# Latest papers in fluid mechanics

### Dynamo effect in decaying helical turbulence

Author(s): Axel Brandenburg, Tina Kahniashvili, Sayan Mandal, Alberto Roper Pol, Alexander G. Tevzadze, and Tanmay Vachaspati

It is shown that decaying helical turbulence amplifies a weak magnetic field, which then itself becomes helical. However, it displays a transient behavior different from any other helical or nonhelical magnetic decaying field in a parametric representation between the temporal scaling exponents p(t) and q(t).

[Phys. Rev. Fluids 4, 024608] Published Fri Feb 22, 2019

### Experiments on the low-Reynolds-number settling of a sphere through a fluid interface

Author(s): Paul A. Jarvis, Heidy M. Mader, Herbert E. Huppert, Katharine V. Cashman, and Jon D. Blundy

Experiments on the low-Reynolds number settling of a sphere through a fluid interface focus on the effect of the viscosity ratio and Bond number on the shape of the sphere position curve as a function of time, whether sinking occurs through a tailing or film drainage mode and the sinking timescale.

[Phys. Rev. Fluids 4, 024003] Published Wed Feb 20, 2019

### Experimental lateral wall boundary layer behavior of a differentially rotating split-cylinder flow

Author(s): J. O. Rodríguez-García and J. Burguete

The cylindrical wall boundary layer of a closed cylinder split in two halves at the equator is studied experimentally. When these two parts rotate in exact corotation the internal flow is essentially in solid-body rotation at the angular velocity of both halves. When a slight difference between the ...

[Phys. Rev. E 99, 023111] Published Tue Feb 19, 2019

### Complex behavior very close to the pinching of a liquid free surface

Author(s): M. Rubio, A. Ponce-Torres, E. J. Vega, M. A. Herrada, and J. M. Montanero

High speed observations are made of the pinching off of a pendant drop.

[Phys. Rev. Fluids 4, 021602(R)] Published Tue Feb 19, 2019

### Condensate in quasi-two-dimensional turbulence

Author(s): S. Musacchio and G. Boffetta

Three-dimensional numerical simulations of forced turbulence in a thin layer show the transfer from the forcing to larger scales is by two-dimensional modes, and to smaller scales by three-dimensional modes, the latter providing a viscosity-independent dissipation.

[Phys. Rev. Fluids 4, 022602(R)] Published Tue Feb 19, 2019

### Two-dimensional isotropic turbulent inflow conditions for vortex particle method

Author(s): Sparsh Sharma and Ennes Sarradj

A genetic algorithm is used to optimally populate a random flow with two-dimensional Gaussian-shaped vortices subject to certain constraints.

[Phys. Rev. Fluids 4, 022701(R)] Published Tue Feb 19, 2019

### Can small-scale turbulence approach a quasi-universal state?

Author(s): Shunlin Tang, Robert A. Antonia, Lyazid Djenidi, and Yu Zhou

Analytical considerations, based on the Navier-Stokes equations, which take into account the finite Reynolds number effect, together with all available experimental laboratory data, confirm a tendency towards the universal predictions of K41 as the Reynolds number continues to increase.

[Phys. Rev. Fluids 4, 024607] Published Tue Feb 19, 2019

### Assessment of two vortex formulations for computing forces of a flapping foil at high Reynolds numbers

Author(s): A. Martín-Alcántara and R. Fernandez-Feria

Impulse-based vortex formulas are found appropriate for computing propulsion forces from experiments only when the vortices remain close to the moving foil, as in starting motions or in hovering flight, but are inaccurate in forward flight or swimming, where projection methods perform much better.

[Phys. Rev. Fluids 4, 024702] Published Tue Feb 19, 2019

### Separation regimes of two spheres falling in shear-thinning viscoelastic fluids

Author(s): D. Freire, L. G. Sarasúa, A. Vernet, S. Varela, G. Usera, C. Cabeza, and A. C. Martí

Experiments show that the separation between two spheres settling down in viscoelastic shear-thinning solutions exhibit two different regimes depending on the fluids’ rheological properties. The existence of a previously reported elusive regime is corroborated.

[Phys. Rev. Fluids 4, 023302] Published Fri Feb 15, 2019

### Shape factor of the turbulent boundary layer on a flat plate and the Reynolds shear stress in the outer region

Author(s): P. Phani Kumar and J. Dey

From the linear equation for the velocity defect in the outer boundary layer, the product of mean wall normal velocity and free stream velocity is shown to vary linearly with the Reynolds shear stress.

[Phys. Rev. Fluids 4, 024605] Published Fri Feb 15, 2019

### Properties of the scalar variance transport equation in turbulent channel flow

Author(s): Ang Zhou, Joseph Klewicki, and Sergio Pirozzoli

We study the total scalar variance equation for fully developed turbulent channel ﬂow subject to uniform scalar generation. We find a total scalar variance leading balance four-layer structure similar to total kinetic energy balance. Diﬀerences in the two Karman constants, k and k_θ, are clarified.

