What happens to the vortex structures when the rising bubble transits from zigzag to spiral?

2017 ◽  
Vol 828 ◽  
pp. 353-373 ◽  
Author(s):  
Jie Zhang ◽  
Ming-Jiu Ni
Keyword(s):  
Angular Velocity ◽  
Magnetic Fields ◽  
Physical Mechanism ◽  
Critical Value ◽  
Theoretical Explanation ◽  
Vortex Structures ◽  
Direct Numerical Simulations ◽  
Vortex Pairs ◽  
Spiral Path ◽  
Rising Bubble

It has been demonstrated by many experiments carried out over the last 60 years that in certain liquids a single millimetre-sized bubble will rise within an unstable path, which is sometimes observed to transit from zigzag to spiral. After performing several groups of direct numerical simulations, the present work gives a theoretical explanation to reveal the physical mechanism causing the transition, and the results are presented in two parts. In the first part, in which a freely rising bubble is simulated, equal-strength vortex pairs are observed to shed twice during a period of the pure zigzag path, and this type of motion is triggered by the amounts of streamwise vorticities accumulated on the bubble interface, when a critical value is reached. However, when the balance between the counter-rotating vortices is broken, an angular velocity is induced between the asymmetric vortex pairs, driving the bubble to rise in an opposite spiral path. Therefore, although there is no preference of the spiral direction as observed in experiments, it is actually determined by the sign of the stronger vortex thread. In the second part, external vertical magnetic fields are imposed onto the spirally rising bubble in order to further confirm the relations between the vortex structures and the unstable path patterns. As shown in our previous studies (Zhang & Ni, Phys. Fluids, vol. 26 (10), 2014, 102102), the strength of the double-threaded vortex pairs, as well as the imbalance between them, will be weakened under magnetic fields. Therefore, as the vortex pairs become more symmetric, the rotating radius of the spirally rising bubble is observed to decrease. We try to answer the question, put forward by Shew et al. (2005, Preprint, ENS, Lyon), ‘what caused the bubble to transit from zigzag to spiral naturally?’

Numerical Simulations of Electric Field Driven Self-Assembly of Monolayers of Mixtures of Nanoparticles

2017 ◽  
Author(s):  
E. Amah ◽  
N. Musunuri ◽  
Ian S. Fischer ◽  
Pushpendra Singh
Keyword(s):  
Electric Field ◽  
Physical Properties ◽  
Numerical Simulations ◽  
Self Assembly ◽  
Composite Particle ◽  
Critical Value ◽  
Direct Numerical Simulations ◽  
Composite Particles ◽  
Liquid Interfaces ◽  
Induced Dipole

We numerically study the process of self-assembly of particle mixtures on fluid-liquid interfaces when an electric field is applied in the direction normal to the interface. The force law for the dependence of the electric field induced dipole-dipole and capillary forces on the distance between the particles and their physical properties obtained in an earlier study by performing direct numerical simulations is used for conducting simulations. The inter-particle forces cause mixtures of nanoparticles to self-assemble into molecular-like hierarchical arrangements consisting of composite particles which are organized in a pattern. However, there is a critical electric intensity value below which particles move under the influence of Brownian forces and do not self-assemble. Above the critical value, when the particles sizes differed by a factor of two or more, the composite particle has a larger particle at its core and several smaller particles forming a ring around it. Approximately same sized particles, when their concentrations are approximately equal, form binary particles or chains (analogous to polymeric molecules) in which positively and negatively polarized particles alternate, but when their concentrations differ the particles whose concentration is larger form rings around the particles with smaller concentration.

Scission and Healing in a Spinning Elastomeric Cylinder at Elevated Temperature

2002 ◽  
Vol 69 (5) ◽  
pp. 602-609 ◽  
Author(s):  
A. S. Wineman ◽  
J. A. Shaw
Keyword(s):  
Elevated Temperature ◽  
Angular Velocity ◽  
Critical Value ◽  
Anomalous Behavior ◽  
Molecular Networks ◽  
Material System ◽  
Elastomeric Material ◽  
Angular Velocities ◽  
Macromolecular Network ◽  
Temperature Responses

