Sandstorms are extreme, uncontrollable natural phenomena that pose significant hazards. Accurate forecasting, which is key to hazard mitigation, depends on a deep understanding of their formation and evolution. The complexity of sandstorm dynamics arises from the coexistence of and competition between thermal and shear turbulence; the interactions among the wind, dust, temperature, and electric fields; the spatiotemporal multiscale nature of turbulence and sand movement; and the multiple processes of heat/mass transfer, electrification, and collision—each responding differently throughout the nonstationary and nonequilibrium evolution of sandstorms. These complexities present important case studies and new challenges for contemporary turbulence research. Based on a critical review, this article summarizes recent advances in the turbulent characteristics of various physical fields in sandstorms, their dominant factors and origins, and improvements in sandstorm forecasting. From the perspective of sandstorm fluid dynamics, issues for future research are proposed, including full-scale field measurements, controlled experiments, and numerical modeling.
Convection in stellar cores hosts a wide range of fluid dynamical phenomena. Characterized by high Reynolds and Peclet numbers, the convection is rotationally constrained with low Rossby numbers and has very low Prandtl and Mach numbers. This review describes the main properties of this turbulent convection, as well as how associated fluid processes affect our broader understanding of stars. Numerical simulations have given new insights into the dynamics of convection in stellar cores. As the convection has very high Rayleigh numbers and is internally heated and internally cooled, it appears to be in the ultimate, or diffusion free, regime. The convection drives a magnetic dynamo, leads to mixing at the boundary of the convection zone, and generates internal waves that propagate into the stably stratified stellar envelope. While we are beginning to understand some of these aspects of stellar convection, many properties of stellar cores remain a mystery, and new efforts are required to determine how these fluid processes affect stellar evolution.
Magnetic resonance velocimetry provides quantitative three-component three-dimensional velocity data but has poor spatial resolution, poor signal-to-noise ratio, and prohibitively long acquisition times for many applications. When combined with physical knowledge such as conservation of mass, the Navier–Stokes equations, and a constitutive model for the viscosity, however, this data's high information content can generate superresolved fields that include unmeasured variables such as the pressure and wall shear stress. This combination is achieved by asserting a physical model, creating a well-posed Bayesian inverse problem with a small search space, and solving it with gradient-based optimization accelerated with adjoint methods. Different physical models can be proposed and the likelihood of each model can be calculated, given the data. With a similar method, the information content of the data can be calculated, given each model. This careful probabilistic representation of uncertainty allows models to be ranked, data quality to be assessed, and future experiments to be designed. We outline the history and principles of this method, provide two toy problems, and show three applications to magnetic resonance velocimetry data.
The Atlantic Ocean circulation is susceptible to large-scale instabilities with potentially disrupting climate and societal impacts. These instabilities are caused by the sensitivity of the zonally integrated flow, the Atlantic meridional overturning circulation (AMOC), to surface freshwater perturbations. The fluid dynamics of this stability problem is addressed here by focusing on the processes that set the pattern and amplitude of the equilibrium flow state, by identifying the feedback mechanisms responsible for the instabilities, and by connecting the resulting flow changes to variability and transition behavior. This framework is then used to explore why current Earth system models show such a diversity in simulating the present-day AMOC state, its multidecadal to multicentennial variability, and its projections under global warming.
Three-dimensional massively separated flows around finite wings at high angles of attack exhibit complex dynamics. The wakes feature large-scale flow structures, including the separation bubbles, stall cells, shear layers, tip vortices, leading- and trailing-edge vortices, arch vortices, and ram's horn vortices. Smaller vortices also emerge from the breakdown of the larger structures through various instability mechanisms. The collection of the large-scale flow features strongly influences the three-dimensional nature of the flow and the forces exerted on the wing. The planform parameters of aspect ratio, sweep, taper ratio, and twist determine the structures and instabilities that are accentuated in the wake, giving rise to a wide variety of behaviors. These wake dynamics have been studied extensively over the last decade in coordinated experimental, computational, and theoretical investigations across a range of Reynolds numbers. In particular, modal and nonmodal linear global stability analyses of the three-dimensional separated flows have provided enhanced physical insights and supported the design of flow control strategies. The effectiveness of control approaches for massively separated flows has been verified by companion experiments for both laminar and turbulent regimes. This review summarizes research efforts that have elucidated large-scale separated flow characteristics and universal flow features across different wake regimes in translating wings from low to moderate Reynolds numbers. The findings from these efforts provide a foundation for a deeper understanding of massively separated flows at higher Reynolds numbers and suggest potential opportunities for wake analysis of wings experiencing dynamic motion and gusts.
Decarbonization of buildings is one of the main challenges for the energy transition. In particular, the provision of heating, cooling, and ventilation to maintain a comfortable and healthy interior environment can be very energy intensive. Three approaches to help with the decarbonization of buildings are ( a ) upgrading the building envelope, especially the insulation, to reduce heat flow to or from the exterior; ( b ) improving the efficiency of the heating or cooling system, including the design and operation of ventilation flows; and ( c ) decarbonization of the heating and cooling systems, typically through electrification using heat pumps, and possibly the development of heat networks and interseasonal heat storage. This review touches on different elements of these challenges, mainly those related to ventilation, exploring some of the complexities of the fluid mechanics involved.
