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 multiple camera, multiple resolution PIV system is used to capture a large streamwise, wall-normal field of view (2δ × 1.5δ), while simultaneously resolving the near-wall flow down to a height of y+ ≈ 3 at Reynolds numbers of Reτ ≈ 8, 000 to 20, 000. The high spatial resolution in the near-wall vicinity is used to provide a direct estimate of the wall shear stress and subsequent analysis of both the small near-wall and larger outer flow structure conditional on the local friction velocity and conversely on larger scale fluctuations. Conditional two-point correlations, and conditional mean structures will be presented.

Experimental investigation of the near and far field structure of high Reynolds number turbulent boundary layers

The structure of the wake behind a submersed body greatly affects the performance and design of many engineering applications. The behaviour of the wake partially dictates the drag experienced by the body, therefore optimisation of this wake behaviour can have a significant effect on the energy required to move the body relative to the fluid. The formation of wakes over smooth bodies is well understood, however, the implications of surface roughness on the near wake structure are not well known. Surface roughness is known to greatly affect the behaviour of a turbulent boundary layer and the nature of the structures within. The subsequent interaction of these modified boundary layers in the wake modifies the formation of the wake. Roughness can be introduced into a system in a range of ways, such as manufacturing processes, surface finishes and degradation and accumulation of fouling. Therefore, the prevalence of roughness in combination with the importance of wake behaviour on performance is the motivation for the investigation of the implications of surface roughness on plane wake flows. To investigate these implications a series of 2-component 2dimensional particle image velocimetry (2C-2D PIV) measurements were performed on a smooth and rough flat plate in the LTRAC large horizontal water tunnel facility. The boundary layer was tripped at the leading edge and measurements were acquired in the near wake of the blunt trailing edge of the plate. The surface was roughened using commercially available sandpaper adhered to the plate surface.

PIV measurements of a wake from a rough flat plate

The turbulent boundary layer in a region of increasing adverse pressure gradient (APG) at the rear of an axisymmetric generic submarine body is investigated using a series of high resolution planar particle image velocimetry (PIV) measurements. Profiles of mean and fluctuating velocity are presented in coordinates aligned with the wall at stations ranging from weak favourable pressure gradients (FPG) through to strong APGs. The degree to which equilibrium based pressure velocity scaling and Zagarola and Smits [6] scaling are able to collapse these axisymmetric boundary layer profiles is explored.

Statistics and scaling of axisymmetric turbulent boundary layers under the transition from zero to adverse pressure gradients

Video: Direct numerical simulation of turbulent boundary layer with localised heat source: an analogy to simulate bushfire

Transport of scalars by turbulent fluid motion is of fundamental importance and is encountered in applications such as pollutant formation, mass and heat transfer and chemical reactions. The physics of many of these applications is extremely complex and not well understood due to the intriguing complex topology of the fluid motions and scalar fields. In this paper, a new formulation is proposed to study the detailed topology of diffusing scalars and their relationship to the turbulent flow topology. Direct numerical simulations (DNS) are used to study those topologies in homogeneous isotropic turbulence, using an object-oriented code (Crespo 1994).

Julio Soria

Papers

Experimental investigation of the near and far field structure of high Reynolds number turbulent boundary layers

PIV measurements of a wake from a rough flat plate

Statistics and scaling of axisymmetric turbulent boundary layers under the transition from zero to adverse pressure gradients

Video: Direct numerical simulation of turbulent boundary layer with localised heat source: an analogy to simulate bushfire

Topology of Dynamically Passive Scalar Fields in Isotropic Homogeneous Turbulence