Latest papers in fluid mechanics

Author(s): Zeeshan Saeed, Yasaman Farsiani, and Brian R. Elbing

Drag reduction within a turbulent boundary layer was achieved with dilute polymers that were controlled such that the Weissenberg number (Wi) was uniform in the flow. Two conditions were selected for comparison: (i) different Wi at similar drag reduction (DR) levels and (ii) different DR at comparable Wi. While the mean velocity had a secondary Wi dependence, the fluctuating statistics had a strong Wi dependence. Similarly, proper orthogonal decomposition showed the polymers deplete energy for the small scales. These trends underscore the importance of Wi when seeking a universal scaling of polymer-modified flows

[Phys. Rev. Fluids 9, 104606] Published Fri Oct 18, 2024

Author(s): Yang Liu and Yifeng Zhu

We investigate flow instability of the free convection boundary layer evolving along a vertical cylinder with a broad spectrum of surface curvatures. The random perturbation calculation shows that the boundary layer could filter disturbances for a high frequency band (HFB) and the single mode perturbation calculation determines the flow characteristic frequency fc. The results suggest that as curvature A increases, the characteristic frequency of the thermal boundary layer increases, while the disturbance amplitude decreases. The flow Reynolds stress is strongly amplified toward the downstream at fc and dramatically decreases at a decaying frequency fc-3.5∆.

[Phys. Rev. Fluids 9, 103901] Published Thu Oct 17, 2024

Author(s): A. Movaghar, R. Chiodi, M. Oevermann, O. Desjardins, and A. R. Kerstein

A simple computational model simulating the evolving shape of the interface between two immiscible fluids such as oil and water in a turbulent flow has been validated using high-fidelity numerical simulations. Established theory is extended to predict that the dependencies of the smallest scale of interface wrinkling on turbulence intensity and surface tension collapse to dependence on a single parameter, involving two power-law regimes. The computational model reproduces all these features and the predicted power-law exponents. This and the other validations indicate that the model accurately represents the interaction between surface tension and turbulent fluid motion.

[Phys. Rev. Fluids 9, 104003] Published Thu Oct 17, 2024

Author(s): Alberto Vela-Martín

The limitations of data-driven extreme-event forecasting are examined by finding the minimum computational cost of producing accurate forecasts. For this purpose, the information bottleneck method is applied to a very large dataset of direct numerical simulations of turbulent trajectories in two-dimensional Kolmogorov flow. This method is used to construct optimal models to predict extreme dissipation bursts, exploring the trade-off between model complexity and predictive skill. The results show that model complexity must increase exponentially with the forecast horizon to produce accurate predictions and that this is connected with uncertainty in the causal origin of extreme events.

[Phys. Rev. Fluids 9, 104603] Published Thu Oct 17, 2024

Author(s): Tian-Yang Han, Jie Zhang, and Ming-Jiu Ni

The maximum spreading radius of liquid metal droplets under magnetic fields is a key parameter for characterizing their spreading behaviors, which garners close attention in fusion engineering. While the scaling law for the maximum spreading of metal droplets under a vertical magnetic field has been established, no relevant studies have addressed the effects of horizontal magnetic fields due to anisotropic spreading induced by the Lorentz force. In this paper, we conduct a three-dimensional numerical simulation to investigate the maximum spreading features of GaInSn droplets under a horizontal magnetic field and propose a theoretical model to predict their maximum spreading area.

[Phys. Rev. Fluids 9, 103703] Published Wed Oct 16, 2024

Author(s): Xuelian Bao, Huaxiong Huang, Zilong Song, and Shixin Xu

This work presents a micro-macro model for polymeric fluids incorporating an Elastic-Plastic (EP) potential to examine irreversible bond breaking in polymer chains. Using numerical simulations, we reveal how microscopic polymer behaviors under shear flow, including elongation, rotation, and bond breaking, impact shear stresses and velocities at the macro scale. Comparisons with classical potentials (Hookean, FENE, Morse) highlight distinct stress and flow responses, with shear-thinning behavior observed at high shear rates due to polymer rotation.

[Phys. Rev. Fluids 9, 103301] Published Tue Oct 15, 2024

Author(s): Marion Berry, Christophe Josserand, Anniina Salonen, and François Boulogne

We explore the dynamics of alkane drops impacting a brine bath, where the temperature is tuned to freeze the drops on impact. Depending on the thermal shock and drop velocity, the result is either a flat or cupped frozen drop. By analyzing the competition between the impact dynamics and the formation of a thin layer of ice at the interface between the drop and the bath, we explain this morphological transition.

