For sedimenting nonspherical particles at finite Reynolds numbers, very small offsets in the center of mass (less than 0.05% of particle length) can dramatically alter settling behavior. Nonuniformity in mass distribution enhances lateral dispersion and alters overall settling velocity; small changes in particle orientation lead to the onset of wake features which can either stabilize or destabilize the particle’s trajectory, bifurcating over a relatively narrow range of Reynolds number. These results carry implications for a variety of natural and engineered processes, such as the transport and settling of microplastics and/or multimaterial aggregates in the environment.

Red blood cell aggregation significantly affects blood flow, especially in microcirculatory conditions. Our study employs hyperbolic microfluidic constrictions and a convolutional neural network for image analysis to investigate how these aggregates dissociate under extensional flow, in conditions relevant to microcirculation. This research provides crucial data for validating models, and the proposed technique could enhance evaluations of red blood cell aggregability.

In this work, we derive analytical solutions for a unique electrokinetic phenomenon, being termed as transport-induced-charge electroosmosis (TICEO), which does not originate from electric double layers, but is due to the local ion separation in a nanopore filled with an electrolyte solution in the presence of a salinity gradient. We show that the direction of TICEO is independent of the applied electric field, and thus suitable for alternating current (AC) pumping applications. Using a transient model, we examine the time scale, length scale, and operating frequency range for TICEO in a thin nanopore, providing useful guidance for nanopore design in AC nanofluidic technology.

In this Letter, we report a phenomenon where two overlapping heating peaks are observed over a slightly blunted cone instead of just one in a hypersonic flow. Through optical measurements and direct numerical simulations, we confirm that the former peak originates from dilatational effects of second mode while the latter emerges due to high shear viscous dissipation. The convergence between the saturation location of second mode and onset location of transition leads to the overlap of peaks. This study not only highlights the additional heating regions over blunt models brought by the second mode, but also suggests employing designed bluntness strategies to control them.

The authors explore how solidification and thermo-instability could produce the sunflower shape of paraffin wax confined within a Hele-Shaw cell.

A viscous fluid in contact with a solid channel that has a preferential affinity with respect to a second fluid embedded in the channel leads to a spontaneous imbibition process. Due to the increasing friction of the invading phase, the invading fluid scales diffusively in time. What happens when a third liquid is a lubricant coating the channel? We show that when the lubricant viscosity is decreased, dissipation switches from being localized in the bulk of the invading phase, to the lubricant layers. This leads to a new crossover, below which diffusive dynamics are not observed. Our results open up the possibility of using this mechanism in SLIPS and LIS to control capillary flows.

Droplet formation is an important research topic with applications in geophysics, where fragmentation is key in the exchange between atmosphere and ocean, and in industrial processes, with many products obtained by emulsification. Despite its importance this fundamental physical problem remains a puzzle. In this work we examine small droplet formation in turbulent multiphase flows. By surgically acting on the Navier-Stokes equations we show that small-droplet formation is the main cause of high dissipation events characterizing multiphase turbulence, and not vice versa. We also prove that tiny droplets are formed through the internal capillary dynamics of large droplets during break-up.

The follower force model is a fundamental model for active filaments, commonly utilized to model microtubule-motor protein complexes and collections of cilia. In this work we perform a thorough analysis of this model, employing techniques from computational dynamical systems, adapted from high Reynolds number fluid dynamics, to map out the bifurcations in the system and classify emergent states. This approach allows us to bridge the gap between 2D and 3D analyses, in particular establishing the initial buckling as a double Hopf bifurcation. Additionally, we identify the existence of a quasiperiodic solution at the second bifurcation, and categorize the dynamics at higher values of forcing.

Algorithms used to calculate red blood cell damage are often constructed empirically from laminar flows, and the extensibility of these algorithms to turbulent flows is an open question. To examine this question, we investigate the effect of turbulent flows on red blood cell damage by simulating individual red blood cells in turbulent channel flow using a multiscale computational framework. The results from the turbulence simulations are compared to laminar channel flow at equivalent wall shear stress. While the laminar flows generate greater stretch in a time-averaged sense, the turbulent flows produce spikes in deformation associated with short bursts of high-shear conditions.

