Understanding active colloidal systems with nonequilibrium self-organization is a long-standing, challenging area in material sciences and biology. In this work we use a colloidal suspension of Janus particles to investigate the hydrodynamics that underlies their various morphologies (unilamellar, multilamellar, and striated structures) and rheological properties.

A fluid can be heterogeneous in either its density or viscosity - its two basic material properties. In this Letter, we ask how an ambient flow field is disturbed due to a point viscosity variation in the fluid. If we are able to answer this question, we can create an arbitrary spatial distribution of viscosity discretely by placing viscosity sources and sinks of appropriate strength, allowing us to model complex fluids. As an example, we show how the hydrodynamics of a rigid particle is altered due to the presence of point viscosity variations.

The paper focuses on the effect of buoyancy on the spectral characteristics of turbulent flows. We obtain an analytical solution in the nondissipative limit for the spectrum containing both ranges with Obukhov-Bolgiano and Kolmogorov scalings. This leads to the derivation of an expression for the Bolgiano scale through the governing parameters of the problem.

The influence of planform and Strouhal number on the performance of bio-inspired pitching panels is investigated by changing the trailing edge sweep of a trapezoidal planform, resulting in panels of forked (F), straight (S), and pointed (P) trailing edges. Positive mean thrust only develops for S and P panels of reduced aspect ratio. Decomposition of thrust into its normal and axial force components (NF and AF) show that a thrust-producing panel has large propulsive efficiency for small AF, and that thrust is then due primarily to NF. However, once a thrust-producing panel reaches a local efficiency maximum, increases in Strouhal number result in increases in mean thrust due to larger AF.

Airway closure is a surface-tension-driven phenomenon that occurs when the ratio of the pulmonary liquid layer thickness to the airway radius exceeds a certain threshold. In previous studies, it has been found that airway epithelial cells can be lethally or sub-lethally damaged due to the high peak of the wall stresses and stress gradients during the liquid plug formation. We, in the present study, demonstrate that these stresses are also related to the elastoviscoplastic features of the liquid layer. The airway mucus in healthy, asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) conditions are studied using the Saramito-HB model.

The configuration of a polymer in a moving fluid changes drastically from coiled to stretched when the velocity gradient exceeds a critical value. This phenomenon, known as the coil-stretch transition, has been characterized by examining the entropy of the polymer extension. We extend this approach to a set of laminar and random flows that are regarded as benchmarks for the study of polymer stretching. The entropy of the polymer fractional extension emerges as an effective tool for studying the coil-stretch transition and comparing its features in different flows, especially in situations when limited statistics on the dynamics of polymers is available.

Materials made up of soft elastic particles suspended in a fluid form a broad subset of complex fluids, including emulsions, suspensions of microgels, liposomes, or vesicles. Until now, no established model can describe how they flow. Here, we derive from particle-level dynamics a constitutive model describing the rheology of two-dimensional dense soft suspensions below the jamming transition, in a regime where elastic interactions dominate the stress response. Our model successfully captures many observed rheological features, such as shear thinning, normal stresses, and normal stress differences scaling respectively linearly and quadratically with deformation rate in the slow flow limit.

The flow fields in a porous medium under no slip conditions contain all the physical information to predict the Klinkenberg slip correction tensor, without requiring the solution of additional problems. For situations in which higher-order terms are needed to predict the apparent permeability tensor, the flow problem with no-slip and considering a first-order slip are enough to build an excellent estimate using a Padé approximant.

It is well-known that high-intensity acoustic fields in narrow open ducts can give rise to shock waves, where the length of the duct is a key factor, but the geometry of the edges at the opening is not. However, the precise geometry of the inner edges of the duct is critical, together with the value of a Reynolds number associated with the boundary layer, in determining whether or not vortices form at the open end of the duct and whether or not they are radiated outwards. The generation and dynamics of these vortical structures are studied experimentally by means of particle image velocimetry, and numerically using Lattice Boltzmann simulations.

In this work we simulate two-dimensional Rayleigh-Bénard turbulent convection covering four decades in Rayleigh number for two different Prandtl numbers, and find for the first time a clear-cut transition by considering the fluctuations of the heat flux through a horizontal plane, rather than its mean value. In the figure the sharp transition is displayed by a jump in the ratio of the root-mean-square fluctuations of heat flux to its mean value and occurs at $Ra/Pr\approx {10}^9$ . Our finding suggests that an analysis of fluctuations should help to discriminate some open questions concerning regime transitions.

