Cavity Radius Research Articles

Dynamics of Vibration Isolation System with a Ball Vibration Absorber

The forced vibrations of a mechanical vibration isolation system consisting of a ball vibration absorber (BVA) with linear viscous resistance and a movable carrier body under an external harmonic load are considered. Appell’s formalism is used to formulate and numerically solve the equations of combined motion of the heavy ball without sliding in the spherical cavity of the carrier body. The frequency response of the system and curves of the maximum amplitude of vibrations of the carrier body versus the radius of the spherical cavity and the coefficient of viscous resistance of the BVA are plotted. The conditions and constraints on the rolling of the heavy ball in the spherical cavity without sliding are established.

Permeability enhancements of borehole outburst cavitation in outburst-prone coal seams

Borehole outbursts eject coal power, create cavities and relieve in-situ stress while drilling cross-measure boreholes into outburst coal seams for pre-drainage. This paper highlights the borehole outburst cavitation to enhance the permeability of coal for the first time. Borehole outburst is a mechanical behavior, and the outburst trigger and development depend on the status of gas pressure gradient near the opening coal wall. It was testified that the high-pressure water, with 10–15 MPa in pressure and 20–30 m3/h in flow, was feasible to remove the failure coal near the opening wall and stimulate a new set of outbursts, thus creating bigger cavities and better permeability enhancement. Furthermore, an outburst control system was designed for borehole outburst cavitation. The system contains an outburst stimulation subsystem, a gas drainage subsystem and a separation subsystem, thus ensuring the flow and the separation of outburst ejections within a relatively closed environment. The outburst cavity reconstitutes the in-situ stress and permeability of the surrounding coal mass, leading to a failure zone, a plastic zone and a visco-elastic zone. The failure zone and the plastic zone due to borehole outburst cavitation are both effective in permeability enhancement. Finally, a field test in the No. 10 coal seam at the Luling mine proved that the sufficient permeability enhancement scope is approximately twelve times of the cavity radius. The permeability enhancement promotes the increase of borehole distance from 5 m to 12 m, thus reducing 78.7% of boreholes in number for the pre-drainage of the No. 10 coal seam.

Influence of particle contact models on soil response of poorly graded sand during cavity expansion in discrete element simulation

The discrete element method (DEM) has been extensively adopted to investigate many complex geotechnical related problems due to its capability to incorporate the discontinuous nature of granular materials. In particular, when simulating large deformations or distortion of soil (e.g. cavity expansion), DEM can be very effective as other numerical solutions may experience convergence problems. Cavity expansion theory has widespread applications in geotechnical engineering, particularly to the problems concerning in situ testing, pile installation and so forth. In addition, the behaviour of geomaterials in a macro-level is utterly determined by microscopic properties, highlighting the importance of contact models. Despite the fact that there are numerous contact models proposed to mimic the realistic behaviour of granular materials, there are lack of studies on the effects of these contact models on the soil response. Hence, in this study, a series of three-dimensional numerical simulations with different contact constitutive models was conducted to simulate the response of sandy soils during cylindrical cavity expansion. In this numerical investigation, three contact models, i.e. linear contact model, rolling resistance contact model, and Hertz contact model, are considered. It should be noted that the former two models are linear based models, providing linearly elastic and frictional plasticity behaviours, whereas the latter one consists of nonlinear formulation based on an approximation of the theory of Mindlin and Deresiewicz. To examine the effects of these contact models, several cylindrical cavities were created and expanded gradually from an initial radius of 0.055 m to a final radius of 0.1 m. The numerical predictions confirm that the calibrated contact models produced similar results regarding the variations of cavity pressure, radial stress, deviatoric stress, volumetric strain, as well as the soil radial displacement. However, the linear contact model may result in inaccurate predictions when highly angular soil particles are involved. In addition, considering the excessive soil displacement induced by the pile installation (i.e. cavity expansion), a minimum distance of 11a (a is the cavity radius) is recommend for practicing engineers to avoid the potential damages to the existing piles and adjacent structures.