[Phys. Rev. Fluids 9, 103602] Published Tue Oct 15, 2024

Author(s): Yelyzaveta Velizhanina and Bernard Knaepen

Although flows of electrically conducting fluids in circular pipes with applied magnetic fields occur in many engineering applications, the mechanisms behind their transition to turbulence are still not fully understood. In this context, we address the transient algebraic growth of three-dimensional disturbances in MHD pipe flow with a transverse magnetic field. Depending on its intensity, we identify four types of optimal perturbations, characterized by different topologies and varying underlying growth mechanisms. Additionally, we study the nonlinear evolution of the optimal perturbations using direct numerical simulations.

[Phys. Rev. Fluids 9, 103702] Published Tue Oct 15, 2024

Author(s): Fabio Pino, Miguel A. Mendez, and Benoit Scheid

The drag-out problem for small Reynolds numbers (Re) admits the Landau-Levich-Derjaguin (LLD) solution for small capillary numbers (Ca), and Derjaguin’s solution for large Ca. We investigate whether these solutions are absolutely or convectively unstable, solving the Orr-Sommerfeld eigenvalue problem. We show that Derjaguin’s solution is convectively unstable for Ka<17 and absolutely unstable for Ka=0.15 Re1.7 for Re > 10 where Ka is the Kapitza number. For water (Ka=3400), the LLD solution is always convectively unstable. The absolute instability is observed only when the dip-coated film is additionally fed from above.

[Phys. Rev. Fluids 9, 104002] Published Tue Oct 15, 2024

Author(s): Xiaotao Zhao, Huaiyu Cheng, Bin Ji, Linmin Li, and Rickard E. Bensow

To consider fluid compressibility and multiscale features in cavitation simulations, we develop a compressible Euler-Lagrange method in OpenFOAM. Using this method, we investigate the characteristics of sheet/cloud cavitation and tip-leakage cavitation in detail. The results show that the evolution of microscopic bubbles has a great influence on the pressure fluctuations in the flow field. Furthermore, we also study the bubble size distribution and obtain two distinct power laws, namely -4/3 for small bubbles and -10/3 for large bubbles.

[Phys. Rev. Fluids 9, 104304] Published Tue Oct 15, 2024

Author(s): Nelson Poumaëre, Benoît Pier, and Florence Raynal

Residence time distributions (RTDs) of a saturated unstable channel flow are investigated. To this aim, a new Lagrangian method for computing RTDs in any type of open flow is developed. The RTDs obtained display two kinds of pattern: for short travel distances, a pattern of peaks and valleys is observed for long residence times; for longer travel distances, a large probability peak is observed at the travel time of the Tollmien–Schlichting wave.

[Phys. Rev. Fluids 9, 104501] Published Tue Oct 15, 2024

Author(s): Fangbo Li, Weiwei Zhang, and Matthias Ihme

To provide theoretical support for the development of wall models for simulating transcritical flows, we investigate structural properties in the turbulent boundary layers of transcritical flows. Primary innovations include: i) Substantiating the inner-outer interactions in turbulent boundary layers with strong variations in thermodynamic properties; ii) Extending the traditionally incompressible attached eddy model to transcritical flows and provide new scaling for the mixing length model and the wall-attached eddy model by considering inner-outer interactions. This mixing length model can be used in the nonequilibrium wall model for wall-modeled large eddy simulations (WMLES) methodology.

[Phys. Rev. Fluids 9, 104602] Published Tue Oct 15, 2024

Author(s): A. Goerlinger, A. Germa, F. Zoueshtiagh, and A. Duchesne

Jet impacts are a widely used method for surface cooling, prompting significant research into the thermal transfer processes that occur when a jet strikes a heated surface. However, hydrodynamic aspects of the problem have yet to be investigated. We demonstrate that low-inertia jets produce single and centimeter scale drops upon impact, whereas high-inertia jets generate millimeter or submillimeter scale droplets that are ejected axisymmetrically, but at a well-defined angle relative to the horizontal plane. We focus on the second regime and characterize the radius of the contact area below the jet and the properties (ejection angle, speed, radius, energy) of the ejected droplets.