In this paper we explain why bubble chains are stable in champagne, but unstable in other carbonated beverages. The hydrodynamic interactions among bubbles in a chain determine its stability. The wake behind each bubble affects the subsequent ones, producing a lift force. For a clean and small bubble, the lift force is destabilizing; when surfactants are present or the bubble size exceeds a certain value, the additional vorticity production on the surface changes the structure of the wake and the sign of the lift force is reversed resulting in a stable chain.

Bubbles and droplets are often formed by blowing contained gas through a constriction or tube. We show theoretically and experimentally that when the bubble is attached to the tube, its growth can be monotonic and progressive or go through an unstable state of rapid growth. Our analytical model, which reproduces experimental data, uses a dimensionless number B combining the container volume, tube radius, and Laplace and atmospheric pressures. The pressure dependence suggests that the gas compressibility, which induces a nonzero hydrodynamic compliance, is at the origin of the instability and explains the great bubble size disparity of bubbles produced by blowing at an imposed gas flow rate.

The impact of a highly accelerated drop on a thin film of similar fluid is accompanied by the production of secondary droplets, while mixing occurs between the drop and the film. From high-speed imaging of laboratory experiments, we analyze the key aspects of this collision. Using an innovative colorimetry-based technique, we additionally deduce the size of the drop imprint in the film, the amount of water transferred from the drop to the film, and the total volume ejected away during the impact.

Numerical simulation using the coupled level set and volume of fluid (CLSVOF) method was conducted to study the coalescence dynamics of two different-sized drops of miscible liquids. Effects of surface tension ratio, Ohnesorge number (Oh), and diameter ratio on three pinch-off regimes: first- and second-stage, and no pinch-off, are examined. Partial coalescence is characterized by its appearance, disappearance, and subsequent reappearance as the surface tension ratio decreases. A regime map of distinct coalescence outcomes is constructed for different surface tension ratios, where the critical surface tension ratio required for partial coalescence increases monotonically with an increase in Oh.

The Hall effect plays an interesting role in the context of astrophysical dynamos. Using direct numerical simulation, the dynamo action in Hall magnetohydrodynamic turbulence has been systematically studied. Unlike previous studies, the total Hall contribution has been decomposed, separating the contribution of magnetic and current fields in feeding the large-scale magnetic field. Using scale-specific flux relations, it is shown that the small-scale magnetic and current fields contribute to the large-scale magnetic field growth. Unlike small-scale magnetic fields and large-scale current fields, the small-scale current fields appear to be the leading contributors to small-scale dynamo action.

The Electrohydrodynamic conduction mechanism is investigated in non-Newtonian dielectric fluids described by the Oldroyd-B model. We examine flow patterns, oscillation amplitudes, force competition mode, and power-law spectral scaling accompanying oscillatory flow. Commensurability magnitudes of the elastic stress and Coulomb forces alter the convective mode, leading to discrepancies in the electroelastic instability paths. The kinetic and elastic energies of driving parameters are quantified, and five subregions found for flow patterns. Dynamic behavior is related to the strong electroconvective flow within the heterogeneous charge layers.

An inertial supercritical gravity current sustained by a constant source at $x=0$ propagates toward a free drainage edge at $x=x_D$. After the nose spills over the edge, a steady state is possible. We show that the time-dependent process leading to the steady state is important in applications (e.g., fire on the ceiling) and is strongly affected by the mixing/entrainment model between the current and the ambient.

A liquid film coating the inner side of a horizontal tube is prone to the Rayleigh-Plateau instability resulting from surface tension forces. However, when these capillary forces are dominated by gravity forces (i.e. when the Bond number increases), the liquid film possibly stabilizes. In this study, we determine numerically the instability threshold in good agreement with preexisting experiments and show that deformation of the interface during liquid drainage is responsible for flow stabilization.

Surface viscosity triggers surface shear flows in incompressible two-dimensional domains when surface curvature changes, triggering stretching flows and stresses (top) and non-axisymmetric shape instabilities (bottom).

A liquid film flowing down a fiber or thread is well known to destabilize into an axisymmetric bead-like pattern in stagnant air. We demonstrate experimentally how side wind affects such a film flow. Our results show that side wind increases the descent speed of the liquid beads, highlighting that the film flow regime can be changed by controlling the magnitude of the wind speed.

