Showing papers in "Annual Review of Fluid Mechanics in 2019"
TL;DR: In this paper, a review of recent developments in bounding uncertainties in RANS models via physical constraints, in adopting statistical inference to characterize model coefficients and estimate discrepancy, and in using machine learning to improve turbulence models.
Abstract: Data from experiments and direct simulations of turbulence have historically been used to calibrate simple engineering models such as those based on the Reynolds-averaged Navier–Stokes (RANS) equations. In the past few years, with the availability of large and diverse data sets, researchers have begun to explore methods to systematically inform turbulence models with data, with the goal of quantifying and reducing model uncertainties. This review surveys recent developments in bounding uncertainties in RANS models via physical constraints, in adopting statistical inference to characterize model coefficients and estimate discrepancy, and in using machine learning to improve turbulence models. Key principles, achievements, and challenges are discussed. A central perspective advocated in this review is that by exploiting foundational knowledge in turbulence modeling and physical constraints, researchers can use data-driven approaches to yield useful predictive models.
496 citations
TL;DR: This review summarizes the hypothesis that the dominant energy-containing motions in wall turbulence are due to large eddies attached to the wall and the modeling attempts made thereafter, with a focus on the validity of the model's assumptions and its limitations.
Abstract: Modeling wall turbulence remains a major challenge, as a sufficient physical understanding of these flows is still lacking. In an effort to move toward a physics-based model, A.A. Townsend introduced the hypothesis that the dominant energy-containing motions in wall turbulence are due to large eddies attached to the wall. From this simple hypothesis, the attached eddy model evolved, which has proven to be highly effective in predicting velocity statistics and providing a framework for interpreting the energy-containing flow physics at high Reynolds numbers. This review summarizes the hypothesis itself and the modeling attempts made thereafter, with a focus on the validity of the model's assumptions and its limitations. Here, we review studies on this topic, which have markedly increased in recent years, highlighting refinements, extensions, and promising future directions for attached eddy modeling.
252 citations
TL;DR: The leading edge vortex (LEV) is known to produce transient high lift in a wide variety of circumstances as mentioned in this paper, and the underlying physics of LEV formation, growth, and shedding are explored for a set of can...
Abstract: The leading-edge vortex (LEV) is known to produce transient high lift in a wide variety of circumstances. The underlying physics of LEV formation, growth, and shedding are explored for a set of can...
204 citations
TL;DR: In this paper, a review of direct numerical simulations of turbulent flows with droplets or bubbles is presented, where the initial diameter of the bubble/droplet is smaller or larger than the Kolmogorov length scale and the instantaneous surface deformation is fully resolved or obtained via a phenomenological model.
Abstract: This review focuses on direct numerical simulations (DNS) of turbulent flows laden with droplets or bubbles. DNS of these flows are more challenging than those of flows laden with solid particles due to the surface deformation in the former. The numerical methods discussed are classified by whether the initial diameter of the bubble/droplet is smaller or larger than the Kolmogorov length scale and whether the instantaneous surface deformation is fully resolved or obtained via a phenomenological model. Also discussed are numerical methods that account for the breakup of a single droplet or bubble, as well as multiple droplets or bubbles in canonical turbulent flows.
181 citations
TL;DR: The dynamics and deformations of immersed flexible fibers are at the heart of important industrial and biological processes, induce peculiar mechanical and transport properties in the fluids that c... as mentioned in this paper,.
Abstract: The dynamics and deformations of immersed flexible fibers are at the heart of important industrial and biological processes, induce peculiar mechanical and transport properties in the fluids that c...
131 citations
TL;DR: In this paper, the authors describe the behavior of hypersonic flows as energetic and result in regions of high temperature, causing internal energy excitation, chemical reactions, ionization, and gas surface interactions.
Abstract: Hypersonic flows are energetic and result in regions of high temperature, causing internal energy excitation, chemical reactions, ionization, and gas-surface interactions. At typical flight conditi...
119 citations
TL;DR: Bubbles are present in a large variety of emerging applications, from advanced materials to biology and medicine, as either laser-generated or acoustically driven bubbles in this paper.
Abstract: Bubbles are present in a large variety of emerging applications, from advanced materials to biology and medicine, as either laser-generated or acoustically driven bubbles In these applications, th
107 citations
TL;DR: A surprisingly sparse understanding of the process of liquid invasion into a porous medium is known as liquid invasion in both nature and technology as discussed by the authors, despite its enormous importance, there is a surprisingly sparse understand of the processe...
Abstract: Liquid invasion into a porous medium is a phenomenon of great importance in both nature and technology. Despite its enormous importance, there is a surprisingly sparse understanding of the processe...
94 citations
TL;DR: This review summarizes recent developments on the problem of mixing in its lamellar representation, which relies on a near-exact formulation of the Fourier equation on a moving substrate and allows one to bridge the spatial structure and evolution of the concentration field with its statistical content in a direct way.
Abstract: Mixing is the operation by which a system evolves under stirring from one state of simplicity—the initial segregation of the constituents—to another state of simplicity—their complete uniformity B
93 citations
TL;DR: In this paper, large-amplitude internal waves induce currents and turbulence in the bottom boundary layer (BBL) and are thus a key driver of sediment movement on the continental margins.
Abstract: Large-amplitude internal waves induce currents and turbulence in the bottom boundary layer (BBL) and are thus a key driver of sediment movement on the continental margins. Observations of internal ...
