We study experimentally the compaction dynamics of a fully immersed granular packing subjected to a slow upward flow. We consider continuous flows as well as discontinuous flows featuring short flow bursts that act as flow “taps.” We monitor the compaction rate of initial packings prepared by settling, and we find that compaction is possible for a continuous flow with upward velocities below the fluidization threshold. Flow bursts are found strongly to enhance the compaction efficiency, especially when the bursts are above the fluidization threshold.

High-fidelity simulations are performed to study active flow control techniques for alleviating deep dynamic stall of an SD7003 airfoil in plunging motion. The flow Reynolds number is $\text{Re}=60000$ and the freestream Mach number is $\text{M}=0.1$. Numerical simulations are performed with a finite-difference-based solver that incorporates high-order compact schemes for differentiation, interpolation, and filtering on a staggered grid. A mesh convergence study is conducted and results show good agreement with available data in terms of aerodynamic coefficients. Different spanwise arrangements of actuators are implemented to simulate blowing and suction at the airfoil leading edge. We observe that, for a specific frequency range of actuation, mean drag and drag fluctuations are substantially reduced while mean lift is maintained almost unaffected, especially for a two-dimensional (2D) actuator setup. For this frequency range, 2D flow actuation disrupts the formation of the dynamic stall vortex, which leads to drag reduction due to a pressure increase along the airfoil suction side, towards the trailing edge region. At the same time, pressure is reduced on the suction side near the leading edge, increasing lift and further reducing drag.

We present simulations for the steady-shear rheology of a model adhesive dispersion in the dense regime. We vary the range of the attractive interparticle forces $u$ as well as the strength of the dissipation $b$. For large dissipative forces, the rheology is governed by the Weissenberg number $\text{Wi}\sim b\dot{\gamma }/u$ and displays Herschel-Bulkley form $\sigma ={\sigma }_y+c{\text{Wi}}^{\nu }$ with exponent $\nu =0.45$. Decreasing the strength of dissipation, the scaling with Wi breaks down and inertial effects show up. The stress decreases via the Johnson-Samwer law $\mathrm{\Delta }\sigma \sim T_s^{2/3}$, where temperature $T_s$ is exclusively due to shear-induced vibrations. During flow, particles slide past each other such that their relative velocities are primarily directed tangentially to the particle surfaces. This tangential channel of energy dissipation and its suppression leads to a discontinuity in the flow curve and an associated discontinuous shear-thinning transition. We set up an analogy with frictional systems, where the phenomenon of discontinuous shear-thickening occurs. In both cases, tangential forces, frictional or viscous, mediate a transition from one branch of the flow curve with low tangential dissipation to one with larger tangential dissipation.

The driven, cylindrical, free interface between two miscible Stokes fluids with high viscosity contrast have been shown to exhibit dispersive hydrodynamics. A hallmark feature of dispersive hydrodynamic media is the dispersive resolution of wavebreaking that results in a dispersive shock wave. In the context of the viscous fluid conduit system, the present work introduces a simple, practical method to precisely control the location, time, and spatial profile of wavebreaking in dispersive hydrodynamic systems with only boundary control. The method is based on tracking the dispersionless characteristics backward from the desired wavebreaking profile to the boundary. In addition to the generation of approximately steplike Riemann and box problems, the method is generalized to other, approximately piecewise-linear dispersive hydrodynamic profiles, including the triangle wave and $N$ wave. A definition of dispersive hydrodynamic wavebreaking is used to obtain quantitative agreement between the predicted location and time of wavebreaking, viscous fluid conduit experiment, and direct numerical simulations for a range of flow conditions. Observed space-time characteristics also agree with triangle and $N$-wave predictions. The characteristic boundary control method introduced here enables the experimental investigation of a variety of wavebreaking profiles and is expected to be useful in other dispersive hydrodynamic media. As an application of this approach, soliton fission from a large, boxlike disturbance is observed both experimentally and numerically, motivating future analytical treatment.

