Cavity Phenomenon Research Articles

A SUPS formulation for simulating unsteady natural/mixed heat convection phenomena in square cavities under intense magnetic forces

This paper presents stabilized finite element simulations of unsteady natural and mixed heat convection phenomena occurring in square enclosures. It is assumed that the enclosures are subject to intense uniform magnetic fields applied perpendicular to the vertical walls. It is well known that the classical Galerkin finite element method (GFEM) is prone to yielding nonphysical oscillations when simulating convection-dominated flows. Therefore, in order to handle such flows occurring at high Hartmann (0≤Ha≤100\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0 \\le {\\rm Ha} \\le 100$$\\end{document}) and Rayleigh (103≤Ra≤108\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{3} \\le {\\rm Ra} \\le 10^{8}$$\\end{document}) numbers, the stabilization of the GFEM is accomplished with the streamline-upwind/Petrov–Galerkin and pressure-stabilizing/Petrov–Galerkin formulations. Temporal discretizations are performed with the backward Euler formula. At each time step, the nonlinear systems are linearized with the Newton–Raphson (N–R) method, and the linearized systems are solved directly using the sparse lower–upper decomposition. The incompressible flow solvers are developed in-house, run in parallel within the FEniCS ecosystem, and tested on a comprehensive set of numerical experiments with various thermal boundary conditions. Numerical simulations and comparisons reveal that the proposed formulation performs quite well at high Hartmann and Rayleigh numbers without exhibiting any significant localized or globally spread numerical instabilities. Moreover, it achieves this by using uniform meshes and only linear elements, making the formulation much more computationally efficient. Mesh convergence analyses are also performed to demonstrate the reliability of the numerical results obtained.

Research on multiphase flow field characteristics of underwater gun double‐tube parallel firing

AbstractThe underwater muzzle flow field formed by the double‐tube launch will interfere with each other, resulting in large changes in the characteristic parameters of the flow field and shock wave morphology. Therefore, the numerical model of three‐dimensional unsteady multiphase flow for underwater gun‐sealed launching was established. Meanwhile, an experimental platform for the underwater sealed launch was built and the rationality of the model was verified. The interior ballistic process compiled by UDF was coupled with the dynamic mesh technology, and the VOF multiphase flow model integrated with the Schnerr‐Sauer cavitation model was chosen to numerically calculate the muzzle flow field of the 30 mm underwater gun double‐tube parallel launch, and the numerical results were compared to those of the single‐tube launch. The results show that the gas is expelled from the muzzle and rapidly expands to form a gas cavity, and the “necking” phenomenon of the gas cavity occurs at 0.2 ms, while the Mach disk structure has formed. Due to the mutual interference between the flow fields formed by each tube, the diameters of the Mach disk are slightly different, and the flow field structure has a certain asymmetry in the evolution process. The core area of the shock wave is bowl‐shaped of the single‐tube launch, while it is not completely filled of the double‐tube launch. Within 0.5 ms, the diameter of the Mach disk increases monotonously when the single‐tube is launched, while it first increases and then decays by a double‐tube.

Study of a Mode Separation Due to Polarization Existing in a Cavity-Enhanced Absorption Spectroscopy.

A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of αmin = 7.6 × 10−9 cm−1 can be obtained in a single laser scan of 10 s.

Unsteady flow phenomena in turbine shroud cavities

This paper presents those flow parameters at which coherent structures appear in the blade tip cavities of shrouded turbine blades. To the authors’ knowledge, this is reported for the first time in the open literature. The unsteady flow in a shroud cavity is analysed based on experimental data recorded in a labyrinth seal test rig. The unsteady static wall pressure in the shroud cavity inlet and outlet is measured using time-resolving pressure sensors. Sensors are located at staggered circumferential positions to allow cross-correlation between signals. The unsteady pressure signals are reduced using Fourier analysis and cross-correlation in combination with digital filters. Based on the data, a theory is formulated explaining the phenomena reflected in the measurements. The results suggest that pressure fluctuations with distinct numbers of nodes are rotating in the shroud cavity outlet. Moreover, modes with different node numbers appear to be superimposed, rotating at a common speed in circumferential direction. The pressure fluctuations are not found at all operating points. Further analysis indicates that the pressure fluctuations are present at operating points matching distinct parameters correlating with the cavity flow coefficient. Unsteady RANS simulations predict similar flow structures for the design operating point of the test rig.

Steam cavity and explosion intensity of molten copper column interaction with water

The steam explosion is one of the most serious accidents in the metallurgical industry. In this paper, an experimental study on the interaction of molten copper column with cooling water was carried out using high-speed camera and transient pressure measurement system. The effects of temperature, falling height, diameter and mass of molten copper column on the interaction were investigated. The steam cavity phenomenon was observed in the interaction and the percentage of steam cavity volume affects the probability of fragmentation of molten copper column. The maximum percentage of steam cavity volume increases firstly and then decreases with the increase of temperature and falling height. It increases with the increase of diameter of molten copper column, but the mass has a weak effect. The pressure wave generated by the interaction of molten copper column with cooling water gradually increases with an increase in the temperature of molten copper column. The energy conversion rate in the interaction of molten copper column with cooling water was calculated. The rate of energy conversion from the thermal energy of molten copper column to the energy of pressure wave is low because most of the thermal energy is dissipated into the cooling water by heat transfer.