Inspired by industrial processes in oil and gas well plugging and abandonment (P&A) operations, this study investigates the injection of heavy, thick fluids into lighter fluids. By experimenting and analyzing flow behaviors using dimensionless numbers such as Reynolds number, Froude number, inclination angle, Bingham number, and viscosity ratio, we identified different flow regimes such as stable and unstable slumping, separation, and mixing. These findings offer valuable insights for improving fluid flow analysis in applications like 3D printing and other industrial processes.

We show that the apparent relaxation time inferred from the exponential thinning regime in viscoelastic pinch-off is not necessarily a material property, as was assumed so far, but depends on the system size for various polymer solutions and filament thinning techniques. It depends on the plate size in Capillary Breakup Extensional Rheometry (CaBER) with both slow and fast plate separation protocols, and on the nozzle size in Dripping-onto-Substrate (DoS), corroborating recent observations with the dripping technique. It is not due to artifacts such as solvent evaporation or polymer degradation and it cannot always be rationalized by finite extensibility effects.

We simulate a spheroidal swimmer through a complex fluid. A spheroidal swimmer model is developed and exerted in a direct-forcing fictitious domain method framework. We find an inertial spheroidal puller with a small swimming intensity swims faster than the counterpart subjected to the Stokes flow regime, a departure from observations of spherical pullers. Within the Giesekus fluid medium, an augmented mobility factor correlates with an increased squirmer velocity, while a larger aspect ratio contributes to neutral squirmer speed enhancement in the presence of fluid inertia. Meanwhile, a slenderer, inertial squirmer with a vigorous swimming intensity expends more energy.

The 2nd moment correlations between thermodynamic fluctuations in shock wave/turbulent boundary layer interaction flows are numerically studied. By introducing Kovasznay modal decomposition, it is shown that the correlations are functions of inter-modal competition and inter-modal correlation. A simplified model is proposed to eliminate the impact of inter-modal correlation, and thus the correlations can be determined merely by the root-mean-square values of thermodynamic fluctuations.

The mean temperature profile is measured in a three-layer water-FC77-mercury system (FC77 is a type of Fluorinert electronic liquid). The mean temperature profile is measured across (a) the mercury-FC77 interface and (b) the FC77-water interface. The three-layer system provides slippery boundary conditions for the middle FC77 layer in an otherwise standard Rayleigh-Bénard convection system. This results in greatly enhanced heat transport across the FC77 layer, both in magnitude and in the scaling exponent with the Rayleigh number Ra.

Our experiments demonstrate that pancake-shaped bubbles in thin tilted Hele-Shaw cells rise slower than what is expected by simply correcting the effective gravity. This effect is shown to be more important for larger inclination. A careful study highlights an asymmetry for the lubrication film between the bubble and the top and bottom walls. As the cell inclination increases, this asymmetry also increases. We propose that it induces an additional “friction” due to fluid motion between the surrounding of the bubble and the Poiseuille flow further away.

Impacting nanodroplets on pillared surfaces are investigated with molecular dynamics simulations. The difference between the liquid spreading dynamics on flat and pillared surfaces is shown to arise from the intrusion effect of liquid into pillar gaps. The altered dynamics can be understood by modeling the maximum spreading factor β${}_{max}$. The scaling laws of β${}_{max}$ on flat surfaces can successfully predict β${}_{max}$ on pillared surfaces when the volume term of the bulk droplet is properly accounted for. For a six parameter group of the initial problem we propose a universal phase space which contains six outcome regimes for impacting nanodroplets.

Ion selective concentration shocks have been shown to develop in electrochemical cells with homogeneous porous media of low surface charge. Here we demonstrate through a set of simulations that heterogeneity in the porous structure can lead to substantial differences in separation performance. Both variation in surface charge and characteristic pore size, result in vortical flow in the depleted area, affecting multiple metrics such as energy consumption, water recovery, and desalination. These effects can also be observed for hierarchical media and may be exploited in future designs of porous materials in electrochemical applications beyond shock electrodialysis

This work presents a reduced-order model-based feedback control strategy for suppressing vortex-induced vibration (VIV) of a spring-mounted cylinder using the balanced proper orthogonal decomposition (BPOD) method. The BPOD model, closely aligned with the full-order model (FOM), is employed to design an active flow control strategy with blowing and suction actuators, effectively suppressing VIV up to Re = 100 by adjusting or eliminating unstable eigenmodes. The optimal control strategies, robust to variations in Reynolds numbers, highlight significant gain margins when positioning velocity probes near x/D = 3.0, though probe placement in wavemaker regions might be suboptimal.

