Physical Review Fluids
Tail length influences swimming speed of helical swimmers in granular media
Author(s): Rogelio Valdés, Elsa de la Calleja, Roberto Zenit, and Francisco A. Godínez
We experimentally investigate the effects of helical tail length on the swimming efficiency of artificial robots in granular matter. Using magnetically driven swimmers, we found that longer tails boost forward velocity, challenging the traditional behavior observed in Newtonian fluids. We reveal the crucial role of head size in these dynamics through a modified Resistive Force Theory model. Our results demonstrate the intricate relationship between head drag and tail morphology, showing long-range effects linked to force chain formation and buckling. This important discovery broadens our understanding of locomotion in granular systems, an area where current theories are limited.
[Phys. Rev. Fluids 9, 124303] Published Fri Dec 06, 2024
Granular flows over normally vibrated inclined bases
Author(s): Prasad Sonar, Ashish Bhateja, and Ishan Sharma
We investigate granular flows over an inclined, normally vibrated rigid base using the discrete element method, systematically varying the inclination angle (θ), vibration frequency (f), and amplitude (A). Our findings demonstrate that vibrated bases can amplify the mass flow rate (Q) by 25–100 times compared to fixed bases depending upon the choice of parameters and, further, it is possible to find conditions that maintain Q nearly constant. Finally, Q is characterized by a dimensionless parameter S, also known as the shaking strength, which represents the ratio of vibrational to gravitational energies.
[Phys. Rev. Fluids 9, 124304] Published Fri Dec 06, 2024
Effects of wall groove misalignment on viscoplastic flow dynamics in superhydrophobic channels
Author(s): A. Joulaei, H. Rahmani, and S. M. Taghavi
In viscoplastic Poiseuille flows over superhydrophobic surfaces, misalignment between lower and upper grooves alters flow characteristics. Adjusting groove misalignment along with key dimensionless parameters—offset number, Bingham number, slip number, groove periodicity, and slip area fraction—affects velocity distributions, plug morphology, and yielded/unyielded zones. Misalignment intensifies velocity and strain rate deviations, leading to plug deformation, asymmetry, and potential breakage. Four distinct regimes of center plug morphology emerge, highlighting the complex interplay between misalignment and viscoplastic flow behavior.
[Phys. Rev. Fluids 9, 123301] Published Wed Dec 04, 2024
Combined parabolic and elliptic velocity profile-based low-dimensional model in falling film
Author(s): Arghya Samanta
Based on the assumption of a combined parabolic and elliptic velocity profile, the simplified second-order depth-averaged equations are derived. As the parameter A relating to the eccentricity of the ellipse increases, new results adequately capture available findings. However, A = 2.23219 provides a relatively more accurate result. Maximum amplitude and speed of the steady-state traveling wave increase with rising values of A. The backflow phenomenon occurs in the capillary regime. Interestingly, the combined velocity profile detects the point of inflection in the capillary region at A = 2.23219, but it disappears at higher values of A, signaling a strong influence of the elliptic part.
[Phys. Rev. Fluids 9, 124002] Published Wed Dec 04, 2024
Flows over backward-facing steps with different spanwise widths
Author(s): Ke Zheng, Heri Setiawan, Jimmy Philip, Junghoon Lee, and Jason P. Monty
The effect of spanwise aspect ratio (AR) on flow characteristics over backward-facing steps with extended streamwise length and external corners is investigated experimentally. For intermediate AR, a unique wake pattern is observed, where there is no separation bubble, and the flow after reattachment ejects from, rather than impinging on, the central plane bottom floor, leading to intensified fluctuations and a broader region of high turbulence. This flow feature may be attributed to strong interactions of separated flows from all open edges with possible contributions from corner vortices that develop alongside the step. We also discuss the influence of these corner vortices for varying AR.
[Phys. Rev. Fluids 9, 124601] Published Wed Dec 04, 2024
Self-exploring automated experiments for discovery, optimization, and control of unsteady vortex-dominated flow phenomena
Author(s): Karen Mulleners
This paper discusses the transformative potential of self-exploring automated experiments for the discovery, optimization, and control of unsteady vortex-dominated flow phenomena. By minimizing experimentalists’ input in the actual performance of fluid experiments, the potential for scientific discovery is maximized.
[Phys. Rev. Fluids 9, 124701] Published Wed Dec 04, 2024
Predictability of isotropic turbulence by massive ensemble forecasting
Author(s): Alberto Vela-Martín
Turbulent flows are difficult to predict due to chaos, which amplifies any uncertainty in the initial conditions. The way this uncertainty grows and propagates is key to understanding the emergence of complexity in turbulence and to assessing the reliability of turbulence forecasts. In this Letter, a novel approach based on massive ensembles of simulations reveals that uncertainty propagates in isotropic turbulence following a simple law that depends only on the average properties of the flow. This result opens avenues to improve current forecasting techniques by efficiently and accurately modeling uncertainty propagation.