[Phys. Rev. Fluids 4, 024606] Published Fri Feb 15, 2019

### Dynamics of bubble-bubble interactions experiencing viscoelastic drag

Author(s): Ekaterina Zilonova, Maxim Solovchuk, and T. W. H. Sheu

The subject of the present theoretical study is the dynamics of bubble-bubble interactions in a viscoelastic medium. First, new equations for calculating the viscoelastic drag exerted on bubbles during their translational motion in a viscoelastic medium are derived. The drag equations are incorporat...

[Phys. Rev. E 99, 023109] Published Thu Feb 14, 2019

### Rotating polygonal depression soliton clusters on the inner surface of a liquid ring

Author(s): Hamid Ait Abderrahmane, Pooya Soltanian Sedeh, Hoi Dick Ng, and Georgios H. Vatistas

We report an experimental observation of rotating depression soliton sets on the inner surface of a viscous liquid ring, carrying background waves. These occur within a rotating shallow layer of oil inside a stationary cylindrical container. The solitons are organized either in single, two, or regul...

[Phys. Rev. E 99, 023110] Published Thu Feb 14, 2019

### Dynamics of long gas bubbles rising in a vertical tube in a cocurrent liquid flow

Author(s): M. Magnini, S. Khodaparast, O. K. Matar, H. A. Stone, and J. R. Thome

Long gas bubbles in a vertical tube do not rise in stagnant liquid if Bo < 0.842. An investigation shows that long bubbles rising in co-current liquid flow exhibit different asymptotic regimes at small liquid capillary numbers, as Bo transitions across this critical value.

[Phys. Rev. Fluids 4, 023601] Published Wed Feb 13, 2019

### Sharp-edged geometric obstacles in microfluidics promote deformability-based sorting of cells

Author(s): Zunmin Zhang, Wei Chien, Ewan Henry, Dmitry A. Fedosov, and Gerhard Gompper

The application of the deterministic lateral displacement approach in microfluidics to deformability-based sorting of cells is explored. Mesoscale hydrodynamics simulations are employed to show that this requires large flow-induced cell deformation in the trajectory-distinguishing flow region.

[Phys. Rev. Fluids 4, 024201] Published Wed Feb 13, 2019

### Turbulence and jet-driven zonal flows: Secondary circulation in rotating fluids due to asymmetric forcing

Author(s): M. J. Burin, K. J. Caspary, E. M. Edlund, R. Ezeta, E. P. Gilson, H. Ji, M. McNulty, J. Squire, and G. R. Tynan

We report on experiments and modeling on a rotating confined liquid that is forced by circumferential jets coaxial with the rotation axis, wherein system-scale secondary flows are observed to emerge. The jets are evenly divided in number between inlets and outlets and have zero net mass transport. F...

[Phys. Rev. E 99, 023108] Published Tue Feb 12, 2019

### Wake characteristics of a utility-scale wind turbine under coherent inflow structures and different operating conditions

Author(s): Xiaolei Yang and Fotis Sotiropoulos

Large-eddy simulations show the similarity of turbine wakes for different tip speed ratios and suggest the coexistence of inflow large eddies and wake shear layer instability as the mechanism for the onset of wake meandering.

[Phys. Rev. Fluids 4, 024604] Published Tue Feb 12, 2019

### Minimum enstrophy principle for two-dimensional inviscid flows around obstacles

Author(s): F. Muller, A. Burbeau, B.-J. Gréa, and P. Sagaut

Large-scale coherent structures emerging in two-dimensional flows can be predicted from statistical physics inspired methods consisting in minimizing the global enstrophy while conserving the total energy and circulation in the Euler equations. In many situations, solid obstacles inside the domain m...

[Phys. Rev. E 99, 023105] Published Mon Feb 11, 2019

### $1/f$ noise and long-term memory of coherent structures in a turbulent shear flow

Author(s): M. Pereira, C. Gissinger, and S. Fauve

A shear flow of liquid metal (Galinstan) is driven in an annular channel by counter-rotating traveling magnetic fields imposed at the end caps. When the traveling velocities are large, the flow is turbulent and its azimuthal component displays random reversals. Power spectra of the velocity field ex...

[Phys. Rev. E 99, 023106] Published Mon Feb 11, 2019

### Shape oscillations of a nonspherical water droplet

Author(s): Mounika Balla, Manoj Kumar Tripathi, and Kirti Chandra Sahu

The dynamics of a nonspherical droplet falling in air is investigated via three-dimensional numerical simulations of the Navier-Stokes and continuity equations. The main focus of this work is to study the effects of the relative influence of surface tension force over the inertial force and the orie...

[Phys. Rev. E 99, 023107] Published Mon Feb 11, 2019