Latest papers in fluid mechanics

Numerical and experimental investigation of the stability of a drop in a single-axis acoustic levitator

Physics of Fluids - Sun, 12/01/2019 - 08:00
Physics of Fluids, Volume DFAL2019, Issue 1, December 2019.
Acoustic levitation can be employed to hold liquid drops in midair, enabling novel applications in X-ray scattering of proteins, amorphous crystallization of solutions, or contactless mixing. Multiple studies have characterized the physical behavior of a levitated drop inside an acoustic field. Here, we present a numerical and experimental study on the acoustic levitation of water drops in a single-axis acoustic levitator consisting of an ultrasonic transducer and an opposing reflector. Instead of modeling an abstract incident acoustic field, our model considers the shape of the drop as well as the real geometry of the levitator. We also use a high-speed camera to observe the disintegration and the undesired oscillations of the drops. Our results show that the insertion of a drop in the levitator provokes a shift in its resonant frequency that depends on the shape of the drop. Second, the levitation behavior depends on whether the levitator operates slightly below or above the resonance. Third, if the levitator is driven above the resonant frequency, it is possible to levitate with more strength and avoid disintegration of the drop. This research provides an insight on how to achieve more stable experiments that avoid the bursting and undesired oscillations of the levitated sample. We hope that it will facilitate numerous experiments involving acoustically levitated liquid drops.

Inverse energy cascade and vortical structure in the near-wall region of turbulent channel flow

Physical Review Fluids - Wed, 11/27/2019 - 10:00

Author(s): Fujihiro Hamba

An inverse cascade is found in the energy transfer in scale space of channel flow. The conditional average of the velocity field associated with the inverse cascade or negative subgrid-scale production reveals a long streamwise vortex near the wall with a short vortex in the upstream region.


[Phys. Rev. Fluids 4, 114609] Published Wed Nov 27, 2019

Generation of bursting magnetic fields by nonperiodic torsional flows

Physical Review E - Tue, 11/26/2019 - 10:00

Author(s): Juan Sánchez Umbría and Marta Net

A mechanism for the cyclic generation of bursts of magnetic fields by nonlinear torsional flows of complex time dependence but very simple spatial structure is described. These flows were obtained numerically as axisymmetric solutions of convection in internally heated rotating fluid spheres in the ...


[Phys. Rev. E 100, 053110] Published Tue Nov 26, 2019

Evolution of Gaussian wave packets in capillary jets

Physical Review E - Tue, 11/26/2019 - 10:00

Author(s): F. J. García, H. González, F. J. Gómez-Aguilar, A. A. Castrejón-Pita, and J. R. Castrejón-Pita

A temporal analysis of the evolution of Gaussian wave packets in cylindrical capillary jets is presented through both a linear two-mode formulation and a one-dimensional nonlinear numerical scheme. These analyses are normally applicable to arbitrary initial conditions but our study focuses on pure-i...


[Phys. Rev. E 100, 053111] Published Tue Nov 26, 2019

Effects of artery size on the hydrodynamic diffusivity of red cells and other contained particles

Physical Review Fluids - Tue, 11/26/2019 - 10:00

Author(s): Lydia I. Kolitsi and Stergios G. Yiantsios

A numerical study finds that shear induced diffusivities increase with distance from the channel walls because of increasing mobility, up to an artery-size dependent maximum due to the diminishing local shear rate. At the center they remain finite owing to crossflow sweeps generated from the interaction of near-wall eddies.


[Phys. Rev. Fluids 4, 113103] Published Tue Nov 26, 2019

Dynamics of two-dimensional flow around a circular cylinder with flexible filaments attached

Physical Review E - Mon, 11/25/2019 - 10:00

Author(s): Jian Deng, Xuerui Mao, and Fangfang Xie

A direction adaptive approach for the reduction of drag and the suppression of lift fluctuation in flow passing a circular cylinder is developed. Flexible filaments are attached to the surface of the cylinder, and different configurations, including the number, lengths, and angles of attachment of t...


[Phys. Rev. E 100, 053107] Published Mon Nov 25, 2019

Two jets during the impact of viscous droplets onto a less-viscous liquid pool

Physical Review E - Mon, 11/25/2019 - 10:00

Author(s): Quan Ding, Tianyou Wang, and Zhizhao Che

It is generally accepted that a Worthington jet occurs when a droplet impacts onto a liquid pool. However, in this experimental study of the impact of viscous droplets onto a less-viscous liquid pool, we identify another jet besides the Worthington jet, forming a two-jet phenomenon. The two jets, a ...


