The objective of this work is to establish a unified theoretical framework for the rheology of dense granular suspensions across different flow regimes. By conducting rheological measurements at imposed volume fractions or at imposed values of particle normal stress, a unified granular rheology can be proposed across the viscous to inertial flow regime. This granular rheology, identified for suspensions of hard spheres, can be extended to a soft granular rheology for soft particles.

Margination, the migration of leukocytes to the vessel wall, is a well-known physiological process. However, the role of cell stiffness in this phenomenon remains unclear. Our study investigates the segregation of stiffened red blood cells (RBCs) within healthy cell suspensions. Unexpectedly, we found central and corner peaks in the stiffened RBC distribution and a nonmonotonic dependency on the flow rate. Length scales for margination are found to be much longer than typical distances between bifurcations in-vivo, questioning the importance of margination in physiological situations.

Cavitation near the critical point is unusual because the compressibility becomes very high and the density difference between liquid and vapor becomes small and vanishes completely in the single phase supercritical region. We have investigated laser-induced cavitation in this unusual regime with high speed video at up to 5 million frames per second using liquid helium as the working fluid. Our theoretical analysis shows that the pressure in the liquid outside a bubble can be much lower than the ambient pressure. Near the critical point, the low pressure liquid becomes unstable and generates a cloud of microbubbles, which is consistent with predictions of nucleation theory near the spinodal.

Dynamic mixed scale-similarity/Smagorinsky type models (DMM) are promising as they typically have a high $a$ $priori$ correlation with subgrid stresses, and provide sufficient subgrid dissipation to be robust for practical applications. However, past DMMs require two or more levels of test filtering, making them unattractive for production codes. We propose an efficient DMM with a single level of test filtering, and through $a$ $posteriori$ LES tests of turbulent channel flow and wall-modeled LES of turbulent smooth-body separation (see figure, for a Gaussian bump), demonstrate their robustness and improved accuracy with under 5% additional cost compared to the standard Dynamic Smagorinsky Model.

We experimentally investigate vortex-induced forces of an oscillating bluff body across subcritical to critical Reynolds number (Re) regimes, and reveal hydrodynamic component variations versus Re, thereby addressing a gap in understanding at high Re. Specifically, the drag crisis cannot be mitigated by vibration, the excitation force is sensitive to both Re and vibration parameters, and the added mass coefficient sensitivity to vibration decreases at high Re. The maximum vortex-induced-vibration (VIV) amplitude can reach 3.0 times the diameter. We confirm that current prediction theory at lower Re are not applicable to the full-scale one, underscoring the effect of Re on hydrodynamic inputs.

The interaction of a pair of tandem bubbles is investigated rising in elastic yield stress fluids. The bubble radii have a wide range of relative sizes in 3.6𝑚𝑚 ≤ 𝑅 ≤ 16𝑚𝑚. Three distinct patterns are predicted: bubble approach, bubble separation and formation of an equilibrium distance. The latter appears when material elasticity comes into play. A distorted negative wake is formed and sustained behind the leading bubble, with two stagnation points, repelling the trailing bubble and maintaining the bubble distance. Maps of the trailing bubble radius versus bubble radii ratio are presented showing the three patterns for various initial separation distances and material properties.

This study examines the interaction of two spanwise, transversally injected jets with a supersonic crossflow using experimental and numerical methods. The results reveal that smaller jet spacings suppress crossflow entrainment into the inter-jet region and intensify the interaction between the upstream bow shocks generated by each jet, resulting in larger pressure jumps in the passage between the jets. Conversely, larger jet spacings promote greater crossflow entrainment and enhance mixing efficiency, driven by more pronounced vortex interactions between the jets. Computational fluid dynamics (CFD) simulations also reveal shear layer unsteadiness, which induces shock oscillations.