This paper studies the rise of a Taylor bubble in a long, vertical liquid-filled conduit such as the riser of an oil production or exploration well. It is shown that, as the bubble rises and expands due to the gradually decreasing hydrostatic pressure, a situation may be reached in which the character of the expansion abruptly transitions from moderate to very violent. The phenomenon can be particularly dangerous with very deep wells, in which a “gas kick,” i.e., the intrusion of gaseous hydrocarbons in the riser, can result in a violent and destructive “blowout.” As an example, with its expansion, a 1-m-long bubble rising in a 1000-m-long tube causes the liquid to spill out of the top of the tube with a velocity of the order of cm/s for the better part of an hour, until it rises to the order of 10 m/s in the last several seconds. This process is considered to have been responsible for the DeepWater Horizon accident in the Gulf of Mexico in 2010. The same phenomenon underlies volcanic eruptions of the Strombolian type, which are characterized by the intermittent ejection of material up to heights of tens or even hundreds of meters. In this paper the process is studied with a quasiequilibrium model, a dynamic model, and a drift-flux model, all of which give results in substantial agreement with each other. The quasiequilibrium model leads to a very simple relation which, while approximate, gives a robust criterion for the occurrence of violent bubble expansions. The results of an illustrative calculation with OpenFOAM are also presented.

When a sphere, suspended in an otherwise quiescent fluid, is rotated, the resulting flow exhibits a wide range of fundamental fluid physics, ranging from the collision of boundary layers through to the development of radial jets and subsequent absolute instabilities in the spatially and temporally developing flow. In this way, such a flow provides a paradigm for the study of instabilities in temporally and spatially developing boundary layers. There is a body of experimental work on this problem Y. Kohama and R. Kobayashi, J. Fluid Mech. 137, 153 (1983); T. Hada and A. Ito, Trans. Vis. Soc. Jpn. 23, 231 (2003) that suggests that the flow within the sphere's boundary layer can be unstable to spiral vortices for moderate rotation rates; however, other experiments F. P. Bowden and R. G. Lord, Proc. R. Soc. London A 271, 143 (1963); F. Kreith et al., Int. J. Heat Mass Transfer 6, 881 (1963); S. A. W. Calabretto et al. (unpublished) demonstrate that the flow over the sphere remains laminar until much higher rotation rates. We consider this problem from a computational perspective, seeking to understand how, when, and where such instabilities may develop. Our results show good agreement with the experiments of Kreith et al. and Calabretto et al. in that the flow over much of the boundary layer remains undisturbed and, additionally, the spiral vortex instabilities that develop in the flow are convective in nature.

A combined theoretical and experimental study is presented for the flow-induced compaction of a one-dimensional fibrous porous medium near its gel point for deformation at low and high rates. The theory is based on a two-phase model in which the permeability is a function of local solid fraction, and the deformation of the solid is resisted by both a compressive yield stress and a rate-dependent bulk viscosity. All three material properties are parameterized and calibrated for cellulose fibers using sedimentation, permeation, and filtration experiments. It is shown that the incorporation of rate-dependence in the solid stress significantly improves the agreement between theory and experiment when the drainage flow is relatively rapid. The model is extended to rates outside the range where it was calibrated to understand the dynamics of a standard test for pulp suspensions: the Canadian Standard Freeness test. The model adequately captures all of the experimental findings, including the score of the freeness test, which is found to be sensitively controlled by the bulk solid viscosity and to a lesser degree by the permeability law, but depends only weakly on the compressive yield stress.

Practically all flows are turbulent in nature and contain some kind of irregularly shaped particles, e.g., dirt, pollen, or life forms such as bacteria or insects. The effects of the particles on such flows and vice versa are highly nontrivial and are not completely understood, particularly when the particles are finite sized. Here, we report an experimental study of millimetric fibers in a strongly sheared turbulent flow. We find that the fibers show a preferred orientation of $-0.38\pi \pm 0.05\pi$ ($-68\pm 9^{\circ }$) with respect to the mean flow direction in high-Reynolds-number Taylor-Couette turbulence, for all studied Reynolds numbers, fiber concentrations, and locations. Despite the finite size of the anisotropic particles, we can explain the preferential alignment by using Jefferey's equation, which provides evidence of the benefit of a simplified point-particle approach. Furthermore, the fiber angular velocity is strongly intermittent, again indicative of point-particle-like behavior in turbulence. Thus large anisotropic particles still can retain signatures of the local flow despite classical spatial and temporal filtering effects.

