Annual Review of Fluid Mechanics - Volume 43, 2011

Mammalian spermatozoa motility is a subject of growing importance because of rising human infertility and the possibility of improving animal breeding. We highlight opportunities for fluid and continuum dynamics to provide novel insights concerning the mechanics of these specialized cells, especially during their remarkable journey to the egg. The biological structure of the motile sperm appendage, the flagellum, is described and placed in the context of the mechanics underlying the migration of mammalian sperm through the numerous environments of the female reproductive tract. This process demands certain specific changes to flagellar movement and motility for which further mechanical insight would be valuable, although this requires improved modeling capabilities, particularly to increase our understanding of sperm progression in vivo. We summarize current theoretical studies, highlighting the synergistic combination of imaging and theory in exploring sperm motility, and discuss the challenges for future observational and theoretical studies in understanding the underlying mechanics.

The use of particle image velocimetry (PIV) to study the spatial and temporal features of unsteady fluid flow has increased dramatically in the past five to ten years. One particular application of PIV is to examine how shear-layer instabilities and turbulence lead to radiated sound. In this review, the basic operation of a PIV system is provided along with an introduction to the equations that relate unsteady fluid motion to sound. The references then illustrate how PIV is currently used in a number of canonical flow problems of interest in which the phenomena are dominated by shear-layer instabilities that lead to radiated noise. Specifically, cavity flows, flow over airfoils and cylinders, and finally jet flows are considered.

Rip currents are fast-moving flows, traveling “outward almost at right angles to the shore” (Shepard 1936), creating a natural hazard for beachgoers, who suddenly find themselves in deep water. Field measurements and instrumentation, laboratory techniques, and numerical modeling have improved with time, enabling a more complete description of rip currents now. Surprisingly, there are many types of rip currents that can occur on beaches, and these currents are created by a wide variety of mechanisms that are presented here, along with numerical, physical, or field validation. We also show the potential for prediction schemes for use by lifeguards and beach managers.

The magnetic fields of the planets, including the Earth, are generated by dynamo action in their fluid cores. Numerical models of this process have been developed that solve the fundamental magnetohydrodynamic equations driven by convection in a rotating spherical shell. New results from these theoretical models are compared with observations of the geomagnetic field and magnetic data gathered from space missions. The mechanism by which a magnetic field is created is examined. The effects of rotation and magnetic field on the convection are of paramount importance in the simulations. A wide range of simulations with different convection models, varying boundary conditions, and parameter values have been performed over the past 10 years. The effects of these differences are assessed. Numerical considerations mean that all dynamo simulations use much enhanced values of the diffusivities. We consider to what extent this affects results and show how the asymptotic behavior at low diffusion is starting to be inferred using scaling laws. The results of specific models relating to individual planets are reviewed.

Small amounts of surfactant can drastically change bubble behavior. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a nonuniform concentration distribution of surfactant along the bubble surface. In other words, a tangential shear stress appears on the bubble surface due to the surface tension variation caused by the surface concentration distribution, which results in the reduction of the rising velocity of the bubble. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, we review the recent studies related to these interesting behaviors of bubbles caused by the surfactant adsorption/desorption on the bubble surface.

Experimental observations indicate that, at sufficiently high cell densities, swimming bacteria exhibit coordinated motions on length scales (10 to 100 μm) that are large compared with the size of an individual cell but too small to yield significant gravitational or inertial effects. We discuss simulations of hydrodynamically interacting self-propelled particles as well as stability analyses and numerical solutions of averaged equations of motion for low Reynolds number swimmers. It has been found that spontaneous motions can arise in such systems from the coupling between the stresses the bacteria induce in the fluid as they swim and the rotation of the bacteria due to the resulting fluid velocity disturbances.

In this review, we consider and unify all aspects of the dynamics of Newtonian and viscoelastic liquid drops in high-speed gas flows, including shock waves. The path to understanding is opened by novel, laser-induced fluorescence visualizations at spatial resolutions of up to 200 pixels for millimeter and exposure times as low as 5 ns. The central role of the competition between Rayleigh-Taylor and Kelvin-Helmholtz instabilities is assessed in the frame of rich aerodynamics, from low subsonic to supersonic, and the multitude of characteristic length scales and timescales at play with varying liquid properties. Acceleration and liquid redistribution (drop deformation) early in the evolution set the stage for this competition, and we insist on an interpretation of the drag coefficient that is physically meaningful. Two principal breakup regimes (patterns of bodily loss of coherence) are identified depending on whether the gas finds its way through the liquid mass, causing gross disintegration, or goes around to induce, through shear, a surface-layer peeling-and-ejection action. Corresponding criticalities are quantified in terms of key physics, consistent with experiments. This covers in a unified fashion all liquids, independent of viscosity and elasticity, and the potential role of direct numerical simulations in supporting further advances is forecast. The resulting particle-size distributions (in a final equilibrium cloud) depend crucially on the pattern of breakup, although in this respect the role of elasticity obtains a special significance in terms of the underlying entangled-polymer-chain dynamics. From a more general perspective, we explain the canonical significance of this fundamental problem and summarize the wide range of its practical relevance, including the recently renewed interest in predicting shock-induced fluidization (or high-speed, atmospheric dissemination) of large masses of liquid agents (so-called weapons of mass destruction).