[Phys. Rev. E 100, 053108] Published Mon Nov 25, 2019

Interfacial dynamics and pinch-off singularities for axially symmetric Darcy flow

Physical Review E - Mon, 11/25/2019 - 10:00

Author(s): Liam C. Morrow, Michael C. Dallaston, and Scott W. McCue

We study a model for the evolution of an axially symmetric bubble of inviscid fluid in a homogeneous porous medium otherwise saturated with a viscous fluid. The model is a moving boundary problem that is a higher-dimensional analog of Hele-Shaw flow. Here we are concerned with the development of pin...


[Phys. Rev. E 100, 053109] Published Mon Nov 25, 2019

Normal impact force of Rayleigh jets

Physical Review Fluids - Mon, 11/25/2019 - 10:00

Author(s): Benjamin R. Mitchell, Joseph C. Klewicki, Yannis P. Korkolis, and Brad L. Kinsey

The normal impact force of Rayleigh jets is investigated for three possible impact scenarios: a) steady-state jet, b) wavy jet, and c) droplet train. Owing to momentum conservation, the peak force experienced by the droplet train is over three times greater than that of the steady jet.


[Phys. Rev. Fluids 4, 113603] Published Mon Nov 25, 2019

Direct numerical simulation of low Reynolds number turbulent swirling pipe flows

Physical Review Fluids - Mon, 11/25/2019 - 10:00

Author(s): Rey C. Chin and Jimmy Philip

The axial and azimuthal mean momentum equation for swirling pipe flows are derived. Increasing the swirl strength increases the extent of the inertial region by pushing the beginning of the inertial region closer to the wall. This is due to the axial viscous forces rather than the azimuthal ones.


[Phys. Rev. Fluids 4, 114607] Published Mon Nov 25, 2019

Lyapunov spectrum of forced homogeneous isotropic turbulent flows

Physical Review Fluids - Mon, 11/25/2019 - 10:00

Author(s): Malik Hassanaly and Venkat Raman

A local point of view is adopted on the chaoticity of turbulent flow, using the Lyapunov analysis. Among other findings, it is found that chaotic perturbation growth is a highly localized phenomenon that occurs in regions of high-velocity gradient but not in regions of high turbulence intensity.


[Phys. Rev. Fluids 4, 114608] Published Mon Nov 25, 2019

Flow past a rotating hydrophobic/nonhydrophobic circular cylinder in a flowing soap film

Physical Review Fluids - Mon, 11/25/2019 - 10:00

Author(s): Navya Geethika Chikkam and Sanjay Kumar

Flow past a rotating circular cylinder is investigated experimentally in flowing soap film at Re of 200 to 250. Vortex shedding suppression is not found. A mode of single signed vortices dependent on surface hydrophobicity, with a rotation sense opposite to that of cylinder rotation, is observed.


[Phys. Rev. Fluids 4, 114802] Published Mon Nov 25, 2019

Predicting droplet velocity in a Hele-Shaw cell

Physical Review Fluids - Fri, 11/22/2019 - 10:00

Author(s): Benjamin Reichert, Isabelle Cantat, and Marie-Caroline Jullien

We study the motion of a low viscous non-wetting droplet in a Hele-Shaw cell while it is pushed by an external phase at low capillary numbers. The velocity can be strongly affected by a stagnant cap at the rear of the drop and we propose a model that remarkably reproduces the experimental data.


[Phys. Rev. Fluids 4, 113602] Published Fri Nov 22, 2019

Bifurcations to turbulence in transitional channel flow

Physical Review Fluids - Fri, 11/22/2019 - 10:00

Author(s): Masaki Shimizu and Paul Manneville

In channel flow, the transition from turbulence displays a crossover from laminar-turbulent patterns following a two-dimensional directed-percolation scenario to a regime with localized turbulent bands statistically propagating along a single direction. A simple model accounts for this bifurcation.


[Phys. Rev. Fluids 4, 113903] Published Fri Nov 22, 2019

Marine crustaceans with hairy appendages: Role of hydrodynamic boundary layers in sensing and feeding

Physical Review Fluids - Fri, 11/22/2019 - 10:00

Author(s): Kaitlyn Hood, M. S. Suryateja Jammalamadaka, and A. E. Hosoi

Crustaceans use their hairy appendages to sense and track food in flows with intermediate Reynolds number. A reduced order model is developed to predict the flow phase and generate a design principle for engineering flows past hairy surfaces.