The natural frequency of a fluid overlying on a wavy wall in general reduces. This reduction is observed by a shift in the minimum of the Faraday threshold, i.e., in the parametric acceleration versus frequency plot.

The influence of a surface covered with riblets on laminar-turbulent transition in channel flow is investigated through modal and non-modal stability analysis of n-periodic systems, allowing to take into account detuned long-wavelength instabilities. For large riblet spacing, a wavenumber lock-in regime is observed, in which the wavelength of the optimal streaks is controlled by the riblet spacing, and the streaks amplitude is modulated in the spanwise direction via a beating mechanism, as in geometric frustration. Moreover, in the presence of harmonic forcing, riblets lead to the development of oblique waves that may trigger an early transition.

In highly viscous environments, an object’s dynamics is perturbed in the presence of soft confining boundaries due to the coupling of the lubricated flow with the boundary’s elasticity. Since the deformability of the boundary is central to the elastohydrodynamic forces induced, altering its material nature to that of a capillary fluid interface can drastically alter the magnitude and direction of these forces. Based on a model system with two infinite cylinders rotating near a fluid interface, such changes are theoretically and numerically explored in detail. New scaling results and a reversal in the sign of the generated normal forces, unseen with classical elastic substrates, are revealed.

How does a cavity-embedded n-dodecane droplet behave under near-critical thermodynamic conditions? We present a comprehensive three-dimensional simulation of such a droplet subjected to a normal shock wave, utilizing an effective resolution of 0.87 billion finite volumes. This represents the largest simulation of its kind to date. A novel configuration is introduced to incorporate the influence of the cavity on the droplet dynamics. This study advances our understanding of droplet behavior under extreme conditions, potentially aiding in resolving specific transcritical flow challenges encountered in scramjets, ramjets, and liquid rocket engines.

The time evolution of the liquid relative permeability of grain packings initially filled with liquid foam is measured to increase until a constant value is eventually observed. We demonstrate that this evolution is directly related to the coarsening of the liquid foam confined in the pore space. Using the measured bubble size evolution combined with the intrinsic liquid permeability of the bulk foam, we have determined values corresponding to the function that describes the foam confinement effects as a function of the bubble-to-grain size ratio.

As liquid films on solid substrates approach submicron thickness, they tend to become naturally unstable due to the action of long-range dispersion forces. Interestingly, the presence of surfactants and gravitational drainage alters the stability characteristics of these films. Using linear and nonlinear stability analyses in the lubrication limit, we show that draining films containing surfactants exhibit greater stability compared to stationary films with surfactants, as well as draining films with clean interfaces. Our findings can have potential implications for the stability of a wide variety of thin films, ranging from precorneal tear films to industrial coatings.

The dynamics and energy-harvesting performance of piezoelectric plates in oscillatory flows have been studied numerically. The simulations show that when these plates are arranged in an array with certain distance between neighbors, the average energy-harvesting capacity of each individual plate may be increased by as much as 110% within the range of parameters considered. The underlying physical mechanism has been identified as wake energy recovery - a plate in such a formation is able to extract energy from the wakes of its neighbors that will otherwise be dissipated. This finding can be used in the development of environmental-friendly soft-body wave energy harvesters.

We study numerically and analytically the effect of Navier slip on the orientational dynamics and effective shear viscosity of a semi-dilute suspension of two-dimensional particles with either circular or elongated (plate-like) shape, interacting only via hydrodynamic and contact forces. We show that at dilute concentrations slip causes the elongated particles to align in the flow direction, whilst for large concentrations tumbling of the particles occurs due to particle-particle interactions. We show this change in orientational microstructure directly impacts the effective viscosity of the suspension: a minimum in the effective viscosity occurs at a threshold concentration.

