Cavitation is a phenomenon in which bubbles form and collapse in liquids due to pressure or temperature changes. Even common tools like a rubber popper, which is a children's toy that rapidly changes its shape when snapped, can be used to create cavitation at home. As a rubber popper toy slams a solid wall underwater, toroidal cavitation forms. As part of this project, we aim to explain how an elastic shell causes cavitation and to describe the bubble morphology. High-speed imaging reveals that a fast fluid flow between a snapping popper and a solid glass reduces the fluid pressure to cavitate. Cavitation occurs on the popper surface in the form of sheet cavitation. Our study uses the axisymmetric Rayleigh-Plesset equation and the energy balance to capture the relationship between the bubble lifetime and the popper deformability. The initial distance between the popper and the wall is an important parameter for determining the cavitation dynamics. Presented results provide a deeper understanding of cavitation mechanics, which involves the interaction between fluid and elastic structure.

The main causes of energy dissipation in micro- and nanoscale wetting are viscosity and liquid-solid friction localized in the three-phase contact line region. Theoretical models predict the contact line friction coefficient to correlate with the shear viscosity of the wetting fluid. Experiments conducted to investigate such correlation have not singled out a unique scaling law between the two coefficients. We perform molecular dynamics simulations of liquid water-glycerol droplets wetting silicalike surfaces, aimed to demystify the effect of viscosity on contact line friction. The viscosity of the fluid is tuned by changing the relative mass fraction of glycerol in the mixture and it is estimated both via equilibrium and nonequilibrium molecular dynamics simulations. Contact line friction is measured directly by inspecting the velocity of the moving contact line and the microscopic contact angle. It is found that the scaling between contact line friction and viscosity is sublinear, contrary to the prediction of molecular kinetic theory. The disagreement is explained by accounting for the depletion of glycerol in the near-wall region. A correction is proposed, based on multicomponent molecular kinetic theory and the definition of a rescaled interfacial friction coefficient.

The nonlinear decay of oscillations of a liquid column in a U-shaped tube is investigated within the theoretical framework of the projection method formalized by Bongarzone et al. [Chaos 31, 123124 (2021)]. Starting from the full hydrodynamic system supplemented by a phenomenological contact line model, this physics-inspired method uses successive linear eigenmode projections to simulate the relaxation dynamics of liquid oscillations in the presence of sliding triple lines. Each projection is shown to eventually induce a rapid loss of total energy in the liquid motion, thus contributing to its nonlinear damping. A thorough quantitative comparison with experiments by Dollet et al. [Phys. Rev. Lett. 124, 104502 (2020)] demonstrates that, in contradistinction with their simplistic one-degree-of-freedom model, the present approach not only describes well the transient stick-slip dynamics, but also correctly captures the global stick-slip to stick transition, as well as the residual exponentially decaying bulk motion following the arrest of the contact line, which has been so far overlooked by existing theoretical analyses but is clearly attested experimentally. This study offers a further contribution to rationalizing the impact of contact angle hysteresis and its associated solidlike friction on the decay of liquid oscillations in the presence of sliding triple lines.

The effects of stagnation enthalpy on a hypersonic boundary layer developing over a compression ramp are analyzed in this work using direct numerical simulations. Separate sets of simulations with different values of free-stream temperature, wall-cooling rate, and edge Mach number are carried out to evaluate and isolate stagnation enthalpy effects. Moreover, these sets of calculations are performed with and without vibrational excitation to further characterize the impact of this thermodynamic phenomenon on the flow. The presented calculations show that the variation of stagnation enthalpy and the presence of vibrational excitation are not able to qualitatively alter the structure of the flow. From a quantitative point of view, it is shown that the variation of the thermodynamic state of the gas can cause a 10% variation in the skin-friction coefficient, a 20% decrease in wall heat flux, and a shift in the typical frequencies of wall-pressure fluctuations by a factor of 2 toward higher frequencies.

