This paper is associated with a poster winner of a 2020 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 poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2020.GFM.P0017.

This paper is associated with a video winner of a 2020 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.2020.GFM.V0067.

This paper is associated with a video winner of a 2020 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion 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.2020.GFM.V0026.

This paper is associated with a poster winner of a 2020 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 poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2020.GFM.P0004.

This paper is associated with a poster winner of a 2020 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 poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2020.GFM.P0027.

This paper is associated with a video winner of a 2020 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.2020.GFM.V0020.

This paper is associated with a video winner of a 2020 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.2020.GFM.V0052.

This paper is associated with a video winner of a 2020 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion 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.2020.GFM.V0074.

Transcritical shock-droplet interactions (TSDIs) occur in a spectrum of high-speed propulsion systems involving liquid fuel injection. “Transcritical” behavior refers to a condition at which the combustion chamber pressure nears the critical pressure of the fuel-air mixture and, by increasing the temperature, a transition from liquidlike to gaslike state is observed. Our understanding of TSDI is significantly less developed than its gas-phase (ideal-gas or supercritical) or liquid-phase (subcritical) counterparts, which are referred to as shock-bubble interactions (SBIs) and shock-droplet interactions (SDIs), respectively. In this paper, we investigate the interaction of a shockwave with an n-dodecane droplet at supercritical pressures. A fully conservative diffuse-interface framework coupled with the Peng-Robinson equation of state is developed to accurately determine the state of the fluid and the resulting interfacial instabilities as the shock propagates through the droplet. The influence of varying the initial temperature of the fuel, the ambient pressure, and the shockwave strength on the shock structure and the droplet morphological deformation is delineated. The dynamics of the TSDI cases are then compared to the subcritical SDI and supercritical or ideal-gas SBI counterparts. It is shown that, depending on the preshock temperature and pressure, the TSDIs exhibit some common features observed in classical cases of SDIs and SBIs, bridging the gap between the sub- and supercritical problems.

Flow-focusing devices have gained great interest in the past decade, due to their capability to produce monodisperse microbubbles for diagnostic and therapeutic medical ultrasound applications. However, up-scaling production to industrial scale requires a paradigm shift from single chip operation to highly parallelized systems. Parallelization gives rise to fluidic interactions between nozzles that, in turn, may lead to a decreased monodispersity. Here we study the velocity and pressure field fluctuations in a single flow-focusing nozzle during bubble production. We experimentally quantify the velocity field inside the nozzle at $100\mathrm{n}\mathrm{s}$ time resolution, and a numerical model provides insight into both the oscillatory velocity and pressure fields. Our results demonstrate that, at the length scale of the flow-focusing channel, the velocity oscillations propagate at fluid dynamical timescale (order of $\mu \mathrm{s}$) whereas the dominant pressure oscillations are linked to the bubble pinch-off and propagate at a much faster timescale (order of $\mathrm{n}\mathrm{s}$).

We present a numerical study of settling and clustering of small inertial particles in homogeneous and isotropic turbulence, in the dilute regime in which particles do not interact with each other or affect the fluid. Particles are denser than the fluid, but not in the limit of being much heavier than the displaced fluid. At fixed Reynolds and Stokes numbers, we vary the fluid-to-particle mass ratio and the gravitational acceleration. The effect of varying one or the other is similar but not quite the same. We report nonmonotonic behavior of the particles' velocity skewness and kurtosis with the second parameter, and an associated anomalous behavior of the settling velocity when compared to the free-fall Stokes velocity, including some cases of loitering. Clustering increases for increasing gravitational acceleration and for decreasing fluid-to-particle mass ratio.

We study properties of the turbulent/nonturbulent interface (TNTI) between two layers of stratified fluids through direct numerical simulations (DNSs). Zero mean shear forcing creates moderate turbulence in one of the layers with the Taylor microscale Reynolds numbers in the mixed region of ${\text{Re}}_{\lambda }=35,44$. We focus on the similarities and differences of the properties of stratified TNTIs due to two distinct types of forcing: (a) convection due to a boundary heat source and (b) agitation resembling a vertically oscillating grid experiment. Similarly to other stratified flows, the small scale dynamics of the TNTI in the present DNSs differ from what would be expected in comparable yet unstratified TNTIs. The interface cannot be indeed uniquely identified by the commonly used vorticity $\omega$. Instead, the potential enstrophy ${\mathrm{\Pi }}^2$ is shown to be the most appropriate marker in these flow cases. It is emphasized that the Kolmogorov lengthscale ${\eta }_K\sim \sqrt{\nu /\omega }$ is not representative of the small scale dynamics of the interface. Hence, an alternative lengthscale, ${\eta }_{\mathrm{\Pi }}$, is defined, in analogy to the Kolmogorov scale, based on the potential enstrophy, ${\eta }_{\mathrm{\Pi }}={({\nu }^3/{\mathrm{\Pi }}_{*})}^{1/6}$, being ${\mathrm{\Pi }}_{*}=|g/{\rho }_0\mathrm{\Pi }|$. The conditionally averaged profiles of potential enstrophy ${\mathrm{\Pi }}^2$, enstrophy ${\omega }^2$, and turbulent kinetic energy dissipation $\epsilon$ of the two distinctly different turbulence forcing cases collapsed when scaled by ${\eta }_{\mathrm{\Pi }}$ at different time instants in each simulation. This implies not only the self-similarity of the small scale statistics of the TNTI in either of the two cases, but also the similarity between the statistics of the two different turbulent flows in the proximity of TNTI.

