According to the definition by $Oxford$ $Languages$, style is $a$ $particular$ $procedure$ $by$ $which$ $something$ $is$ $done$ and thus perfectly fits with what we express in a technical paper; but it also designates $a$ $distinctive$ $appearance,$ $typically$ $determined$ $by$ $the$ $principles$ $according$ $to$ $which$ $something$ $is$ $designed.$ This, that might be less clear in a scientific context, is precisely what we discuss in our paper devoted to the question of style in science. The figure shows a tear by van der Weyden, circa 1435.

Filaments or elongated drops are common in daily life, nature, and technological applications. They contract due to surface tension. It is known from experiments that the velocity at which their tips retract can be increased in viscoelastic filaments that contain polymer additives. Here, it is shown from simulations that the velocity with which the tips of prestressed filaments retract is greatly increased compared to filaments in which the polymer molecules are relaxed. The enhancement can be understood in terms of a quantity that is positive when the flow does work on the polymer molecules but negative when the molecules do work on the flow, i.e. when elastic recoiling or unloading takes place.

We develop a rigorous and robust framework which allows for control of interfacial flows. In particular, we use a Model Predictive Control methodology that computes Optimal Controls based on a high fidelity Reduced Dimensional Model initialized with observations of an experimental proxy. The results are excellent and, while the actuation mechanism used here is an electric field, the versatility of the framework affords easy extension to other flows, control mechanisms, and geometries.

Transitional flow within an arteriovenous fistula (AVF) leads to vascular disease, which can be treated by implanting a stent. Large Eddy Simulations of blood flow within a patient-specific AVF revealed the significantly higher turbulent behavior in the Stent-absent AVF when compared to the Stented AVF. This finding provides key understanding of the reasons behind the success of this treatment strategy from a fluid dynamics perspective.

This article explores the fluid dynamics underlying voice production. The key parameters explored in this paper are frequency (from adult male to children and adult females) and the degree of closure of the vocal folds during phonation. The latter can be a pathological condition, but is common in children and females without being problematic. The findings of this study indicate that changes in jet dynamics across the human frequency range may explain the fundamental differences between male and female voices.

We present an analysis of pulsatile shear-thinning flows in two-dimensional channels. Characteristic viscosity is determined based on steady-state analysis of non-Newtonian flows and is used to non-dimensionalize the flow system by introducing the non-Newtonian Womersley number. Numerical analyses on various Carreau fluids reveal the existence of master curves related to the amplitude and phase lag of the flows, where the shape of the master curve is determined by the degree of shear-thinning. Furthermore, the pulsatile flow dynamics can be predicted accurately using pre-computed master curves, thus predicting shear-thinning pulsatile flow dynamics without explicit transient computations.

Surface waves have been observed to be brought to rest in finite time when a layer at the surface is impregnated with floating particles. A theoretical model is presented to explore whether a similar nonlinear damping process operates if the surface is covered by a thin plate of yield-stress fluid. The model demonstrates that the yield stress again arrests motion in finite time, although the final decay of the wave amplitude is faster than with floating particles.

Using the Reynolds scaling analysis, we simplify the Landau-de Gennes (LdG) equations for the flow of liquid crystals (LCs) in thin gaps. Then, both the simplified LdG equations and the full LdG equations are applied to a Couette geometry and a slider bearing geometry for the evaluation and verification of the simplified LdG equations by comparing results. The use of the simplified LdG equations results in a large decrease in wall time, which has the potential to significantly reduce the computational resources needed for simulating LC flows in complex geometries.

In this article we provide a comparative study of rate laws for internal energy excitation and dissociation of nitrogen. These thermochemical properties are obtained by studying molecular interactions on two independently developed $ab$ $initio$ potential energy surfaces developed at the University of Minnesota and NASA Ames Research Center. Furthermore, a comparison of a canonical hypersonic flow field is provided, where the only modeling input to the flow simulation are the $ab$ $initio$ potential energy surfaces.

The geometrical properties of the sets of points in the Poincaré maps associated to the Lagrangian orbits are quantified using topological data analysis. The number of segments in the sets of points and the occurrence of holes were determined with the 0- and 1- persistent homologies, respectively. Left: Poincaré map for Lagrangian orbits obtained with Ra = ${10}^4$, Right: Corresponding Vietoris-Rips radii of 1-homologies for points with several average distances to the center of the point distribution.

In this work we present an inexpensive model that unifies bubble collapse properties in arbitrary geometries. The model produces a single value that quantifies the asymmetry known as the ‘anisotropy parameter’. We show that bubble collapse properties, such as bubble displacement, depend primarily on this parameter.

In the study of sprays, a theoretical gap exists between droplet modeling and reality, especially in high pressure applications like jet engine combustion. In this paper, we use interface-capturing direct numerical simulations of freely deforming droplets to demonstrate the impact of droplet deformation and liquid internal circulation on drag. An increase of over 200% in drag can be seen, depending on the conditions studied. We further showed that these phenomena can be quantified by the pressure and the Weber number, and a correlation for droplet drag was obtained based on these factors.