Reverse Current Model for Coronal Mass Ejection Cavity Formation

Abstract We report here a new model for explaining the three-part structure of coronal mass ejections (CMEs). The model proposes that the cavity in a CME forms because a rising electric current in the core prominence induces an oppositely directed electric current in the background plasma; this eddy current is required to satisfy the frozen-in magnetic flux condition in the background plasma. The magnetic force between the inner-core electric current and the oppositely directed induced eddy current propels the background plasma away from the core, creating a cavity and a density pileup at the cavity edge. The cavity radius saturates when an inward restoring force from magnetic and hydrodynamic pressure in the region outside the cavity edge balances the outward magnetic force. The model is supported by (i) laboratory experiments showing the development of a cavity as a result of the repulsion of an induced reverse current by a rising inner-core flux-rope current, (ii) 3D numerical magnetohydrodynamic (MHD) simulations that reproduce the laboratory experiments in quantitative detail, and (iii) an analytic model that describes cavity formation as a result of the plasma containing the induced reverse current being repelled from the inner core. This analytic model has broad applicability because the predicted cavity widths are relatively independent of both the current injection mechanism and the injection timescale.

Improved Omnidirectional 2D Photonic Crystal Selective Emitter for Thermophotovoltaics

Hafnia-filled, two dimensional (2D) tantalum (Ta) photonic crystals (PhCs) are promising emitters for high performance thermophotovoltaic (TPV) systems because they enable, for a wide range of incidence angles, efficient spectral tailoring of thermal radiation. However, fabricating these PhCs to the required tolerances has proven to be a challenging task. In this paper, we use both focused ion beam (FIB) imaging and simulations to investigate the effects of fabrication imperfections on the emittance of a fabricated hafnia-filled PhC and to identify critical geometric features that drive the overall PhC performance. We demonstrate that, more so than uniform cavity filling, the key to the best filled PhC performance is the precise cavity period and radius values and thickness of the top hafnia layer.

The influence of helium on cavity evolution in ion-irradiated T91

The effect of helium pre-implanted in concentrations spanning 0 to 1000 appm on the swelling evolution in ferritic-martensitic alloy T91 was explored. Irradiation with 5.0 and 4.4 MeV Fe2+ ions at 460 °C up to damage levels of 350 dpa were performed with pre-implanted helium levels of 0, 1, 10 and 1000 appm. Low concentrations of helium were found to enhance nucleation at low damage levels. However, at high damage levels, differences in nucleation became insignificant, and nearly identical cavity distributions were created. At low helium levels, the cavity evolution was facilitated by similar cavity sink strengths. High helium concentrations reduced the critical cavity radius and resulted in the formation of a high-density bubble population. The bubbles were <4 nm and remained stable even up to high damage levels, while the growth of larger cavities was suppressed. The suppression of swelling was found to be a result of helium trapping or alteration of cavity and dislocation bias. Dislocation and precipitate evolution was also studied and exhibited relative insensitivity to helium concentration, and did not significantly influence the cavity evolution.

Size-dependent finite strain analysis of cavity expansion in frictional materials

This paper presents unified solutions for elastic–plastic expansion analysis of a cylindrical or spherical cavity in an infinite medium, adopting a flow theory of strain gradient plasticity. Previous cavity expansion analyses incorporating strain gradient effects have mostly focused on explaining the strain localization phenomenon and/or size effects during infinitesimal expansions. This paper is however concerned with the size-dependent behaviour of a cavity during finite quasi-static expansions. To account for the non-local influence of underlying microstructures to the macroscopic behaviour of granular materials, the conventional Mohr–Coulomb yield criterion is modified by including a second-order strain gradient. Thus the quasi-static cavity expansion problem is converted into a second-order ordinary differential equation system. In the continuous cavity expansion analysis, the resulting governing equations are solved numerically with Cauchy boundary conditions by simple iterations. Furthermore, a simplified method without iterations is proposed for calculating the size-dependent limit pressure of a cavity expanding to a given final radius. By neglecting the elastic strain increments in the plastic zone, approximate analytical size-dependent solutions are also derived. It is shown that the strain gradient effect mainly concentrates in a close vicinity of the inner cavity. Evident size-strengthening effects associated with the sand particle size and the cavity radius in the localized deformation zone is captured by the newly developed solutions presented in this paper. The strain gradient effect will vanish when the intrinsic material length is negligible compared to the instantaneous cavity size, and then the conventional elastic perfectly-plastic solutions can be recovered exactly. The present solutions can provide a theoretical method for modeling the size effect that is often observed in small-sized sand-structure interaction problems.

Axisymmetric nonstationary elastic contact problem for conforming surfaces

A numerical analytic approach has been described to investigate the process of impact interaction of a solid spherical body with the inner surface of a spherical cavity in elastic space. A nonstationary mixed initial boundary problem is stated with generally a priori unknown movable boundaries. The methods of integral transforms and expansion of sought for values in Fourier series and the quadrature method are used to reduce the problem in solving a system on integral equations jointly with body motion equations and a relationship for determining the contact area boundary. Concrete numerical computations were done for different relationships of body and cavity radii. The impact of the relationship of body radii and mass on the characteristics of contact interaction have been analysed.

