Physical Review Fluids

Author(s): Jordi Casacuberta, Stefan Hickel, and Marios Kotsonis

A physical mechanism has been identified through which a small surface add-on to a swept (angled-back) aircraft wing can delay the development of turbulent disorganized airflow. To reduce the aerodynamic drag of an aicraft, it is important to maximize the extent of smooth laminar airflow around its wings. The mechanism, termed “reverse lift-up effect”, dampens out the critical instability waves leading to turbulent airflow over swept aircraft wings under certain conditions. This may be achieved through particular design of a surface pattern on the aicraft wings. Overall, the present findings aim to contribute to novel design methodologies for future generations of laminar aicraft wings.

[Phys. Rev. Fluids 9, 043903] Published Thu Apr 04, 2024

Author(s): Kevin Cherng, Sanjiva Lele, and Daniel Livescu

We systematically examine how heat conduction between two fluids at different temperatures, large contrasts in transport properties, and sudden changes in transport properties can affect the fully compressible Rayleigh-Taylor instability (RTI) using direct numerical simulations. These variations cause departures from the classical self-similar development of the RTI, along with misalignment between regions of mixing and regions of most intense turbulent activity. Under certain conditions, dynamical quantities such as vorticity and dissipation appear to depend only on the transport properties and not on past flow history.

[Phys. Rev. Fluids 9, 043904] Published Thu Apr 04, 2024

Author(s): Frank G. Jacobitz and Kai Schneider

Taylor’s hypothesis of frozen flow is revisited in homogeneous turbulent shear flow by examining the cancellation properties of Eulerian and convective accelerations at different spatial flow scales. The anti-alignment in the joint pdf given in the figure shows that the Eulerian and convective accelerations cancel for small scales of the turbulent motion, indicating that Taylor’s hypothesis holds. This is not so for the large-scale motion of homogeneous turbulent shear flow.

[Phys. Rev. Fluids 9, 044602] Published Wed Apr 03, 2024

Author(s): Mandeep Saini, Youssef Saade, Daniel Fuster, and Detlef Lohse

Cavitation bubbles are present in a plethora of industrial and medical applications, and their understanding proves crucial to the development and tuning processes. In this study, we perform three dimensional direct numerical simulations (DNS) of multiple cavitation bubbles driven by pressure waves. Similarly observed in previously conducted experiments, it is found that these bubbles can exhibit an asymmetry in the direction of wave propagation. We show that this asymmetry is a consequence of the force induced by the wave on the bubbles, due to the finite speed of sound in the liquid medium.

[Phys. Rev. Fluids 9, 043602] Published Tue Apr 02, 2024

Author(s): Jihoo Moon, Ehsan Mahravan, and Daegyoum Kim

This work investigates the interaction of a viscoelastic solid and a nearby collapsing bubble, using numerical simulations. The Deborah number, which quantifies the relative time scales of solid deformation and bubble dynamics, is found to effectively characterize the bubble-solid interaction. Analyses on the temporal distribution of energy components and the imbalance in pressure distribution surrounding the bubble reveal the rationale behind changes in bubble behaviors with respect to the viscosity and elasticity of the solid. Furthermore, the solid deformation induced by an expanding and collapsing bubble and the shape of the crater formed by a bubble jet are examined.

[Phys. Rev. Fluids 9, 043603] Published Tue Apr 02, 2024

Author(s): Antoine Jouin, Nicola Ciola, Stefania Cherubini, and Jean Christophe Robinet

A three-dimensional boundary layer is the seat of a spatially amplified instability, the crossflow instability, which is encountered for example on swept wings. Beyond a certain amplitude, the flow destabilizes again through a secondary instability. The objective of this work is to show that by using a discrete spatial Floquet method (Block method) from a certain transversally periodic elementary pattern, there can exist a large-scale modulation linked to amplification mechanisms involving detuned modes.

[Phys. Rev. Fluids 9, 043901] Published Tue Apr 02, 2024

Author(s): Alicia Aguilar-Corona, Micheline Abbas, Matthieu Mercier, and Laurent Lacaze

The concept of apparent coefficient of restitution is extended, to describe the wall-bouncing of a cylinder falling in a viscous fluid. When the Stokes number (St) is increased, the coefficient of restitution increases from 0 to 1 over two decades of St, with a critical Stc=75±25 (compared to Stc≈10 for spheres). While rationalizing results from experiments and numerical simulations, we evidence the relevance of lumping the complex details of physical phenomena involved during contact into a simple concept based on the contact apparent roughness and elasticity. The increase of dissipation associated with the contact elasticity is well captured by a model based on solid contact time scale.

