# Latest papers in fluid mechanics

### In-phase synchronization between two auto-oscillating bubbles

Author(s): Dang Minh Nguyen, Muttikulangara Swaminathan Sanathanan, Jianmin Miao, David Fernandez Rivas, and Claus-Dieter Ohl

Two bubbles communicate with each other acoustically without being affected by any external sound field. We have performed an experiment showing that at the right distance, their connection becomes strong enough that they synchronize perfectly and eventually merge as one.

[Phys. Rev. Fluids 4, 043601] Published Mon Apr 22, 2019

### Comparing free surface and interface motion in electromagnetically driven thin-layer flows

Author(s): Benjamin C. Martell, Jeffrey Tithof, and Douglas H. Kelley

Particles at the top and bottom of a thin fluid layer are simultaneously tracked to measure how well such experiments model two-dimensional flow. Top and bottom flow directions align well. The bottom-to-top speed ratio nearly matches laminar predictions, even for chaotic flow, except in misaligned regions.

[Phys. Rev. Fluids 4, 043904] Published Mon Apr 22, 2019

### Dynamics of potential vorticity staircase evolution and step mergers in a reduced model of beta-plane turbulence

Author(s): M. A. Malkov and P. H. Diamond

Quasigeostrophic turbulence can self-organize into “staircases”—periodic arrays of turbulence separated by layers of potential vorticity (PV) gradients. These patterns are recovered from a reduced model that couples mean PV and potential enstrophy. Sharpening occurs due to bistable mixing of mean PV.

[Phys. Rev. Fluids 4, 044503] Published Mon Apr 22, 2019

### Harmonic to subharmonic transition of the Faraday instability in miscible fluids

Author(s): Antoine Briard, Benoît-Joseph Gréa, and Louis Gostiaux

Numerical simulations of the turbulent Faraday instability in miscible fluids reveal that when the mixing layer grows in size, there exists a highly anisotropic harmonic regime before the subharmonic one, in which background potential energy increases as a signature of irreversible mixing.

[Phys. Rev. Fluids 4, 044502] Published Fri Apr 19, 2019

### Boundary layer turbulence and freestream turbulence interface, turbulent spot and freestream turbulence interface, laminar boundary layer and freestream turbulence interface

We study the boundary-layer turbulence and freestream turbulence interface (BTFTI), the turbulent spot and freestream turbulence interface (TSFTI), and the laminar boundary-layer and freestream turbulence interface (LBFTI) using direct simulation. Grid spacings in the freestream are less than 1 Kolmogorov length scale during transition. Probability density functions of temperature and its derivatives are used to select the interface identification threshold, corroborated by a vorticity-based method. The interfaces so detected are confirmed to be physical a posteriori by the distinctive quasi-step-jump behavior in the swirling strength and temperature statistics along traverses normal to the BTFTI and TSFTI. No interface-normal inflection is detected across the LBFTI for either swirling strength, temperature, vorticity magnitude, Reynolds shear stress, streamwise velocity, normal velocity, or turbulence kinetic energy. The present direct numerical simulation data thus cast serious doubts on the shear-sheltering hypothesis/theory, which asserts that a subset of freestream fluctuations is blocked by the LBFTI. In the early stage of transition, quasi-spanwise structures exist on the LBFTI. The TSFTI shape is dominated by head prints of concentrated hairpin vortices. Further downstream, the BTFTI geometry is strongly modulated by groves of hairpin vortices (the boundary layer large-scale motions) with a distinct streamwise preferential orientation. Streamwise velocity and turbulence kinetic energy only exhibit minor plateaus (rather than quasi-step-jump) across the BTFTI and the TSFTI. We emphasize that it is more meaningful and important to acquire reproducible and reliable interface-normal statistics prior to considering any plausible substructures and elusive transient dynamics of the BTFTI, TSFTI, and LBFTI.

### Breakup of a leaky dielectric drop in a uniform electric field

Author(s): Qingming Dong and Amalendu Sau

Electrohydrodynamic (EHD) breakup phenomena for a leaky dielectric drop suspended in another immiscible viscous dielectric and subjected to a uniform electric field are examined using the leaky dielectric theory and the explicit forcing lattice Boltzmann method, by taking into account full nonlinear...

