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

Application of digital holographic microscopic PIV to a water jet

Vertical axis wind turbines (VAWT) are an alternative to horizontal axis wind turbines (HAWT) for wind and tidal energy harvesting applications. Studies of vertical axis wind turbines have heavily invested in understanding the behaviour of the complete turbine, but often with only cursory assessment of the contributions of unsteady aerodynamics of individual blades. This paper experimentally explores the process of unsteady flow separation, or dynamic stall, of the flow around the blades of a gyromill-style vertical axis wind turbine, and the factors upon which its appearance and evolution rely. Introduction Wind and tidal energy harvesting have long been mainstays of a growing renewable energy generation industry. Horizontal axis wind turbines (HAWT) have been the dominant turbine form for decades, but there has at times been an interest in alternative configurations. Amongst these, the vertical axis wind turbine (VAWT) presents a number of potential advantages, but also significant technical challenges. Chief amongst the aerodynamic challenges posed by vertical axis wind turbines is the dynamic stall phenomenon. This unsteady flow separation is thought to induce undesirable structural vibrations, contribute to the production of noise, and have a deleterious effect on the efficiency of the turbine, and is a crucial problem to understand and address for more widespread adoption of vertical axis wind turbine technology. The history of dynamic stall research is rich, but the highly nonlinear nature of the phenomenon, and the large parameter space within which the experimentalist must work, mean that previous results must be considered with caution when approaching the question of dynamic stall in a new application or context. Much previous work has been published on dynamic stall of helicopter blades, and of flapping wings or oscillating fins [1, 2, 3], but the combination of rotation and rapid blade incidence angle changes particular to vertical axis wind turbines warrants greater attention. Insufficient study of the relative importance of the kinematic parameters involved has been performed. Buchner and Soria [4] addresses the dynamic stall’s dependence on Reynolds number, while Ferreira et al [5] studies the effect of tip speed ratio, and also touches on the dependence of the flow on Reynolds number, but the dimensionless pitching rate, and its relevance to the onset and evolution of the dynamic stall are not addressed. Dynamic Stall and its Governing Parameters The bulk of the literature regarding the aerodynamics of vertical axis wind turbine blades considers the effect of tip speed ratio λ,

Scaling of vertical axis wind turbine dynamic stall

Data obtained from direct numerical simulations of isotropic homogeneous turbulence and the diffusion of a scalar with an applied mean gradient is analysed using the topological techniques developed by Chong, Perry and Cantwell [ Physics of Fluids A , 2 :765-777, 1990]. Simulations were run at two different Taylor--Reynolds numbers and two different Schmidt numbers. Comparing the numerical results obtained, relationships between the scalar characteristics and the topological features of the flow are identified.

https://journal.austms.org.au/ojs/index.php/ANZIAMJ/article/download/1013/911

The topology of homogeneous isotropic turbulence with passive scalar transport

PIV Measurements of a Backward Facing Step Flow

This project extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The DNS data of wall bounded flows studied here are from the zero pressure gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached eddy hypothesis.

https://ntrs.nasa.gov/api/citations/19970014676/downloads/19970014676.pdf

Julio Soria

Papers

Application of digital holographic microscopic PIV to a water jet

Scaling of vertical axis wind turbine dynamic stall

The topology of homogeneous isotropic turbulence with passive scalar transport

PIV Measurements of a Backward Facing Step Flow

A study of the turbulence structures of wall-bounded shear flows