388 research outputs found
Sound Scattering and Its Reduction by a Janus Sphere Type
Copyright © 2014 Delyia Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Sound scattering by a Janus sphere type is considered. The sphere has two surface zones: a soft surface of zero acoustic impedance and a hard surface of infinite acoustic impedance. The zones are arranged such that axisymmetry of the sound field is preserved. The equivalent source method is used to compute the sound field. It is shown that, by varying the sizes of the soft and hard zones on the sphere, a significant reduction can be achieved in the scattered acoustic power and upstream directivity when the sphere is near a free surface and its soft zone faces the incoming wave and vice versa for a hard ground. In both cases the size of the sphere’s hard zone is much larger than that of its soft zone. The boundary location between the two zones coincides with the location of a zero pressure line of the incoming standing sound wave, thus masking the sphere within the sound field reflected by the free surface or the hard ground. The reduction in the scattered acoustic power diminishes when the sphere is placed in free space. Variations of the scattered acoustic power and directivity with the sound frequency are also given and discusse
Self-thermophoresis of Laser-heated spherical Janus particles
An analytical framework is presented for calculating the self-induced thermophoretic velocity of a laser-heated Janus metamaterial micro-particle, consisting of two conducting hemispheres of different thermal and electric conductivities. The spherical Janus is embedded in a quiescent fluid of infinite expanse and is exposed to a continuous light irradiation by a defocused laser beam. The analysis is carried under the electrostatic (Rayleigh) approximation (radius small compared to wave-length). The linear scheme for evaluating the temperature field in the three phases is based on employing a Fourier-Legendre approach, which renders rather simple semi-analytic expressions in terms of the relevant physical parameters of the titled symmetry-breaking problem. In addition to an explicit solution for the self-thermophoretic mobility of the heated Janus, we also provide analytic expressions for the slip-induced Joule heating streamlines and vorticity field in the surrounding fluid, for a non-uniform (surface dependent) Soret coefficient. For a ‘symmetric’ (homogeneous) spherical particle, the surface temperature gradient vanishes and thus there is no self-induced thermophoretic velocity field. The ‘inner’ temperature field in this case reduces to the well-known solution for a laser-heated spherical conducting colloid. In the case of a constant Soret phoretic mobility, the analysis is compared against numerical simulations, based on a tailored collocation method for some selected values of the physical parameters. Also presented, are some typical temperature field contours and heat-flux vectors prevailing in the two-phase Janus as well as light-induced velocity and vorticity fields in the ambient solute, and a new practical estimate for the self-propelling velocity
Tsunamis Induced by Submarine Slumpings off the Coast of Israel
In the course of history several extreme changes in the sea level along the coast of the Levant have occurred. As these events have always been associated with earthquakes they were often described as tsunami or seismic sea waves. Reviewing the historical descriptions of such events at Israel’s coast, one finds more often a recession of the sea than a flooding of the shore. Such events may have been caused by slumpings on the continental slope. Based on data of actual submarine scars, a quantitative evaluation of this hypothesis was made. It was found that the slumping of a mass 6 km long, 2 km wide and about 50 m deep would cause the formation of a shock-induced solitary wave of about 10 m in height at the edge of the continental slope. The accompanying draw-down of the sea level at the coast would last aboul hours, and lay the sea floor bare for a distance of about ½-1½ km, in agreement with some historical descriptions. Though possibly occurring only once or twice in a millennium, earthquake-induced slumpings may constitute a danger to nuclear power plants, and not just to maritime vessels and installations
Light-induced heat-conducting micro/nano spheroidal particles and their thermoosmotic velocity fields
Electro-Orientation and Electro-Rotation of Metallodielectric Janus Particles
The electro-rotation (EROT) and electro-orientation (EOR) behavior of
metallodielectric (MD) spherical Janus particles (JP) are studied analytically
and verified experimentally. This stands in contrast to previous either
heuristic or numerically computed models of JP dipoles. First, we obtain
frequency-dependent analytic expressions for the corresponding dipole terms for
a JP composed of a dielectric and metallic hemispheres, by applying the
standard (weak-field) electrokinetic model and using a Fourier-Legendre
collocation method for solving two sets of linear equations. EROT and EOR
spectra, describing the variation of the JP angular velocity on the forcing
frequency of a rotating and non-rotating spatially uniform electric field,
respectively, are explicitly determined and compared against experiments
(different JP size and solution conductivity). A favorably good qualitative
agreement between theory and experimental measurements was found
The effect of a finite roll rate on the miss-distance of a bank-to-turn missile
AbstractWe consider a three-dimensional pursuit-evasion situation where a highly maneuverable evader, which we model as a “pedestrian” á la Isaacs, is engaged by a faster-pursuer. The pursuer has limited maneuverability, that is, the pursuer has a minimal turning radius, and in order to change the spatial direction of his velocity vector, he must first re-align his thrust vector in a similar manner to a bank-to-turn missile. The state space of the ensuing differential game is three-dimensional and its complexity is intermediate between Isaac's [1] classical “Homicidal Chauffeur” and “Two Car” differential games. This new DG is solved as a game of kind, and a capture criterion for a faster but less maneuverable pursuer is analytically established in terms of the game parameters
Sound Scattering by an Elastic Spherical Shell and its Cancellation using a Multi-pole Approach
The scattering and transmission of sound by an elastic spherical shell is considered when it is subject to an incoming monochromatic planar wave. It is aimed to cancel the sound scattering using combinations of multi-pole sources located at the centre of a shell filled with compressible fluid. Assuming linear acoustics and structural dynamics, exact solutions are derived for total elimination of the sound scattering for three cases: a free-space, near a hard ground or near a free-surface, where in the last two cases it is assumed that the incoming wave propagates normal to the interface to maximize sound reflection back unto the source of the incoming wave. An elastic spherical shell of 1 m radius embedded in water and filled with air or oil is analysed to show the dominance of low-mode numbers for frequencies of less than 10 kHz and thus demonstrate the ability of this approach to damp acoustic scattering by means of low-order multi-poles inside the shell. Contour and mode distribution plots are also given and analysed
Optically Modulated Propulsion of Electric Field Powered Photoconducting Janus Particles
Herein we demonstrate the ability to optically tune the mobility of
electrically powered Janus particles (JP) that are half coated with various
Zinc Oxide (ZnO) semiconducting layers, i.e. polycrystalline, amorphous and
amorphous with a SiO2 passivation layer. The ZnO semiconductor photo-response
enables increase in its electrical conductivity with light having wavelengths
of sufficient photon energy with respect to the semiconductor bandgap. This
effect, termed optically modulated electrokinetic propulsion (OMEP), can be
harnessed to increase the contrast in polarizability between the dielectric and
semiconducting hemispheres, which in turn, results in an increased
electrokinetic mobility. The addition of optical activation to the electrical
field enables an additional degree of control of JP mobility. We also
demonstrate optical control of collective behavior and particle-particle
interactions for dense semi-conducting Janus particle populations
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