# Latest papers in fluid mechanics

### Absolute instability of impinging leading edge vortices in a submodel of a bileaflet mechanical heart valve

Author(s): Hadi Zolfaghari and Dominik Obrist

Mechanical heart valves have been linked to the production of unphysiological turbulent blood flow. As a first step toward a model for laminar-turbulent transition in this flow, an absolute instability in the impinging leading-edge vortices that develop between the valve leaflets is identified.

[Phys. Rev. Fluids 4, 123901] Published Fri Dec 06, 2019

### Mechanisms of airfoil noise near stall conditions

Author(s): Giovanni Lacagnina, Paruchuri Chaitanya, Tim Berk, Jung-Hoon Kim, Phillip Joseph, Bharathram Ganapathisubramani, Seyed Mohammad Hasheminejad, Tze Pei Chong, Oksana Stalnov, Kwing-So Choi, Muhammad Farrukh Shahab, Mohammad Omidyeganeh, and Alfredo Pinelli

A study of mechanisms of noise radiation from airfoils at high angles of attack (partial to full separation) identifies four different noise generation flow regimes. The relationship between noise, surface pressure, and shear layer turbulence is explored.

[Phys. Rev. Fluids 4, 123902] Published Fri Dec 06, 2019

### Mitigation of turbophoresis in particle-laden turbulent channel flows by using incident electric fields

Author(s): M. Di Renzo, P. L. Johnson, M. Bassenne, L. Villafañe, and J. Urzay

The influence of electric forces on inertial particles in turbulent channel flows is explored using direct numerical simulations. It is found that the near-wall buildup of particle concentration due to turbophoresis can be reduced up to 2 orders of magnitude by imposing an ac electric field.

[Phys. Rev. Fluids 4, 124303] Published Fri Dec 06, 2019

### Dispersion and reaction in random flows: Single realization versus ensemble average

Author(s): Antoine Renaud and Jacques Vanneste

The dispersion and reaction of a scalar patch by a random flow is investigated. A comparison of single realizations to the ensemble average shows explicitly that the two concentrations have very different tails, leading to different reaction front velocities in the fast-reacting problem.

[Phys. Rev. Fluids 4, 124502] Published Fri Dec 06, 2019

### Genuine compressibility effects in wall-bounded turbulence

Author(s): Ming Yu, Chun-Xiao Xu, and Sergio Pirozzoli

Finite dilatational effects in wall-bounded compressible turbulence are investigated. We find that finite correlation between the solenoidal and the dilatational parts of the velocity field account for a nonnegligible fraction of the turbulent shear stress near walls.

[Phys. Rev. Fluids 4, 123402] Published Thu Dec 05, 2019

### Effects of phase lag on the hovering flight of damselfly and dragonfly

Author(s): Pei-Yi Zou, Yu-Hsiang Lai, and Jing-Tang Yang

In this work we studied the differences in flight kinematics and aerodynamics that could relate to differences in wing morphologies of a dragonfly and a damselfly. The damselflies and dragonflies normally fly with the fore wing or hind wing in the lead, respectively. The wing of the damselfly is pet...

[Phys. Rev. E 100, 063102] Published Wed Dec 04, 2019

### Rayleigh-Taylor-like instability in a foam film

Author(s): Evgenia Shabalina, Antoine Bérut, Mathilde Cavelier, Arnaud Saint-Jalmes, and Isabelle Cantat

A foam film is prepared in a controlled way, so that the top part of the film is much thicker than the bottom part. This situation is unstable under gravity and Rayleigh-Taylor-like fingers grow.The wavelength and the growth rate depend on the effective gravity and on the thick-film width.

[Phys. Rev. Fluids 4, 124001] Published Wed Dec 04, 2019

### Micron-sized double emulsions and nematic shells generated via tip streaming

Author(s): Kunyun He, Francisco Campo-Cortés, Martyna Goral, Teresa López-León, and José Manuel Gordillo

The smallest compound droplets reported up to date, with outer diameters comparable with a red blood cell, are produced in a continuous way at frequencies exceeding 10 kHz from the capillary breakup of the coaxial cylindrical jets issued from the tip of the conical drops formed using an outer coflow.

[Phys. Rev. Fluids 4, 124201] Published Wed Dec 04, 2019

### Systematic direct simulation Monte Carlo approach to characterize the effects of surface roughness on accommodation coefficients

Author(s): Kishore K. Kammara, Rakesh Kumar, Amit K. Singh, and Arun K. Chinnappan

The effects of surface roughness on accommodation coefficients are studied using the direct simulation Monte-Carlo (DSMC) approach. DSMC simulations are carried out by varying the roughness parameters. An empirical relationship between surface roughness and accommodation coefficients is established.

[Phys. Rev. Fluids 4, 123401] Published Tue Dec 03, 2019

### Membrane filtration with multiple fouling mechanisms

Author(s): Pejman Sanaei and Linda J. Cummings

A simplified mathematical model, which characterizes membrane internal pore structure accounts for multiple simultaneous membrane fouling mechanisms (adsorption, blocking, and cake formation) and is able to predict the optimum permeability.

[Phys. Rev. Fluids 4, 124301] Published Tue Dec 03, 2019

### Inertial drag in granular media

Author(s): Shivakumar Athani and Pierre Rognon

We find evidence that the granular drag force on an object moving through a granular material is strongly enhanced when the object is accelerating. This effect can be explained by showing that the accelerating object accelerates the grains nearby, inducing an inertial resistance.

