We show that a microswimmer trapped thermodynamically at fluid-fluid interfaces experiences a torque arising from the hydrodynamic boundary condition at the interface. This turns force dipoles corresponding to pullers perpendicular and pushers parallel to the interface. We demonstrate that in the general case, when there is a viscosity contrast across the interface, the steady state orientation is given by the interplay between the torques arising from the force dipoles and the self-propulsion which is sensitive to the viscosity difference between the two fluids.

Non-Newtonian fluids are found in many applications related to porous or fractured media. At a macroscopic scale, inelastic non-Newtonian fluid obeys a nonlinear Darcy’s equation. This paper show thats the solution of such a nonlinear equation in a heterogeneous permeability field obeys a minimum principle similar to the minimum dissipation principle of Stokes flow.

Knowledge of the onset of turbulence in particle-laden pipe flows is important for a range of practical applications. Therefore, in the current study we consolidate both existing and new experimental data to investigate the exact role of size and volume fraction of the suspended particles on the stability of suspension flows. By introducing a new parameter, based on the particle-to-pipe diameter ratio and volume fraction, a wide range of particle-laden flows are united on one single curve, being a function of the suspension Reynolds number. Moreover, this parameter allows us to distinguish between the different transition mechanisms: classical, intermediate, or particle-induced.

The flow of liquid food bolus in different intestinal contraction regimes is studied experimentally, analytically, and numerically. We show that a particle subjected to a peristaltic wave has a nonintuitive propulsion-reflux motion. When multiple waves are generated sequentially, as happens in the gut, reflux is found to be maximized for an inter-wave length corresponding to that observed physiologically in animals, indicating a possible evolutionary bolus absorption optimization. We find that counter-propagating waves generate a high-pressure region from which high-velocity bolus jets emerge. As a result, these waves generate 80 times more mixing than waves going in the same direction.

We perform simulations of propagation dynamics of cellular detonations in disturbed media. Upstream density variations impose periodic oscillations of the average leading shock pressure and detonation cell scales. Mode-locking of detonation dynamics still occurs in cellular detonations both for regular and irregular detonations under a large disturbance wavelength.

Spectroscopic measurements form the basis of understanding material properties characterized by the linear susceptibility. In this paper, the authors discuss how such linear susceptibilities are affected in the presence of odd viscosity. The authors also discuss a natural framework where odd viscosity arises due to the presence of injected torque in a fluid with a spin degree of freedom.

When subjected to a radial temperature gradient, Taylor-Couette flows undergo evolution of flow states with increasing rotations: a first transition from asymmetric vertical convection to spiral vortices, and a second transition to turbulence characterized by Taylor vortices. In the latter turbulent flow state, the radial transport of heat and angular momentum exhibits a similar power-law scaling. Our data analysis of the spatial distributions of the flux densities reveals mutual turbulent structures through which both heat and angular momentum are transported efficiently.

We investigate the influence of thermal, mechanical, and magnetic boundary conditions (BCs) on convective dynamos in a rapidly rotating plane fluid layer using direct numerical simulations. While the velocity BCs largely control whether large-scale flows and fields are generated, the magnetic BCs affect the magnetic field topology. The role of the thermal BCs is of note: For no-slip boundaries, the Nusselt number increases significantly when a fixed heat flux is imposed instead of a given temperature difference. We explain this effect, which applies to both dynamos and nonmagnetic, rotating convection, by an interplay of Ekman pumping and the internal structure of the thermal boundary layer.

As sea ice grows from sea water, salt initially dissolved in the liquid is rejected from the solid. Buoyancy-driven flows develop under the ice layer in two ways: 1) interfacial boundary layer convection resulting in small-scale fingers and 2) internal convection originating from brine drainage channels inside the ice, flushing out longer-scale convective streamers. We study these dynamics experimentally by freezing salt water from above in a quasi-2D Hele-Shaw cell, observing with Schlieren and direct imaging systems. Interfacial fingers turn out more significant as a salt-transport pathway than previously thought, persisting throughout ice growth, whereas streamers show on-off behavior.