92 citations
TL;DR: A review of recent experimental and theoretical studies in the area of fluid particles (drop and vesicles) in electric fields, with a focus on the transient dynamics and extreme deformations is provided in this paper.
Abstract: The 1969 review by J.R. Melcher and G.I. Taylor defined the field of electrohydrodynamics. Fifty years on, the interaction of weakly conducting (leaky dielectric) fluids with electric fields continues to yield intriguing phenomena. The prototypical system of a drop in a uniform electric field has revealed remarkable dynamics in strong electric fields such as symmetry-breaking instabilities (e.g., Quincke rotation) and streaming from the drop equator. This review summarizes recent experimental and theoretical studies in the area of fluid particles (drop and vesicles) in electric fields, with a focus on the transient dynamics and extreme deformations. A theoretical framework to treat the time evolution of nearly spherical shapes is provided. The model has been successful in describing the dynamics of vesicles (closed lipid membranes) in an electric field, highlighting the broader range of applicability of the leaky dielectric approach.
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TL;DR: Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries as discussed by the authors, hence, convective processes are often the domin...
Abstract: Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries. Hence, convective processes are often the domin...
TL;DR: Tide-locked planets are planets in which tidal stresses from the host star have spun down the planet's rotation to the point where its length of sidereal day equals the length of year.
Abstract: Tide-locked planets are planets in which tidal stresses from the host star have spun down the planet's rotation to the point where its length of sidereal day equals its length of year In a nearly
TL;DR: The presence of electric fields in immiscible multifluid flows induces Maxwell stresses at sharp interfaces that can produce electrohydrodynamic phenomena of practical importance as discussed by the authors, which can be found in the literature.
Abstract: The presence of electric fields in immiscible multifluid flows induces Maxwell stresses at sharp interfaces that can produce electrohydrodynamic phenomena of practical importance Electric fields c
TL;DR: Models describe jets of maternal blood emerging from spiral arteries into a disordered and deformable porous medium, as well as solute uptake by fetal blood flowing through elaborate three-dimensional capillary networks.
Abstract: The placenta is a multifunctional organ that exchanges blood gases and nutrients between a mother and her developing fetus. In humans, fetal blood flows through intricate networks of vessels confin...
TL;DR: The very low–Reynolds number fluid mechanics of this system is reviewed; important features include how cilia rotation combines with tilt to produce asymmetric flow, boundary effects, time dependence, and the interpretation of particle tracking experiments.
Abstract: The systematic breaking of left–right body symmetry is a familiar feature of human physiology. In humans and many animals, this process originates with asymmetric fluid flow driven by rotating cilia, occurring in a short-lived embryonic organizing structure termed the node. The very low–Reynolds number fluid mechanics of this system is reviewed; important features include how cilia rotation combines with tilt to produce asymmetric flow, boundary effects, time dependence, and the interpretation of particle tracking experiments. The effect of perturbing cilia length and number is discussed and compared in mouse and zebrafish. Whereas understanding of this process has advanced significantly over the past two decades, there is still no consensus on how flow is converted to asymmetric gene expression, with most research focusing on resolving mechanical versus morphogen sensing. The underlying process may be more subtle, probably involving a combination of these effects, with fluid mechanics playing a central role.
TL;DR: In this paper, nonlinear stability theories have been developed to explain laminar-turbulent transition processes in boundary and free shear layers, and a review of these theories can be found.
Abstract: This article reviews the nonlinear stability theories that have been developed to explain laminar–turbulent transition processes in boundary and free shear layers. For such spatially developing she...
TL;DR: A remarkable number of different flow phenomena contribute critically to the proper functioning of the hearing and balance senses, both of which are hosted by the inner ear as mentioned in this paper, including quasi-steady and high-frequency Stokes flow, incompressible wave guides, unsteady boundary layers, and fluid-structure interactions between viscous fluids, soft membranes, and hair cell bundles.
Abstract: A remarkable number of different flow phenomena contribute critically to the proper functioning of the hearing and balance senses, both of which are hosted by the inner ear. This includes quasi-steady and high-frequency Stokes flow, incompressible wave guides, unsteady boundary layers, and fluid–structure interactions between viscous fluids, soft membranes, and hair cell bundles. We present these phenomena, review recent results, and discuss how they relate to the physiology of the vestibular system and the mechanics of hearing. In addition, we study flow phenomena, including gravity-driven particulate flow, magnetohydrodynamics, buoyancy, and steady streaming, that are related to pathologies of the inner ear and relevant to diagnosis and treatment of these diseases.
TL;DR: Brownian motion at long timescales has been extensively studied both theoretically and experimentally for over a centu... as discussed by the authors, and has been shown to be stable at long time scales.
Abstract: Since the discovery of Brownian motion in bulk fluids by Robert Brown in 1827, Brownian motion at long timescales has been extensively studied both theoretically and experimentally for over a centu...
TL;DR: Study of fungal life histories by fluid mechanicians has shown their exquisite capability for engineering and revealed new organizing ideas for understanding fungal diversity.
Abstract: Fungi are the dark matter of biology, typically leading cryptic lives, buried in soil or inside of plants or other organisms, and emerging into the light only when they build their elegantly engine...
TL;DR: Subrahmanyan Chandrasekhar (1910−1995) is justly famous for his lasting contributions to topics such as white dwarfs and black holes, stellar structure and dynamics,...
Abstract: Subrahmanyan Chandrasekhar (1910–1995) is justly famous for his lasting contributions to topics such as white dwarfs and black holes (which led to his Nobel Prize), stellar structure and dynamics, ...