Long, flexible fibers transported by a turbulent channel flow sample nonlinear variations of the fluid velocity along their length. As the fibers tumble and collide with the boundaries, they bounce off with an impulse that propels them toward the center of the flow, similar to pole vaulting. As a result, the fibers migrate away from the walls, leading to depleted regions near the boundaries and more concentrated regions in the bulk. These higher concentrations in the center of the channel result in a greater net flux of fibers than what was initially imposed by the fluid. This effect becomes more pronounced as fiber length increases, especially when it approaches the channel height.

We are carrying out the experimental study of the emptying of a hermetically sealed cylindrical tank in a rotating frame. We determine the optimal rotational velocity for the tank, which minimizes the emptying time. Moreover, there exists a critical rotation velocity at which the emptying time exceeds that of the nonrotating case, depending on the relative duration of each of the three emptying regimes identified in this flow, namely, (i) a bubble regime, (ii) a vortical air jet regime, and (iii) a bathtub vortex regime. The three regimes, easily distinguished by the shape of the air-water interface, have different draining velocities. The lifetime of each regime, as well as the transitions between them, depend on both the water level and the rotating velocity of the platform.

Bubbles have always captivated our curiosity with their aesthetics and complexities alike. While the act of blowing bubbles is familiar to everyone, the underlying physics of these fleeting spheres often eludes reasoning. In this Letter, we discuss the dynamics of inflating a soap bubble using controlled airflow through a film-coated nozzle. We assess and predict the rate of inflation by varying the source pressure. Visualizing the previously unexplored internal flow reveals that air enters the bubble as a round jet, emerging from the nozzle opening and impinges on the expanding concave bubble interface to form a toroidal vortex. Several scaling laws of the associated vortical flow spanning the entire bubble and the vortex core are reported. The observed dynamics of this bubble-confined vortex ring formation indicate universality in certain aspects when compared to the free laminar vortex rings.

In this paper, we investigate the exit dynamics of a sphere launched underneath a liquid bath surface at a prescribed impact velocity. Spheres with radii approximate or smaller than the capillary length are considered. Following our previous work of a ligament drawn of a liquid bath [J. Fluid Mech. 922, A14 (2021)], a two-dimensional model is applied to describe the liquid dynamics, and the whole exit dynamics up to the descent or the pinch-off moments is considered. The process can be sequenced into a partial exit stage that forms a coated layer and a full exit stage with an attached ligament. A bouncing-off regime, a lower pinch-off penetration regime, and an upper pinch-off penetration regime are identified, separating by a penetration Weber number and a switching Weber number. The phase diagram is revealed, where the two critical Weber numbers are functions of the Bond number. By considering the energy evolutions, we show that the impact energy is mainly converted into the surface energy and the gravitational potential energy for the low- and large-gravity cases, respectively. The coated layer is mainly formed in the partial exit stage, whose maximum volume increases with the impact velocity but decreases with gravity effect. Stretching motion is shown to have negligible influence on the local pinch-off behavior, while it determines the appearance and the location of pinch-off. Our results can help to understand the exit behaviors of aquatic animals, and the design of microamphibious aircraft or energy collection devices.

We present a numerical study of a thin elastic sheet with small extensibility freely sedimenting in a viscous fluid. Two scenarios are investigated: sedimentation in free space and near an infinite wall, where the wall may be vertical or tilted. Elastic sheets with a rest shape of a square are modeled with a finite-element-based continuum model that accounts for in-plane stretching and out-of-plane bending. The fluid motion is computed by the method of regularized Stokeslets in free space and regularized Blakelets near a wall. During sedimentation, the interplay between gravity and the elastic response of sheets gives rise to complex deformation and reorientation dynamics, measured by a dimensionless elastogravitational number $\left({\mathrm{E}}_{\mathrm{G}}\right)$. In free space, sheets attain a stable orientation by aligning perpendicular to gravity. Sheets with larger deformability adopt more compact conformations and experience smaller hydrodynamic drag, thereby sedimenting faster. A sheet with a random initial orientation reorients to align perpendicular to gravity, accompanied by lateral drift due to the symmetry-breaking in conformations. We identified two reorientation mechanisms depending on flexibility. When a sheet is placed near an infinite wall, sedimentation is hindered compared to that in free space due to wall-induced hydrodynamic drag. Near a vertical wall, sheets exhibit asymmetric conformations that cause the sheet to drift, with the drifting dynamics determined by ${\mathrm{E}}_{\mathrm{G}}$. The difference in flexibility leads to a nonmonotonic trend in the evolution of wall-normal distance. Near a tilted wall, sheets show qualitatively different dynamics when the wall angle is large: they either deposit on or slide along the wall with a fixed wall-normal distance.

