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Sommerfeld's affiliation with fluid mechanics started when he began his career as an assistant of the mathematician Felix Klein at Göttingen. He always regarded fluid mechanics as a particular challenge. In 1904, he published a theory of hydrodynamic lubrication. Four years later, he conceived an approach for the analysis of flow instability (the Orr-Sommerfeld approach) as an attempt to account for the transition from laminar to turbulent flow. The onset of turbulence also became a major challenge for some of his pupils, in particular Ludwig Hopf and Fritz Noether. Both contributed considerably to elaborate the Orr-Sommerfeld theory. Heisenberg's doctoral work was another attempt in this quest. When Sommerfeld published his lectures on theoretical physics during World War II, he dedicated one of the six volumes to the mechanics of continuous media. With chapters on boundary layer theory and turbulence, it exceeded the scope of contemporary theoretical physics-revealing Sommerfeld's persistent appreciation of fluid mechanics. He resorted to Prandtl's Göttingen school of fluid mechanics in order to stay abreast of the rapid development of these specialties. [ABSTRACT FROM AUTHOR]

Particle image velocimetry (PIV) has evolved to be the dominant method for velocimetry in experimental fluid mechanics and has contributed to many advances in our understanding of turbulent and complex flows. In this article we review the achievements of PIV and its latest implementations: time-resolved PIV for the rapid capture of sequences of vector fields; tomographic PIV for the capture of fully resolved volumetric data; and statistical PIV, designed to optimize measurements of mean statistical quantities rather than instantaneous fields. In each implementation, the accuracy and spatial resolution are limited. To advance the method to the next level, we need a completely new approach. We consider the fundamental limitations of two-pulse PIV in terms of its dynamic ranges. We then discuss new paths and developments that hold the promise of achieving a fundamental reduction in uncertainty. [ABSTRACT FROM AUTHOR]

This article surveys the contributions of Philip Geoffrey Saffman to our knowledge of fluid-dynamical phenomena both in nature and in the laboratory. We begin with Saffman's first work on fluid mechanics in Cambridge, England, in the mid-1950s and then describe the evolution of his ideas and research, over many diverse areas in fluid mechanics until his final paper in 2002. It is argued that Saffman brought a unique perspective to our interpretation of fluid mechanics as a broad scientific discipline that remains with us today. [ABSTRACT FROM AUTHOR]

This article presents a brief account of the life and work of Hans W. Liepmann, a distinguished fluid dynamicist, an outstanding teacher and leader, and the third Director of the Graduate Aeronautical Laboratories, California Institute of Technology. [ABSTRACT FROM AUTHOR]

Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its radiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in the vortical, turbulent region; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic jets. The resulting unified view of wave packets provides insights that can help guide control strategies. [ABSTRACT FROM AUTHOR]

Forest canopies are important components of the terrestrial carbon budget, which has motivated a worldwide effort, FLUXNET, to measure CO2 exchange between forests and the atmosphere. These measurements are difficult to interpret and to scale up to estimate exchange across a landscape. Here we review the effects of complex terrain on the mean flow, turbulence, and scalar exchange in canopy flows, as exemplified by adjustment to forest edges and hills, including the effects of stable stratification. We focus on the fundamental fluid mechanics, in which developments in theory, measurements, and modeling, particularly through large-eddy simulation, are identifying important processes and providing scaling arguments. These developments set the stage for the development of predictive models that can be used in combination with measurements to estimate exchange at the landscape scale. [ABSTRACT FROM AUTHOR]

Recent remarkable progress in computing power and numerical analysis is enabling us to fill a gap in the dynamical systems approach to turbulence. A significant advance in this respect has been the numerical discovery of simple invariant sets, such as nonlinear equilibria and periodic solutions, in well-resolved Navier-Stokes flows. This review describes some fundamental and practical aspects of dynamical systems theory for the investigation of turbulence, focusing on recently found invariant solutions and their significance for the dynamical and statistical characterization of low-Reynolds-number turbulent flows. It is shown that the near-wall regeneration cycle of coherent structures can be reproduced by such solutions. The typical similarity laws of turbulence, i.e., the Prandtl wall law and the Kolmogorov law for the viscous range, as well as the pattern and intensity of turbulence-driven secondary flow in a square duct can also be represented by these simple invariant solutions. [ABSTRACT FROM AUTHOR]

We review studies of the statistics of isotropic turbulence in an incompressible fluid at high Reynolds numbers using direct numerical simulation (DNS) from the viewpoint of fundamental physics. The Reynolds number achieved by the largest DNS, with 40963 grid points, is comparable with the largest Reynolds number in laboratory experiments. The high-quality DNS data in the inertial subrange and the dissipative range enable the examination of detailed statistics at small scales, such as the normalized energy-dissipation rate, energy and energy-flux spectra, the intermittency of the velocity gradients and increments, scaling exponents, and flow-field structure. We emphasize basic questions of turbulence, universality in the sense of Kolmogorov's theory, and the dependence of the statistics on the Reynolds number and scale. [ABSTRACT FROM AUTHOR]

This article reviews the main established ideas on the influence of a magnetic field on turbulence in electrically conducting fluids. We limit our discussion to the asymptotic range of very small values of the magnetic Reynolds number, characterized by the fact that the induced magnetic field remains very small in comparison with the applied magnetic field. We consider three kinds of flows here. The simplest one is freely decaying homogeneous turbulence, which serves as a test bed to analyze the development of anisotropy resulting from the linear damping by the Lorentz force. We then discuss flows between walls perpendicular to the magnetic field and emphasize the influence of the Hartmann layers that develop in their vicinity. We then review the main features of the possible quasi-two-dimensional regime that can arise in that context. Finally, we consider magneto-hydrodynamic turbulent shear flows. These are frequent in industrial applications involving molten metals, such as in metal processing or in the blanket of future nuclear fusion reactors. We pay particular attention to recent attempts to develop specific RANS (Reynolds-averaged Navier-Stokes) models for these flows. [ABSTRACT FROM AUTHOR]

Following The Royal Society's decision to allow the release of hitherto confidential documents concerning referees' reports and other material relating to the publication of historical manuscripts, the present paper examines the exchanges preceding the publication of the two papers on turbulent flow by Osborne Reynolds that have so greatly influenced the development of Engineering Fluid Mechanics over the past century. The documents cited reveal that, although the earlier experimental paper was warmly welcomed, the referees were critical of the subsequent analytical contribution, it appears that the publication of the latter paper was due mainly to the considerable standing Reynolds had by then acquired, in part from the impact of his experimental paper published 12 years earlier. The paper also provides a summary of Reynolds' career and research prior to his embarking on the research published in the two seminal papers. [ABSTRACT FROM AUTHOR]

The ability of turbulent flows to effectively mix entrained fluids to a molecular scale is a vital part of the dynamics of such flows, with wide-ranging consequences in nature and engineering. It is a considerable experimental, theoretical, modeling, and computational challenge to capture and represent turbulent mixing which, for high Reynolds number (Re) flows, occurs across a spectrum of scales of considerable span. This consideration alone places high-Re mixing phenomena beyond the reach of direct simulation, especially in high Schmidt number fluids, such as water, in which species diffusion scales are one and a half orders of magnitude smaller than the smallest flow scales. The discussion below attempts to provide an overview of turbulent mixing; the attendant experimental, theoretical, and computational challenges; and suggests possible future directions for progress in this important field. [ABSTRACT FROM AUTHOR]

Reviews studies on the mechanisms that control neutrally unstable turbulent boundary-layer flow over hills and waves. Comparison of calculations based on various analytical models; Discussion on the practical applications of the studies.