The present paper conducted a large-eddy simulation of turbulent flow and mass transfer over two-dimensional wavy walls with different wave slopes (ratio of amplitude to wavelength ranging from 0.0375 to 0.1) to investigate the generation and interaction of vortices and the effect on turbulent mass transfer. The present wavy wall induces flow separation and reconstructs a turbulent shear layer having a concave feature, and thus triggers streamwise vortices via the centrifugal instability mechanism. We confirm that these vortices originate from separation and reattachment points. The convex wall enhances the spanwise vorticity traced to the upstream trough, such that spanwise vortices form via the Kelvin-Helmholtz instability mechanism deviating from the crest. There are four vortex modes between crests controlling the turbulent scalar statistics. (1) The interaction between spanwise and streamwise vorticity activates strong shear, high-level turbulent kinetic energy (TKE) and Reynolds shear stress (RSS) near the separation point while upstreaming the origins of high streamwise and vertical turbulent scalar fluxes (VTSF). (2) The shear effect in the separation zone maintains the TKE and RSS, whereas the trapped separation bubble causes dominated scalar diffusion on the leeward side. (3) Vortex pairs are generated at the reattachment point to eject low-momentum and low-scalar fluids upward into the average flow. (4) The layout of vortices on the windward side is reversed to that in the separation point, and the restriction of the convex wall on the streamwise vorticity weakens the shear and forms a region of low RSS and VTSF.

The interaction between turbulent axisymmetric wakes plays an important role in many industrial applications, notably in the modeling of wind farms. While the nonequilibrium high-Reynolds-number scalings present in the wake of axisymmetric plates has been shown to modify the averaged streamwise scalings of individual wakes, little attention has been paid to their consequences in terms of wake interactions. We propose an experimental setup that tests the presence of nonequilibrium turbulence using the streamwise variation of velocity fluctuations between two bluff bodies facing a laminar flow. We have studied two different sets of plates (one with regular and another with irregular peripheries) with hot-wire anemometry in a wind tunnel. We show that the wake-interaction length, which quantifies the streamwise position where the wakes start interacting, can be deduced from the streamwise profiles of turbulence intensity, a quantity that is easy to resolve even in challenging field measurements. By acquiring streamwise profiles for different plate separations and identifying the wake-interaction length for each separation it is therefore possible to study the interaction between two axisymmetric turbulent wakes in terms of its energy cascade and in this case to show that the interaction between them is consistent with nonequilibrium scalings. This work also generalizes previous studies concerned with the interaction of plane wakes to include axisymmetric wakes. We find that a simple mathematical expression for the wake-interaction length based on nonequilibrium turbulence scalings can be used to collapse the streamwise developments of the second, third, and fourth moments of the streamwise fluctuating velocity.

Living microorg anisms in confined systems typically experience an affinity to populate boundaries. The reason for such affinity to interfaces can be a combination of their directed motion and hydrodynamic interactions at distances larger than their own size. Here we will show that self-propelled Janus particles (polystyrene particles partially coated with platinum) immersed in droplets of water and hydrogen peroxide tend to accumulate in the vicinity of the liquid/gas interface. Interestingly, the interfacial accumulation occurs despite the presence of an evaporation-driven flow caused by a solutal Marangoni flow, which typically tends to redistribute the particles within the droplet's bulk. By performing additional experiments with passive colloids (flow tracers) and comparing with numerical simulations for both particle active motion and the fluid flow, we disentangle the dominating mechanisms behind the observed interfacial particle accumulation. These results allow us to make an analogy between active Janus particles and some biological microswimmers concerning how they interact with their environment.

A simple model is set up to describe the growth of hydrogen bubbles at the cathode of a water electrolyzer operated at constant current. The cathode is assumed to be a porous horizontal plug covered with a thin porous catalytic layer where the hydrogen evolution reaction occurs. A dilute solution of a strong acid in water is sucked through this plug, and hydrogen bubbles grow quasistatically at its upward-facing surface. After an initial stage, the radius of the bubbles increases as $t^{1/3}$ when the spacing of the bubbles is small, and approaches a power-law intermediate between $t^{1/3}$ and $t^{1/2}$ when it is larger. The onset current density at which bubbles first appear increases with the value of the supersaturation at which nucleation at the electrode is assumed to occur. As the current density increases above this onset, the spacing of the bubbles decreases, which may lead to bubble coalescence (not analyzed), and the time of growth of a bubble increases slightly. The current-voltage characteristic of the electrode shows that the increase of the activation overpotential and the Ohmic losses due to attached bubbles nearly balance the decrease of the concentration overpotential due to these bubbles.