A slip boundary is usually realized by micro-structured surfaces, where primary fluid flows over the microstructures being filled with a secondary immiscible fluid. We establish a theoretical framework for interfacial coupled flow on a slip boundary over a micro-structured surface. Coupled flow of two fluids, which is the key problem for the study of slip boundary, is solved by finding the approximate integral relation between the slip velocity and the slip velocity gradient.

Stokesian Dynamics is a well-established computational method for simulating dynamics of many particles suspended in a conventional passive fluid medium such as water. Active fluids composed of self-driving microscopic particles with broken time reversal symmetry permit the emergence of a so-called odd viscosity. In this work, we extended the conventional Stokesian Dynamics formalism to incorporate the additional hydrodynamic effects due to odd viscosity under a simplified quasi-two-dimensional case. This demonstrates that Stokesian Dynamics can be generalized to simulate collective behaviors of many particles suspended in an active fluid medium with both even and odd viscosity.

A study of the thermophoretic motion of a spherical aerosol particle in a microchannel with permeable walls bounding a porous medium is presented. The field equations of energy and momentum for the system are solved using a semi-analytical method with the help of a boundary collocation technique. The study includes the effect of thermal stress slip at the surface of the particle in addition to temperature jump, thermal creep, and frictional slip.

The transition of flow from laminar to turbulent has been a topic of considerable interest for fluid dynamicists. Evolution of spanwise three-dimensionalities in the wake of an elongated bluff body occurs before the wake becomes turbulent. Several engineering structures such as industrial chimneys, marine risers, and off-shore structures have tapered geometry. In this study, we show the three-dimensional (spanwise) modes in the wake of a rotationally oscillating tapered cylinder.

Quartz crystal microbalance with dissipation monitoring (QCM-D) is a standard analytical tool in soft matter and life sciences; its resonant frequency and bandwidth are very sensitive to a tiny amount of substance (e.g., molecular assemblies, proteins, cells) adhering to the quartz resonator surface. Since the resonator is often immersed in aqueous solution set in motion by surface oscillations, viscous forces also impact measurements. Thus, reliable interpretation of QCM-D results requires quantitative study of the shear stress due to the complex flow affected by particles. We examine theoretically the hydrodynamic stress due to suspended and adsorbed particles and their impact on QCM-D response.

We report a novel droplet breakup regime in the microfluidics T-junction, where new droplet interfaces are created in the two arm channels, rather than at the center of the junction. We show that there is a different hydrodynamic root for this new regime, and propose a simple geometric rule which enables this regime compared to the classical T-junction. A dynamic model for lower capillary number (Ca) is developed to predict the breakup process. We also show that at higher Ca the classical regime of central breakup is recovered in the same T-junction.

Euler-Lagrange simulations are often required to study complex compressible multiphase problems. Such systems include volcanic explosions and supernovas. We developed an unsteady neighboring particle interaction framework with the help of the compressible Maxey-Riley-Gatignol equation and the scattering theory, which further improves force predictions used in Euler-Lagrange simulations.

We consider the Maxey-Riley equation for small neutrally buoyant inertial particles in the Boussinesq approximation. The main motivation is the study of phytoplankton and oceanic particles in turbulent flows. We show that particles behave as forced damped oscillators, with different regimes depending on the particles Stokes number and the fluid Brunt-Väisälä frequency. Using direct numerical simulations we study their dynamics and show that small neutrally buoyant particles in these flows tend to cluster in regions of low local vorticity.

This study investigates the impacts of fluid inertia and solute diffusion on the reactive transport involving bimolecular fluid-solid reactions in rough channel flows. The critical role of flow fields on fluid-solid reactions is highlighted. Further, a parsimonious upscaled model that successfully captures the reactive transport process is developed.

Theoretical and numerical turbulent flow studies either consider turbulence as generated by fluid-solid interactions or focus on intrinsic, bulk properties. There is another group of flows for which homogeneity and isotropy are broken by the body force that sustains the flow, not by physical boundaries. In this class we consider the cellular flow produced by a two-dimensional spatially periodic forcing whose laminar state corresponds to a regular array of counter-rotating vortices. We find that, despite the developed turbulence recovering isotropic features at small scales, the signature of forcing remains in the mean flow, allowing us to quantify the efficiency of the work done by the force.