[Phys. Rev. E 99, 043106] Published Thu Apr 18, 2019

### Dynamics of deformable straight and curved prolate capsules in simple shear flow

Author(s): Xiao Zhang, Wilbur A. Lam, and Michael D. Graham

A computational study of the motion of slightly deformable curved prolate capsules—a model for sickle red blood cells—in shear flow, shows that the average of the long-time orbit of the capsules, regardless of the initial orientation, can be well approximated using the Jeffery orbit solution.

[Phys. Rev. Fluids 4, 043103] Published Thu Apr 18, 2019

### Spirals and ribbons in counter-rotating Taylor-Couette flow: Frequencies from mean flows and heteroclinic orbits

Author(s): Yacine Bengana and Laurette S. Tuckerman

In simulations of counter-rotating Taylor-Couette flow, both spirals and ribbons are found to obey the RZIF property. We find two heteroclinic cycles, one whose excursions resemble ribbons, and the other which remains axisymmetric. Both cycles connect the same two stacked Taylor-vortex pairs.

[Phys. Rev. Fluids 4, 044402] Published Thu Apr 18, 2019

### Effect of shear on coherent structures in turbulent convection

Author(s): Prafulla P. Shevkar, G. S. Gunasegarane, Sanal K. Mohanan, and Baburaj A. Puthenveettil

In the presence of shear, the mean spacing between coherent structures on horizontal surfaces in turbulent Rayleigh-Bénard convection, in excess of the no-shear spacing, is shown to be proportional to Re3Zsh/(DRa), where Zsh is the viscous-shear length and D a nonmonotonic function of Pr.

[Phys. Rev. Fluids 4, 043502] Published Wed Apr 17, 2019

### Are extreme dissipation events predictable in turbulent fluid flows?

Author(s): Patrick J. Blonigan, Mohammad Farazmand, and Themistoklis P. Sapsis

A method that combines dynamics and statistics for the underlying attractor allows for the extraction of characteristic states that precede laminarization events which subsequently lead to extreme dissipation episodes.

[Phys. Rev. Fluids 4, 044606] Published Wed Apr 17, 2019

### Relaxation processes in carbon dioxide

The present study is aimed in providing a framework for applying different continuum models of relaxation processes in carbon dioxide flows. Kinetic equations for the distribution function are written taking into account the CO2 structure and various mechanisms of vibrational relaxation; collision operators for different internal energy transitions are derived. For weak non-equilibrium conditions, a one-temperature model is developed with emphasis to the bulk viscosity phenomenon. For strong non-equilibrium conditions, multi-temperature models are introduced, and their advantages and limitations are discussed. A general algorithm for calculating vibrational relaxation time in polyatomic molecules is proposed. Bulk viscosity coefficients are studied in the temperature range 200–2500 K; it is shown that uncoupling rotational and vibrational modes results in essentially overpredicted values of the bulk viscosity coefficient at low temperatures. The shock wave structure in CO2 is studied using the continuum models and compared with the solution obtained in the frame of the model kinetic approach; the effect of bulk viscosity on the shock wave width and temperature profile is evaluated. It is concluded that well justified choice of the model extends considerably the range of applicability of the continuum approach for non-equilibrium flow simulations.

### Modifications to the gradient schemes on unstructured cell centered grids for the accurate determination of gradients near conductivity changes

Gradient schemes for the cell centered finite volume method on unstructured grids, namely, the divergence theorem and the least squares schemes, have been widely adopted because they have reached a high precision for most applications. These schemes assume continuously differentiable fields for the calculation of the gradients. However, this assumption is violated in the vicinity of conductivity jumps between cells. It is shown that this deficiency leads to a wrong calculation of the gradients and thus the flux density in cells near conductivity changes. For large conductivity jumps, the error of the flux density can exceed several orders of magnitude. Based on theoretical considerations, flux conservative versions of the schemes are derived for the central gradient scheme and extended to the divergence theorem and least squares schemes. The modified schemes named flux conservative divergence theorem and flux conservative least squares take the nonlinearity of a conservation variable near conductivity changes into account and eliminate the error made by the assumption of a continuously differentiable field. The schemes are demonstrated on Cartesian and highly skewed grids with different grid resolutions with a large conductivity jump. The error of the flux density is shown to be reduced by several orders of magnitude up to machine precision for Cartesian grids.