A Hele-Shaw cell is a device used to study fluid flow between two parallel plates separated by a small gap. The governing equation of flow within a Hele-Shaw cell is Darcy's law, which also describes flow through a porous medium. In this work, we derive a generalization to Darcy's law starting from a three-dimensional fluid with a parity-broken viscosity tensor with no isotropy. We discuss the observable effects of parity-odd fluids in various physical setups relevant to Hele-Shaw experiments, such as channel flow, flow past an obstacle, bubble dynamics, and the Saffman-Taylor instability. In particular, we show that when such a fluid is pushed through a channel, a transverse force is exerted on the walls, and when a bubble of air expands into a region of such fluid, a circulation develops in the far field, with both effects proportional to the parity-odd viscosity coefficients. The Saffman-Taylor stability condition is also modified, with these terms tending to stabilize the two-fluid interface. Such experiments can in principle facilitate the measurement of parity-odd coefficients in both synthetic and natural active matter systems.

Lipid-shelled microbubbles exhibit a strain-softening behavior and thus are characterized by preferential excursion from equilibrium during expansion at insonation. However, experimental studies have reported a counterintuitive behavior, identified as the compression-only behavior, where they pulsate mainly in the compression phase. We construct bifurcation diagrams of lipid and polymer-shelled microbubbles indicating the existence of a parameter range for which buckled shapes that are characterized by significantly smaller volume and lower total energy level, in comparison with the spherosymmetric state, are expected to spontaneously arise. The timescale for such a transition depends on the amplitude and frequency of the initial disturbance but more importantly on shell rheology in terms of the bending vs area dilatation modulus and shear vs dilatational viscosity. We show by performing stability analysis and constructing phase diagrams of coated microbubbles that an initially prestressed shell facilitates the onset of buckling at relatively small sound amplitudes. Moreover, low values of the shell bending modulus and shear viscosity in comparison with the area dilatation modulus and dilatational viscosity, respectively, facilitate the onset of shape modes that characterize bifurcating branches leading to deformed shapes with significant volume compression at relatively low sound amplitudes. By performing dynamic simulations for lipid-shelled microbubbles, we capture the onset of compression-only effect during which the shell tends to oscillate around compressed buckled shapes when subject to an acoustic disturbance. When phase diagrams of polymeric shells are examined the amplitude threshold for dynamic buckling to occur typically arrives before the onset of parametric shape mode excitation. Therefore the shell cohesion is compromised before it achieves a steady pulsation around a compressed shape, and this is verified by our simulations.

The effect of streamwise-traveling waves of spanwise wall velocity (StTW) on the quasistreamwise vortices (QSV) populating the near-wall region of turbulent channels is studied via a conditional averaging technique applied to flow snapshots obtained via direct numerical simulation. The analysis by Yakeno, Hasegawa, and Kasagi [Phys. Fluids 26, 085109 (2014)], where the special case of spatially uniform wall oscillation (OW) was considered, is extended to the general case of StTW, which yield both reduction and increase of turbulent skin-friction drag. StTW are found to significantly impact the wall-normal distribution of the vortex population. The conditionally averaged velocity field around the vortices shows that the contributions of the QSV to the quadrant Reynolds shear stresses change significantly during the control cycle. On the one hand, as for OW, the suppression of Q2 events (with upwelling of low-speed fluid away from the wall) dominates the drag-reduction process. On the other hand, the enhancement of Q2 and also Q4 events (with downwelling of high-speed fluid toward the wall) is related to drag increase. Based on the link identified between the phase changes of the Reynolds stresses and the principal directions of the rate-of-strain tensor induced by the StTW, a predictive correlation for drag reduction by StTW is proposed which uses physically significant parameters to overcome the shortcomings of existing models.