Spatiotemporally chaotic dynamics of transitional plane Couette flow may give rise to regular turbulent-laminar stripe patterns with a large-scale pattern wavelength and an oblique orientation relative to the laminar flow direction. A recent dynamical systems analysis of the oblique stripe pattern demonstrated that the Navier-Stokes equations have unstable equilibrium solutions that capture the three-dimensional spatial structure of the oblique stripe patterns. While equilibrium solutions are embedded in the turbulence-supporting set and capture spatial features of the flow, a description of the dynamics requires evolving time-periodic solutions. These periodic orbits are unstable, expected to lie dense in the invariant set supporting turbulence, are shadowed by the chaotic trajectory and may allow for a quantitative description of turbulent statistics via periodic orbit expansions. Here we identify unstable periodic orbits that not only show oblique large-scale amplitude modulation in space but also have a characteristic time evolution. The periodic orbits represent standing waves that slowly propagate across wavy velocity streaks in the flow on viscous diffusion timescales. The unstable periodic orbits are embedded in the edge of chaos in a symmetry subspace of plane Couette flow and thereby may mediate transition to and from turbulent flows with oblique patterns.

The compressibility effect on opposition drag control is studied via direct numerical simulation of turbulent channel flows at a bulk Reynolds number ${\text{Re}}_b=3000$ for three different bulk Mach numbers: ${\text{M}}_b=0.3$, 0.8, and 1.5. For all ${\text{M}}_b$, the drag reduction (DR) has a similar trend as that of the strictly incompressible case; namely, DR first increases and then decreases with the sensing plane location $y_d^{+}$. With increasing ${\text{M}}_b$, DR slightly decreases at small $y_d^{+}$ but increases at large $y_d^{+}$. Consequently, $y_d^{+}$ for achieving maximum drag reduction (${\text{DR}}_{max}$) shifts to larger values, namely, from $y_d^{+}=12.5$ for ${\text{M}}_b=0.3$ to 20 for ${\text{M}}_b=1.5$, consistent with the outward shift of the peaks of Reynolds stresses at higher ${\text{M}}_b$. By rescaling the sensing plane with semilocal units, a better collapse of DR is achieved among different ${\text{M}}_b$, particularly for small $y_d^{*}$. The optimal sensing plane is found to be $y_d^{*}\approx 15$ with ${\text{DR}}_{max}\approx 23\%$. Interestingly, for large $y_d^{+}$ cases, a resonance buffer layer characterized by a streamwise periodic array of spanwise-coherent rollers is established, one of the main reasons for the deterioration of drag reduction performance. This layer of hydroacoustic instability resonance results from the intense interaction of wall-normal wave propagation with the background mean shear. Space-time correlation of wall-normal velocity reveals near-wall organized spanwise structures with a well-defined streamwise wavelength ${\lambda }_x$, decreasing with increasing ${\text{M}}_b$.

The sloshing motion of a confined liquid inside a vertically moving tank is analyzed in the present series of paper. The main objective of the study is to understand the multiple resulting energy dissipation mechanisms, namely wall-liquid impacts and free surface phenomena, among others. This analysis is connected to the damping effects on the aircraft wings caused by the liquid action inside the fuel tanks. Due to the complexity and nonlinearity of the flow generated inside the tank, only experiments or efficient numerical solvers are suitable for studying the problem. In this paper, the tank-fluid system is periodically excited with a prescribed law of motion and the nonlinear features are observed, the force between the wall and the fluid and the global energy balance are computed. A weakly compressible smoothed particle hydrodynamics model has been reformulated and adapted to this kind of violent and turbulent flow. The evolution of the different terms that appear in the energy conservation law are computed, and the formulation is compared to other alternatives where the advantages of the present formulation are indicated. The comparison with the experimental results and the fluid-structure interaction case is carried out in Part II [Marrone et al., Numerical study on the dissipation mechanisms in sloshing flows induced by violent and high-frequency accelerations. II. Comparison against experimental data, Phys. Rev. Fluids 6, 114802 (2021)] of this work.