The collision behavior of droplet pairs subjected to confined shear flows at high density and viscosity ratios remains unexplored and unquantified, and therefore, the critical density and viscosity ratios for the collision mode of droplets under geometric parameters were unknown. Simulations have been performed using a free-energy-based lattice Boltzmann method, with the aim of determining new regimes compared to the previous study. The results demonstrate that the interaction between viscous and inertia forces plays a vital role in the coalescence nature of droplets, their deformation, course of trajectory, as well as their expedition toward the steady state.

The addition of polymers is known to fundamentally affect capillary singularities such as drop pinch-off. In this study, we wonder how polymeric drops spread onto a solid. Although the dynamics of the liquid bridge connecting the drop to the solid is similar to that of pure water, we find that the interface gets significantly sharper, which is quantified by measuring the curvature of the bridge. These findings are reminiscent of what is found for drop coalescence, even though the spreading-coalescence analogy breaks down with viscoelastic liquids.

Focusing on the evolution of wake structure in a flow around an impulsively stopped sphere in an incompressible viscous fluid with a streamwise magnetic field, a complicated vortex structure system containing a primary vortex ring, a fragmented secondary vortex, and an accompanying vortex is discussed, summarized in the {N, Re} phase diagram. The scaling laws of drag coefficient and peak azimuthal vorticity are given and their behaviors are investigated under the influence of a magnetic field.

Experiments are performed on gas entrainment and leakage mechanisms of various ventilated cavity flow patterns around a conical axisymmetric body. As the ventilation rate increases, the size of shedding foam grows while the shedding frequency reduces, leading to balanced gas injection and leakage. The mechanism controlling cavity hysteresis is quantitatively explained using a validated gas leakage model in which the foam is periodically shed as a vortex ring. The closure discrepancy between fully developed cavities (recirculating vortex) and free-standing supercavities (continuous vortex tube) is explained by the adverse pressure gradient difference in the cavity streamwise direction.

The presented work introduces a novel cancelation technique, based on the linear superposition of stationary crossflow instabilities (CFI) through the application of a streamwise series of optimally positioned discrete roughness elements (DRE) arrays on a swept wing surface. The DRE arrays are designed and arranged with suitable amplitude and phase shift to induce velocity disturbance systems that destructively interact, ultimately damping the developing CFI. This results in the suppression of monochromatic CFI, reducing their amplitude and growth and delaying the boundary layer transition to turbulence.

The drag past spherical particles attached to a deformable fluid-fluid interface in the limit of small Capillary number and small deviation of contact angle from 90 degrees is determined. Analytical results are given for a single particle in flow while an analytical investigation supplemented with a straightforward numerical investigation is done for pairs of particles on an interface. Asymptotic results are given for large Bond number.

We develop a parallel-plate hydrodynamic model to study the influence of thermal Marangoni flow on dynamic wetting failure. We show that flow toward the dynamic contact line delays wetting failure to a higher critical capillary number by changing stress gradients in the vicinity of the contact line. Through this work, we provide a potential framework for developing faster coating processes using thermal Marangoni flow.

A local rise of the surface temperature of a liquid layer of only a few tenths of degrees results in a thermocapillary flow, which can decrease the thickness of a supported liquid film down to tens of nanometers over millimetric distances. We show that accurate measurements of the thinning dynamics of this ultra-thin film can be used to finely probe the molecular interactions at stake.

With experiments and theory the effect of acoustic parameters on the stability and break up of a planar liquid sheet under the influence of a standing acoustic wave are studied. Atomization characteristics of slit nozzles and the effects of acoustic pressure and injector position are analyzed. Above a critical acoustic pressure the standing wave acoustic field generates a Faraday wave on the liquid sheet surface. At a specific flow rate an empirical coefficient is found which enables linear instability theory to predict the wavelength of the Faraday waves and the breakup length of the liquid sheet.

The flow field around an axisymmetric slender body at an angle of attack is complex and typically contains extensive regions of three-dimensional crossflow separation; although the body is axisymmetric the side force induced by the flow on the body is significant and may change greatly along the body. This study provides a new perspective on the development of flow unsteadiness and the side-force distribution along the body for a wide range of angles of attack.

Surface acoustic waves (SAWs) are used in droplet microfluidics for operations on sessile droplets, such as internal mixing, directional motion, or nebulization. An unsolved problem has been to understand how high frequency (MegaHz) SAWs can counterintuitively cause low frequency (10-500 Hz) oscillations on the drop free surface, which is known to enhance drop mobility. Using simultaneous measurements of the free-surface dynamics and internal acoustic pressure, we find that the drop oscillation instability results from coupling between an intracavity acoustic mode excited by the SAW, and an inertia-capillary surface eigenmode, through amplitude modulation and delayed radiation pressure feedback.