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.

In this paper, we report on the outer-layer self-similarity of the turbulent boundary layer (TBL) from the perspectives of the turbulent/non-turbulent (T/NT) interface. We receive support from the experimental database of the TBL. The database is measured with particle image velocimetry (PIV) at low to moderate Reynolds number range $\left(\mathrm{R}{\mathrm{e}}_{\tau }=389\text{–}1000\right)$. The T/NT interface is detected by the local turbulent kinetic energy method. The intermittency in the outer layer is considered in the velocity decomposition. From our interface perspective, the exponential self-similarity from the mean streamwise velocity is discovered in the outer region. This indicates the consistency of turbulence characteristics in the turbulent region. The outer-layer self-similarity exists above the logarithmic region and below the region linked to the T/NT interface. Based on the two-point correlation, the consistency of the structural spatial characteristics is revealed. Interface-based turbulent fluctuations suggest that the wall-normal scale of the outer-layer structure not only relates to the height from the wall but also needs to consider the distance to the interface. The energy superposition effect of the outer-layer structure is modeled by the structural spatial characteristics. According to the energy superposition effect, for different interface conditions, the structural-based reconsidered turbulent kinetic energy in the outer region is consistent under the interface-based self-similar frame. The proposed interface-based perspective emphasizes that study in the outer intermittent region should not unilaterally consider the wall reference frame. At the same time, the turbulence characteristics in the intermittent region is self-similar from this interface-based reference framework.

The continuous time random walk (CTRW) approach has been widely applied to model large-scale non-Fickian transport in the flow through disordered media. Often the underlying microscopic transport mechanisms and disorder characteristics are not known, and their effect on large-scale solute dispersion is encoded by a heavy-tailed transition time distribution. Here we study how the microscale physics manifests in the CTRW framework and how it affects solute dispersion. To this end, we consider transport in disordered media with random sorption and random flow properties. Both disorder mechanisms can give rise to anomalous particle transport. We present the CTRW models corresponding to each of these physical scenarios to discuss the different manifestations of microscale heterogeneity on large-scale dispersion depending on the particle injection modes. The combined impact of random sorption and advection is studied with a CTRW model that explicitly represents both microscale disorder mechanisms. While random advection and sorption may show similar large-scale transport behaviors, they can be clearly distinguished in their response to uniform injection conditions and, in general, to initial particle distributions that are not flux weighted. These findings highlight the importance of the microscale physics for the interpretation and prediction of anomalous dispersion phenomena in disordered media.

We develop a neural-network interpolation (NNI) to improve the prediction of preferential concentration in simulations of particle-laden turbulence. The NNI uses the particle position and velocity on neighboring grid points to estimate the fluid velocity at the particle position via fully connected neural networks. By avoiding the requirement for superresolution of the entire field and additional interpolations, the NNI offers computational efficiency and simplifies implementation. To evaluate the effectiveness of NNI and compare it with other interpolation methods, we conduct simulations on two-dimensional homogeneous isotropic turbulence subjected to high-wave-number forcing. This specific turbulent flow has a long inertial range and rich small-scale structures, posing a challenge for velocity interpolations and subsequently accurate prediction of preferential concentration. We compare the results on flow fields, energy spectra, and preferential concentration against the reference data obtained from direct numerical simulations at a range of the Stokes number from 0.1 to 5.0. The comparison demonstrates that the NNI can recover the effect of small-scale motion on particle distribution, so it improves the prediction accuracy of the preferential concentration from a priori test results of the large-eddy simulation on coarse grids.