Singlet vs. triplet interelectronic repulsion in confined atoms

Hund’s multiplicity rule invariably holds for the ground configurations of few-electron atoms as well as those of multi-electron quantum dots. However, the ordering of the corresponding interelectronic repulsions exhibits a reversal in the former but not in the latter system, upon varying the system parameters. Here, we investigate the transition between these two types of behaviour by studying few-electron atoms confined in spherical cavities. “Counter-intuitive” ordering of the interelectronic repulsions is confirmed when the nuclear charge is low enough and the cavity radius is large enough.

Acoustic radiation force of a solid elastic sphere immersed in a cylindrical cavity filled with ideal fluid

An expression for the acoustic radiation force function on a solid elastic spherical particle placed in an infinite rigid cylindrical cavity filled with an ideal fluid is deduced when the incident wave is a plane progressive wave propagated along the cylindrical axis. The acoustic radiation force of the spherical particle with different materials was computed to validate the theory. The simulation results demonstrate that the acoustic radiation force changes demonstrably because of the influence of the reflective acoustic wave from the cylindrical cavity. The sharp resonance peaks, which result from the resonance of the fluid-filled cylindrical cavity, appear at the same positions in the acoustic radiation force curve for the spherical particle with different radii and materials. Relative radius, which is the ratio of the sphere radius and the cylindrical cavity radius, has more influence on acoustic radiation force. Moreover, the negative radiation forces, which are opposite to the progressive directions of the plane wave, are observed at certain frequencies.

Spectral and physicochemical properties of difluoroboranyls containing N,N-dimethylamino group studied by solvatochromic methods

The solvatochromism of the dyes was analyzed based on the four-parameter scale including: polarizability (SP), dipolarity (SdP), acidity (SA) and basicity (SB) parameters by method proposed by Catalán. The change of solvent to more polar caused the red shift of absorption and fluorescence band position. The frequency shifts manifest the change in the dipole moment upon excitation. The ground-state dipole moment of the difluoroboranyls was estimated based on changes in molecular polarization with temperature. Moreover, the Stokes shifts were used to calculate the excited state dipole moments of the dyes. For the calculation, the ground-state dipole moments and Onsager cavity radius were also determined theoretically using density functional theory (DFT). The experimentally determined excited-state dipole moments for the compounds are higher than the corresponding ground-state values. The increase in the dipole moment is described in terms of the nature of the excited state.

Tits as bryologists: patterns of bryophyte use in nests of three species cohabiting a primeval forest

Numerous birds use bryophytes as nest construction material but the species used and their biology remain largely unknown. Therefore, questions related to the selectivity of birds in their bryophyte use, and why they use/ignore particular species, remain unanswered. We studied the composition of bryophytes, including both mosses (Bryophyta) and liverworts (Marchantiophyta), in nests of Marsh Tit Poecile palustris, Blue Tit Cyanistes caeruleus, and Great Tit Parus major breeding sympatrically in tree cavities in pristine patches of the Białowieża National Park, Poland. We checked whether the bryophyte composition of the nests differed, compared usage of different bryophytes with their availability in the surroundings (10 m radius) of the tree cavities, and looked for characteristics that may have resulted in their selection as nesting material. The birds appeared to be highly selective; of 54 bryophyte species found near cavities, 21 were never used. Individual nests contained 4.0–5.5 bryophyte species on average, and only 2.3–2.7 species exceeded 5% of the total bryophyte volume. In each tit species the three most abundantly used bryophytes belonged to mosses and comprised ca. 80–89% of the total bryophyte volume. All of the tits utilized pleurocarpous bryophytes, growing as wefts, mats or pendants mostly on tree trunks. Simulations of plucking showed that the mosses employed as nest constituents yielded larger bundles with longer shoots when plucked. The tit species differed in the sets of bryophyte species collected. Great Tit nests contained an almost completely different assemblage of mosses from that used by Marsh and Blue Tits. This variation was related to the varying mass of their broods; more robust mosses provided support for heavier broods of Great Tits, while the finest moss species were sufficient to form a cushion for the much lighter Marsh Tit broods.

A Simulation Study on the Effect of Cavity Shapes on the Penetration of Linear Shaped Charges with Curved Liners

The finite element models of curved-liner shaped charges penetrating aluminum-alloy thin plates were constructed with the ANSYS/LS-DYNA software to analyze the effect of cavity shapes on the charge penetration with curved liners. The cavity height, radius, and spatial dimensions were numerically simulated. Simulation results indicate that these properties are important factors in determining the penetration of curved-liner charges. The penetration depth exhibits an initial increase followed by a decrease with the fixed cavity radius. When the cavity height is fixed, the penetration depth decreases with the cavity radius. When the cavity is semicircular, the penetration depth first increases and then decreases with the cavity radius and height.