We present a simple extension of the suspension balance model (SBM) for viscous Stokes flow that allows for modeling of suspensions of particles of bidisperse size. In this work, we show a range of simulations to justify the assumptions made in the polydisperse suspension balance model. The aim is to study the effects of moderate size variation on the rheology of the suspension.

Systematic experiments of particle clouds settling in quiescent water are performed in the absence and in the presence of background rotation, varying the size of particles to assess the influence of particle inertia on the clouds’ dynamics. In the absence of rotation, the particle inertia enhances the growth rate of turbulent clouds until particles decouple from eddies. In the presence of background rotation, the latter interrupts the clouds’ growth as it constrains clouds to become vortical columns.

The formation of large drops via collisions of droplets in warm clouds constitutes a major unsolved problem in rain prediction studies. Often typical turbulent accelerations of parcels of air that generate collisions are much smaller than gravitational acceleration. We manage to provide a complete theory in the limit of small accelerations and confirm it by direct numerical simulations of the Navier-Stokes turbulence. However, the theory is limited to Reynolds numbers much smaller than those in clouds, where, as we show, huge Reynolds number can overturn many existing paradigms.

Leveraging tools from graph signal processing and work on Lagrangian coherent structures, we introduce a graph-based scale transform that is fully Lagrangian and based on the transport properties of the flow. Applying this tool to several different flows, we are able to assess the dynamic consequences of kinematic coherence in a variety of contexts. This approach allows us to extend traditional Eulerian techniques (filtering, compression) to the analysis of Lagrangian quantities.

Composition gradients in the interior of Jupiter can affect and even suppress convective motions. In some situations, a composition gradient can trigger the formation of multiple convective layers separated by sharp diffusive interfaces, preventing further mixing. This fluid state, called layered convection, has been proposed to occur in giant planets. However, it is not guaranteed that secondary convective layers can form and survive underneath a turbulent convection zone. Our simulations find that below an evolving convection zone, layer formation is difficult and the fluid always fully mixes. This may have bearing on the survival of composition gradients in Jupiter’s interior.

Turbulent diffusion and mixing of heat, introduced passively by a thin ribbon in grid-generated turbulence, was enhanced drastically by a small array of thin cylinders, positioned closely downstream of the ribbon. A multi-structure flow region was formed behind the array and relaxed towards grid turbulence, albeit maintaining a turbulence intensity and an integral length scale that were twice as large as those in the absence of the array. This configuration has potential for use in applications that would benefit from intense turbulent mixing.

Turbulence models for scalar and momentum transport are often analogous to each other, but roughness breaks this similarity. The turbulent Prandtl number, the momentum and scalar time-scales, and the dissipation budget are among the central quantities in the modeling of scalar transport. This study examines how roughness affects them by analyzing the results of direct numerical simulations of channel flow with resolved roughness. The flow field below the crest of the elements, which is very difficult to access experimentally, provides answers to long-standing questions.

Localized wall injection exists both in naturally occurring flows and in industrial applications, such as seepage of water from permeable soil layers in a river and film cooling of turbine blades. In these cases, the incoming flow is practically always subjected to disturbances, which in turn increases the turbulence level. In this experimental study, the combined effects of localized wall injection and background turbulence on the structure of turbulent channel flow are investigated for the first time.

Modeling molecular mixing remains a key issue for the filtered density function (FDF) method. We propose a new closure for the mixing timescale of each individual chemical species. A posteriori tests demonstrate the advantages of the proposed model, highlighting its potential to be employed in FDF simulations of turbulent reacting flows.

The purpose of this work is to investigate whether a cascading process can be associated with the rotational motions of compressible three-dimensional turbulence. This question is examined through the lens of circulicity, a concept related to the angular momentum carried by large turbulent scales. Point-splitting and coarse-graining analyses are performed in order to uncover some properties of the circulicity nonlinear transfer flux. These properties are established independently from the expression of the equation of state.

A Lagrangian stochastic model for the orientation evolution of inertialess, rod-shaped particles is proposed and studied in a two-dimensional homogeneous shear turbulent flow. The model consists of superposing a short-correlated random component to the steady large-scale mean shear and also accounts for the anisotropies of velocity gradient fluctuations. Analytical results on a few key orientation statistics are derived, highlighting the model’s ability to reproduce the long-term effects and elucidate its limits at intermediate times to catch violent fluctuations when compared to direct numerical simulations.

We conduct an experimental study of axisymmetric gravity-capillary standing waves generated by a vertically vibrating ring on a fluid surface. Different regimes of standing waves are highlighted at the basin center: linear, nonlinear, and ejection regimes. For moderate forcing, the spatial profile of standing waves breaks the up-down symmetry as predicted by a third-order nonlinear theory, whereas for stronger forcing, the maximum height reached by the axisymmetric gravity-wave crest, at the basin center, is found to increase linearly with its wavelength, as a consequence of the saturation of its steepness.