In this experimental study on a methane-hydrogen fired laboratory-scale pressurized annular combustor, various steady and transient operating conditions were explored. Under steady operation, the thermoacoustic stability depended strongly on the equivalence ratio, but only weakly on the mass flow rate and hydrogen power fraction. During transient equivalence-ratio ramps, the predominant instability of the system switched between standing and spinning azimuthal modes. The ramping rate influenced the thermoacoustic amplitude during the switching, as well as the onset and decay of the different modes. The strong acoustic reflection at the choked outlet produced significant harmonic content during both steady and transient operation. Nonlinear time-series analysis and clustering algorithms showed that the Jensen-Shannon complexity and the permutation entropy could accurately classify the modal dynamics in an unsupervised manner. The classification revealed that standing azimuthal modes, with several preferred orientations, dominated under steady operation. The amplitude of the heat-release-rate (HRR) oscillations around the annulus depended on the mode orientation. The energy contribution from the harmonic components was isolated via bandpass filtering, revealing that the orientation of the first harmonic ($n=2)$ followed the fundamental mode, eliminating negligible HRR oscillation regions at the pressure node of the fundamental mode. By filtering out the higher-order harmonic content, intermittent switching between high-amplitude periodicity and low-amplitude chaos in the fundamental azimuthal mode was uncovered. Tools from dynamical systems theory confirmed the existence of chaos and demonstrated that the intermittency belongs to the type-II Pomeau-Manneville class. This study provides evidence of the type-II intermittency route to chaos in an annular combustor with longitudinal and azimuthal modes.

While predicting the flow behavior for a well-defined particulate microstructure is fairly tractable, systematic tailoring of the particle properties to yield desired flows remains illusive. For example, we still do not know how to precisely control rheological response by tuning particle types and properties or how to tailor porous media for filtration applications. Here we show that the recent advances in automatic differentiation provide a platform for methodologically addressing such questions. We optimize the packing of a polydisperse system of periodically arranged circular rods to maximize flow rate in the direction of applied pressure gradient. We show that the optimum topology of the structured porous medium depends on the solid volume fraction and approaches a staggered square with the particles' diameter ratio of $\sqrt{2}-1$ as the solid volume fraction reaches $\pi /4[1+{(\sqrt{2}-1)}^2]$. Regardless of the porous medium topology, the divergence exponent of the pressure gradient is universal and is equal to $5/2$, as one finds from lubrication theory for a simple square lattice. These results are independent of the flow's Reynolds number and can provide a foundation to study inverse problems for suspension systems.

Here we discuss how the use of textured liquid-infused solids can expedite the dynamics of spreading of viscous drops, when the subjacent oil layer is less viscous. We characterize the nature and amplitude of the effect, and the laws governing the spreading at short time. More generally, our findings affirm the universality of the early dynamics of viscous contact, found to hold for drops that coalesce, spread, or even, like here, slip on their substrate.

The exploration of a two-dimensional wind-driven ocean model with no-slip boundaries reveals the existence of a turbulent asymptotic regime where energy dissipation becomes independent of fluid viscosity. This asymptotic flow represents an out-of-equilibrium state, characterized by a vigorous two-dimensional vortex gas superimposed onto a western-intensified gyre. The properties of the vortex gas are elucidated through scaling analysis for detached Prandtl boundary layers, providing a rationalization for the observed anomalous dissipation. The asymptotic regime demonstrates that boundary instabilities alone can be strong enough to evacuate wind-injected energy from the large-scale oceanic circulation.

A self-aligning particle (SAP) attains a permanent orientation without application of external torques in a low Reynolds number simple shear flow in contrast with the continuous rotation exhibited by most rigid bodies including thin fibers and disks. SAPs align close to the fluid lamellae, and we characterize the robustness of this flow alignment to secondary disturbances such as perturbation to the shear flow, Brownian motion, interparticle interactions, or the presence of a wall using dynamic simulations of representative SAP geometries. The robustness of the flow alignment of SAPs at all concentrations generates highly aligned microstructures inaccessible to traditional particle suspensions.

The presence of a dispersed phase substantially modifies small-scale turbulence. However, there has not been a comprehensive mechanistically based understanding to predict turbulence modulation. Based on the energy flux balance, we propose a theoretical model to predict the turbulent kinetic energy modulation in isotropic turbulence due to the dispersed phase. The comparison between model predictions and results from prior particle-resolved simulations and existing high-fidelity experiments supports the performance of the model over a range of turbulence and particle parameters. The model is then used to explore turbulence modulation characteristics over a wider parameter space formed by five independent system-controlling parameters, showing rather complicated dependence on the particle size and particle-to-fluid density ratio.