Dynamic characteristics of parallel water-entry cavity based on potential flow theory

During the water-entry of the parallel axisymmetric bodies, the water-entry cavities are asymmetrical due to the mutual interference between the cavities. In the current study of the cavity dynamic models, the relatively perfect models of axially symmetric dynamic calculation of low speed single water entry have been established. These models mainly focus on the evolution of cavitation, and thus simplifying the flow pattern. However, due to the particularity of parallel water, the fluid forms a relative flow during the evolution of the cavitation in the inner region of the axisymmetric body axis. As a result, the flow is no longer axisymmetric but develops into a complex three-dimensional flow with strong nonlinearity, making the the theoretical model more difficult to establish. In order to analyze the evolution of the parallel cavities of the parallel axisymmetric body water-entry, the flow of the water-entry cavity interference region is simplified by the existing single water-entry calculation model based on the potential flow theory. The constraint of the relative flow to the cavity is simplified into a constraint potential, and the variation of cavity shape is analyzed. Based on the nonlinear hypothesis, the influence function is introduced to establish the calculation model of three-dimensional cavity and the three-dimensional evolution characteristics of parallel cavities are analyzed. The obtained results show that the velocity potential of the axisymmetric body water-entry can be regarded as the superposition of a point source and a line source located on the axis of the cavity. The expansion of the cavity is affected mainly by the point source, while the shrinkage of the cavity is influenced mainly by the line source. During the parallel water-entry of the axisymmetric body, the evolution of the parallel cavities in space is mirror symmetric and the mutual interference between cavities can be analyzed by introducing the potential wall surface. The potential wall has an inhibitory effect on the evolution of the cavities. The variation of the parallel water-entry cavity radius with the polar angle is related to the depth of the cavity cross section. In the inhibition evolution area near the closed point, the cavity radius decreases gradually with the increase of the polar angle, and the void section radius in the inhibition evolution region far from the closed point increases with the polar angle increasing, and is opposite to the radius law. In the shallower depth of the water-entry, the excessive evolution is formed in the expansion process of the cavity, and the excessive evolution will gradually weaken and disappear in the contraction process of the cavity.

Low-Spatial Coherence High-Power Electrically Injected 6xx nm Dual-Emitter Laser

Laser-based displays have attracted much attention owing to large-size screen and full-color gamut compared with other displays such as liquid crystal display and light emitting diode. However, there exists a phenomenon, speckle, limits the applications of laser display because of the high coherence of laser. In this work, we developed an electrically injected 6xx nm dual-emitter laser which combines the low-spatial coherence with the high-power. The output power of the dual-emitter laser exceeds 500 mW under 20?C pulse operation. The single emitter consists of D-shaped section used to obtain more independent spatial modes thus reduces coherence and a stripe area to obtain the high power. The radius of the D-shaped cavity is 500 μm and the length of stripe is 1000 μm. We used the standard photolithography and inductively coupled plasma (ICP) process to fabricate the device. The speckle contrast was measured to be 5%. It exhibits a great potential of reducing speckle from the source directly for laser display.

Equilibrium structure and diffusion in concentrated hydrodynamically interacting suspensions confined by a spherical cavity

The short- and long-time equilibrium transport properties of a hydrodynamically interacting suspension confined by a spherical cavity are studied via Stokesian dynamics simulations for a wide range of particle-to-cavity size ratios and particle concentrations. Many-body hydrodynamic and lubrication interactions between particles and with the cavity are accounted for utilizing recently developed mobility and resistance tensors for spherically confined suspensions (Aponte-Rivera &amp; Zia, Phys. Rev. Fluids, vol. 1(2), 2016, 023301). Study of particle volume fractions in the range $0.05\leqslant \unicode[STIX]{x1D719}\leqslant 0.40$ reveals that confinement exerts a qualitative influence on particle diffusion. First, the mean-square displacement over all time scales depends on the position in the cavity. Additionally, at short times, the diffusivity is anisotropic, with diffusion along the cavity radius slower than diffusion tangential to the cavity wall, due to the anisotropy of hydrodynamic coupling and to confinement-induced spatial heterogeneity in particle concentration. The mean-square displacement is anisotropic at intermediate times as well and, surprisingly, exhibits superdiffusive and subdiffusive behaviours for motion along and perpendicular to the cavity radius respectively, depending on the suspension volume fraction and the particle-to-cavity size ratio. No long-time self-diffusive regime exists; instead, the mean-square displacement reaches a long-time plateau, a result of entropic restriction to a finite volume. In this long-time limit, the higher the volume fraction is, the longer the particles take to reach the long-time plateau, as cooperative rearrangements are required as the cavity becomes crowded. The ordered dynamical heterogeneity seen here promotes self-organization of particles based on their size and self-mobility, which may be of particular relevance in biophysical systems.