[Phys. Rev. Fluids 9, 044301] Published Tue Apr 02, 2024

Author(s): Yongyun Hwang

Two possible and physically meaningful scalings for near-wall streamwise turbulence intensity are derived using velocity spectra. First, if viscous wall effect on the wall-attached inactive motions from the log and outer layers is negligible enough so that the related spectrum does not diminish with Reynolds number, the near-wall turbulence intensity grows indefinitely and scales as ln Reτ. Whereas if it is strong enough so that the near-wall turbulence intensity remains finite at all Reynolds numbers, it scales as 1/ln Reτ. The existing measurement data favors the latter scaling, but the issue will be settled by the availability of such data at higher Reynolds numbers.

[Phys. Rev. Fluids 9, 044601] Published Tue Apr 02, 2024

Author(s): Tristan Gilet

The dynamics of a liquid drop attached to a solid and subjected to arbitrary accelerations can be qualitatively reproduced with a low-dimensional model in which the drop is replaced by a cuboid. The cuboid deforms, vibrates and slides similarly to a drop, while being much simpler to describe mathematically.

[Phys. Rev. Fluids 9, 043601] Published Mon Apr 01, 2024

Author(s): Tianyi Bai, Cheng Cheng, and Lin Fu

This work explores the streamwise inclination angle of attached eddies in both the streamwise velocity and temperature fields using direct numerical simulations of turbulent channel flows. Though it has been well studied in incompressible flows, scarce work exists in compressible flows. A high statistical similarity is found between the velocity and temperature fluctuations. The increasing trend of the inclination angle with Reynolds numbers observed in incompressible flows remains. On the contrary, the current database shows a minor Mach-number effect on it. These conclusions could serve as references for extending the attached eddy model from incompressible to compressible flows.

[Phys. Rev. Fluids 9, 034611] Published Fri Mar 29, 2024

Author(s): Xueqing Zhang, Jeremy D. Newton, Serhiy Yarusevych, and Sean D. Peterson

Liquid droplets depinning due to shear flow aerodynamic loading is relevant to a range of engineering applications. Factors influencing depinning include droplet volume and submergence, shear flow velocity profile and acceleration, and surface wettability. A series of experiments are performed to assess the critical depinning velocity of liquid droplets subjected to an accelerating (i) laminar boundary layer, and (ii) slot jet at varying impingement angles. Critical depinning velocity, cast into a nondimensional critical Weber number, is found to be strongly related to a newly introduced volumetric shape factor, which encapsulates droplet shape and related aspects of substrate wettability.

[Phys. Rev. Fluids 9, 034004] Published Thu Mar 28, 2024

Author(s): T. Desclaux, L. Santana, I. Verdeille, P. Duru, P. Joseph, M. Delarue, and O. Liot

Bioclogging, the clogging of pores with living particles, is a complex process that involves various coupled mechanisms such as hydrodynamics, particle deformability, and polydispersity. This article aims to study a yeast clog’s permeability in a microfluidic device. We can finely measure the clog’s permeability for different hydrodynamic forcing using an original and precise on-chip flow rate meter. It reveals that yeast clog’s permeability decreases when the applied pressure increases, with saturation at high pressure. A semi-empirical model based on a double-porosity structure accurately captures the experimental observations.

[Phys. Rev. Fluids 9, 034202] Published Thu Mar 28, 2024

Author(s): Ghansham Rajendrasingh Chandel, Vishal Sankar Sivasankar, and Siddhartha Das

Active drops through active stresses alter drop evaporation dynamics, puncture evaporating drops, and induce an inside-out evaporation. For an active drop with vortex defect, activity can extend contractile drop evaporation lifetimes by 50% and can accelerate extensile drop evaporation by 33%. Non-intuitive deposition patterns emerge due to evaporation induced fluid flows where the fluid is dragged towards both outer and newly formed inner contact lines. Potential applications of such active drop evaporation include enhanced drop longevity in biological contexts and customizable thin film deposits.