[Phys. Rev. Fluids 9, 104802] Published Tue Oct 15, 2024

Author(s): Jeffrey R. Carpenter and Anirban Guha

Development of offshore wind farms in shallow coastal seas is becoming increasingly widespread, and raises questions as to the potential extent of hydrodynamic impacts to the oceanic environment. This work presents an idealized analytical model to study the potential alterations to the mean ocean currents due to the presence of the offshore wind farm foundation structures and their increased friction. We find that this “blocking” of ocean currents has a simple scaling that depends primarily on the ratio of the friction inside and outside the farms, and depending on the farm characteristics can either be negligible, or change mean currents by around 10% in existing farms in the North Sea.

[Phys. Rev. Fluids 9, 103802] Published Fri Oct 11, 2024

Author(s): Satoshi Matsumoto, Masanobu Inubushi, and Susumu Goto

We have constructed a machine learning-based turbulence model for a shell model. Without ad hoc stabilizations, the constructed model becomes unstable when the cutoff wavenumber is lower than 0.2η−1, where η is the Kolmogorov length. This wavenumber characterizes the subordination of smaller to larger scales in turbulence. This finding highlights a theoretical limitation of turbulence modeling without stabilization techniques.

[Phys. Rev. Fluids 9, 104601] Published Fri Oct 11, 2024

Author(s): Charles W. Wolgemuth

Previous theoretical work predicted that the variance of the velocity in a suspension of spherical particles should diverge with system size, but experiments find the variance asymptotes to a finite value at large system size. Accounting for the inertia of the suspending fluid is shown to resolve this discrepancy and provides predictions for how the variance depends on the volume fraction and densities of the suspended particles and the densities of the fluid.

[Phys. Rev. Fluids 9, L102301] Published Fri Oct 11, 2024

Author(s): Hansol Wee, Ajay Harishankar Kumar, Xiao Liu, and Osman A. Basaran

Liquid sheets are common in technology and nature, and can rupture due to intermolecular van der Waals (vdW) forces if they are sufficiently thin. Recent work by Burton and Taborek has demonstrated that contracting inviscid liquid sheets can break even without vdW forces. Here, we have used two-dimensional simulations and theory to show that when fluid viscosity is small but finite, contracting liquid sheets can escape from pinch-off in the absence of vdW forces due to two distinct mechanisms that depend on Ohnesorge number (a dimensionless group that is proportional to viscosity).

[Phys. Rev. Fluids 9, 103601] Published Wed Oct 09, 2024

Author(s): Yunfan Huang and Moran Wang

Liquid-liquid interfaces typically exhibit a diffuse nature, with an interface thickness that is comparable with the electric double layer under practical conditions. In two-liquid electrokinetics, effects of solvent mixing become particularly pronounced when imbalanced ion partitioning dominates the interface charging, which is intricately linked to the interfacial physico-chemical properties. Our study presents a holistic framework to incorporate the general charging mechanisms into a diffuse interface description, paving the way for future research in two-liquid physico-chemical hydrodynamics, where the constitutive relationship of two-liquid interface charging is pivotal.

[Phys. Rev. Fluids 9, 103701] Published Wed Oct 09, 2024

Author(s): Yang Shen, Hao Zhang, Wei Huang, Chao-yang Liu, and Zhen-guo Wang

Aerodynamic modeling for aircraft often incurs high costs and time-intensive simulations. Our research addresses this challenge by utilizing a transfer learning model that effectively harnesses existing historical aerodynamic data. By leveraging point cloud data from previous simulations, we significantly reduce the need for new, costly simulations while achieving accurate predictions. This approach enables near-real-time aerodynamic analysis across different configurations, offering a solution that maximizes sample efficiency for future aircraft design and optimization.

[Phys. Rev. Fluids 9, 104101] Published Wed Oct 09, 2024

Author(s): Ming Yu, Yibin Du, Qian Wang, Siwei Dong, and Xianxu Yuan

Direct numerical simulations of two-way force-coupled particle-laden compressible turbulent boundary layers are performed to investigate the effects of particles on momentum and kinetic energy transport. The presence of particles suppresses turbulent fluctuations, as reflected in reduced Reynolds stresses, and diminished skin friction and turbulent kinetic energy production. Particle dissipation, due to the relative velocity between fluid and particles, accounts for under 1% of mean viscous dissipation and 10% of turbulent dissipation in the highest mass loading case. The near-wall mean temperature is elevated and influences the particle feedback force and reduced turbulent diffusion.

[Phys. Rev. Fluids 9, 104303] Published Wed Oct 09, 2024