[Phys. Rev. Fluids 9, L122601] Published Wed Dec 04, 2024
Particle-in-liquid compound drops impact on solid wall: Spreading and retraction
Author(s): Rui Wang and Chun-Yu Zhang
When a particle-laden droplet impacts a flat surface, the presence of the particles inhibits its maximum spreading and reduces the rebound time.
[Phys. Rev. Fluids 9, 123601] Published Tue Dec 03, 2024
Self-similar and universal dynamics in drainage of mobile soap films
Author(s): Antoine Monier, François-Xavier Gauci, Cyrille Claudet, Franck Celestini, Christophe Brouzet, and Christophe Raufaste
We experimentally investigated the drainage of vertical rectangular soap films as they thin under gravity. Drainage dynamics were measured by tracking isothickness interference fringes. We showed that the downward motion of these fringes is self-similar with a power-law time evolution, or equivalently, that the thickness profiles exhibit space-time separation. By combining our data with previous studies, we collapsed all profiles onto a single curve, demonstrating the universality of this phenomenon. These findings are important for studying liquid foams and marginal regeneration instability.
[Phys. Rev. Fluids 9, 124001] Published Tue Dec 03, 2024
Shock-induced instability of dual-layer dilute gas-particle mixture
Author(s): Yifeng He, Baoqing Meng, Baolin Tian, and Yue Yang
We report the mechanism and modeling for the two-dimensional shock-induced instability of a dual-layer gas-particle mixture. In the mixture, the instability is triggered by the pressure perturbation near the perturbed interface instead of the baroclinic vorticity. The velocity difference of gas induced by the pressure perturbation drives the particle interface to grow via drag coupling effects. Inspired by this interfacial instability mechanism, we estimate the growth of the particle interface in the linear stage.
[Phys. Rev. Fluids 9, 124301] Published Tue Dec 03, 2024
Interface-induced turbulence in viscous binary fluid mixtures
Author(s): Nadia Bihari Padhan, Dario Vincenzi, and Rahul Pandit
We uncover interface-induced turbulence, a striking nonequilibrium statistically steady state with spatiotemporal chaos, emerging from interfacial fluctuations in low-Reynolds-number binary-fluid mixtures. Using direct numerical simulations of the Cahn-Hilliard-Navier-Stokes equations, we reveal a power-law energy spectrum indicative of turbulence without a conventional inertial cascade.
[Phys. Rev. Fluids 9, L122401] Published Tue Dec 03, 2024
Melancholia states of the Atlantic Meridional Overturning Circulation
Author(s): Johannes Lohmann and Valerio Lucarini
A dynamically unstable state of the Atlantic ocean circulation in a global ocean model is constructed via an edge tracking algorithm. Such an unstable state is relevant as it mediates the potential future tipping point of the meridional overturning circulation from its present-day state to a collapsed state as a result of climate change. We identify the physical characteristics of the unstable state, which gives insights into the physical mechanisms necessary to induce a collapse, as well as potential fingerprints and early-warning signals of the tipping point.
[Phys. Rev. Fluids 9, 123801] Published Mon Dec 02, 2024
Pattern formation of freezing infiltration in porous media
Author(s): Nathan D. Jones, Adrian Moure, and Xiaojing Fu
Gravity-driven flow of water into unsaturated porous media can form preferential pathways due to the gravity fingering instability. Here, we consider this process in a subfreezing porous medium using numerical simulations. We find that the macroscopic infiltration rate can be well predicted by the freezing Damköhler number. In contrast to the classical instability, the introduction of freezing in this problem gives rise to a new flow regime in which secondary flow pathways form in between the initial channels. This secondary instability homogenizes the otherwise channelized flow field and decreases the effective infiltration velocity.
[Phys. Rev. Fluids 9, 123802] Published Mon Dec 02, 2024
Dynamics of two-dimensional water flow in angstrom-scale mono and hybrid channels
Author(s): Chengzhen Sun, Qiyuan Wang, Mehdi Neek-Amal, Runfeng Zhou, and Bofeng Bai
In angstrom-scale channels, where water forms a two-dimensional (2D) monolayer, interlayer shear force decreases, making traditional viscosity definitions unsuitable. Our findings demonstrate that in channels below 1 nm, water flow does not follow conventional viscosity-driven principles; instead, friction between water molecules and channel walls governs flow behavior. We extend the Hagen-Poiseuille equation by excluding viscosity and emphasizing friction, providing a more accurate model for flow in ultra-narrow 2D spaces. This approach improves understanding of water flow in angstrom-scale channels, with applications in desalination and energy conversion.