We analyze the consolidation of freshly deposited cohesive and noncohesive sediment by means of particle-resolved direct Navier-Stokes simulations based on the immersed boundary method. The computational model is parametrized by material properties and does not involve any arbitrary calibrations. We obtain the stress balance of the fluid-particle mixture from first principles and link it to the classical effective stress concept. The detailed data sets obtained from our simulations allow us to evaluate all terms of the derived stress balance. We compare the settling of cohesive sediment to its noncohesive counterpart, which corresponds to the settling of the individual primary particles. The simulation results yield a complete parametrization of the Gibson equation, which has been the method of choice to analyze self-weight consolidation.

We investigate the impact of a solid-body rotation ${\mathbf{\Omega }}_0$ on the large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in presence of a background magnetic field ${\mathbf{B}}_{\mathbf{0}}$ and at low Rossby number. Three-dimensional direct numerical simulations are performed in a periodic box, at unit magnetic Prandtl number and with a forcing at intermediate wave number $k_f=20$. When ${\mathbf{\Omega }}_0$ is aligned with ${\mathbf{B}}_{\mathbf{0}}$ (i.e., $\theta \equiv \widehat{\left({\mathbf{\Omega }}_0,{\mathbf{B}}_{\mathbf{0}}\right)}=0$), inverse transfer is found for the magnetic spectrum at $k<k_f$. This transfer is stronger when the forcing excites preferentially right-handed (rather than left-handed) fluctuations; it is smaller when $\theta >0$ and becomes weak when $\theta \ge {35}^{\circ }$. These properties are understood as the consequence of an inverse cascade of hybrid helicity which is an inviscid/ideal invariant of this system when $\theta =0$. Hybrid helicity emerges, therefore, as a key element for understanding rotating dynamos. Implication of these findings on the origin of the alignment of the magnetic dipole with the rotation axis in planets and stars is discussed.

An important class of fluid-structure problems involves the dynamics of ordered arrays of immersed, flexible fibers. While specialized numerical methods have been developed to study fluid-fiber systems, they become infeasible when there are many, rather than a few, fibers present, and these methods do not lend themselves to analytical calculation. Here, we introduce a coarse-grained continuum model, based on local-slender-body theory, for elastic fibers immersed in a viscous Newtonian fluid. It takes the form of an anisotropic Brinkman equation whose skeletal drag is coupled to elastic forces. This model has two significant benefits: (1) the density effects of the fibers in a suspension become analytically manifest and (2) it allows for the rapid simulation of dense suspensions of fibers in regimes inaccessible to standard methods. As a first validation, without fitting parameters, we achieve very reasonable agreement with three-dimensional (3D) immersed boundary simulations of a bed of anchored fibers bent by a shear flow. Second, we characterize the effect of density on the relaxation time of fiber beds under oscillatory shear and find close agreement to results from full numerical simulations. We then study buckling instabilities in beds of fibers, using our model both numerically and analytically to understand the role of fiber density and the structure of buckling transitions. We next apply our model to study the flow-induced bending of inclined fibers in a channel, as has been recently studied as a flow rectifier, examining the nature of the internal flows within the bed, and the emergence of inhomogeneous permeability. Finally, we extend the method to study a simple model of metachronal waves on beds of actuated fibers, as a model for ciliary beds. Our simulations reproduce qualitatively the pumping action of coordinated waves of compression through the bed.

Three-wave resonant interactions constitute an essential nonlinear mechanism coupling capillary surface waves. In a previous work [Haudin et al. Phys. Rev. E 93, 043110 (2016)], we characterized experimentally the generation by this mechanism of a daughter wave, whose amplitude saturates due to the viscous dissipation. Here, we show experimentally the generation of a daughter wave verifying the resonant conditions, but not the dispersion relation. By modeling the response of the free surface at the lowest nonlinear order, we explain this observation as a forced interaction. The bandwidth of the linear transfer function of the free surface is indeed increased by the significant viscous dissipation. The observation of free surface excitations not following the linear dispersion relation then becomes possible. This forced three-wave interaction mechanism could have important consequences for wave turbulence in experimental or natural systems with nonnegligible dissipation.