The rebound of liquid droplets falling on a hot substrate, known as the Leidenfrost phenomenon, is a well-documented and widely studied topic. However, research on the rebound of droplets on cold surfaces, particularly regarding the inverse Leidenfrost phenomenon, remains relatively limited. In this paper, we experimentally investigate the rebound dynamics of droplets on dry ice surfaces, explore the influencing factors of the inverse Leidenfrost phenomenon, and develop a theoretical model to predict its occurrence.

Polymer flooding is a popular method for enhancing oil recovery in the field of oil extraction. Experimental studies have shown that during injection, polymers experience significant viscosity loss due to shear degradation. To address the issues, a method of encapsulating polymers by synthesizing micro-nano capsules is proposed. The variation of viscosity of capsule suspension during the process of capsule rupture and polymer release are explored in details and an available law that relates suspension viscosity is established.

This study elucidates the origin of traveling waves observed on the lower surface of a levitating droplet rolling on a rotating cylindrical drum. The research begins with a simplified model of the lubrication flow beneath the droplet and examines the linear stability of this base state to Tollmien–Schlichting-type perturbations. By solving the Orr-Sommerfeld equation perturbatively, the study predicts the wavelength and phase velocity of the most unstable mode, yielding good agreement with experimental observations.

A new and interesting phenomenon is found during the interaction between cavitation bubbles and elastic membranes: The collapse of the spark-induced bubble generates a high-speed jet. When the jet impacts the elastic membrane, significant membrane deformation occurs, accompanied by secondary cavitation. To analyze the mechanism of the secondary cavitation, fluid acceleration is introduced through PIV experiments to define the dimensionless inertial force. Secondary cavitation is triggered when the dimensionless inertial force surpasses the dimensionless pressure difference.

In general, there are no stable multiple-bubble states in dispersed two-phase flows. Instead, separation and coalescence are the prevalent types of long-term behaviors. We show that the introduction of a small elevation along the centreline of a Hele-Shaw channel allows groups of propagating bubbles to arrange themselves into a wide variety of stable multiple-bubble states. The bubbles are arranged in alternation on opposite sides of the elevation and propagate steadily with fixed separations. The states are all led by the smallest bubble, whilst the trailing bubbles can be arranged in any order. The interchangeability of the trailing bubbles results in a factorial increase in the number of such states as the number of bubbles increases.

Motivated by the application of spot-fire spread, this experimental study explores how various wildfire-relevant flow properties impact particle transport. We used model firebrands and a model wildfire system to examine the role of plume momentum and buoyancy, canopy-induced turbulence, and canopy wake dynamics on the transport and landing of inertial particles. Our findings suggest that both plumes and canopy turbulence significantly impact the mean transport and dispersion of the particles. Our study emphasizes the need to better parameterize these flow properties in firebrand transport models.

In this work, the results of a linear stability analysis of air-water stratified flows in horizontal circular pipes are presented. The computed stability boundaries are found to compare well with available experimental data. The stability analysis considered three-dimensional infinitesimal perturbations of all possible wavelengths and took into account deformations of the air-water interface. Comparing stability boundaries obtained in pipe, square duct, and two-plate geometries, it is shown that there are cases where the simplified geometry of two parallel plates can be useful to model the stability boundary in a realistic geometry reasonably well.

This study uses deep reinforcement learning to design airfoil pitch control for minimizing lift variations in disturbed flows. Tested in both classical unsteady and nonlinear viscous flow environments, the reinforcement learning controller, enhanced with wake information from pressure sensors and memory of past observations, matches or exceeds the performance of traditional linear controllers. The findings highlight the potential of reinforcement learning for improved aerodynamic control during random disturbances.

This figure displays the iso-surfaces of a turbulent puff’s azimuthal velocity. We have demonstrated that azimuthal motion is the primary cause of triggering the onset of turbulence, a chaotic state. Puffs abruptly break out of a chaotic state at Reynolds numbers Re < 1870, then decay exponentially. We found that the decay rate is entirely consistent with the cubic expression Sreenivasan used in 1979, but only after adding a constant.