Surface slip was employed to control flow separation induced by small-scale Gaussian-shaped surface deformations on a two-dimensional flat plate. Single-surface deformations, including bumps and gaps, were modeled, which generated separated flow along the rear side of the bump and within the gap concavity for a Reynolds number $\text{Re}=100000$ when the plate surface was subject to the no-slip condition. Surface slip was modeled using a Navier-slip condition and quantified by a slip length $\lambda$. Bump deformations had a greater impact on the flow dynamics than gap concavities, generating more intense regions of reversed flow and requiring larger slip lengths to inhibit flow separation. In addition, double-bump configurations were modeled, with the location of the two bumps playing a critical role in the evolution of the flow. When the bumps were close together, the first bump controlled the size of the separation bubble that developed downstream of the second bump. Whereas when bumps ware far apart, a moderate slip length $\lambda$ excited nonlinear oscillatory flow. However, increasing the slip length suppressed this phenomenon and ultimately eliminated all pockets of separated flow.

Understanding the velocity slip of fluids in channels of molecular length scales is essential for accurately manipulating their flow in engineering applications, such as in water desalination or, by reversing the process, osmotic power generation. This study addresses two major open questions, namely (a) the range of validity of the Navier-Stokes equations with slip boundary conditions in describing fluid flows within channels of molecular confinement, and (b) the effect of fluid density and confinement as well as surface properties, such as microscopic roughness and curvature, on fluid slippage along walls. Using a hard-sphere fluid model and describing the fluid-wall interactions by the Maxwell scattering kernel, an event-driven molecular dynamics simulation study was performed in planar and cylindrical channels of different sizes and accommodation coefficients with varying fluid densities. The results show that the well-known criterion for the slip solution, which is valid in rarefied conditions, also holds for a dense gas, i.e., the predictive accuracy improves with decreasing Knudsen numbers, as long as the channel size is larger than about ten molecular diameters. We also find that the key quantity influencing the friction coefficient of a fluid is the peak density at the walls, rather than the nominal density, the fluid confinement, or the channel curvature, as found in previous studies. In particular, higher nominal densities lead to higher density peaks, and therefore a decrease in slip, while tighter confinements lead to lower density peaks, and therefore an increase in slip, regardless of channel geometry. Furthermore, the velocity slip depends on the microscopic roughness via the Smoluchowski factor. These findings are a stepping stone towards a deeper understanding of the molecular mechanisms underlying fluid velocity slip in molecular-scale channels.

We study the effect of confinement on the three-dimensional linear instability of fast-rotating two-dimensional turbulent flows. Using large-scale friction to model the effect of rigid boundaries at the top and bottom, we study the onset of three-dimensional perturbations on a rapidly rotating flow. The friction term is taken to affect both the evolution of the two-dimensional turbulent flow and the perturbations that evolve on top of it. Using direct numerical simulations, the threshold for the onset of three-dimensional perturbations is traced out as a function of the control parameters. As reported in the earlier work of Seshasayanan et al. [J. Fluid Mech. 901, R5 (2020)], we find that the two different mechanisms, namely the centrifugal and parametric-type instabilities, are responsible for the destabilization across the wide range of parameters explored in this study. In the turbulent regime, we find that the large-scale friction term does not affect the threshold in the case of centrifugal instability, while in the case of parametric instability, the large-scale friction makes the flow stable for a wider range of parameters. For the parametric instability, the length scale of the unstable mode is found to scale as the inverse square root of the rotation rate and the growth rate of the unstable mode is found to be correlated with the minimum of the determinant of the strain rate tensor of the underlying two-dimensional turbulent flow, showing resemblance with elliptical type instabilities. Results from the turbulent flow are then compared with the oscillatory Kolmogorov flow, which also undergoes parametric instability resulting into inertial waves. The dependence of the threshold on the aspect ratio of the system is discussed for both the turbulent and the oscillating Kolmogorov flows.