In Part I of this series [Marrone et al., Numerical study on the dissipation mechanisms in sloshing flows induced by violent and high-frequency accelerations. I. Theoretical formulation and numerical investigation, Phys. Rev. Fluids 6, 114801 (2021)], a theoretical formulation and the numerical model were developed in order to obtain a complete perspective of the energy balance of a violently accelerated flow confined inside a rectangular tank. The tank-fluid system was periodically excited with a predetermined law of motion and the force between the wall and the fluid and the global energy balance were computed. In this second part, the experimental validation of the previous formulation is presented. In order to make a comparison with a previous experimental campaign, where the tank moves along a single degree of freedom mechanical guide, two numerical problems have been studied: in the first, the decaying movement of the tank is prescribed according to the experimental measurements, and in the second the tank is coupled to a mass-spring-damper equation, and the sloshing force produced by the confined fluid acts as an external force. Both problems have been studied for two different fluids, water and oil, which implies a difference of two orders of magnitude in terms of Reynolds number. A complete description of the energy balance inside the fluid tank is performed and the complexity of the fluid dynamic behavior that takes place inside the tank is explained. The results are compared to the experimental measurements in terms of fluid-wall interaction and energy dissipation.

Three-dimensional turbulence is usually studied experimentally by using a spatially localized forcing at large scales (e.g., via rotating blades or oscillating grids), often in a deterministic way. Here, we report an original technique where the fluid is forced in volume, randomly in space and time, using small magnetic particles remotely driven. Such a forcing generates almost no mean flow and is closer to those of direct numerical simulations of isotropic homogeneous turbulence. We compute the energy spectra and structure functions using local and spatiotemporal flow velocity measurements. The energy dissipation rate is also evaluated consistently in five different ways. Our experimental results confirm the stationary, homogeneous, and isotropic features of such turbulence, and, in particular, the Tennekes' model for which the Tennekes' constant is experimentally estimated.

The thinning of the liquid films separating bubbles in a foam or in a bubbly liquid controls the coalescence process and the foam stability, and is highly relevant in many industrial processes. The spatiotemporal evolution of the film thickness is governed by highly nonlinear equations, the solution properties of which are still mostly unknown. For a flat film in contact with a meniscus at a lower pressure, a classical theoretical solution is the appearance, deepening and widening of a pinched region, uniform along the meniscus. However, film thinning has been shown to be mainly controlled by nonuniform marginal regeneration, for which no clear explanation has been provided to date. In this article, we establish experimentally that the theoretical uniform film thickness profile is unstable, even in a horizontal film, and we measure the instability wavelength along the meniscus. We show that the Poiseuille flow can be neglected during the destabilization process, and, taking advantage of this scale separation, we build an original equation set leading to the prediction of the instability wavelength.

We show the occurrence of two distinguished classical regimes of wetting, namely, Laplace and solutal Marangoni, during the spreading of oil drops on a surfactant-laden aqueous phase in a single surfactant-oil-water system. The spreading kinetics is found to follow a power-law behavior not only in the Laplace and Marangoni regimes, but also in the transition regime. Our experimental findings are corroborated with the scaling laws. The results demonstrate that increasing the surfactant concentration across the critical micelle concentration is instrumental to obtain the Laplace to Marangoni transition. Moreover, this transition does not depend on surfactant chemistry; instead, it depends on the adsorption/desorption kinetics of surfactant molecules to/from the interfaces that are created or annihilated during drop spreading.

This paper reports an experimental and theoretical investigation of the hydrodynamic interaction of a deformable structure with gravity waves. High-speed imaging and particle image velocimetry measurements were employed to analyze the oscillation characteristics of a cylindrical filament placed transversely in the flow generated due to gravity waves in a water channel. Based on the Euler-Bernoulli equation, we derive and solve a theoretical model to predict the oscillation dynamics of the elastic structure in two limiting cases: the inertia-dominated regime and drag-dominated regime. In the inertia-dominated regime the system exhibits resonance, while the frequency response of oscillations in the drag-dominated regime shows no resonance. Our results from experiments carried out on aquatic plants and silicone rubber fibers further corroborate the findings from the model in the inertialess regime. The quality factor of the oscillation response is used to explain the absence of resonance in the drag-dominated regime. We also address the question as to when the inertial effects can be neglected from the analysis.