Construction of efficient and generalizable reduced order models of developed turbulence is a formidable challenge. Bridging the gap between physics-based models and data-driven methods, we introduce a novel approach that involves constructing a hierarchy of learnable and parameterized reduced Lagrangian models based on Smoothed Particle Hydrodynamics (SPH) and machine learning. By training these models on high fidelity Direct Numerical Simulation data sets and comparing their generalizability, we demonstrate the effectiveness of SPH-informed models at predicting statistical and field-based quantities of turbulent flows across a range of resolutions, time scales, and turbulent Mach numbers.

Angular momentum and moment of total enthalpy integral (AMI and MTEI) equations provide a quantitative mapping between flow phenomena such as turbulence on skin friction and surface heat flux for high-speed boundary layers relative to laminar flows. Using the AMI equation, the effect of edge Mach number (Ma) and wall-cooling are measured on the turbulent torque that enhances the skin friction coefficient. Moreover, the AMI equation introduces stress-weighted density that yields a more robust collapse of the dataset with different configurations. The MTEI equation distinguishes the impact of Ma and wall-cooling on the turbulent flux of mean kinetic energy and enthalpy, respectively.

Thanks to the recent unprecedented leap in computational power, we can now delve into the fascinating realm of finite-temperature superfluid turbulence and study the intricate interplay of two-fluid dynamics. Our analysis focuses on two-fluid vorticity locking where turbulent normal and superfluid flows interact and create complex pressure dynamics. Through careful examination, we showcase how the flow statistics seamlessly transfer and manifest in the mutual pressure field, unravelling new insights into the fundamental laws of fluid mechanics.

We describe a hidden scaling symmetry in developed turbulence which is restored in the inertial interval and, as a consequence, substantiates basic properties of intermittency, such as anomalous scaling. In this work we focus on how this symmetry gets broken in the forcing and dissipation ranges. The study is based on a shell model.

Flow control can be achieved by inducing external pulsations on wall jet impingement. It can enhance the mixing ability and heat transfer characteristics. Evidence of streamwise vortices have been found in oblique jet impingement subjected to external pulsations.

Using theoretical and computational methods we obtain these findings: 1) In micro-confined high-pressure transcritical turbulence, higher than typical levels of vorticity are observed in the viscous sublayer; 2) Flow topologies typical from far-wall regions are observed in the vicinity of the supercritical gas-like wall; and 3) Tube-like flow motions are favored at scales comparable to the density-gradient scale, resulting in a smaller number of vortex sheet structures.

The flowfields associated with chamfered, low aspect ratio cylindrical pins cantilevered into a laminar boundary layer were investigated experimentally using oil flow visualizations and stereoscopic particle image velocimetry. Two chamfered pins were used, one where the chamfer encompassed half of the pin’s planform and one where the chamfer encompassed the entire planform. The chamfered pins resulted in two additional counter-rotating streamwise vortices, named Chamfered Induced Vortices (CIVs), and increased downwash. Changing the chamfer skew angle resulted in a change in the strength and direction of these vortices making them of possible use as flow control devices.

While understanding breaking waves is crucial for the development of parameterizations used in modeling ocean and climate, the complete description of wave breaking is not well understood. We present direct numerical simulations of two-dimensional solitary waves that break on a uniform beach in shallow water, including storm surge represented by an inshore region.

Turbulence experiments in a stratified fluid driven by a set of internal gravity waves commonly lead to a discretization of the energy in frequency and wavenumber due to the emergence of eigenmodes of the fluid domain. In this article, we present an experimental means to inhibit this process and to direct the flow towards the regime described by the weak internal-wave turbulence theory which aims at modeling the oceanic dynamics at small scales.

The paper compares four existing wave breaking models applied in the modified non-linear Schquotodinger equation and examines their potential extension to waves with a continuous spectrum. The authors propose a modification to the breaking model by Kato and Oikawa (1995) in order to model breaking in focused wave groups. The modified model also predicts breaking in perturbed plane waves well.

Rogue waves are individual ocean surface waves much larger than the prevailing wave field. In high sea states they can be a danger to marine operations. Rogue waves are well studied for unstratified water. However, on a two-layer fluid the surface wave spectra are modified due to triad interactions with interfacial waves. Such triad interactions are not possible in homogeneous water. We find the modified surface spectrum reduces rogue waves. These findings are relevant for the changing polar oceans where prolonged ice melt is expected to facilitate open water with higher sea states while also creating a surface layer of freshwater overlaying salt water, as in our idealized model calculations.