Recent work has demonstrated that Lagrangian chaos can arise at the Darcy scale in even simple poroelastic media, relevant across geophysical, biophysical, and industrial domains. Key factors controlling the onset of chaotic advection in poroelastic media flow include medium heterogeneity, compressibility, and transient forcing. Here we focus on how the range of Lagrangian coherent structures (LCSs) present in poroelastic flows influences the transport of diffusive solutes. We use a minimal two-dimensional sinusoidally forced poroelastic computational model to show that LCS interactions impact the dispersion of solute plumes via the establishment of minimum flux manifolds, leading to strongly anomalous plume moments and solute residence time distributions. We interpret these results with fluid element stretching distributions and local Lyapunov exponents. Anomalous features persist even at low Péclet number. Our data show that solute dispersion in poroelastic systems subject to transient forcing can only be understood by resolving the LCSs of these complex flows.

To tackle the challenging near-wall turbulence modeling while preserving low computational cost, the near-wall nonoverlapping domain decomposition (NDD) method is proposed, incorporating the machine-learning technique. Using recently proposed implicit NDD (INDD), the solution can be calculated with a Robin-type (slip) wall boundary condition on a relatively coarse mesh and then corrected in the near-wall region at every iteration through an estimated turbulent viscosity profile obtained from a neural network. To maintain a reasonable complexity with acceptable accuracy, only the near-wall field properties, i.e., wall-normal distance, near-wall velocities, streamwise pressure gradients, and one near-wall scale parameter, are employed as the input features for the neural network. The benefit of incorporating machine learning is twofold. First, the near-wall turbulent viscosity is predicted more accurately than by the traditional algebraic functions used in the approximate approach. Second, similarly to the conventional NDD, the present simulations save one order of computational cost over the fully resolved one-block simulations. The accuracy and efficiency of the method are demonstrated on test cases with the $k\text{−}\varepsilon$ model, including turbulent flows in a channel and an asymmetric diffuser at different Reynolds numbers. Results compare favorably with the one-block benchmark solutions and show a better agreement when compared with approximate NDD predictions. In the latter case, a variation of diffuser geometry is also considered to test the model performance on engineering design tasks with another turbulent model (i.e., Spalart-Allmaras), showing good generalization capability on different turbulence closures.

Three-dimensional direct numerical simulations are used to revisit the experiments on multibubble cavitation performed by Bremond et al., Phys. Fluids 18, 121505 (2006); Phys. Rev. Lett. 96, 224501 (2006). In particular, we aim at understanding the asymmetry observed therein during the expansion and collapse of bubble clusters subjected to a pressure pulse. Our numerical simulations suggest that the asymmetry is due to the force applied by the imposed pressure pulse and it is a consequence of the finite effective speed of sound in the liquid. By comparing our numerical results to the experiments, we found that the effective speed of sound under the experimental conditions was smaller than that of degassed water due to microbubbles in the system which resulted from prior cavitation experiments in the same setup. The estimated values of the effective speed of sound are consistent with those derived from the classical theory of wave propagation in liquids with small amounts of gas. To support this theory, we also present evidence of tiny bubbles remaining in the liquid bulk as a result of the fragmentation of large bubbles during the prior cavitation experiments. Furthermore, we find that this asymmetry also alters the direction of the liquid jet generated during the last stages of bubble collapse.

In convective fluids, small vortices develop between neighboring convective cells. Familiar in the atmosphere in the form of dust devils and water spouts, these convective vortices have been seen in simulations of oceanic convection where the vortices exhibit an unexplained bias towards cyclonic rotation despite having a large Rossby number. Here we use large eddy simulations (LES) of an idealized oceanic convective mixed layer and vary independently the Coriolis acceleration and the surface buoyancy flux to investigate the development of the cyclonic bias of convective vortices. While large convective vortices are biased for sufficiently large values of the Coriolis parameter, small convective vortices do not exhibit a clear bias. Using Lagrangian particles, we find that the large convective vortices develop through the merger of many small convective vortices. We propose a statistical theory to predict the cyclonic bias of large convective vortices composed of many small unbiased convective vortices and test the theory using LES results. We apply the theory to typical convective conditions and find that convective vortices in upper ocean mixed layers are expected to exhibit a bias, while convective vortices in the terrestrial and Martian atmospheres are expected to be largely unbiased.

We present a physically motivated low-dimensional model for the dynamics of two interacting large-scale circulations (LSC) in two-layer turbulent convection. Inspired by our experimental results of the flow dynamics and coupling in two-layer turbulent convection [J. Fluid Mech. 728, R1 (2013)], the model extends previous studies of single-LSC dynamics to incorporate four stochastic ordinary differential equations describing the strength $\delta$ and azimuthal orientations $\theta$ of two vertically aligned LSCs. The interaction terms of the two LSCs, i.e., thermal and viscous coupling terms, are predicted based on the influence of the fluid temperature by the other LSC through heat advection and thermal diffusion, and the enhanced (reduced) viscous damping across the interface between the two LSCs. Our model produces two stable LSC rolls and predicts their preferred flow states for the thermal and viscous couplings. The model describes properly the diffusive motion of both $\delta$ and $\theta$ of the two LSCs, and the Poissonian distribution of time interval between LSC cessations. More importantly, our study reveals that flow reversals and cessations in two-layer convection can be achieved when turbulent fluctuations drive the azimuthal diffusion of the two LSCs into a flow state that the two LSC planes are orthogonal to each other, the strength of the LSC in the fluid layer with a relatively larger Rayleigh number reduces to zero deterministically, owing to the unbalanced buoyancy forcing. Our model provides accurate predictions for the enhanced occurrence frequency of flow reversals observed in the experiment, and it suggests a new dynamical process of flow reversals in multilayer turbulent convection.