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.
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.
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.
In liquid filtration, a particulate-laden feed solution is passed through a porous material (the filter), often a membrane, designed to capture the particulate matter. Usually, the filter has a complex interior structure of interconnected pores, through which the feed passes, and in many cases of interest, it may be reasonable to approximate this interior structure as a network of interconnected tubes. This idea, which dates back about 70 years, greatly simplifies the modeling and simulation of the filtration process. In this article, we review the use of networks as a framework for modeling and investigating filtration, describing the key ideas and milestones. We also discuss some promising areas for future development of this field, particularly concerning the design of next-generation filters.
Direct-ink writing (DIW) has rapidly become a versatile 3D fabrication method due to its ability to deposit a wide range of complex fluids into customizable 3D geometries. This review highlights key fundamental fluid mechanics and soft matter challenges across the different stages of the DIW printing process. The rheology of fluids and suspensions governs the flow behavior through narrow nozzles, posing questions about extrudability, confined flow dynamics, and clogging mechanisms. Downstream, the formation and deposition of extruded filaments involve extensional flows and potential instabilities, while postdeposition dynamics introduces complexities related to yield stress and structural stability. These stages are inherently interdependent, as optimizing material composition without considering filament stability risks compromising the final structure. As DIW applications expand through advanced ink formulations, developing fundamental fluid mechanics frameworks is essential to replace trial-and-error approaches with predictive design methodologies to enable more precise control and improved reliability of the printing process.
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.
Particulate suspensions, consisting of solid particles dispersed in a fluid, exhibit complex flow behaviors influenced by multiple factors, including particle interactions, concentration gradients, and external forces. Suspensions play an important role in diverse processes, from sediment transport to food processing, and display instabilities triggered by shear-driven effects, frictional interactions, and viscous forces. These instabilities can often be understood by identifying the key mechanical quantities that govern the dynamics. Following hydrodynamic tradition, such mechanics can be characterized by dimensionless numbers, which encapsulate the interplay between geometric, kinematic, and mechanical factors. Many of these numbers represent competitions between opposing pairs of mechanical quantities, which we discuss in detail while also considering a few phenomena that require more complex combinations. By emphasizing the underlying mechanical principles, this review provides a perspective for understanding pattern formation and flow instabilities in confined particulate suspensions across different flow geometries.
This review first examines how urban wind flow impacts the sustainability and resilience of cities and identifies the three main challenges in predictive modeling of urban flows: the complexity of the flow physics, the variability and uncertainty in the flow conditions, and the diversity and multiscale nature of urban geometries. To review the complexity of the flow physics, the typical flow patterns observed in canonical urban flows are summarized, and related modeling challenges and opportunities in both wind tunnel experiments and simulations are highlighted. Next, opportunities to predict realistic urban flows by addressing the other challenges are explored through the lens of a modeling framework with uncertainty quantification. The important role of field measurements, supporting the more accurate characterization of uncertainties in the flow conditions, as well as enabling validation with real-world data, is emphasized. The review concludes with two specific examples that demonstrate how integrated use of field measurements and computational models can improve the understanding and modeling of real urban flows to ultimately support sustainable development goals for urban areas.
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.
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.
The objective of this contribution is to review more than 80 years of experimental measurements of the settling of snow particles and surrogates in natural and laboratory settings and suggest viable directions for future research. Under the broad category of frozen hydrometeors, snow particles are characterized by a variety of shapes and inertial properties that we broadly refer to as snow morphology attributes and depend on the micrometeorology of the air column, including temperature, relative humidity, wind speed, and turbulence. The uncertainty in the prediction of snow settling velocity is partly due to the significant variability in snow crystal shape, density, and drag properties, as well as the modulating effect of ambient turbulence, which has been observed to affect particle orientation and falling style and enhance or reduce the terminal velocity, as compared to quiescent flow conditions. Because of the complexity of finite-size, nonspherical particles’ interaction with turbulent flows at high Reynolds numbers, we stress the need for simultaneous flow and snow morphology measurements in the field and we review past and current experimental techniques and methodologies.
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.
Porous media flows are generally viewed as inefficient mixers, where solutes may be dispersed yet poorly mixed, making mixing a critical limiting factor for a wide range of processes. The complexity and opacity of porous structures have long made these dynamics difficult to observe. With emerging experimental techniques, concepts and models of mixing in porous media are rapidly evolving. Recent advances link mixing dynamics to fluid deformation arising in flow through porous materials. Unlike diffusion and dispersion, which only dissipate chemical gradients, fluid shear and stretching amplify and sustain them. This review explores the role of fluid deformation in governing mixing, chemical reactions, and biological processes in porous media. We begin by highlighting key experimental observations that have improved our understanding of mixing in these systems. We then examine the fundamental concepts, models, and open questions surrounding fluid deformation and mixing in porous media, emphasizing their dependence on material structure, heterogeneity, dimensionality, and transient flow phenomena, as well as their interaction with chemical and biological processes.
