discoveries of several new phenomena in wakes, are presented in this review.

Vortex Dynamics in the Cylinder Wake

Digital particle image velocimetry (DPIV) is a non-intrusive analysis technique that is very popular for mapping flows quantitatively. To get accurate results, in particular in complex flow fields, a number of challenges have to be faced and solved: The quality of the flow measurements is affected by computational details such as image pre-conditioning, sub-pixel peak estimators, data validation procedures, interpolation algorithms and smoothing methods. The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.

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PIVlab – Towards User-friendly, Affordable and Accurate Digital Particle Image Velocimetry in MATLAB

The structure of energy-containing turbulence in the outer region of a zero-pressure-
 gradient boundary layer has been studied using particle image velocimetry (PIV) to measure the instantaneous velocity fields in a streamwise-wall-normal plane. Experiments performed at three Reynolds numbers in the range 930 0) that occur on a locus inclined at 30–60° to the wall.In the outer layer, hairpin vortices occur in streamwise-aligned packets that propagate with small velocity dispersion. Packets that begin in or slightly above the buffer layer are very similar to the packets created by the autogeneration mechanism (Zhou, Adrian & Balachandar 1996). Individual packets grow upwards in the streamwise direction at a mean angle of approximately 12°, and the hairpins in packets are typically spaced several hundred viscous lengthscales apart in the streamwise direction. Within the interior of the envelope the spatial coherence between the velocity fields induced by the individual vortices leads to strongly retarded streamwise momentum, explaining the zones of uniform momentum observed by Meinhart & Adrian (1995). The packets are an important type of organized structure in the wall layer in which relatively small structural units in the form of three-dimensional vortical structures are arranged coherently, i.e. with correlated spatial relationships, to form much longer structures. The formation of packets explains the occurrence of multiple VITA events in turbulent ‘bursts’, and the creation of Townsend's (1958) large-scale inactive motions. These packets share many features of the hairpin models proposed by Smith (1984) and co-workers for the near-wall layer, and by Bandyopadhyay (1980), but they are shown to occur in a hierarchy of scales across most of the boundary layer.In the logarithmic layer, the coherent vortex packets that originate close to the wall frequently occur within larger, faster moving zones of uniform momentum, which may extend up to the middle of the boundary layer. These larger zones are the induced interior flow of older packets of coherent hairpin vortices that originate upstream and over-run the younger, more recently generated packets. The occurence of small hairpin packets in the environment of larger hairpin packets is a prominent feature of the logarithmic layer. With increasing Reynolds number, the number of hairpins in a packet increases.

Vortex organization in the outer region of the turbulent boundary layer

Small-scale turbulence has been an area of especially active research in the recent past, and several useful research directions have been pursued. Here, we selectively review this work. The emphasis is on scaling phenomenology and kinematics of small-scale structure. After providing a brief introduction to the classical notions of universality due to Kolmogorov and others, we survey the existing work on intermittency, refined similarity hypotheses, anomalous scaling exponents, derivative statistics, intermittency models, and the structure and kinematics of small-scale structure—the latter aspect coming largely from the direct numerical simulation of homogeneous turbulence in a periodic box.

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The phenomenology of small-scale turbulence

Originally introduced in the fluid mechanics community, dynamic mode decomposition (DMD) has emerged as a powerful tool for analyzing the dynamics of nonlinear systems.
 However, existing DMD theory deals primarily with sequential time series for which the measurement dimension is much larger than the number of measurements taken.
 We present a theoretical framework in which we define DMD as the eigendecomposition of an approximating linear operator.
 This generalizes DMD to a larger class of datasets, including nonsequential time series.
 We demonstrate the utility of this approach by presenting novel sampling strategies that increase computational efficiency and mitigate the effects of noise, respectively.
 We also introduce the concept of linear consistency, which helps explain the potential pitfalls of applying DMD to rank-deficient datasets, illustrating with examples.
 Such computations are not considered in the existing literature but can be understood using our more general framework.
 In addition, we show that our theory strengthens the connections between DMD and Koopman operator theory.
 It also establishes connections between DMD and other techniques, including the eigensystem realization algorithm (ERA), a system identification method, and linear inverse modeling (LIM), a method from climate science.
 We show that under certain conditions, DMD is equivalent to LIM.