In wall turbulence, meandering behaviors of large-scale structures observed in the logarithmic layer are a crucial spatial feature to understand the spatial organization of these structures and to improve the structure-based turbulence model. These structures extend from the near-wall region to the edge of boundary layers $\left(\delta \right)$. Thus, their meandering motions leave an imprint on the two-point turbulence statistics across the flow, especially in the logarithmic region. Herein, we demonstrate the influence of the meandering motions of wall-attached structures on the two-point correlation and the premultiplied two-dimensional spectra by analyzing the direct numerical simulation data of the turbulent boundary layer at ${\text{Re}}_{\tau }\approx 1000$. The meandering magnitudes of wall-attached structures increase with their height $\left(l_y\right)$ but significantly differ among the identified structures even at a given $l_y$. In addition, the wall-attached structures of streamwise velocity fluctuations $\left(u\right)$ are found to be aligned along with a preferred spanwise offset regardless of the meandering motion. This feature is confirmed through the conditional two-point correlations of $u$ within the meandering structures, which leads to a distinct X pattern in the logarithmic region. We further examine the spectral behavior of the meandering structures across the streamwise and spanwise wavelengths (${\lambda }_x$ and ${\lambda }_z$, respectively) through the premultiplied two-dimensional spectra. With increasing meandering magnitudes, the energy spectra are more inclined to the ${\lambda }_z$ direction. Furthermore, the energy spectra of the strong meandering structures are bounded by a linear relationship ${\lambda }_x\sim {\lambda }_z$ over the large-scale range, reminiscent of self-similar scaling predicted by the attached-eddy hypothesis. In particular, the upper bound is aligned along ${\lambda }_x\approx 2{\lambda }_z$ over ${\lambda }_z>\delta$, which is absent in the energy spectral of the total $u$. This result reveals that the meandering motions allow the large-scale structures to contain wide spanwise scale energy that is self-similar in the logarithmic region. Finally, a discussion on the near-wall parts of the meandering structures is given with an emphasis on Townsend's inactive eddies.

In this work, we study the scale characteristics of the log- and outer-region motions and structures in subsonic and supersonic turbulence. To this end, a series of direct numerical simulations of the compressible turbulent channel flow at medium Reynolds numbers are performed. Based on this database, the streamwise and spanwise lengthscales of the outer-region motions are investigated by the two-point correlations and the one-dimensional spectra. The energy distribution among the multiscale structures in the outer region is found to be dominated by the semilocal friction-Reynolds-number effects rather than the Mach-number effects. This conclusion not only holds for the velocity fluctuations, but also the fluctuations of the thermodynamic variables. In addition, the streamwise and spanwise lengthscales of the outer motions do not alter significantly when the flow passes the sound barrier as reported by a previous experimental study [Bross, Scharnowski, and Kähler, J. Fluid Mech. 911, A2 (2021)]. On the other hand, the self-similar structures populating the logarithmic region are investigated by adopting a linear coherence spectrum. The streamwise/wall-normal aspect ratio of the self-similar wall-attached structures of the streamwise velocity and temperature fluctuations is approximately 15.5 and the counterpart of density and pressure fluctuations is 1.8. The present study confirms the existence of self-similar structures in compressible wall turbulence and assesses their geometrical characteristics.

The transmission and reflection of finite-amplitude, three-dimensional, internal gravity wave packets across a reflection level are examined numerically in a Boussinesq fluid with a nonuniform retrograde shear flow. We derive the critical amplitude for wave packets to transmit partially above the reflection level predicted by linear theory, occurring when the magnitude of the vertical shear associated with their wave-induced mean flow is locally greater than that of the background shear. We find that transmitted and reflected wave packets corresponding to strongly nonhydrostatic primary waves can interact resonantly to generate quadratically nonlinear secondary wave packets. We propose a weakly nonlinear mechanism, based on self-induced local decreases in buoyancy, to explain the generation of secondary wave packets by nonbreaking moderately nonhydrostatic primary waves, and we predict the critical amplitude for its onset. Numerical simulations are performed for a range of nonhydrostatic wave packets with small to moderately large initial amplitudes with their predicted wave-induced mean flow superimposed. Transmission is quantified using the pseudomomentum corresponding to upward-propagating waves above the reflection level predicted by linear theory. In most cases, the transmission transiently grows and decays as wave packets first cross and then reflect from the reflection level predicted by linear theory. We find that for all but the most strongly nonhydrostatic wave packets, larger-amplitude waves exhibit smaller peak transmission, relative to the total pseudomomentum. The time at which peak transmission occurs is diagnosed. Strongly nonhydrostatic wave packets exhibit continuous transmission well above the reflection level. When we consider the time interval for transmission to decrease to half its peak value, we find that this becomes longer with larger initial amplitude. These behaviors are found to result from the combined effects of modulational instability and the generation and evolution of secondary wave packets. Results are discussed in the context of previous studies of one- and two-dimensional wave-packet transmission and reflection.

Diffusiophoresis is the spontaneous motion of particles under gradients of solutes. In electrolyte-driven diffusiophoresis, the zeta potential of the particles is an important surface property that characterizes diffusiophoretic mobility. However, the zeta potential is not a fixed material property and colloidal surfaces often show varying potentials depending on the physicochemical properties of the surrounding fluid, e.g., solute type, ionic strength, and pH. In this paper, we study experimentally and theoretically pH-dependent diffusiophoresis of polystyrene particles using a dead-end pore geometry. In particular, the influence of the isoelectric point (pI) on diffusiophoresis is demonstrated in the absence and presence of wall diffusioosmosis. Throughout the paper, we show with experiments and model calculations how the pH-dependent diffusiophoresis and diffusioosmosis influence the particle motion in dead-end pore configurations, including changes that occur when there is a sign change in the zeta potential near the pI.

