We employ dense kinetic theory extended to include velocity correlations, contact duration, and the presence of a particle bed for steady, fully developed gravitational flow of grains in a vertical pipe with a circular cross section and bumpy boundaries. We determine the variation of average flow properties with both pipe radii and total flow rate. There are two solutions associated with high and low pressures for a given pipe radius and total flow rate.

The generation of leading-edge vortices (LEV) by the caudal fin (CF) has long been recognized as playing a key role in efficient propulsion in fish-like swimming. This study focuses on LEV enhancement due to vortex shedding of the median anal fin (AF), utilizing trout’s unique morphology and fin configurations. The shed anal-fin vortex (AFV) is found to stabilize and strengthen the LEV of the CF through its shearing with the CF leading edge, creating stronger leading-edge suction, resulting in stronger thrust production and increased propulsive efficiency in trout-like swimming.

Rotating Rayleigh-Benard convection is the fundamental flow system to understand many geophysical and astrophysical flows. Typically studied in a confined cylindrical geometry, it was found in recent years that in such confined set-ups, a strong flow structure can develop near the side wall, obscuring observations of the bulk dynamics that we are interested in. Here we study different properties of this wall bounded flow, focussing on the interaction between the bulk and side wall region, to advance our understanding of how to disentangle the two.

When gently putting down a volatile immiscible drop of low viscous liquid onto a small more viscous liquid layer, it spreads under the layer for favorable positive spreading coefficient from the action of Marangoni forces. In this work however, the opposite is observed after a finite receding time: a reversed flow associated with the volatile droplet recession. This paper precisely analyzes this phenomenon both experimentally and theoretically and the physics behind it.

The Leidenfrost effect causes liquid droplets, like water or ethanol, to levitate over hot surfaces due to an insulating vapor layer. This process limits heat transfer, leading researchers to find ways to suppress it. Remarkably, our experimental findings show that ionic liquids do not exhibit the Leidenfrost phenomena, despite similar impact speed and surface temperature. Their superior thermal stability prevents the Leidenfrost effect, opening exciting possibilities for enhancing a variety of thermal processes, including cooling and coating applications.

Various patterns appear when plugs rupture in channels. With the assumption of a quasi- equilibrium process, the effects of surface tension on plug rupture are specified. The entropic lattice Boltzmann method used to simulate the rupture process exhibits great potential in the study of multiphase fluid flows.

We study experimentally the behavior of a bubble placed within a horizontal high-speed solid-body rotating flow. We focus on the stretching and breakup of the bubble, when surfactant (TTAB) is introduced in the solution. When the concentration of surfactant is below the critical micelle concentration, the bubble behaves as if it were seeing an effective surface tension equal to that in the saturated solution. In addition, we present evidence that the bubble could breakup via a resonance mechanism, when the rotation rate of the tank becomes of the order of the eigenfrequency of the bubble.

The Basset-Boussinesq history force is shown to have a significant impact on droplet dynamics depending on the viscosity ratio and the Stokes Number. An accurate analytic modeling is proposed and validated by both direct numerical simulations and spectral analysis in the Fourier space.

The physical mechanism behind the beauty of the iconic image of a drop detaching from the tip of the jet produced when a drop or a stone falls on a pond, is identical to the one driving the 1000 m/s jets impacting a solid substrate when a liquid cavitates near a solid, causing structural damage, or when the micron-sized bubbles ingested by breaking waves burst at the ocean’s surface, ejecting nanometric jets with velocities up to 100 m/s that produce part of the sea spray aerosols which favors cloud formation. We show that, due to the large liquid inertia, the radial flow rate established initially remains mostly constant in time during the short instant along which these jets are issued.

Developed to address the drawbacks of electro-wetting, electro-dewetting is a digital microfluidic technique that makes a liquid dewet on a substrate using an electric potential. In this paper, we propose an electrohydrodynamic model to describe the dynamic evolution of a charged slender drop under the influence of an external electric field. We introduce lubrication theory to simplify the model and reproduce fundamental microfluidic operations, including dewetting, rewetting, and droplet shifting.

The coupling effect of physical and thermodynamic wall roughness is investigated using nonlinear parabolized stability equations. In-phase wall temperature cooling superimposed on a patch of wavy wall results in a synergetic coupling effect that delays the transition to turbulence.