Experimental measurements and numerical simulations of turbulent flows are characterized by a tradeoff between accuracy and resolution. In this study, we combine accurate sparse pointwise mean velocity measurements with the Reynolds-averaged Navier-Stokes (RANS) equations using data assimilation methods. Importantly, we bridge the gap between data assimilation (DA) using physics-informed neural networks (PINNs) and variational methods based on classical spatial discretization of the flow equations, by comparing both approaches on the same turbulent flow case. First, by constraining the PINN with sparse data and the underdetermined RANS equations without closure, we show that the mean flow is reconstructed to a higher accuracy than a RANS solver using the Spalart-Allmaras (SA) turbulence model. Second, we propose the SA turbulence model augmented PINN (PINN-DA-SA), which outperforms the former approach by up to 73% reduction in mean velocity reconstruction error with coarse measurements. The additional SA physics constraints improve flow reconstructions in regions with high velocity and pressure gradients and separation. Third, we compare the PINN-DA-SA approach to a variational data assimilation using the same sparse velocity measurements and physics constraints. The PINN-DA-SA achieves lower reconstruction error across a range of data resolutions. This is attributed to discretization errors in the variational methodology that are avoided by PINNs. We demonstrate the method using high-fidelity measurements from direct numerical simulation of the turbulent periodic hill at $\text{Re}=5600$.

When a flat plate impacts water at small deadrise angles, the air cushion underneath plate bottom is asymmetrical about plate center and produces great asymmetrical impact pressure. In such an occasion, the motion response of plate should be very complicated and is highly coupled with air cushioning effect, particularly, the pitching motion which will alter the transient deadrise angle. However, the existing studies only considered the vertical translational motion, both the horizontal translational motion and pitching motion are restricted. In this paper, the complete motion behavior of plate induced by air cushioning effect during the water impact at small deadrise angles is numerically studied. In the regime of impact velocity and deadrise angle, four typical plate motion patterns have been discovered based on the variation characteristics of pitch angle: pitching-down, fluctuating-pitching-down, pitching-up-down, and pitching-up. To elucidate the underlying mechanism, the water impact process of plate at small deadrise angles is investigated, which contains four distinct stages: keel compression, edge compression, fluid expansion, and reloading. The pitching motion patterns are mainly determined by the keel compression and edge compression stages. In the keel compression stage, the air underneath plate keel is compressed and produces a pitching-down moment; in the following edge compression stage, the air underneath plate edge is heavily compressed and produces a strong pitching-up moment. Further, the influence of impact velocity and deadrise angle on the pitching motion has been investigated.

In the study of particle suspensions, away from the jamming threshold, it is common to interpret the effective viscosity in terms of the volume fraction, neglecting roughness effects. Here we show that particle roughness can significantly modify viscous dissipation in configurations that represent fixed volume-fraction conditions. We derive a hydrodynamic model for the forced interaction of a two-dimensional particle, where roughness is represented by a periodic corrugation, with an adjacent wall. In particular, we address the limit of small nominal particle-wall separation, with the corrugation amplitude comparable with said separation. A lubrication analysis provides the rectilinear and angular velocities of the particle as functions of the instantaneous angular configuration. The particle may either translate while rotating or become “locked” in a specific phase and translate without rotation. The time-averaged rectilinear velocity, which is the object of interest, is a purely geometric quantity, obtained without the need to address any time dynamics.

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.

Flow field measurements from a Couette-Poiseuille experiment are used to examine quantitatively certain steps of the self-sustained process (SSP) of wall-bounded transition to turbulence. Although the different parts of the SSP have been discussed at large in the literature, direct measurements from experiment are scarce and the present results show, using a local analysis of the turbulent patterns, that (1) the amplitude of streamwise rolls is related to streak waviness, bringing a quantitative picture to one of the main physical mechanisms of Waleffe's model of SSP, and (2), at low waviness, direct measurements of the correlation between the streak and roll amplitudes, respectively probed by the streamwise and wall-normal velocity perturbations, quantify the lift-up effect. This analysis method of the SSP does not rely on the specificity of Couette-Poiseuille flows and can be used to investigate this mechanism in other flows.