The paper is concerned with the capillary flow of water in tubes that are partially prefilled with sunflower oil. We study the system's dynamics as a function of the oil column's length in horizontal and vertical alignments. The study shows that, in a horizontal flow, a spontaneous process of water-droplet formation occurs at the level of the water-oil interface. We investigate this process as a function of the prefilling liquid's volume and describe how to manipulate the location of the breakup event by simply increasing the oil content that prefills the capillary tube. After droplet breakup, experimental observations on capillary intake reveal that for short length scales the liquid-liquid system moves with a constant velocity. We also show how theoretical models can be adapted to predict the position of the water-oil meniscus in time.

Nonlinear Rayleigh-Bénard convection in an infinite-Prandtl-number fluid layer between poorly conducting boundaries is considered as a model for convection in the earth's upper mantle. It is shown that accounting for the generally neglected impact of viscous dissipation may lead to the development of large-scale spatiotemporal chaotic dynamics governed by the familiar Kuramoto-Sivashinsky equation ${\mathrm{\Phi }}_{\tau }+{\nabla }^4\mathrm{\Phi }+2{\nabla }^2\mathrm{\Phi }-{\left(\nabla \mathrm{\Phi }\right)}^2+\alpha \mathrm{\Phi }=0$, known to occur in various physical systems.

Recent developments in the transition to zero-carbon fuels show that ammonia is a valid candidate for combustion. However, liquid ammonia combustion is difficult to stabilize due to a large latent heat of evaporation, which generates a strong cooling effect that adversely affects the flame stabilization and combustion efficiency. In addition, the slow burning rate of ammonia enhances the undesired production of ${\mathrm{NO}}_x$ and ${\mathrm{N}}_2\mathrm{O}$. To increase the flame speed, ammonia must be blended with a gaseous fuel having a high burning rate. In this context, a deeper understanding of the droplet dynamics is required to optimize the combustor design. To provide reliable physical insights into diluted ammonia sprays blended with gaseous methane, direct numerical simulations are employed. Three numerical experiments were performed with cold, standard, and hot ambient in nonreactive conditions. The droplet radius and velocity distribution, as well as the mass and heat coupling source terms are compared to study the effects on the evaporation. Since the cooling effect is stronger than the heat convection between the droplet and the environment in each case, ammonia droplets do not experience boiling. On the other hand, the entrainment of dry air into the ammonia-methane mixture moves the saturation level beyond 100% and droplets condense. The aforementioned phenomena are found to strongly affect the droplet evolution. Finally, a three-dimensional Voronoi analysis is performed to characterize the dispersive or clustering behavior of droplets by means of the definition of a clustering index.

This article describes the dynamics of small inertial particles centrifuging out of a single vortex. It shows the importance of caustics formation in the vicinity of a single vortex: both for particle collisions and void formation. From these single-vortex studies we provide estimates of the role of caustics in high Reynolds number turbulence, and in the case of clouds, estimate how they may help in rain initiation by bridging the droplet-growth bottleneck. We briefly describe how the Basset-Boussinesq history force may be calculated by a method which does not involve huge memory costs, and we provide arguments for its possible importance for droplets in turbulence. We discuss how phase change could render cloud turbulence fundamentally different from turbulence in other situations.

A previously developed method for large eddy simulations (LESs), based on spectral eddy viscosity models obtained using analytical theories of turbulence, is reevaluated with a goal of maximizing its dependence on information available directly from actual LES data and minimizing necessary input from theories of turbulence. The method computes the subgrid scale (SGS) energy transfer among resolved scales and its wave number distribution from the evolving LES velocity fields. This information is supplemented by asymptotic properties of the energy flux in the inertial range leading to the form of a spectral eddy viscosity that allows self-contained simulations without use of extraneous SGS models. The method is tested in LESs of isotropic turbulence at high Reynolds number where the inertial range dynamics is expected and is observed in LESs using the proposed method.

This paper is associated with a video winner of a 2021 American Physical Society's Division of Fluid Dynamics (DFD) Milton Van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2021.GFM.V0069.

This paper is associated with a video winner of a 2021 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2021.GFM.V0038.

This paper is associated with a video winner of the 2021 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2021.GFM.V0036.