We consider a swimmer consisting of a collinear assembly of three spheres connected by two slender rods. This swimmer can propel itself forward by varying the lengths of the rods in a way that is not invariant under time reversal. Although any non-reciprocal strokes of the arms can lead to a net displacement, the energetic efficiency of the swimmer is strongly dependent on the details and sequences of these strokes, and also the sizes of the spheres. We define the efficiency of the swimmer using Lighthill's criterion, i.e., the power that is needed to pull the swimmer by an external force at a certain speed, divided by the power needed for active swimming with the same average speed. Here, we determine numerically the optimal stroke sequences and the optimal size ratio of the spheres, while limiting the maximum extension of the rods. Our calculation takes into account both far-field and near-field hydrodynamic interactions. We show that, surprisingly, the three-sphere swimmer with unequal spheres can be more efficient than the equally sized case. We also show that the variations of efficiency with size ratio is not monotonic and there exists a specific size ratio at which the swimmer has the highest efficiency. We find that the swimming efficiency initially rises by increasing the maximum allowable extension of the rods, and then converges to a maximum value. We calculate this upper limit analytically and report the highest value of efficiency that the three-sphere swimmer can reach.

Many intriguing patterns are driven by fluid instabilities, even down to micrometer and nanometer scales, with numerous technological and industrial applications. Here, we propose a novel physical mechanism for filament formation in a stretched viscous sheet by combining stretching and van der Waals forces. We show that a transverse instability is enhanced while the longitudinal instability is suppressed, resulting in filamentation. We find a close agreement between the experimental data and the predictions of a linear-stability analysis. These results offer a pathway to achieving sophisticated microstructuring–nanostructuring for functional devices in either a single fiber or in integrated fabrics for large-scale textiles.

The study of thermocapillary driven flows is typically restricted to “open” systems, i.e., ones where a liquid film is bounded on one side solely by another fluid. However, a large number of natural and engineered fluidic systems are composed of solid boundaries with only small open regions exposed to the surrounding. In this work we study the flow generated by the thermocapillary effect in a liquid film overlaid by a discontinuous solid surface. If the openings in the solid are subjected to a temperature gradient, the resulting thermocapillary flow will lead to a nonuniform pressure distribution in the film, driving flow in the rest of the system. For an infinite solid surface containing circular openings, we show that the resulting pressure distribution yields dipole flows which can be superposed to create complex flow patterns, and demonstrate how a confined dipole can act as a thermocapillary motor for driving fluids in closed microfluidic circuits. For a mobile, finite-size surface, we show that an inner temperature gradient, which can be activated by simple illumination, results in the propulsion of the surface, creating a thermocapillary surface swimmer.

Experiments and simulations were carried out to contribute to a better understanding and prediction of high-pressure injection into a gaseous environment. Specifically, the focus was put on the phase separation of an initially supercritical fluid due to the interaction with its surrounding. N-hexane was injected into a chamber filled with pure nitrogen at 5 MPa and 293 K. Three different test cases were selected based on a priori calculations applying the adiabatic mixture assumption considering phase separation. The cases selected cover regimes in which real-gas effects and phase separation due to multicomponent mixing are likely to occur. Simultaneous shadowgraphy and elastic light scattering experiments were conducted to capture both the flow structure as well as the phase separation. In addition, large-eddy simulations with a vapor-liquid equilibrium model were performed. Both experimental and numerical results show phase separation for the identical cases. Moreover, qualitative characteristics of the formation process agree well between experiments and numerical simulations. The transition behavior from a dense-gas to a spraylike jet was captured by both.