In this paper, we investigate the concentration instabilities that arise in vertical dense particulate flows during hydraulic conveying. The widely used two-fluid model for dense particulate flows is employed, and existing closure relations for drag correlation, stress tensors, etc., found in the literature are utilized. The ratio between the pipe diameter and particle size should be greater than 60 in this model. Then, an investigation is conducted to analyze the linear stability of the two-fluid model in a homogeneous and bounded pipe to determine the growth rates, frequency, and propagation velocity of small disturbances. Results show that the vertical dense particulate flows in hydraulic conveying exhibit instability across a broad spectrum of controlling parameters. We also obtain a fully nonlinear transient numerical solution for the two-fluid system using the finite difference method. The numerical solution of the model shows a high level of concordance with the linear stability analysis conducted, accurately capturing the corresponding growth rates in the region of wave formation. In addition, it has been found that small disturbances transform into saturation waves with finite amplitudes when the nonlinear effect becomes dominant. The saturation waves are asymmetrical due to the imbalance of inertia, wall shear stress, particle pressure, and particle viscosity mechanisms. The instable mode corresponds to plug flow or slug flow.

We propose a dimensionality reduction and unsupervised clustering method for the automatic classification and reduced-order modeling of density-stratified turbulence in laboratory experiments. We apply this method to 113 long shadowgraph movies collected in a “stratified inclined duct” experiment, where turbulence is generated by instabilities arising from a sheared buoyancy-driven counterflow at Reynolds numbers $\mathrm{Re}\approx 300–5000$, tilt angles $\theta =1^{\circ }–6^{\circ }$, and Prandtl number $\mathrm{Pr}\approx 700$. The method automatically detects edges representative of discrete density interfaces, extracts a low-dimensional vector of statistics representative of their morphology, projects these statistics onto a two-dimensional phase space of principal coordinates, and applies a clustering algorithm. Five clusters are detected and interpreted physically based on their typical interface morphology and an examination of representative frames, revealing distinct types of turbulence and mixing: laminarizing, braided, overturning, granular, and unstructured, as well as some intermediate types. The ratio of time spent in each cluster varies gradually across the $(\mathrm{Re},\theta )$ space. At intermediate values of $\mathrm{Re}\theta$, intermittent turbulence cycles between clusters in phase space and reveals at least two distinct routes to stratified turbulence. These insights demonstrate the potential of this method to reveal the underlying physics of complex turbulent systems from large experimental datasets.

We present an experimental setup that utilizes a pump-probe laser configuration to investigate the interplay between light momentum and fluid mechanics in a two-layer liquid system. By inducing a nanometric bulge on the upper liquid layer, we observe the emergence of a transient bulge on the liquid-liquid interface propelled by viscous stress towards the liquid with a higher refractive index. Through the integration of noninvasive measurements and extensive numerical simulations, we successfully validate our experimental findings and elucidate the intricate interplay between light momentum theories [Minkowski, Math. Phys. Klasse 1908, 53 (1908); M. Abraham, Rend. Circ. Mat. Palermo 28, 1 (1909)] and fluid mechanics. Importantly, we demonstrate that the transient deformation height can serve as a valuable indicator of surface tension and viscosity, enabling precise nanoscale manipulation and holding great potential for advancements in sensors, actuators, and optical devices. The profound insights gained from this study offer significant promise for the advancement of microfluidics and optofluidics, driving progress in these fields.

We use complex network analysis to investigate the vortical interactions in a bluff-body stabilized combustion system containing a turbulent lean premixed swirling flame. Using time-resolved vorticity measurements, we construct time-varying weighted spatial networks whose node strength distribution is derived from the Biot-Savart law. We find widespread evidence of scale-free topology in the vortical networks, with the most coherent flow structures acting as the primary network hubs. Crucially, we find that even after the onset of thermoacoustic instability, the scale-free topology can persist continuously in time, contrary to some suggestions from the literature. This discovery could have important implications for the design of flow controllers that rely on destroying the primary hubs of vortical networks.