The Rayleigh-Plateau instability occurs when a liquid film flows down a cylindrical surface. Using long-wave theory and stability analysis, we show that this instability can be modulated by coating the cylindrical surface with a thin layer of soft solids. In particular, we find that the elasticity of the soft layer can cause the film flow from being absolutely to convectively unstable. Our findings are also verified by transient numerical solutions of the full asymptotic model.

Only under certain conditions does a drop falling onto a bath entrap an air bubble. We propose a phenomenological law that describes these bubbling conditions in terms of Froude, Weber, and capillary numbers.

Through numerical simulations we reveal the three phase flow as a newtonian droplet comes in contact with an immiscible viscoelastic liquid film. The droplet dynamics becomes insensitive to the film height when the viscoelastic effects dominate. A viscoelastic ridge forms at the moving contact line, which evolves with a power-law dependence on time.

We present an open-source Direct Numerical Simulation framework to analyze surfactant-laden flows. With adaptive mesh refinement and parallelization, this tool enables researchers to explore the effects of surfactants on interfacial flows, particularly their impact on rising bubbles. The simulations show that surfactants slow down bubbles and alter their trajectory. Such numerical frameworks on the solutal Marangoni effect are crucial for understanding and predicting the behavior of multiphase flows in natural and industrial processes.

This paper describes how the drainage of capillary tubes in the presence of surfactants drive spontaneous thin film climbing events which are limited by the competition among advection, diffusion, and adsorption/desorption kinetics.

We investigate the role that vapor diffusion plays in the evolution of an evaporating liquid film using a coupled liquid-vapor system in which the evaporation rate is dictated by both the film’s thickness and its curvature. Under this kinetic-diffusion model, the thermocapillary Marangoni effect is split into two distinct components: the first results from surface tension gradients driven by uneven heating while the second arises from surface tension gradients caused by imbalances in vapor diffusion. Notably, these two components interact with evaporative mass loss and vapor recoil in a rich and complex manner, which we analyze within the temporal and spatiotemporal frameworks.

Evaporation on rough or structured surfaces is of interest from both fundamental and practical points of view. In this work, we consider volatile droplets and rivulets with contact lines pinned to wedge-shaped elements of the substrate topography. We develop a local analytical model of the evaporation-induced flow in the vicinity of the contact line in both liquid and gas phases, analyze the influence of gas viscosity as well as of an opening angle of the wedge on the flow patterns, and obtain critical contact angles, above which the eigenmode solution dominates the evaporation-induced solution.

The way a foam flows is dependent on the surfactant used. Why? We use numerical simulations of an elementary bubble rearrangement to resolve at the bubble scale the surfactant distribution and dynamics. We rationalize the results by extending the classical Lucassen model from a thermodynamic perspective. Our framework reveals the origin of surfactant-induced dissipation in sheared foams and is applicable to other surfactant-controlled systems.

In fluids with broken parity and time-reversal symmetry, odd viscosity introduces the possibility of unique behaviors such as lift force on an obstacle in a symmetric geometry. As incompressible odd viscous fluids are known to not show lift on cylinders under no-slip boundary conditions, this study examines the effect of a finite slip length. While it follows from the Lorentz reciprocal theorem that lift does not arise at first order in slip length, we find that lift does arise at second order upon solving for the fluid profile explicitly. Extending to Oseen flow, we derive a fluid profile that offers new insights into lift force on a cylinder at low Reynolds numbers.

The traditional Kozeny-Carman formulation does not predict the permeability of complex porous structures. We develop an original model for the characterization of the intrinsic permeability of porous media with spatially heterogeneous pore size distributions. By conceptualizing the medium as a collection of smaller-scale porous units in series, our model captures spatial variability and aligns with microfluidics experiments on designed complex structures. Our model offers a fresh perspective beyond the traditional Kozeny-Carman formulation, enhancing our understanding of how pore size variability influences the overall medium permeability.