Previous results of discrete simulations of steady, unidirectional particle flows, here collected and critically reanalyzed, permit to make the case that the kinetic theory of granular gases, extended to include the correlations in the velocity fluctuations and the role of friction in collisions, provides the long-sought universal framework to predict the flow of realistic particles over the entire range of solid volume fraction from dilute to very dense—the upper limit being the critical value at which rate-independent components of the stresses arise. The case is made even stronger by the explicit derivation of the popular inertial rheology and its nonlocal extension to deal with heterogeneities based on the granular fluidity concept as special limits of the kinetic theory. In the process, common statements about the frictional-collisional duality in the granular stresses and the importance of long-lasting contacts creating a percolating network are shown to be greatly exaggerated for granular flows in practical applications.

The stresslet in a dilute suspension of rigid spheres in an Oldroyd-B fluid under imposed shear and uniaxial exentional flow is studied using a combination of analytical and numerical techniques. Previous work has focused on the stresslet contribution to the per-particle viscosity of a suspension. This contribution has been shown to decrease with increasing suspending fluid elasticity in shear and decrease with increasing strain in transient uniaxial extension. By considering the surface tractions in the Newtonian limit, we propose analytical scalings for the three components of the stresslet—pressure, viscous stress, and polymer stress—as functions of the appropriate Weissenberg or Deborah number. We conduct direct numerical simulations of a dilute sphere in viscoelastic fluid flow to compare against analytical scalings and see good agreement for weak elasticity, with increasing deviations as the fluid becomes increasingly elastic. Finally, we discuss the consequences of these trends on the tractions and stresses experienced by the suspended particles.

We conducted experiments using a typical centimeter swimmer, A. salina, and an electromagnetically driven quasi-two-dimensional flow to study the interaction between active matter and flow. The flow spanned from time-independent cellular flow to weakly time-dependent regimes. We focused on the impact of swarming active matter on hyperbolic Lagrangian coherent structures (LCSs) that mark the most straining regions in the flow. There is one decade of scale separation between active matter agents and the length scale of LCSs. We illustrated that the impact of active matter on LCSs was much more significant compared to localized random noise with similar energy input. In addition, we revealed that the perturbation generated by active matter could couple with the background flow and further deform the LCSs. In addition to the impact on the most straining hyperbolic regions of flow, we also revealed that the rotational elliptical region of the flow was much more susceptible to active matter perturbation in a Lagrangian perspective. We further described how the influence of active matter changed with their number densities and background flow intensities. We revealed that the LCSs could be decently altered even at a small number density of active matter. Through this work, we aim to provide valuable insights and draw attention to the problem regarding the interaction between active matter and external flow structures.

The reconstruction of incomplete information in flow fields is a pervasive challenge in numerous turbulence-related applications. This paper proposes a framework for the high-fidelity reconstruction of large-area damaged turbulent fields with high resolution based on a physically constrained generative adversarial network. The network incorporates some special designs, such as leveraging complete or sparse fields of velocity components as physical constraints, employing a ResNet-like network with a fast Fourier convolution module, and adopting a PatchGAN discriminator network. To train the network, we employ a combination of loss functions, which comprise mean absolute error, network-level loss, and feature matching loss. We validate our method on a dataset of compressible isotropic turbulent flow and investigate three distinct damaged distributions with gap ratios of $39.06\%$ and $56.25\%$. The proposed reconstruction framework has been shown to achieve excellent reconstruction performance. The reconstructed flow fields are consistent with the raw flow fields in terms of magnitude, power spectrum, and two-point correlation function. We also provide extensive ablation studies to validate our approach. The results indicate that the utilization of physical constraints significantly improves the reconstruction performance in damaged regions, particularly in cases that involve a large damaged area. Furthermore, in the proposed reconstruction framework, the size of patch in the PatchGAN discriminator network can be flexibly adjusted according to the scale of turbulence structures, and the perceptual loss function plays a crucial role in evaluating differences between feature maps of flow fields at network levels based on a pretrained network. These special designs ensure consistency across diverse scales of turbulence structures and improve the accuracy and efficiency of network training.