Physical Review Fluids

Cilia are ancient cell organelles that generate fluid flow by beating periodically. They play four key roles: swimming, feeding, pumping, and sensing. This study explores how cilia generate flow and perform these functions. Swimming efficiency peaks when the number of cilia scales with body length squared, matching biological scaling. In choanoflagellates, inward flagella enhance feeding, and outward motion aids swimming. In mouse embryos, nodal flow from motile cilia is sensed by immotile cilia to establish left-right body asymmetry. These findings underscore the diverse roles of ciliary flow and the significance of fluid mechanics in biology.

For a long time, weather forecasting was based on empirical correlations, producing sayings like “Rain before seven, fine by eleven”. The modern weather forecast uses supercomputers and many ground observations. How does it work? Why is our weather app displaying scores for predictions over more than 3 days? Why is it failing sometimes even for shorter periods? Will it improve if we use larger computers and artificial intelligence? The answer to all these questions is now available, thanks to recent progress in mathematics, and involves possible singularities of the inviscid limit of the primitive equations.

While biological systems like dragonflies and schooling fish achieve remarkable performance through coordinated hydrodynamic interactions, the current understanding of the underlying mechanisms remains incomplete. This review examines how vortex dynamics, structural flexibility, and 3D effects influence performance in tandem flapping wing systems. It is shown that for tandems of spanwise-flexible wings, the forewing achieves maximum thrust through fluid-structure resonance while moderately stiff hindwings effectively capture upstream wake structures leading to increased overall performance. The mechanisms by which self-propelled systems achieve energy savings are also discussed.

When a floating body is internally or externally vibrated, its self-generated wavefield can lead to steady propulsion along the interface. In this article, we review several related and recently discovered systems that leverage this propulsion mechanism and interact hydrodynamically with one another via these surface waves. These accessible, tunable, and visually appealing systems motivate future investigations into a number of outstanding questions in fundamental fluid mechanics, while potentially also informing advances in the fields of active matter, hydrodynamic quantum analogs, and robotics.

We review the efforts during the past 50 years to characterize turbulent flows in terms of coherent structures, but remark that, in the same way as the cycle of water on Earth cannot be fully described by ‘coherent’ rivers or storms, 80% of the volume of turbulent flows cannot yet be represented in terms of structures. The objects in the accompanying figure (originally from A. Lozano-Durán, 2011) are Reynolds-stress structures and vortices in a turbulent channel, but most of the volume is empty. We point to specific problem areas, and discuss what the future role of new analysis techniques could be.

Upon the impact of a flat disk on a boiling liquid, i.e., a liquid that is in thermal equilibrium with its vapor, a thin vapor pocket is entrapped under the disk. We experimentally investigate the dynamics of the entrapped vapor pocket, focusing on its time evolution and its subsequent influence on the hydrodynamic loads under various conditions. We found that the dynamics of the entrapped vapor pocket is primarily governed by the phase change process, where condensation (vaporization) will induce (frustrate) its rapid collapse, impairing the cushioning. This differs significantly with that of a non-condensable air pocket, which is known to always provide a load reducing cushioning effect.

Data from experiments or simulations enables tools to accelerate simulations and develop accurate predictions of important turbulence-chemistry interactions in turbulent combustion flows. Several methods designed to exploit this data are presented and discussed. They are motivated by and rooted in traditional paradigms in turbulent combustion that rely heavily on the existence of a low-dimensional manifold for the composition space and its coupling with turbulent transport. These methods include surrogate DNS with principal component transport, the extraction of closure models from multiscalar measurements, and deep operator networks for chemistry integration and acceleration.

This article briefly reviews recent developments in understanding and utilizing bubble dynamics in complex fluids. Bubble dynamics impart deformations and probe properties on time scales as short as the relaxation times of complex fluids containing suspended particles or macromolecules. Examples from our research group with increasing complexity are presented: from linear rheology of soft solids using ultrasound-driven bubbles, to bubble removal from yield-stress fluids, to self-assembly in colloidal gels driven by bubble dynamics. The growing synergy between the communities of cavitation and rheology will help address new challenges in characterization and manipulation of complex fluids.

Experimental high-speed visualization techniques are evolving rapidly and provide valuable tools for learning about ultrafast bubble dynamics that cause unwanted but also desirable damage, such as cavitation and acoustically driven bubbles and droplets relevant for biomedical applications. This article offers my personal perspective on the quest to illuminate the hidden physics of externally stimulated bubbles that have a remarkable ability to focus energy. The focus is given to advanced experimental techniques including ultrafast videomicroscopy and synchrotron x-ray imaging to characterize bubble jetting, vapor bubble nucleation and shape deformations of periodically driven bubbles.

From inkjet printers to irrigation lines, particle-laden flows can fail abruptly by clogging. This Perspective reviews recent work on particulate suspensions in confined geometries and the key control parameters behind clogging. Some general guidelines are provided: particles can be too large (sieving), too crowded (bridging), or too sticky (aggregation). We highlight recent efforts to characterize, model, and delay clogs, and suggest some future research questions.