Dense granular mixtures consisting of particles of different sizes tend to segregate based on size during shear flow, yet predicting the evolution of the composition of a granular mixture in general geometries remains a challenge. This paper systematically studies a key aspect of the three-dimensional nature of segregation and diffusion in dense, bidisperse granular mixtures: segregation and diffusion acting along the direction perpendicular to the plane of shearing, referred to as the anti-plane mode. We utilize discrete-element method simulations to inform, calibrate, and test constitutive equations for the segregation and diffusion fluxes in their anti-plane modes.

We investigate breaking and fragmentation of a floating particle raft by water waves. These laboratory experiments study the fragmentation of a two-dimensional floating solid by surface waves, a situation that also occurs for sea ice. We simultaneously observe oblique fractures on the intact part of the raft and polygonal fragments of different sizes. Observed from above, the graphite raft appears dark and the water white. The wavelength is very large in front of the raft thickness. The image size is 20x20 cm² and the time 114 s after the onset of the waves.

To find a blow-up solution to the Euler equations in an infinite domain is related to the zero viscosity limit of the Clay Institute’s sixth Millennium Problem. Evidence of a point-like, self-similar, blow-up solution occurring in an axisymmetric flow with infinite domain is provided by solving a nonlinear eigenvalue problem for a self-similar velocity with an unknown exponent. An angular expansion is used to map the problem into an infinite hierarchy of ODEs, which shows a convergent scenario of solutions for different truncations. This approach suggests an exponent of approximately 2. This solution contrasts with the previously found annular blow-up at a cylindrical domain boundary, which exhibited an exponent of 2.91.

Turbulence is by definition an out-of-equilibrium phenomenon, with energy flowing continuously from the large, input scales, to the small, dissipative ones. Temporal fluctuations in this energy flux produce corrections to the Kolmogorov energy spectrum which can be predicted using a multiscale perturbative approach. Here we investigate this problem in the Surface Quasi-Geostrophic model of turbulence, comparing the theoretical predictions with the outcome of high-resolution direct numerical simulations.

From Townsend’s attached-eddy model (AEM), the scalings of velocity moments of attached eddies have been widely investigated in literature. We derive analytically the scalings of the moments of velocity gradients of attached eddies by using the AEM, indicating -2 and -3 scalings for second- and third-order moments respectively, which are in agreement with direct numerical simulation (DNS) data. In addition, non-negligible influences of the small-scale eddies on the velocity gradients in the logarithmic region are discussed.

To study the evolution of large-scale helical structures in natural and engineering flows, we investigate the dual-channel characteristics of large- scale helicity transfer in compressible turbulent flows theoretically and numerically. Theoretical analysis and well-resolved direct numerical simulations indicate an alternative route to sustain the large-scale helical structures, through the cross-scale interaction at inertial scales and inverse dissipation at small scales under the expansion conditions.

A universal energy cascade is studied for incompressible ferrofluid turbulence by means of exact relations. Under weak external magnetic field, kinetic and total energy cascades occur at similar rates. Upon increasing the strength of the external magnetic field, the total energy cascade becomes nonstationary and occurs at a rate different from that of the kinetic energy cascade. However, the scale independent nature of the cascade remains universal.

This study investigates the focusing of inertial waves (IW) generated by an axisymmetric torus oscillating in a rotating fluid. A full range of vertical kinetic energy propagation angles at the focal point was explored using direct numerical simulations (DNS). A systematic comparison was made between linear DNS and nonlinear DNS. It was found that there is an optimal angle that maximizes energy transfer from the torus to the focal zone. In addition, triadic IW resonances were identified as a source of turbulence and a large central vertical vortex was also identified in agreement with the theory of Davidson et al. (2006).