We constructed a control law based on the Wiener-Hopf technique to perform an experimental boundary layer control over a NACA 0008 wing profile, aiming at attenuating Tollmien-Schlichting waves and delaying transition. Under such an approach, causality can be enforced when building the control kernel, such that an optimal control kernel is obtained directly from transfer functions obtained in experiments. We show that the optimality property provided by the Wiener-Hopf kernel can significantly improve the attenuation of Tolmien-Schlichting waves when compared with a typical wave-cancelling approach.

Asymptotic and numerical methods are used to show that pretransitional disturbances, such as Klebanoff modes and Görtler vortices, are attenuated in supersonic boundary layers exposed to free-stream vorticity and flowing over porous surfaces when their spanwise wavelength is much larger than the boundary-layer thickness. The growth rate of highly-oblique Tollmien-Schlichting waves, triggered by a leading-edge adjustment receptivity mechanism, is instead enhanced.

We investigated the buoyancy-driven flow of a non-Newtonian fluid over an inclined flat plate. This paper explores the stability of three different modes: the transverse traveling Tolmien-Schlichting waves, the stationary longitudinal rolls, and the oblique rolls. Our study reveals that the nature of the stability depends significantly on the rheological properties.

This study investigates capillary-driven self-assembly, addressing the challenges of accurately measuring and modeling capillary forces between close particles. To account for the tilting of particles, induced capillary dipoles are introduced to extend the existing capillary charge representation. Experimental results in magneto-capillary self-assemblies demonstrate the significant impact of induced dipoles on their cohesive behavior. These findings offer valuable insights into the design and fabrication of functional structures through capillary-driven self-assembly.

We elucidate bridge height scaling relationships in coalescing liquid lenses using extensive lattice Boltzmann simulations to improve the understanding of interfacial behavior in microfluidics and beyond. In the viscous regime, we observe a proportional scaling between bridge height and time $t$ in both two and three dimensions (2D and 3D). Moving into the inertial regime, we uncover intriguing variations in the scaling behavior: in 2D, the scaling transitions to $t^{2/3}$, whereas in 3D we reveal a different scaling, characterized by $t^{1/2}$.

In this work, we provide a quantitative understanding of the organization and the cell-free layer dynamics of red blood cells in a stenosed microchannel. We present a microfluidic experimental method that allows us to resolve the spatiotemporal evolution of the cell-free layer under time-dependent driving of the flow with arbitrary waveform. Our work reveals a phase inversion of the temporal cell-free area before the constriction due to symmetry breaking at finite inertia, as demonstrated by complementary numerical simulations.

Large particles migrate upward in gravity-driven granular shear flows, which results in size segregation, a phenomenon of considerable practical importance. In this study, we show using extensive discrete element method (DEM) computations that this effect is not caused by a granular lift force or Archimedean buoyancy, rather it stems from a buoyancy force that exceeds the weight of the large particle, which is a result of patches of high-stress concentration on the surface of the large particle caused by strong flow-induced layering.

In confirmation of our previous experimental observations, two-way coupled Eulerian numerical simulations evidence that particle clouds settling in a quiescent ambient have a maximum growth rate for a finite Rouse number $R$, which is the ratio of the individual particle settling velocity to the typical cloud velocity based on its initial radius and total buoyancy. Simulations of laminar clouds recover this phenomenon, hence turbulence is not essential to the phenomenon. It is due to the disruption of the cloud structure by particles, weakening its circulation and therefore enhancing the cloud growth.

Particle confinement in porous media under laminar conditions affects the macroscopic fluid velocity which, in turn, affects finite-size colloidal transport itself. By means of a novel numerical simulation scheme, we show that even under small confinement conditions (particle diameter about 2% of average pore size), fluid and transported particles are dynamically rerouted towards more permeable paths. This leads to the emergence of ephemeral laminar vortices at pore throat entrances and affects the variance and mean fluid velocity.

Fluid flow structures that enhance heat transfer are important in many natural and technological settings. Here we compute the steady planar incompressible fluid flows that maximize the rate of heat transfer from a solid surface, for various rates of viscous energy dissipation. We find a transition from rectangular convection rolls to flows that branch near the boundary. These flows may be related to branching flows that have been predicted theoretically and computed in three dimensions.