[Phys. Rev. Fluids 4, 124302] Published Tue Dec 03, 2019

### Generation of a large-scale vorticity in a fast-rotating density-stratified turbulence or turbulent convection

Author(s): Igor Rogachevskii and Nathan Kleeorin

We find an instability resulting in generation of large-scale vorticity in a fast-rotating small-scale turbulence or turbulent convection with inhomogeneous fluid density along the rotational axis in anelastic approximation. The large-scale instability causes excitation of two modes: (i) the mode wi...

[Phys. Rev. E 100, 063101] Published Mon Dec 02, 2019

### Bretherton's buoyant bubble

Author(s): Wassim Dhaouadi and John M. Kolinski

Direct observation of the liquid film binding buoyant Bretherton bubbles reveals a van-der-Waals-stabilized thickness.

[Phys. Rev. Fluids 4, 123601] Published Mon Dec 02, 2019

### Rotational separation after temporary coalescence in binary droplet collisions

Author(s): Kuo-Long Pan, Kuan-Ling Huang, Wan-Ting Hsieh, and Chi-Ru Lu

In binary drop impact, in addition to reflexive separation and stretching separation, which occur at low and high impact angles (B), respectively, rotational separation is found at intermediate B. A generic model is developed to examine the scenarios and the interplay of various factors.

[Phys. Rev. Fluids 4, 123602] Published Mon Dec 02, 2019

### Deep learning of turbulent scalar mixing

Author(s): Maziar Raissi, Hessam Babaee, and Peyman Givi

Based on recent developments in physics-informed deep learning and deep hidden physics models, we put forth a framework for discovering turbulence models from scattered and potentially noisy spatio-temporal measurements of the probability density function.

[Phys. Rev. Fluids 4, 124501] Published Mon Dec 02, 2019

### Impact of capillary drops of complex fluids on a solid surface

The drop impact on a solid surface is studied in the context of complex fluids that exhibit viscoplastic, viscoelastic, and thixotropic behavior. The effects of rheology and surface tension are investigated for a range of corresponding dimensionless numbers associated with each phenomenon. Two usual quantities are employed to understand the drop dynamics, namely, the maximum spreading diameter and the time the drop remains in contact with the solid. Another result is the drop shape evolution, captured by displaying selected instants. The first part of the work is dedicated to examine the influence of capillary effects for more real fluids, in the present case, solutions of Carbopol, kaolin, and bentonite whose mechanical properties are taken from experimental measurements reported in the literature. In the second part, we conduct parametric studies varying the dimensionless numbers that govern the problem. We have shown that the influence of surface tension in yield stress materials is less significant and can be negligible when real parameters are input in the model. On the other hand, Newtonian and viscoelastic fluids are more susceptible to surface tension effects. This quantity tends to decrease maximum spreading diameter and decrease contact time due to its resistance in the spreading stage. While inertia, elasticity, and plastic effects favor the drop to spread and to increase its contact time with the solid substrate, a more thixotropic behavior leads to the opposite trend.

### On collinear steady-state gravity waves with an infinite number of exact resonances

In this paper, we investigate the nonlinear interaction of two primary progressive waves traveling in the same/opposite direction. Without loss of generality, two cases are considered: waves traveling in the same direction and waves traveling in the opposite direction. There exist an infinite number of resonant wave components in each case, corresponding to an infinite number of singularities in mathematical terms. Resonant wave systems with an infinite number of singularities are rather difficult to solve by means of traditional analytic approaches such as perturbation methods. However, this mathematical obstacle is easily cleared by means of the homotopy analysis method (HAM): the infinite number of singularities can be completely avoided by choosing an appropriate auxiliary linear operator in the frame of the HAM. In this way, we successfully gain steady-state systems with an infinite number of resonant components, consisting of the nonlinear interaction of the two primary waves traveling in the same/opposite direction. In physics, this indicates the general existence of so-called steady-state resonant waves, even in the case of an infinite number of resonant components. In mathematics, it illustrates the validity and potential of the HAM to be applied to rather complicated nonlinear problems that may have an infinite number of singularities.

### Up-down asymmetric exact solutions to the Navier-Stokes equations

Exact up-down asymmetric solutions to the Navier-Stokes equations for a viscous and incompressible fluid with time-dependent viscosity ν(t) are derived. Transformations of the exact solutions are defined that produce an infinite sequence of new solutions from each known one. The solutions are presented in terms of elementary functions and have no singularities. Three infinite-dimensional families of new exact axisymmetric unsteady solutions to the viscous magnetohydrodynamics equations are derived. Dynamics of vortex rings and vortex blobs is studied for some exact up-down asymmetric incompressible viscous fluid flows and viscous plasma flows.

### Turbulent drag reduction by polymer additives: Fundamentals and recent advances

A small amount of polymer additives can cause substantial reduction in the energy dissipation and friction loss of turbulent flow. The problem of polymer-induced drag reduction has attracted continuous attention over the seven decades since its discovery. However, changes in research paradigm and perspectives have triggered a wave of new advancements in the past decade. This review attempts to bring researchers of all levels, from beginners to experts, to the forefront of this area. It starts with a comprehensive coverage of fundamental knowledge and classical findings and theories. It then highlights several recent developments that bring fresh insights into long-standing problems. Open questions and ongoing debates are also discussed.

### Turbulent drag reduction by polymer additives: Fundamentals and recent advances

A small amount of polymer additives can cause substantial reduction in the energy dissipation and friction loss of turbulent flow. The problem of polymer-induced drag reduction has attracted continuous attention over the seven decades since its discovery. However, changes in research paradigm and perspectives have triggered a wave of new advancements in the past decade. This review attempts to bring researchers of all levels, from beginners to experts, to the forefront of this area. It starts with a comprehensive coverage of fundamental knowledge and classical findings and theories. It then highlights several recent developments that bring fresh insights into long-standing problems. Open questions and ongoing debates are also discussed.