Experimental demonstration of critical coupling of whispering gallery mode cavities on a Bloch surface wave platform.

We experimentally demonstrate critical coupling of whispering gallery mode (WGM) disk resonators implemented on a Bloch surface wave platform using scanning near-field optical microscopy. The studied structure is a 60nm thick TiO2 WGM disk cavity (radius of 100μm) operating within the C-band telecommunication wavelength. An extinction ratio of 26dB and a quality factor of 2200 are measured. Such a high extinction ratio verifies the critical coupling of the WGM resonator. This result paves the way to planar optical signal processing devices based on the proposed geometry, for which a critical coupling condition is a guarantee of optimum performance.

A possible reason behind the initial formation of pentagonal dodecahedron cavities in sI-methane hydrate nucleation: A DFT study

In this letter, a possible reason behind selective host-guest organization in the initial stage of sI methane hydrate nucleation is provided, through density functional theory based calculations. In doing so, we have connected earlier experimental and theoretical observations on the structure and energetics of sI methane hydrate to our findings. Geometry and relative stability of small (H2O)5 and (H2O)6 clusters, presence of CH4 guest, integrity and cavity radius of (H2O)20 and (H2O)24, as well as the weak van der Waals type of forces, particularly dispersion interaction, are major factors responsible for initial formation of methane encapsulated dodecahedron cavity over tetrakaidecahedron.

Phase-matching of multiple-cavity detectors for dark matter axion search

Conventional axion dark matter search experiments employ cylindrical microwave cavities immersed in a solenoidal magnetic field. Exploring higher frequency regions requires smaller size cavities as the TM010 resonant frequencies scale inversely with cavity radius. One intuitive way to make efficient use of a given magnet volume, and thereby to increase the experimental sensitivity, is to bundle multiple cavities together and combine their individual outputs, ensuring a phase-matching of the coherent axion signal. We perform an extensive study for realistic design of a phase-matching mechanism for multiple-cavity systems and demonstrate its experimental feasibility using a double-cavity system.

Experimental determination of the effective point of measurement in electron beams using a commercial scintillation detector

PurposeA commercial plastic scintillation detector was used to determine the effective point of measurement (EPOM) of three cylindrical ionization chambers in four clinical electron beams. The experimentally determined EPOM values were compared to Monte Carlo (MC)-calculated EPOM values from the literature. The energy dependence of the EPOM was also investigated. MethodsPercent depth dose (PDD) curves were measured with an Exradin W1 scintillation detector and were used as a representative PDD to water. Depth dose curve measurements were also acquired with the Exradin A18, A1SL, and A28 cylindrical ionization chambers. To obtain a depth dose to an equivalent chamber volume made of water (DDCVW), the raw ionization chamber depth dose data were corrected by the chamber fluence correction factor and restricted mass collisional stopping power ratio at each measurement depth. A non-linear least squares fitting technique was utilized to determine the EPOM for the three chambers. The W1 PDD curve was shifted by small increments, Δz, until the ratio of the W1 PDD to the ionization chamber DDCVW was depth independent. The optimal shift was used as an estimate of the EPOM for each chamber. ResultsThe ratios of the unshifted W1 PDD to the chamber DDCVW decreased with depth. After shifting by the optimal Δz value, the ratios of the W1 PDD to the chamber DDCVW were constant with depth with an average value near unity, which implies that the two profiles are in good agreement with each other. The calculated EPOM shifts and associated uncertainties (expressed as a fraction of the chamber cavity radius, rcav) for the A18, A1SL, and A28 ionization chambers were 0.21±0.04, 0.10±0.05, and 0.22±0.03, respectively. ConclusionsThe EPOM shifts determined in this work agree with calculations by other investigators to within one standard deviation for the A18 and A1SL ionization chambers. This agreement confirms that the AAPM TG51-recommended EPOM shift of 0.5×rcav is not accurate for the A18 and A1SL chambers for electron beam dosimetry. Contrary to previous works, there was no observable trend in the measured EPOM values with electron energy for each chamber.