[Phys. Rev. Fluids 9, 124201] Published Mon Dec 02, 2024
Contrasting flow dynamics between stationary and moving clapping bodies
Author(s): Suyog V. Mahulkar and Jaywant H. Arakeri
Flow dynamics of pulse-jetting aquatic animals are often studied with apparatus kept stationary. This paper examines whether the stationary configuration captures the mechanics of the swimming animal. A clapping body, simulating a pulse-jet animal, is tested in freely propelled (dynamic) and forward motion constrained (stationary) conditions. Significant differences are observed in body motion and wake structure: clapping motion is nearly twice as fast in dynamic cases, while mean thrust coefficient and vortex circulation are higher in stationary cases, varying with body depth but which remain constant in dynamic cases. Care is needed in extrapolating data from stationary to swimming bodies.
[Phys. Rev. Fluids 9, 114702] Published Wed Nov 27, 2024
Rayleigh-Taylor unstable flames: The effect of two-mode coupling
Author(s): Mingxuan Liu (刘明轩) and Elizabeth P. Hicks
We explore how adding a reaction to a Rayleigh-Taylor unstable interface affects the way that two short wavelength modes couple to generate longer wavelength modes. Using simulations, we identify five distinct flame growth solution types. Depending on the greatest common divisor of the wavenumbers of the two modes, the flame may stall, develop coherent pulsations, or even become a metastable traveling wave. We also compare our results with two-mode coupling in ablative and classical Rayleigh-Taylor and show that all three systems may follow the same mode coupling dynamics.
[Phys. Rev. Fluids 9, 113203] Published Tue Nov 26, 2024
Giant superhydrophobic slip of shear-thinning liquids
Author(s): Ory Schnitzer and Prasun K. Ray
Recent experiments and simulations have suggested that flows of strongly shear-thinning liquids can be greatly enhanced by superhydrophobicity, above and beyond the enhancements familiar for Newtonian liquids. Using asymptotics and numerics, we illuminate the singular mechanism underlying such enhancement, considering the prototypical problem of shear-driven flow over a grooved superhydrophobic surface. A key finding is that the singular scaling of the effective slip length at small solid fractions transitions from logarithmic to algebraic as the viscosity at infinite shear is reduced relative to its value at zero shear.
[Phys. Rev. Fluids 9, L112201] Published Tue Nov 26, 2024
Aerodynamic bag breakup of a polymeric droplet
Author(s): Navin Kumar Chandra, Shubham Sharma, Saptarshi Basu, and Aloke Kumar
When liquid droplets encounter high-speed gas flows, they fragment through various modes, with bag-mediated breakup mode observed at lower Weber numbers. This study shows that even minute concentrations of long-chain polymer molecules in a Newtonian solvent can drastically modify its fragmentation characteristics. By investigating the role of liquid elasticity, we uncover how polymeric additives control the fragmentation dynamics of a droplet specifically in the bag breakup regime.
[Phys. Rev. Fluids 9, 113303] Published Mon Nov 25, 2024
Bubble oscillation and effects of dynamic behaviors on forces and mass transfer in photoelectrochemical water splitting
Author(s): Yonglu She, Qiang Xu, Tengfei Nie, Xinyi Luo, Mengsha Wang, Xingmiao Ye, Dengwei Jing, and Liejin Guo
The evolution and accumulation of bubbles formed by gas release at the reaction interface present a considerable challenge for improving the efficiency of photoelectrochemical water splitting. Bubble oscillation enhances mass transfer and influences flow dynamics within the system. This study successfully regulates the amplitudes of bubble oscillations by adjusting system temperature and operating voltage and analyzes the mechanism and the mass transfer theory of oscillating bubbles, offering a new approach to managing bubble dynamics.
[Phys. Rev. Fluids 9, 114305] Published Mon Nov 25, 2024
Interaction of an inner-scaled Helmholtz resonator with boundary-layer turbulence
Author(s): Abdelrahman Hassanein, Davide Modesti, Fulvio Scarano, and Woutijn J. Baars
This work provides key insights into the interaction between grazing turbulence and a Helmholtz resonator. By tuning the resonator’s frequency to align with the scales of energetic pressure fluctuations in turbulent boundary layers, the device amplifies small-scale velocity fluctuations while reducing large-scale turbulence energy. This frequency-specific modulation suggests that Helmholtz resonators could serve as scalable, wall-embedded surfaces for targeted turbulence control, offering new pathways in flow control.
[Phys. Rev. Fluids 9, 114610] Published Mon Nov 25, 2024