This study examines the effect that misalignment has on the viscosity measurements of Newtonian fluids in a parallel-plate rotational rheometer. Our theoretical results show that at small gap heights, misalignment can produce additional secondary velocity components and pressures in the fluid, which affect the forces and moments in the rheometer. These forces and moments on the top plate were found to increase as the misalignment tilt angle increased.

In this study we have extended the mean-field based model to analyze the layered structure of the electric double layer and found that oscillations in charge density arise in multivalent counterions. Formation of the counterion condensed layer and overscreening creates a larger accumulation of coions in the electric double layer, leading to an electroosmotic flow reversal and attenuation in ion selectivity of the pore. The present model shows that the counterion size has an impact on the condensed layer and hence, overscreening. We demonstrate that the electroosmotic flow reversal of multivalent electrolytes can be suppressed by mixing with monovalent slats.

Cohesive forces occur at the particle scale and have effects up to the macroscopic scale. Using the canonical configuration of a column of grains collapsing under its own weight in air, this paper reports that a bulk description framework of cohesive effects can account for the macroscopic observations. Experiments are reported with two different cohesion sources, capillary bridges or a polymer coating, for the collapse of rectangular and cylindrical granular columns. The bulk framework is shown to capture the effects of cohesion on the final deposit for both sources of cohesion.

Uncharged micron-sized water droplets flying toward each other do not always coalesce due to the cushioning effect of the air between them.For oppositely charged droplets, we discover a regime for which droplets always collide when they move inside the stable manifolds of a saddle point of the relative droplet dynamics. A consequence is that only for small electrical charges does the droplet coalescence rate depend primarily upon the Knudsen number (Kn), the ratio of the mean-free-path of air to the mean droplet radius. For much larger charges, coalescence does not depend upon Kn. Our theory predicts the critical charge at which the transition between the two regimes occurs.

The dynamics of a magnetic particle of moment m in an oscillating magnetic field H has a qualitative dependence on the magnetization model. There are sustained oscillations for a particle with a permanent dipole. The particle is static for an induced dipole with no hysteresis. For an induced dipole with hysteresis, there is a transition between oscillations with a magnetic moment oscillating about a particle axis, and a magnetic moment flipping relative to the particle axis.

We explore stably stratified flows in V-shaped triangular cavities, following Prandtl’s mountain and valley flow model. Our study identifies five distinct steady states. A zero-flow state bifurcates into symmetry-conjugated asymmetric circulation states forming a pitchfork bifurcation. Additionally, we identify two symmetric states characterized by upslope and downslope flows, respectively. These states, although not symmetry-conjugated, originate from the same eigenmode, leading to a novel bifurcation pattern that deviates from traditional canonical forms. Our findings illuminate the complex bifurcation structure of stratified flows in such cavities, which has previously been overlooked.

Flexible structures subjected to varying flow conditions are often more resilient than their rigid equivalents. We exploit this bio-inspired strategy, called reconfiguration, to improve the efficiency of a scale model underwater propeller. Using flexible blades that deform passively, we show experimentally that it is possible to program the deformation of the rotor such that the efficiency of the propeller is improved in off-design conditions. We also show that the deformation of the blades can be predicted using a simple model.

Our experimental study explores ice melting rates in quiescent water and in turbulent flow. Particle image velocimetry measurements allow us to visualize and characterize flows generated by meltwater plumes and to non-invasively measure melt rate of an ice sphere fixed in place in the center of our isotropic turbulence tank, in which randomly actuated synthetic jets produce a core of homogeneous isotropic turbulence. We present relationships between ambient water temperature and turbulent kinetic energy on melt rates.

This study is establishing a new method to characterize the thermal boundary layer in turbulent Rayleigh-Bénard convection. Transition from the convection dominated bulk domain to the diffusion dominated boundary layer can be identified by extrema in the time-variation of the temperature along the fluid-parcel pathlines. We demonstrate that our method reveals the spatiotemporal structure of the boundary layer.

We use large-eddy simulations to study the effects of helical-shaped blades on the wake flow characteristics of vertical axis wind turbines (VAWTs) at low tip speed ratios. Compared with the straight-bladed VAWT, our study shows that the helical-bladed VAWT generates near-wake flow structures with more three-dimensional features, which accelerate the wake transition to turbulence, enhance the small-scale turbulent dissipation, and result in a more rapid decay of the wake turbulence intensity. Moreover, the helical-bladed VAWT also exhibits much smaller temporal variations for the torque and power coefficients than the straight-bladed VAWT, resulting in smoother wind power generation.