[Phys. Rev. Fluids 4, 114102] Published Fri Nov 22, 2019

Laboratory study of the wave-induced mean flow and set-down in unidirectional surface gravity wave packets on finite water depth

Physical Review Fluids - Fri, 11/22/2019 - 10:00

Author(s): R. Calvert, C. Whittaker, A. Raby, P. H. Taylor, A. G. L. Borthwick, and T. S. van den Bremer

We derive a multiple-scales solution for the Eulerian mean flow under wavepackets, which is driven by the divergence of Stokes drift and the setdown. This solution is valid for all water depths and is validated by flume experiments and recovers all previous solutions in their respective limits.


[Phys. Rev. Fluids 4, 114801] Published Fri Nov 22, 2019

Chaos and mixing in self-propelled droplets

Physical Review Fluids - Thu, 11/21/2019 - 10:00

Author(s): Reiner Kree and Annette Zippelius

An investigation shows that simple flow inside a droplet can propel it along regular trajectories while simultaneously leading to chaotic motion of tracer particles. The resulting advective mixing can accelerate and even dominate transport by diffusion for biologically plausible Péclet and Batchelor numbers.


[Phys. Rev. Fluids 4, 113102] Published Thu Nov 21, 2019

Data-based, reduced-order, dynamic estimator for reconstruction of nonlinear flows exhibiting limit-cycle oscillations

Physical Review Fluids - Thu, 11/21/2019 - 10:00

Author(s): Juan Guzmán-Iñigo, Markus A. Sodar, and George Papadakis

A data-based, linear dynamic estimator is developed to reconstruct the velocity field around an airfoil using measurements from a single sensor point in the wake. The performance of the estimator is very robust to the sensor location and to small changes in Reynolds number.


[Phys. Rev. Fluids 4, 114703] Published Thu Nov 21, 2019

The suction effect during freak wave slamming on a fixed platform deck: Smoothed particle hydrodynamics simulation and experimental study

Physics of Fluids - Thu, 11/21/2019 - 02:58
Physics of Fluids, Volume 31, Issue 11, November 2019.
During the process of wave slamming on a structure with sharp corners, the wave receding after wave impingement can induce strong negative pressure (relative to the atmospheric pressure) at the bottom of the structure, which is called the suction effect. From the practical point of view, the suction force induced by the negative pressure, coinciding with the gravity force, pulls the structure down and hence increases the risk of structural damage. In this work, the smoothed particle hydrodynamics (SPH) method, more specifically the δ+SPH model, is adopted to simulate the freak wave slamming on a fixed platform with the consideration of the suction effect, i.e., negative pressure, which is a challenging issue because it can cause the so-called tensile instability in SPH simulations. The key to overcome the numerical issue is to use a numerical technique named tensile instability control (TIC). Comparative studies using SPH models with and without TIC will show the importance of this technique in capturing the negative pressure. It is also found that using a two-phase simulation that takes the air phase into account is essential for an SPH model to accurately predict the impact pressure during the initial slamming stage. The freak wave impacts with different water depths are studied. All the multiphase SPH results are validated by our experimental data. The wave kinematics/dynamics and wave impact features in the wave-structure interacting process are discussed, and the mechanism of the suction effect characterized by the negative pressure is carefully analyzed.

Numerical simulation of a drop impact on a superhydrophobic surface with a wire

Physics of Fluids - Thu, 11/21/2019 - 02:58
Physics of Fluids, Volume 31, Issue 11, November 2019.
Superhydrophobic surfaces patterned with macroscale (≈1 mm) structures have gained increasing interest in the past years because of their potential in reducing the contact time between impacting liquid drops and the solid surface. The reduced wettability of these surfaces is of interest in numerous technical applications, as, for example, in anti-icing on airplane wings. Several experimental studies have been carried out on this topic in the literature; on the other hand, only very limited numerical investigations are available in the literature. In this paper, we present a numerical study based on a volume of fluid code for direct numerical simulation of incompressible multiphase flows. A necessary condition for the realization of this study was the implementation of arbitrary-shaped boundaries using a Cartesian grid system. Our implementation of embedded boundaries is based on a volume fraction representation of the boundaries and on a piecewise linear approximation of their surface. The discretized boundaries are then cut off from the computational domain, leading to an altered formulation of the discretized governing equations. To validate the method, we show simulation results for different impact velocities for the case of a droplet impacting on a wire, which has been investigated experimentally in the literature. The simulations show good agreement in terms of contact time and impact morphology, thus, showing the validity of the implementation. Moreover, an extensive analysis of the velocity field for this setup is presented, helping us to better understand the underlying physical phenomena.

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