When an elastomeric material is subject to sufficiently high temperature, macromolecular network junctions can undergo time-dependent scission and re-crosslinking (healing). The material system then consists of molecular networks with different reference states. A constitutive framework, based on the experimental work of Tobolsky, is used to determine the evolution of deformation of a solid rubber cylinder spinning at constant angular velocity at an elevated temperature. Responses based on underlying neo-Hookean, Mooney-Rivlin, and Arruda-Boyce models, were solved numerically and compared. Different amounts of healing were studied for each case. For neo-Hookean molecular networks, there may be a critical finite time when the radius grows infinitely fast and the cylinder “blows up.” This time depends on the angular velocity and the rate of re-cross linking. In addition, no solution was possible for angular velocities above a critical value, even without the effects of scission. Such anomalous behavior does not occur for Mooney-Rivlin or Arruda-Boyce network response.

scholarly journals The Magnetic Fields of Pulsars

1974 ◽  
Vol 64 ◽  
pp. 187-187
Author(s):  
D. M. Sedrakian
Keyword(s):  
Magnetic Field ◽  
Angular Velocity ◽  
Magnetic Fields ◽  
Relative Motion ◽  
Kinematic Viscosity ◽  
Charged Particles ◽  
Normal Fluid ◽  
Inertia Effects ◽  
Generation Mechanisms ◽  
Image Position

Two generation mechanisms of magnetic fields in pulsars are considered.If the temperature of a star is more than 108K, the star consists of a normal fluid of neutrons, protons and electrons. Because the angular velocity of pulsars is not constant dω/dt ≠0, inertia effects can occur, and generate magnetic fields through the relative motion of charged particles with different masses. The kinematic viscosity of electrons is 30 times larger than that of protons; hence electrons move with the crust, but the proton-neutron fluid will move relative to the electrons. The magnetic momentum can be calculated by the following formula where Meff = Mp + Mn(Nn/Np), R = radius of the star, σ = conductivity. For typical neutron stars we have dω/dt~ 10-8 s-2, R~106 cm, σ~1029 s-1 and we get a magnetic field of the order of 1010 G.

scholarly journals Some estimates on the space scales of vortex pairs emitted from river mouths

2001 ◽  
Vol 8 (1/2) ◽  
pp. 1-7 ◽  
Author(s):  
V. P. Goncharov ◽  
V. I. Pavlov
Keyword(s):  
Point Vortex ◽  
Vortex Pair ◽  
Vortex Structures ◽  
Shelf Zone ◽  
Vortex Pairs ◽  
Polygonal Boundary ◽  
Dipole Vortex ◽  
Qualitative Classification ◽  
River Mouths

Abstract. Two-dimensional vortex pairs are frequently observed in geophysical conditions, for example, in a shelf zone of the ocean near river mouths. The main aims of the work are to estimate the space scales of such vortex structures, to analyze possible scenarios of vortex pair motion and to give the qualitative classification of their trajectories. We discuss some features of the motion of strong localized vorticity concentrations in a given flow in the presence of boundaries. The analyses are made in the framework of a 2D point vortex mo-del with an open polygonal boundary. Estimations are made for the characteristic parameters of dipole vortex structures emitted from river mouths into the open ocean.

Kinematics of an Elastic Sphere Rolling on a Plane and Between Two Planes

1993 ◽  
Vol 115 (3) ◽  
pp. 476-480
Author(s):  
E. Kingsbury
Keyword(s):  
Angular Velocity ◽  
Elastic Deformation ◽  
General Expression ◽  
Velocity Vector ◽  
Elastic Sphere ◽  
Angular Velocity Vector ◽  
Spiral Path ◽  
Symmetry Argument

A sphere rolling between a stationary and a spinning plane traces out a spiral path, even under quasistatic conditions. Published theory suggests that radial creep due to pivot produces the spiral path. We show experimentally a component of the sphere’s angular velocity not considered in the published analysis, raising questions about pivot in producing the spiral. We give a general expression for the sphere angular velocity vector which accommodates a linear, circular or spiral path, pivot or no pivot, and one or two planes in contact. We show that a sphere can roll in a circle on one or between two plane without pivot, but not between a stationary and a spinning plane. We show that a circumferential component of angular velocity results in a spiral path. A symmetry argument suggests that the spiral might be due to elastic deformation in the planes rather than to pivot, but the question is still open.

scholarly journals Magnetic fields and dynamics of the Sun's interior

2008 ◽  
Vol 4 (S259) ◽  
pp. 147-158
Author(s):  
Alexander G. Kosovichev
Keyword(s):  
Magnetic Fields ◽  
Physical Mechanism ◽  
Active Regions ◽  
Magnetic Activity ◽  
Solar Interior ◽  
Global Dynamics ◽  
The Sun ◽  
New Knowledge ◽  
Solar Magnetic Activity ◽  
Key Questions

