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

A novel method for decoupling the velocity fluctuations in screeching axisymmetric jets

Planar-laser-induced fluorescence (PLIF) is used to investigate scalar mixing of circular zero-net-mass-flux (ZNMF) jets in cross-flow. ZNMF jets are formed using the working fluid of the system without net transfer of fluid mass into the system during one period of oscillation. Ensemble-averaged PLIF images of sixteen different ZNMF jets in cross-flow are studied and compared. Two distinct jet structures are seen: single trajectory jets; and multiple trajectory jets. Single trajectory jets are characterised by a single maximum concentration across the width of the jet along a given trajectory. Multiple trajectory jets are characterised by multiple regions of high concentration issuing from the orifice. Single trajectory jets demonstrate mixing of the bulk of fluid outside the boundary layer, while multiple trajectory jets demonstrate greater penetration. It was observed that there is a critical Strouhal number of Stcrit = 0.02 which separates these two regimes.

Scalar mixing of zero-net-mass-flux jets in crossflow

ABSTRACT Introduction Dry Powder Inhalers (DPIs) continue to be developed to deliver an expanding range of drugs to treat an ever-increasing range of medical conditions; with each drug and device combination needing a specifically designed inhaler. Fast regulatory approval is essential to be first to market, ensuring commercial profitability. Areas covered In vitro deposition, particle image velocimetry, and computational modeling using the physiological geometry and representative anatomy can be combined to give complementary information to determine the suitability of a proposed inhaler design and to optimize its formulation performance. In combination, they allow the entire range of questions to be addressed cost-effectively and rapidly. Expert opinion Experimental techniques and computational methods are improving rapidly, but each needs a skilled user to maximize results obtained from these techniques. Multidisciplinary teams are therefore key to making optimal use of these methods and such qualified teams can provide enormous benefits to pharmaceutical companies to improve device efficacy and thus time to market. There is already a move to integrate the benefits of Industry 4.0 into inhaler design and usage, a trend that will accelerate.

Combining experimental and computational techniques to understand and improve dry powder inhalers

Investigation of large scale motions in zero and adverse pressure gradient turbulent boundary layers using high-spatial-resolution particle image velocimetry

Historically the majority of investigations into the transfer of momentum have been focused on the classification and statistical analysis of single points of above average Reynolds stress. These regions of intense Reynolds stress are responsible for the majority of the wall-normal transfer of momentum and most of the production of turbulent kinetic energy. Single point hot-wire measurements were critical in understanding the roll of the regions of intense Reynolds stress, but provide no information about their formation, spatial extent, orientation, nor their position relative to other types of coherent structures. DNS of turbulent wall-bounded shear flows overcomes this information-limit by providing the full three-dimensional (3D) volumetric flow field information, albeit limited to low Reynolds numbers. Advancements in particle image velocimetry (PIV) have enabled the measurement of High Reynolds number turbulent wall-bounded shear flows, with measurements of two instantaneous velocity components in a single two-dimensional (2D) plane (e.g. streamwise wall-normal plane). However, the deductions and conclusions based on these information limited 2D measurements may be detrimentally affected by the lack of information from the third dimension. This study aims to address this issue by using DNS from a turbulent channel flow at R e τ = 945 and comparing the geometric characterisation of intense Reynolds stress objects computed from the full 3D flow fields with those computed from the information-limited 2D flow fields that span streamwise wall-normal planes.

Julio Soria

Papers

A novel method for decoupling the velocity fluctuations in screeching axisymmetric jets

Scalar mixing of zero-net-mass-flux jets in crossflow

Combining experimental and computational techniques to understand and improve dry powder inhalers

Investigation of large scale motions in zero and adverse pressure gradient turbulent boundary layers using high-spatial-resolution particle image velocimetry

On the identification of intense Reynolds stress structures in wall-bounded flows using information-limited two-dimensional planar data