Convection is the primary driver of magnetic fields in planets and stars. This self-generated magnetic field can then react back on the underlying convection by influencing its structure and dynamics. Numerical simulations are used to explore the convective dynamics in the presence of an externally imposed magnetic field that has a component perpendicular to gravity. New flow regimes are identified and their quantitative behavior is analyzed.

We show that fixed points and their stable manifolds determine collision outcomes of oppositely charged water droplets settling in still air. For droplets with large enough opposite charges, the stable manifold of a saddle forms a separatrix that divides colliding trajectories from those that do not collide. A consequence of our findings is that for large charges, collision outcomes are not sensitive to the breakdown of hydrodynamics at small distances.

When studying multiphase flows, it is customary to add a dye to one of the phases to enhance the contrast between different phases. However, many dyes have surface-active effects which, while minute in stationary measurements, can have a significant influence on interfacial phenomena out of equilibrium. In this study, we quantify the consequences of dye addition for the Marangoni Bursting phenomenon, and we offer some insights on the dramatic effects caused by the dye. Furthermore, we show two different, straightforward approaches to quantify the contrast enhancement gained through dye addition.

Three-dimensional electro-thermo-convective flow between two parallel plates under a simultaneously applied voltage and temperature difference is studied with linear stability analysis, direct numerical simulations, and computation of local entropy generation. An infinitesimal random perturbation is found to grow into a rolls pattern, then partially break up into polygons, and finally evolve into hexagons. The rolls pattern always has a larger total entropy generation and mean-square temperature gradient than the hexagon pattern, indicating that the rolls pattern is relatively more stable under these conditions.

We present an experimental study on controlling electrified jet instabilities via imposing orthogonal perturbations. A steady helicoidal whipping structure is achieved for the first time in air ambience, which is of significant importance in producing uniform fibers using electrospinning. The flexibility of orthogonal perturbations in output also allows more types of perturbation patterns following Lissajous curves, which demonstrate great potential in applications such as film deposition.

We explore the nature of gravity and conservative turbidity currents perturbed from their self-similar state and, in particular, we elucidate the cyclic sequence that universally arise in these rapidly-varying regimes. This sequence comprises four states determined by the bulk Richardson number and the acceleration/deceleration of the flow, and its universal nature is supported by six highly resolved direct and large eddy simulations. We study the details of this complex nonmonotonic rapid evolution and explore the how and why of this unique behavior.

We present a model of the draw resonance instability for glass fiber drawing including inertia, gravity, surface tension, and temperature. Using linear stability analysis, we have evidenced, through an alternative scaling, the crucial role of the fiber aspect ratio as a control parameter. It appears that a strong destabilization of the system occurs as this parameter increases. We also show the significant influence of nonhomogeneous ambient temperature on system stability. Contrary to the film casting problem, the high critical draw ratio in industrial applications could be rationalized only for a heat transfer coefficient dependent on both the velocity and cross-sectional area of the fiber.

We examine the flow configuration in a Hele-Shaw cell, where, initially, the cell is filled with a viscous fluid and then displaces it with other viscous fluid-carrying nanoparticles through the inlet boundary. How such nanoparticles modulate viscosity and impact the miscible viscous fingering dynamics is analyzed, showing the nonmonotonic nature of the onset of nanoparticle diffusion instability. The results provide a basis for controlling the viscous fingering instability using nanofluid displacement processes by delivering fluid-carrying nanoparticles to $in$ $situ$ contaminated areas. Examples are nanoparticle-based drug delivery and soil and groundwater remediation.

Bendotaxis has recently been proposed as a mechanism for self-transport of droplets at small scales. When an active droplet undergoes self-transport via bendotaxis, interfacial, elastic, and active forces jointly determine the droplet motion in a deformable channel. Through simulations of thin-film dynamics and a model reduction based on Onsager’s variational principle, we show that wettability and activity can jointly operate to enhance or weaken the self-transport effect of bendotaxis, depending on the sign of wettability (hydrophobic or hydrophilic) on the channel wall and the sign of activity (contractile or extensile) in the droplet.

A free-slip air-water interface with a fixed contact line boundary condition, when oscillating at certain frequencies with a large amplitude, can efficiently induce a fast-speed, far-propagating streaming jet. The extraordinary characters of the streaming jet can be employed to address engineering challenges across multiple disciplines.