Reynolds-averaged Navier—Stokes (RANS) closure operators are generally nonlocal and anisotropic, for example in wall-bounded turbulence. We introduce a computationally efficient approach to obtain these operators, using an adjoint formulation. We then quantify the streamwise and wall-normal nonlocal eddy viscosity in turbulent channel flow, which can be used to guide closure modeling.

This study proves the effectiveness of deep reinforcement learning (DRL) as a valuable tool for addressing complicated active flow control challenges, especially when employing flow fields characterized by strong nonlinearity and various wind attack angles. It demonstrates that employing multiple jets and surface pressure probes can achieve an ideal control performance, effectively diminishing aerodynamic forces and optimizing flow stability around the square cylinder under different wind attack angles. These findings enhance the potential for the practical application of DRL-based flow control strategies in engineering, and further progress toward real-world applications.

We specifically focus on the terms responsible for decay of the Reynolds stresses which can be isolated and evaluated separately from other terms in a canonical setup of homogeneous turbulence. We show that by using anisotropic forcing of the momentum equation we can access states of turbulence traditionally not probed in a triply-periodic domain. We then consider a variety of model forms for which these data allow us to perform a robust selection of model coefficients and select an optimal model that extends to cubic terms when expressed in terms of the principal coordinate Reynolds stresses.

Based on the analytic homotopy analysis method, steady-state interfacial waves exhibiting class-III triad resonance are obtained for the first time in a two-layer liquid with a free surface. A parameter study examines the influences of nonlinearity (wave steepness) and upper layer thickness (vertical distance from sea surface to density transition layer) on the amplitudes of wave components, wave spatial profiles, and energy distributions of interfacial waves with multiple resonances.

Modulational instability is the major energy transfer mechanism between ocean surface waves in deep water. In this work we explore the effects of nonuniform damping, such as that encountered by waves propagating through sea ice, on this important instability. We relax common assumptions about narrow spectral width but are nevertheless able to capture the dynamics of the unstable triad of waves using dynamical systems techniques. We elucidate the differences between uniform and nonuniform damping and explore the consequences for subsequent spectral broadening.

Experimental efforts to corroborate the theoretical Kolmogorov-Zakharov spectrum of surface gravity wave turbulence (WT) have encountered obstacles in the form of finite-size effects and intermittency. We investigated whether the consequences of these dynamics could be outcompeted by rigorously enforcing the assumption of WT in an idealized environment, in which we supply isotropic forcing and random-phased waves. We find that even under these conditions the wave field is modulated by dissipation and intermittency, which we study with higher-order statistics. Nevertheless, we do observe evidence of a wave-driven energy cascade beneath the strongly nonlinear and dissipative effects.

Floating solar panels installed on water reservoirs are gradually becoming an increasingly popular renewable energy scenario. When the reservoir gate is opened to release water, complex interactions between an incoming current and the floating panels will occur. In this paper, by modeling the entire floating panel structure as a thin elastic plate, a mathematical model based on the linearized potential flow theory is established to investigate such interactions. Extensive analyses are conducted on the wave profile and plate deflection, revealing significant fluid resonance phenomena at certain current speeds.

Accurate prediction of wave interaction with a floating elastic plate of small thickness has considerable significance in ocean engineering. We propose a hybrid boundary element and finite difference method for a plate of arbitrary shape. It is found that the plate has an obvious effect on the three-dimensional wave fields, and a large part of the disturbed wave appears around the direction of the incident wave. Like an elastic plate in vacuum, resonant behavior can occur for a plate floating on water. But, the peaks at the natural frequencies will be very large, instead of infinity, due to the wave radiation damping.

Laboratory experiments are performed by setting up a finite depth nonuniform stratification profile with a pycnocline, and generating internal wave modes from one end of the tank using an internal wave generator. We present the first experimental evidence of superharmonic internal wave modes trapped in a pycnocline, which are excited by the resonant triad interaction of two internal wave modes at the same frequency.