The verification of small-scale isotropy requires three-dimensional information of the flow field, a condition rarely satisfied in experiments. To examine this we develop a framework that considers how the presence of bursts at smaller flow scales generates turbulent kinetic energy differently between the horizontal and vertical directions. This framework can be applied both to flow fields obtained via numerical simulations, and to data from field and laboratory measurements. Moreover, a universal relationship emerges to predict small-scale anisotropy from large-scale flow conditions, thus contributing towards the development of next-generation closure models of wall turbulence.

Laboratory experiments of ocean gravity currents show laminar transport supplemented by spontaneous and intermittent cascading of dense water. Statistical analysis reveals self organized criticality of the downward transport. Cascading intrusions are a major contributor of turbulence and vorticity in the ocean interior.

We address coherent pressure structures in turbulent channel flows through SPOD and resolvent analysis with and without an eddy-viscosity model. The spectral analysis revealed energetic structures in the near-wall region, as well as large-scale and spanwise-coherent structures. Pressure structures are targeted in both SPOD and resolvent analysis by selecting an adequate norm through the quadrature weights and observation operator, respectively. The first SPOD and the leading resolvent modes closely agree and show low-ranking behavior. The analyzed modes comprise quasi-streamwise (for near-wall and large-scale structures) and spanwise vortices with pressure peaking at vortex centers.

Which direction matters more for subgrid-scale (SGS) turbulence? Our analysis reveals that the essence of dynamic SGS models is often condensed in only a few (sometimes, just one) special directions, offering new insights into the success of dynamic large-eddy simulation models.

A large-scale direct numerical simulation of a spatially evolving turbulent round jet flow has been conducted at a Reynolds number of 3500 based on the orifice diameter and the mean bulk velocity at the orifice. A fully turbulent pipe flow profile is used as the inlet condition, and high-quality statistical data is generated by averaging over 200 washouts of a particle. The present data include statistical moments up to the third order and the probability density function of the axial velocity at various distances from the centerline and the orifice, showing remarkable self-similarity. The present data is compared with data of previous numerical and experimental studies.

How fast can a Rayleigh-Taylor unstable flame accelerate? How rapidly can its width increase? To investigate these issues, a link is established between the level of molecular mixing and the self-similar evolution of a reactive Rayleigh-Taylor turbulent flow.

Turbulence generated with a random jet array (RJA) can be homogeneous, isotropic, and exhibit negligible mean flow, making it suitable for many applications. However, achieving these characteristics requires a thorough understanding of the turbulence generation process within RJA facilities. In this study, we investigate the evolution of turbulence within an RJA-driven water tank, from its generation to its decay, and distinct features of the flow. Additionally, we propose a relationship for estimating the energy of the generated turbulence in cases with minimal mean flow, and we introduce an efficiency parameter to compare the input energy with the resulting turbulent flow.

In this study, we explore the complex dynamics of a dam break wave comprising a clay-water mixture under turbulent flow conditions as it interacts with a vertical rigid wall. Utilizing advanced three-dimensional Large Eddy Simulations (LES), we investigate the evolution of the dam break over time as a function of fluid rheology. The study aims to provide useful information for the development of risk mitigation strategies and the design of protective structures by examining the influence of clay concentration and initial fluid depth on the wave’s behavior, bed shear stresses, and impact forces.

Novel formulations involving a control surface at a distance from the body are developed to compute drift loads on structures composed of an impermeable hull and a perforated surface accurately and efficiently. The developed formulation can not only give all six components of the mean wave drift force and moment, but also determine the drift loads on each individual body of a multi-body system.

This paper presents analytical solutions to the theoretical problem proposed by Ellison and Turner. Their equations describe the longitudinal evolution of a miscible gravity current when it reaches a steady state. The solutions here proposed allow the evolution of the velocity, height and density of the current to be calculated solely from the local Richardson number. The latter is obtained using a universal function F that can be tabulated or plotted whatever the release conditions. The solutions are then extended to configurations in which a jump appears, and finally for several entrainment laws available in the literature.