AbstractAdvances in helioseismology provide new knowledge about the origin of solar magnetic activity. The key questions addressed by helioseismology are: what is the physical mechanism of the solar dynamo, how deep inside the Sun are the magnetic fields generated, how are they transported to the surface and form sunspots? Direct helioseismic signatures of the internal magnetic fields are weak and difficult to detect. Therefore, most of the information comes from observations of dynamical effects caused by the magnetic fields. I review results of recent helioseismic observations of the magnetohydrodynamics of the solar interior on various scales, including global dynamics associated with the dynamo processes, and formation of sunspots and active regions.

scholarly journals Пылевая плазма в тлеющем разряде в магнитном поле до 3000 G

2018 ◽  
Vol 44 (19) ◽  
pp. 66
Author(s):  
Е.С. Дзлиева ◽  
Л.А. Новиков ◽  
С.И. Павлов ◽  
В.Ю. Карасев
Keyword(s):  
Magnetic Field ◽  
Angular Velocity ◽  
Glow Discharge ◽  
Magnetic Fields ◽  
Dusty Plasma ◽  
Drag Force ◽  
Applied Magnetic Field ◽  
Magnetic Trap ◽  
Current Channel ◽  
Angular Velocities

AbstractA glow discharge dusty plasma in a magnetic trap in which the current channel narrows is obtained in moderate magnetic fields up to 3000 G. The results of initial experiments are reported. The formation of stable dusty plasma structures rotating at record-high angular velocities up to 15 rad/s is observed. The dependence of the angular velocity on the strength of the applied magnetic field is measured experimentally. We interpret it quantitatively on the basis of the ion drag force.

scholarly journals Direct numerical simulations of helical dynamo action: MHD and beyond

2004 ◽  
Vol 11 (5/6) ◽  
pp. 619-629 ◽  
Author(s):  
D. O. Gómez ◽  
P. D. Mininni
Keyword(s):  
Magnetic Fields ◽  
Numerical Simulations ◽  
Periodic Boundary ◽  
Incompressible Flows ◽  
Magnetic Energy ◽  
Periodic Boundary Conditions ◽  
Direct Numerical Simulations ◽  
Dynamo Action ◽  
Spatial Fluctuations ◽  
Kinetic Effects

Abstract. Magnetohydrodynamic dynamo action is often invoked to explain the existence of magnetic fields in several astronomical objects. In this work, we present direct numerical simulations of MHD helical dynamos, to study the exponential growth and saturation of magnetic fields. Simulations are made within the framework of incompressible flows and using periodic boundary conditions. The statistical properties of the flow are studied, and it is found that its helicity displays strong spatial fluctuations. Regions with large kinetic helicity are also strongly concentrated in space, forming elongated structures. In dynamo simulations using these flows, we found that the growth rate and the saturation level of magnetic energy and magnetic helicity reach an asymptotic value as the Reynolds number is increased. Finally, extensions of the MHD theory to include kinetic effects relevant in astrophysical environments are discussed.

scholarly journals Turbulent drag reduction by anisotropic permeable substrates – analysis and direct numerical simulations

2019 ◽  
Vol 875 ◽  
pp. 124-172 ◽  
Author(s):  
G. Gómez-de-Segura ◽  
R. García-Mayoral
Keyword(s):  
Drag Reduction ◽  
Numerical Simulations ◽  
Critical Value ◽  
Direct Numerical Simulations ◽  
Mean Flow ◽  
Turbulent Drag Reduction ◽  
Turbulent Drag ◽  
Theoretical Predictions ◽  
The Mean ◽  
The Difference

We explore the ability of anisotropic permeable substrates to reduce turbulent skin friction, studying the influence that these substrates have on the overlying turbulence. For this, we perform direct numerical simulations of channel flows bounded by permeable substrates. The results confirm theoretical predictions, and the resulting drag curves are similar to those of riblets. For small permeabilities, the drag reduction is proportional to the difference between the streamwise and spanwise permeabilities. This linear regime breaks down for a critical value of the wall-normal permeability, beyond which the performance begins to degrade. We observe that the degradation is associated with the appearance of spanwise-coherent structures, attributed to a Kelvin–Helmholtz-like instability of the mean flow. This feature is common to a variety of obstructed flows, and linear stability analysis can be used to predict it. For large permeabilities, these structures become prevalent in the flow, outweighing the drag-reducing effect of slip and eventually leading to an increase of drag. For the substrate configurations considered, the largest drag reduction observed is ${\approx}$20–25 % at a friction Reynolds number $\unicode[STIX]{x1D6FF}^{+}=180$.

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