Telescoping boxes are widely used to store and transport, e.g., board games, yet knowledge of the physical processes relevant to the end-user experience is currently unavailable. We combine observations on real product packaging with low-Reynolds-number theory and controlled experiments. Three distinct categories of lid motion are identified, controlled by flow in a thin film of air in the gap separating the lid and the base of the box. Finally, the optimal box design that combines the antagonistic criteria of safety and speed is identified.

The global frequency selection of two-dimensional vortex shedding in the flat plate wake is investigated experimentally. By performing a local stability analysis, and low-dimensional modeling based on the time-averaged mean flow velocity profiles, we have shown that the time-averaged velocity profiles give an accurate estimate of the global shedding frequency. Furthermore, we have analyzed the interaction strengths between the mean flow and the higher harmonics thereby experimentally supporting the theoretical criterion outlined previously by Sipp and Lebedev.

Subaqueous granular collapse on an inclined plane was investigated numerically in densely packed conditions by using a multiphase model. A new set of laboratory experiments were performed to validate the multiphase model with four different particle sizes (from fine sand to very coarse sand). The simulated results reveal that both the volume of the sliding mass in the early stages (initial sliding volume) and the front speed increase with increasing particle size. Additionally, increasing the initial concentration reduces the initial sliding volume and front speed.

We observe a new type of pattern formation at the interface between fluid and heavy granular medium in a horizontal rotating cylinder. Gravitational force perturbs the surface of the granular medium and induces azimuthal motion of suspended granules relative to the rotating fluid. The analysis of the experimental data shows that the length of the observed ripples is consistent with the predictions of the theory of the Kelvin-Helmholtz instability.

The roles of inertia (Re) and particle aspect ratio (Ar) on the rheology of elliptical particle suspensions are investigated. Scaling trends are found between the viscosity and the particle alignment for any Re considered here. Different mechanisms of stress are calculated. Besides the major contribution of stresslet, Reynolds stress contributes more as Ar and Re increase.

In this work, we show how to translate known inertial effects for non-motile organisms to motile ones, from passive to active particles. The method relies on a principle used earlier by Legendre and Magnaudet (1997), to deduce inertial corrections to the lift force on a bubble from the inertial drag on a solid sphere.

Inertial particles can interact in two ways with carrier-phase turbulence, exchanging mass, momentum, and energy. This study proposes a wavelet multiresolution framework that analyzes spectral energy transfer involving two phases and multiple scales simultaneously. Its application to preferentially concentrated particle-laden turbulence shows the role of particle clusters in spectral energy transfer as well as the physical consistency of the subgrid-scale (SGS) Stokes number useful to analyze and develop an SGS model for two-way coupled particle-laden turbulence.

We use a simple linearized Navier-Stokes-based model to study the large-scale quasistreamwise rolls in unstably stratified turbulent channel flows. As the influence of buoyancy-driven mechanisms (relative to shear-driven mechanisms) increases, channel-wide structures with two-peaks in the intensity of temperature emerges from the linear model, consistent with direct numerical simulations.

A Mach 1.45 shock and subsequent reshock interacting with a high Atwood number interface between sulfur hexafluoride and air is studied with an adaptive mesh simulation with more than 4.5 billion cells. Mechanisms governing the variable-density flow after the shocks’ interactions with the interface are analyzed with transport equations. The figure shows the mole fraction fields in the numerical shock tube around the interface just before (left) and after (right) the reshock. Red and blue colors represent heavier and lighter fluids, respectively. The reshock deposits baroclinic vorticity at both large and small scales and thus rapid breakdown to fully developed turbulence ensues.

The generalized log law for the high-order moments of longitudinal velocity with distance from a boundary in the inertial region is derived from the following assumptions: that the random sweeping decorrelation hypothesis applies; that the velocity statistics are near-Gaussian; and that the longitudinal velocity spectrum scales as $k^{-1}$ in the intermediate region. Measurements of longitudinal velocity collected within the first meter from the surface in the western deserts of Utah show good agreement with the proposed theory even under mild thermal stratification.