In this investigation, we present numerical simulations of freely settling and rising cubes in a quiescent Newtonian fluid for various values of the Galileo number ($70⩽\text{Ga}⩽250$) and solid-to-fluid density ratio ($0.2⩽m⩽7$). Ultimately, we obtain a comprehensive two-parameter flow map for a freely moving cube and characterize prominent features of each regime of motion such as trajectories and wake structure. We find that regardless of the density ratio, the cube trajectory transitions from oblique to unsteady vertical, helical and finally random or chaotic. Unlike the case of a sphere, helical motion is observed for all density ratios, marking it as a characteristic type of motion for a cube. Furthermore, we present an in-depth force analysis relevant to the induced lateral motions, and we show that there is a significant jump in the drag coefficient coincident with the onset of the helical regime where large-amplitude lateral displacements appear. We attempt to explain these significant variations of the drag coefficient in connection with the path and vortex structures of the moving cube. Consequently, the enhancement of the drag coefficient is explained to be a combined effect of the vortex-induced drag and the orientation angle of the cube.

Convection in rotating spherical geometries is an important physical process in planetary and stellar systems. Using continuation methods at a low Prandtl number, we find both strong equatorially asymmetric and symmetric polar nonlinear rotating waves in a model of thermal convection in thin rotating spherical shells with stress-free boundary conditions. For the symmetric waves, convection is confined to high latitude in both hemispheres but is only restricted to one hemisphere close to the pole in the case of asymmetric waves. This is in contrast to what is previously known from studies in the field. These periodic flows, in which the pattern is rotating steadily in the azimuthal direction, develop a strong axisymmetric component very close to onset. Using stability analysis of periodic orbits, the regions of stability are determined and the topology of the stable/unstable oscillatory flows bifurcated from the branches of rotating waves is described. By means of direct numerical simulations of these oscillatory chaotic flows, we show that these three-dimensional convective polar flows exhibit characteristics, such as force balance or mean physical properties, which are similar to flows occurring in planetary atmospheres. We show that these results may open a route to understanding unexplained features of gas giant atmospheres, particularly in the case of Jupiter. These include the observed equatorial asymmetry with a pronounced decrease at the equator (the so-called dimple), and the coherent vortices surrounding the poles recently observed by the Juno mission.

This study investigates the influence of the surrounding gas on a droplet impacting a smooth dry glass surface at high Weber and Reynolds numbers. It was performed using a flywheel experiment and different gases at ambient pressure. We analyzed the splashing outcome by measuring the size, velocity, and angle of the secondary droplets and by calculating the total volume ejected. We show that gas entrapment is not the mechanism responsible for splashing at high Weber and Reynolds numbers. We demonstrate that splashing is influenced by the density, followed by the viscosity, and last by the mean free path of the surrounding gas. Furthermore, the surrounding gas primarily affects the number of secondary droplets ejected and their ejection angle, whereas the droplet size and horizontal velocity are independent of the surrounding gas properties. We provide the first theoretical expression for the total volume ejected using the theory of Riboux and Gordillo [Phys. Rev. Lett. 113, 024507 (2014)], which attributes the secondary droplet generation to a lift force experienced by spreading lamella. The relationship between the ejected volume and the splashing parameter is described by a power function.

To effectively apply the superhydrophobic surface for flow control, it is necessary to understand the effect of air-water interface shape on the diffusion characteristics of trapped air pockets. In the present work, using the reflection interference contrast microscopy, we directly measure the temporal variation of the three-dimensional shape of the air-water interface formed on submerged hydrophobic surfaces in a channel flow. The grate and the circular hole are considered a roughness feature, its size is varied between 80 and $200\mu \mathrm{m}$, and the Reynolds numbers of 1850 and 5000 are considered based on channel half-height and bulk velocity. The temporal variation of the height, the curvature of the air-water interface, and the corresponding diffusion length is measured, and the diffusion process is classified into three phases. For each phase, the diffusion characteristics are analyzed in relation to the roughness structure type, size, and Reynolds number. Based on the analysis, we propose a scaling relation for the Sherwood number (dimensionless diffusion length). For the first two phases when the interface is pinned at the roughness edge, the Sherwood number is determined by the interface curvature (i.e., the contact angle between the interface and the roughness sidewall), but it is scaled with the Peclet number (the ratio between the shear rate and the diffusion rate) for the last phase when the interface is depinned and moves downward. These relations are also validated with the data available in the literature, and we think this is useful as a simple guideline to design a roughness geometry for the superhydrophobic surfaces.