In this article, we review recent progress in the fundamental understanding of the motion of flexible fibers in a turbulent flow, made through multiscale experiments and simulations. Emphasis is given to problems involving flexible fibers that can be conveniently described with discrete and continuum models closely related to the slender body theory. Current state-of-the-art measurement and simulation methods, including optical techniques, Euler–Lagrange approaches for tracking large swarms of fibers, and recent methodologies for simulating finite-size fibers, are discussed. The capabilities of simulations and experiments are surveyed in connection with the current physical understanding of how flexible fibers interact with the full spectrum of length scales and timescales of turbulence. We review the phenomenological and statistical features of fiber dispersion and spatial distribution. We also discuss the relevant aspects of fiber rotation and deformation, highlighting their connection with mechanisms such as fragmentation and turbulence modulation, which are known to exhibit peculiar features in the case of flexible fibers. We conclude our analysis by providing an outlook on future research direction paths, open methodological issues, and expected advances, in particular those associated with the study of flexible particles in a broader sense.

Geophysical and astrophysical fluid dynamics (GAFD) is an interdisciplinary field. It encompasses a wide range of fluid systems, from planetary atmospheres and the oceans of Earth and icy moons to the interiors of telluric planets, giant planets, and stars. It also spans vast timescales and space scales. Despite this diversity, GAFD is built on common challenges in fundamental fluid mechanics, requiring a multi-approach strategy that integrates theory, simulations, and experiments to explain observations. This review highlights the role of laboratory experiments in GAFD. We first emphasize recent advances in experimental design, methods, and metrology, including large-scale facilities as well as innovative and analog setups. We then focus on two areas where experiments have driven recent breakthroughs: rotating turbulence and flows involving multiphase and phase-change processes. Finally, we discuss emerging challenges and the potential of outreach experiments to stimulate interest in fluid mechanics among students and the public.

The squeezing of blood cells and vesicles through narrow constrictions, such as splenic slits, pulmonary capillaries, vascular endothelial gaps, and microfluidic channels, is crucial in physiology and biotechnology, with fluid mechanics playing a central role. The diverse geometries of these constrictions, the associated flow conditions, and the unique mechanical properties of cells and vesicles create a rich subject in fluid mechanics emerging from nonlinear dynamics of fluid–structure interactions involving both lubrication and Marangoni flows. Advances in microfluidics, video microscopy, and computational modeling have enabled investigations into these complex processes. This review surveys the key features and approaches, recent prominent studies, and unresolved challenges related to these processes, offering insights for researchers across biomechanics, biomedical engineering, biological physics, hematology, physiology, and applied mathematics.

In-flight ice accretion poses a major safety concern in aviation. It occurs due to the icing from clouds of supercooled water droplets, the accumulation of ice crystals at high altitudes, or snow buildup. Assessing the detrimental effects of ice accretion on aircraft performance and handling qualities is a complex, multidisciplinary task, as it requires modeling the dynamics of the dispersed phase (water droplets, ice crystals, and snowflakes), the particle impact dynamics and its interaction with the aircraft surfaces, the liquid film dynamics, the solidification process, and possibly ice shedding. The ice protection systems must also be modeled. The present review addresses the status of ice accretion models and simulation tools. The intrinsically stochastic nature of ice accretion, combined with substantial operational and modeling uncertainties, makes it challenging to validate these tools against experimental observations and use simulation as a reliable means of compliance for certification in icing conditions, especially for innovative aircraft configurations such as wing-body and advanced urban air mobility vehicles.