Using direct numerical simulations of homogeneous bubbly flows, an analysis of velocity fluctuations is performed and a methodology for development of a bubble-induced agitation (pseudoturbulence) model described. This process is based on separating two causes of velocity fluctuations in the liquid: Agitation resulting from wakes and their collective interactions; and nonturbulent fluctuations due to averaged wakes and potential flows around bubbles. An energy conversion signature is observed, revealing the importance of nonlinear interactions. A model is proposed which gives satisfactory results on our database for a wide range of bubble Reynolds numbers and is consistent with experiments.

We consider fundamental limitations on the performance of eddy-viscosity closure models for turbulent flows focusing on the Leith model for two-dimensional Large Eddy Simulation (LES). Optimal eddy viscosities depending on the vorticity gradient magnitude are determined subject to minimum assumptions by solving partial-differential-equation-constrained optimization problems defined such that the corresponding optimal LES best matches the filtered Direct Numerical Simulation. Since the optimal eddy viscosities do not converge to a well-defined limit as the regularization vanishes, we conclude that the problem of finding an optimal eddy viscosity does not have a solution and is ill-posed.

Originating from the generalized Reynolds analogy theory, an empirical scaling for the central mean temperature in compressible turbulent channel flow with symmetric isothermal boundaries is proposed. The empirical scaling is quite accurate and most of the relative errors are below 1.5% as assessed by available direct numerical simulation data at various Reynolds and Mach numbers. The mean temperature profile can be quantitatively obtained through the mean velocity with empirical scaling.

In this paper three-dimensional direct numerical simulations (3-D DNS) on vortex-induced vibrations of an elastically mounted circular cylinder near a stationary wall at a subcritical Reynolds number of 500 and a gap ratio of 0.8 are conducted. It is found that the three-dimensionality increases linearly with amplitude, leading to substantial variations in the vortex dynamics. The interactions of the vortices with the wall-generated boundary layer play significant roles in altering the cylinder vibration.

Accurate representation of the near-wall flow physics at high Reynolds numbers is computationally prohibitive for many uniform resolution turbulence models. Therefore, a closure with spatially varying resolution is sought which seamlessly transitions from low resolution RANS near-wall to high-resolution PANS in the outer region. This new modeling strategy systematically accounts for the energy exchange terms between the resolved and unresolved scales in the region of resolution change. By ensuring energy conservation and equilibrium boundary layer (EBL) scaling, the model accurately captures the flow behavior near-wall for turbulent channel flow at high Reynolds numbers.

Controlling flow-induced fluttering with morphing surfaces has long been a practical solution. However, the background knowledge of how the structure interacts with the flow has been lacking. Using a tightly-coupled fluid-structure interaction algorithm that utilizes conformal mapping techniques to provide geometrical weighting for multiscale decomposition, we isolate the dynamic effects caused by the surface motion and the flow. The energy ratio between the two effects is proven to be a good indicator of the control efficacy of the morphing flap.

During landing flights, birds often perform a perching maneuver, which allows them to land smoothly. In this work, we investigated the effect of wing sweep on the evolution of the instantaneous forces and the flow field during the perching maneuver. Our results indicate that swept wing generates higher aerodynamic forces, which is contributed by a stable leading-edge vortex (LEV).

The interaction between vortex rings and walls widely exists in natural phenomena and engineering practices. In this paper, we experimentally explore synthetic jet vortex rings impinging on a spherical wall. The impingement behavior is revealed in detail by vortex ring evolution and trajectories. An important observation is that driven by the curve, the strength of the induced vortex ring increases with the decrease in the sphere diameter.

Axisymmetric rectilinear vortex tubes develop twist waves if their core size varies along the centerline. These twist waves propagate, and when two opposite-signed twist waves meet their interaction leads to vortex bursting. We perform numerical simulations of vortex tubes with a range of initial core-size perturbation amplitudes, and provide a detailed analysis of the bursting dynamics. Our results explain the main mechanisms driving and subsequently arresting the bursting process; quantify the accelerated energy dissipation due to bursting; and show that bursting is susceptible to non-axisymmetric instabilities that further accelerate the decay of the vortex tube.