[Phys. Rev. Fluids 9, 033603] Published Wed Mar 27, 2024

Author(s): Théophile Gaichies, Anniina Salonen, Arnaud Antkowiak, and Emmanuelle Rio

The base of a jet impinging on an ultrapure water bath is studied experimentally. At the impact point, a train of capillary waves develops along the jet. A striking observation is the existence of an effective nonzero water/water contact angle between the jet and the meniscus. The rationalization of this finite contact angle requires a full description of the shape of the interface. By doing an analytical matching between the meniscus and the jet, we show that the capillary waves can be considered as reflected waves present to ensure pressure continuity. It is finally shown that the value of the apparent contact angle is fixed by energy minimization.

[Phys. Rev. Fluids 9, 034003] Published Wed Mar 27, 2024

Author(s): Jungyun Kim and S. Balachandar

The present work addresses the effect of non-zero volume fraction in predicting the self-induced velocity of particles in an Euler-Lagrange (EL) simulation. Hundreds of EL simulations of flow over a random distribution of stationary particles, covering a range of Reynolds number and volume fraction, are performed to calculate the undisturbed flow and the self-induced velocities of particles in the presence of neighbors. The most significant finding is that the self-induced correction procedure of an isolated particle can be applied even at finite volume fraction, with a simple volume fraction dependent modification. However, the perturbation induced by neighbors often has a much larger effect.

[Phys. Rev. Fluids 9, 034306] Published Wed Mar 27, 2024

Author(s): Jenny Dingwall and John R. Taylor

We address the long-standing mystery surrounding the rotational bias of convective vortices in the atmosphere (dust devils) and the ocean. We investigate the bias using large-eddy simulations of free convection configured for the ocean, but the idealization of our simulations makes the results more broadly relevant to a wide range of flows. We propose a theory that the addition of many small convective vortices, each of which exhibit a small bias, leads to a much more significant bias for large convective vortices. We apply this new theory to typical convective conditions in the ocean and the terrestrial and Martian atmospheres.

[Phys. Rev. Fluids 9, 033503] Published Tue Mar 26, 2024

Author(s): Daniel B. Shaw and Luc Deike

A bursting bubble’s production of film drops - liquid from the bubble’s cap - is experimentally measured as a function of salinity, temperature, surfactant concentration, and viscosity. Existing theories for the film drainage rate and number of film drops are shown to be robustly consistent with the experimental results, but the lifetime of a bubble across various conditions remains poorly described by existing scalings. The reported relationships describing the importance of various physico-chemical variables on ocean spray emissions are of particular significance to the atmospheric and ocean science community as sea spray aerosols impact radiative transfer and cloud seeding.

[Phys. Rev. Fluids 9, 033602] Published Tue Mar 26, 2024

Author(s): Jiachen Zhao, Zhongzheng Wang, and Emilie Sauret

This study numerically and theoretically investigates the control of viscous fingering using a transverse electric field. Viscous fingering can be delayed in the presence of an electric field due to the electrical force distribution along the fluid interface and completely suppressed when the electric field strength exceeds a certain value. A non-dimensional parameter ϕ is defined based on the force balance, which shows good capability for predicting the transition from an unstable viscous fingering to stable displacement under various fluid properties and flow conditions.

[Phys. Rev. Fluids 9, 033701] Published Tue Mar 26, 2024

Author(s): A. Barrero-Gil and A. Velazquez

This letter revisits the problem of the existence of a universal Strouhal number in the separated flow past a stationary bluff body. Theoretical considerations grounded in flow kinetics yield a concise mathematical expression. The resultant model anticipates a universal Strouhal number equal to 1/2π≈0.159, aligning closely with experimental finding. Furthermore, the model has been expanded to forecast the Strouhal – Reynolds relationship in the supercritical regime, demonstrating a universal nature as it relies solely on critical Strouhal and Reynolds numbers. The circular, square, and triangular cross-section shapes have been used for validation purposes.

[Phys. Rev. Fluids 9, L032701] Published Tue Mar 26, 2024

Author(s): P. D. Ravazzoli, A. G. González, and J. A. Diez

Liquid bridges are present in different four-phase systems, like the one studied here, which is formed by a liquid connecting a horizontal solid substrate with a gas phase while surrounded by another immiscible liquid. We obtain the equilibrium solutions by treating each interface as a simple curve with axial symmetry and constant curvature, and satisfying the boundary conditions given by Neumann’s and Young’s laws. Therefore, the final equilibrium solution is formed by a union of a spherical cap, a catenoid, and portions of onduloids or nodoids. We find that the γ-angle is the control parameter of the problem, as it defines the existence and shape of the final equilibrium solution.

[Phys. Rev. Fluids 9, 033601] Published Mon Mar 25, 2024