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On dynamic mode decomposition: Theory and applications

An adaptation of current image shifting techniques has been developed that permits cross-correlation PIV analysis of non- overlapping images recorded on photographic media. The image acquisition technique utilizes pulsed lasers to produce two singly exposed sub-images of particles in one photographic frame. The technique can also provide images suitable for cross-correlation analysis on digitally recorded images without the need for a specialized camera. An adaptive cross- correlation PIV analysis method is employed to extract the in- plane velocity field information with high velocity sampling resolution and large velocity dynamic range. Further capabilities and limitations of the technique are presented as well as an analysis of the associated measurement errors. Some of the more interesting problems that were encountered during the development of the experimental technique are also discussed. The application of this method to the unsteady flow of a vortex ring will be illustrated as an example. High spatial resolution measurements using film recording have yielded the detailed instantaneous 2-D velocity and vorticity fields. Measurements made from digital recordings using this technique provide the evolution of instantaneous in-plane velocity and vorticity fields.

High-resolution cross-correlation PIV on photographic film

The flow structures of a supersonic impinging jet are investigated via numerical simulation. A converging inlet nozzle is used to accelerate the flow to sonic velocity and the flow impinges on a wall located at a normalized stand-off distance (z/D) of 2.5. The Reynolds averaged Navier-Stokes (RANS) and the Large Eddy Simulation (LES) methodologies are used to simulate the shock structures of the impinged jet. The characteristics of the underexpanded supersonic impinging jet is then visually analysed in the LES model. Next, the results of the numerical simulations are compared to experimental data obtained by [7], which uses Schlieren photography and shadowgraphy technique to visualize the shock structures. Results from the RANS and LES numerical models which are consistent with the experimental data demonstrate the capability of both methodologies in simulating supersonic impinging jet flows.

Numerical simulation of supersonic impinging jet flows using Reynolds averaged Navier-Stokes and Large Eddy Simulation

High spatial resolution stereoscopic DPIV in a turbulent boundary layer

Digital in-line holography has been applied for the study of water droplets generated by a micro dispensing device. The micro dispenser allows the control of the droplet size by changing the pressure applied to a water reservoir. Two pressures have been used in this work: 51.7 and 103.4 kPa. The holograms acquired are mathematically reconstructed and the droplet images are analysed. The droplet sizes are found to be between 140-170 μm for a pressure of 51.7 kPa and around 100 μm for 103.4 kPa.

Digital in-line holography for micro-droplet diagnostics

PIV measurements of the velocity field of a supersonic jet impinging on a flat surface were conducted for a pure convergent nozzle with a nozzle pressure ratio of NPR=3.4 and a nozzle to plate spacing of Z/D = 5.0. The velocity field at a center plane of the jet was resolved at a highspatial resolution of 0.012D in the streamwise and radial directions. The paper reports the complex flow structure as well as the lowerand higher-order statistics. INTRODUCTION The physics of supersonic jets has been under investigation over the last five decades due to their various aerospace and industrial applications. When a jet exits a nozzle with a pressure higher than the pressure of the surrounding area, an underexpanded jet forms. This pressure mismatch generates a series of shock and expansion waves within the jet core. This situation becomes more complicated when the jet interacts with an impingement surface. Understanding of the impinging jet flow is important in the design and control of short vertical takeoff and landing (SVTOL) of aircrafts, rocket/missile launching systems (Henderson, 2002; Kumar et al., 2013), and in the cold spray coating process (Gilmore et al., 1999; Assadi et al., 2003). The flow field in supersonic impinging jets is typically highly unsteady. It contains a feedback loop that was first described by Powell (1953). This mechanism is initiated by instabilities in the shear layer at the nozzle lip. The instability waves grow in size and evolve into large scale vortical structures that convect downstream. Near the plate surface, these large scale structures interact with the stand-off shock formed above the plate. This interaction produces motion in the wall jet, and this motion in turn generates an impulsive acoustic wave that propagates back to the nozzle lip (Henderson et al., 2005). At the nozzle lip, these acoustic waves force the shear layer, creating the incipient instability waves and thus completing the feedback loop. Henderson (2002) and others have however demonstrated that “zones of silence” exist; at some conditions the jet exhibits no feedback loop. The authors have previously shown the cyclic nature of the impingement process and the closed loop instability mechanisms using two sets of high-spatial and hightemporal resolution schlieren images of an impinging jet (Buchmann et al., 2011; Mitchell et al., 2012). Figure 1 visualizes the acoustic waves traveling towards the nozzle and the coherent structures at the shear layer. In the present study, high-spatial resolution measurements of the velocity fields of this phenomenon are reported. The important parameters that affect the flow structure and noise production are the nozzle pressure ratio (NPR), the nozzle to surface 1 June 30 July 3, 2015 Melbourne, Australia 9 P-04

Julio Soria

Papers

High-resolution cross-correlation PIV on photographic film

Numerical simulation of supersonic impinging jet flows using Reynolds averaged Navier-Stokes and Large Eddy Simulation

High spatial resolution stereoscopic DPIV in a turbulent boundary layer

Digital in-line holography for micro-droplet diagnostics

High resolution PIV measurements of an impinging underexpanded supersonic jet