### Linear motion of multiple superposed viscous fluids

Author(s): Magnus Vartdal and Andreas N. Osnes

In this paper the small-amplitude motion of multiple superposed viscous fluids is studied as a linearized initial-value problem. The analysis results in a closed set of equations for the Laplace transformed amplitudes of the interfaces that can be inverted numerically. The derived equations also con...

[Phys. Rev. E 99, 043104] Published Tue Apr 16, 2019

### Contact-angle-hysteresis effects on a drop sitting on an incline plane

Author(s): Pablo D. Ravazzoli, Ingrith Cuellar, Alejandro G. González, and Javier A. Diez

We study the contact-angle hysteresis and morphology changes of a liquid drop sitting on a solid substrate inclined with respect to the horizontal at an angle α. This one is always smaller than the critical angle, αcrit, above which the drop would start to slide down. The hysteresis cycle is perform...

[Phys. Rev. E 99, 043105] Published Tue Apr 16, 2019

### Self-acceleration and global pulsation in expanding laminar ${\mathrm{H}}_{2}\text{−}{\mathrm{O}}_{2}\text{−}{\mathrm{N}}_{2}$ flames

Author(s): Jialong Huo, Abhishek Saha, Tao Shu, Zhuyin Ren, and Chung K. Law

Self-acceleration and global pulsation of spherically expanding hydrogen-air flames, with the pulsation arising from the growing and splitting of cellular structures due to flame instability, is studied.

[Phys. Rev. Fluids 4, 043201] Published Tue Apr 16, 2019

### Numerical study of thermal convection induced by centrifugal buoyancy in a rotating cylindrical annulus

Author(s): Changwoo Kang, Antoine Meyer, Harunori N. Yoshikawa, and Innocent Mutabazi

An investigation shows that thermal convection due to centrifugal buoyancy in a cylindrical annulus occurs in the form of drifting columnar vortices. For Rayleigh number Ra > 105, the Coriolis buoyancy deforms the columns and thermal plumes.

[Phys. Rev. Fluids 4, 043501] Published Tue Apr 16, 2019

### Extreme nonequilibrium electrophoresis of an ion-selective microgranule

Author(s): G. S. Ganchenko, E. A. Frants, V. S. Shelistov, N. V. Nikitin, S. Amiroudine, and E. A. Demekhin

We develop a semi-analytic approach to the electrophoresis of an ion-selective particle and show that it compares well to direct numerical simulations and experimental data. A nested concentration profile and a scaling of the the electrophoretic velocity with electric field are found.

[Phys. Rev. Fluids 4, 043703] Published Tue Apr 16, 2019

### Direct numerical simulations of liquid films in two dimensions under horizontal and vertical external vibrations

Author(s): Sebastian Richter and Michael Bestehorn

A liquid film on a horizontally oscillating solid substrate is studied numerically. Breakup of the film is observed as well as formation and coarsening of isolated drops. For oscillations that violate mirror symmetry in the horizontal direction, a mean flow occurs, and the drops begin to move laterally.

[Phys. Rev. Fluids 4, 044004] Published Tue Apr 16, 2019

### Electrostatic assist of liquid transfer between plates and cavities

Author(s): Chung-Hsuan Huang, Marcio S. Carvalho, and Satish Kumar

Liquid removal from cavities is a major challenge in industrial printing processes. It is shown that applying an electric field causes the contact line on the cavity wall to slip. The effect is more pronounced for a deep cavity, resulting in a larger percentage of liquid transferred compared to a shallow cavity.

[Phys. Rev. Fluids 4, 044005] Published Tue Apr 16, 2019

### Flow in a containerless liquid system: Ring-sheared drop with finite surface shear viscosity

Author(s): Shreyash Gulati, Frank P. Riley, Amir H. Hirsa, and Juan M. Lopez

In microgravity a stationary contact ring can constrain a large liquid drop and a second contact ring in rotation can impose shear. With numerical simulations we show that significant flow occurs for finite surface interfacial viscosity, as for example, when protein is adsorbed to the interface.

[Phys. Rev. Fluids 4, 044006] Published Tue Apr 16, 2019