The influence of stable and unstable stratification on the amplification of coherent structures in turbulent channel flows is investigated by computing the linear response to stochastic forcing near the turbulent mean flow. For all considered stratification levels, the two most energetic proper orthogonal decomposition modes are found to contribute to more than 90% of the variance of the response, except for the thermal response to thermal forcing. The results reveal that a single robust mechanism underlies the amplification of coherent large-scale structures at subcritical Richardson numbers and the onset of the instability of coherent large-scale rolls at the critical Richardson number.

Direct numerical simulation (DNS) data are considered a reliable source of information and are commonly used to develop and validate models. This paper explores the uncertainties in DNS data for channel flow. We examine how the way we discretize the flow in the direction normal to the walls affects the results. Our findings indicate that basic measurements like the mean flow and the turbulence level are very accurate. However, there are uncertainties when looking at more complex measurements like the energy dissipation, which can be influenced by factors such as the numerical methods used, the distribution of points in the simulation, and the grid resolution.

For small particles locally injected in a turbulent flow and used as fluid tracers, the reconstruction of the mean flow is biased by the particle dispersion caused by turbulence. For example, in a turbulent jet with nozzle injection, the entrained flow is inaccurately measured because all the tracked particles come from the jet source. In this work, we present a compensation method that provides the accurate mean velocity for turbulent flows in non-homogeneous seeding conditions.

We identify the effects of density and viscosity fluctuations on total stress balance, leading to the development of a general and robust mean velocity transformation for compressible boundary layers. Our study uses a comprehensive database of compressible turbulent boundary layers developed using direct numerical simulations covering a wide range of semi-local Reynolds numbers (800 to 34,000), Mach numbers up to 12, and incorporating wall-cooling. The new mean velocity transformation successfully collapses all compressible cases onto the incompressible law of the wall, within as little uncertainty as that established for incompressible data, due to the integration of key scaling properties.

Particle image velocimetry is used to assess the effect of water depth and upstream face angle on the flow around a circular cylindrical segment. The mean angular momentum was modeled by analytical equations whose forms were obtained using conservative laws. For all considered test cases, a rigid-body rotation layer forms underneath the free surface, and its thickness increases as the flow evolves over the obstacle.

In this paper we present a Large-eddy-simulation (LES)-informed resolvent-based estimator to obtain estimates on the space-time domain of pipe flows after low-rank measurements of wall shear-stress. It is shown that an LES with approximately one order of magnitude less grid points than a Direct numerical simulation (DNS) has a similar performance regarding the estimates quality.

When a trefoil vortex knot with an algebraic core profile self-reconnects, negative helicity vortex sheets form, with $h\precsim 0$ shown in yellow in the figure. Those sheets then begin to wrap around one another, perhaps in the manner of the proposed Lundgren spirals. During this phase the enstrophy growth accelerates, leading to evidence for a finite-time viscosity-independent dissipation anomaly $\mathrm{\Delta }{E}_{ϵ}$. In contrast, for an instability-prone Gaussian/Lamb-Oseen profile, only thin vortex braids form and enstrophy growth is suppressed.

This research analyzes the dynamic stall progression in cross-flow turbine blades demonstrating the effects of flow curvature and induced flow caused by the 360 degree rotation along an axis perpendicular to the freestream. This knowledge contributes to low-order modeling of stall through a proper orthogonal decomposition of the velocity fields, and also explores an alternative control strategy of an intracycle variation in turbine rotation rate that modifies the dynamic stall properties and enhances the power conversion efficiency.

The orientation of small particles, such as plankton or microplastics, is altered by their interaction with flow beneath surface gravity waves. Using a multiscale expansion, we show how spheroidal particles in waves reach a wave-averaged stable orientation that is a function of their shape. This stable orientation also affects the particle drift and dispersion.

Studying the fluid dynamics of exhalations, from droplet formation to turbulent cloud transport, is vital for understanding respiratory disease transmission. Our research combines experiments and numerical modeling to uncover the interfacial phenomena involved in bioaerosol generation during forceful breathing actions like coughing or sneezing. By investigating these fluid-mechanical processes, we aim to establish a foundation for optimizing mitigation strategies and promoting public health and well-being.