Droplets, which are ubiquitous in nature, are formed through intriguing processes, and one such route is air-assisted atomization or aerobreakup. This review focuses on secondary atomization, particularly the breakup of an individual droplet subjected to high-speed flows. This process involves complex interfacial dynamics with multiscale deformations, ranging from global flattening to local unstable waves. The deformations occur at progressively smaller scales while interacting with the surrounding gas phase, forming a nonlinear cascade. Each local undulation serves as a precursor to a self-similar evolution or subsecondary breakup process that ends with a ligament-mediated mechanism. In practical scenarios, droplets often encounter nonuniform, unsteady, impulsive, or compressible flows, like shock waves, which pose extreme conditions. The spatiotemporal scales of the nonuniformity or unsteadiness of the external flow must be comparable with the drop deformation scales at either global or local levels to influence aerobreakup that cascades across hierarchical deformation scales. The compressible effects at high Mach numbers are interestingly shown to suppress the tendency toward breakup.

Electromagnetically forced flows in shallow electrolyte layers offer a versatile and nonintrusive method for exploring quasi-two-dimensional fluid dynamics. This review focuses on the experimental and theoretical aspects of such flows driven by Lorentz forces generated by the interaction of injected electric currents and the applied magnetic fields. The method is applicable to both liquid metals and electrolytes, with the latter more commonly used due to their wide availability and ease of handling. Experimental aspects of the method and key components of mathematical flow analysis are discussed. Initially developed for geophysical flow modeling, the method has been instrumental in exploring various other physical phenomena including vortex and wake dynamics, spatiotemporal chaos, and mixing processes. The review also addresses the challenges of achieving true two-dimensionality in laboratory settings and discusses the influence of various parameters, such as layer thickness and forcing intensity, on the flow behavior. Future research directions in the field are highlighted.

Characterizing the physical and dynamic meteorology of wildland fires has obvious socioeconomic importance and is necessary to develop not only firefighting but also mitigation strategies such as prescribed burns and effective fuel management practices such as forest thinning. However, despite significant progress over a century, there are shortcomings in our understanding of the physical processes governing wildland fire behavior. Although some research progress has been made in understanding how fires spread on grasslands, several aspects of fire behavior within the forest canopy environment are still not well-understood. This review is an attempt to organize the fluid mechanics of the mass, momentum, and energy transfer during wildland fire events through the lens of vegetation canopy turbulence. The structure, organization, and progress of the flame front and the buoyant plume through the canopy are shown to be intricately related to the coherent structures associated with fire–vegetation–atmosphere interaction, and potential future research directions are identified.

Internal waves, generated by wind and tides, are ubiquitous in the ocean. Their dissipation and the resulting vertical mixing play an important role in setting the ocean circulation, stratification, and energetics. Ocean models usually parameterize many or all of these effects. The current generation of parameterizations often relies on assumptions of uniform or slowly varying stratification profiles. Here, we review the growing theoretical, modeling, and observational evidence that vertical nonuniformity in the stratification profile can significantly modify the assumed wave dynamics. Linear scattering, wave–wave interactions, and solitary-like internal wave generation in idealized nonuniform stratification profiles are discussed. The nonuniform features in oceanic vertical stratification profiles are characterized, followed by a discussion of the validity of the slowly varying stratification assumption for such profiles. A concerted effort is made to synthesize research in both fluid dynamics and oceanography.

Soap films and bubbles are inherently unstable systems that evolve over time. Their thickness is primarily governed by the competition between capillary and viscous forces. The presence of surfactants introduces Marangoni stresses, which limit interfacial extension and significantly increase film lifetime. While the bulk flow is typically well-described by a simple Poiseuille profile between the two interfaces, the interfacial dynamics can induce complex behaviors, even in the simple case of horizontal film drainage, which is used as a paradigmatic example in this review. The interfacial velocity is dictated by the thickness gradients and by the interfacial rheology, which, in many practical cases, reduces to the condition of an incompressible interface. This simplified framework allows for analytical predictions and scaling laws in axisymmetric flows. It is also consistent with the spontaneous symmetry breaking that may be observed in horizontal films—a phenomenon associated with the marginal regeneration process, which remains only partially understood. This review presents the most elementary theoretical frameworks capable of capturing the essential features of these flows and provides quantitative comparisons with available experimental data.