Applicability of nondestructive testing to detect cavities in PVDF polymer

The cavitation phenomenon in polymers, commonly known as whitening, has been quite studied in the last two decades due to its relevance to the plastic deformation process and the overall behavior of the material. Its occurrence has been proposed to be either on equatorial or at polar region of the spherulite during deformation. The detection techniques usually employed are small angle X-ray scattering (SAXS) or light scattering, and neither one is suitable for field inspection. The present study focused on nondestructive methodologies to detect the cavity phenomenon in two grades of poly(vinylidene fluoride), comparing simple visual inspection (VI), optical densitometry (OD) and ultrasound velocity (US). The results showed that the last two techniques were able to identify the existence of voids very earlier along the deformation path, about 2%–5% strain, where visual whitening was not even detected. The OD and US showed to be highly effective in distinguishing the effect of the processing conditions on the occurrence of cavitation, via simple and fast methods. The use of ultrasound technique for cavity detection becomes a very promising approach for the production inspection of the pressure sheath of raisers, polymer pipes and other products, as well as for evaluating the integrity of the component that may be affected by occurrence of cavities.

Nonlinear power dependence of the spectral properties of an optical parametric oscillator below threshold in the quantum regime

Photon pairs and heralded single photons, obtained from cavity-assisted parametric down conversion (PDC), play an important role in quantum communications and technology. This motivated a thorough study of the spectral and temporal properties of parametric light, both above the optical parametric oscillator (OPO) threshold, where the semiclassical approach is justified, and deeply below it, where the linear cavity approximation is applicable. The pursuit of a higher two-photon emission rate leads into an interesting intermediate regime, between above OPO threshold and very low pump power, where the OPO still operates considerably below the threshold but the nonlinear cavity phenomena cannot be neglected anymore. Here, we investigate this intermediate regime and show that the spectral and temporal properties of the photon pairs, as well as their emission rate, may significantly differ from the widely accepted linear model. The observed phenomena include frequency pulling and broadening in the temporal correlation for the down-converted optical fields. These factors need to be taken into account when devising practical applications of the high-rate cavity-assisted spontaneous PDC sources.

Justification of the Lugiato-Lefever Model from a Damped Driven ϕ4 Equation

The Lugiato-Lefever equation is a damped and driven version of the well-known nonlinear Schrödinger equation. It is a mathematical model describing complex phenomena in dissipative and nonlinear optical cavities. Within the last two decades, the equation has gained much attention as it has become the basic model describing microresonator (Kerr) frequency combs. Recent works derive the Lugiato-Lefever equation from a class of damped driven ϕ 4 equations closed to resonance. In this paper, we provide a justification of the envelope approximation. From the analysis point of view, the result is novel and non-trivial as the drive yields a perturbation term that is not square integrable. The main approach proposed in this work is to decompose the solutions into a combination of the background and the integrable component. This paper is the first part of a two-manuscript series.

Investigation of the impact of geometry of the riser on the location and shape of shrinkage cavity

During solidification process the phenomenon of shrinkage cavity is often observed. It has negative impact on the structure and physical properties of the final product. To minimize the negative effect of shrinkage cavity the riser is commonly introduced to the solidifying system. If the riser is properly designed shrinkage cavity rather appears within its volume than in the other parts of the casting. It makes possible to obtain defect-free main part of the casting. Presented work shows the impact of the different shapes of the riser on the localization of the shrinkage cavity. The results are obtained with the use of an original computer program based on the finite element method (FEM). They are presented for three-dimensional case of solidification of three types of the riser. The comparison of the results is discussed in details.

Nematic topological defects positionally controlled by geometry and external fields.

Using a Landau–de Gennes approach, we study the impact of confinement topology, geometry and external fields on the spatial positioning of nematic topological defects (TDs). In quasi two-dimensional systems we demonstrate that a confinement-enforced total topological charge of m > 1/2 decays into elementary TDs bearing a charge of m = 1/2. These assemble close to the bounding substrate to enable essentially bulk-like uniform nematic ordering in the central part of a system. This effect is reminiscent of the Faraday cavity phenomenon in electrostatics. We observe that in certain confinement geometries, varying the correlation length size of the order parameter could trigger a global rotation of an assembly of TDs. Finally, we show that an external electric field could be used to drag the boojum fingertip towards the interior of the confinement cell. Assemblies of TDs could be exploited as traps for appropriate nanoparticles, opening several opportunities for the development of functional nanodevices.

Experimental investigation of vortex-ring cavitation

Vortex-ring cavitation occurs when the pressure inside a torus-shaped core of a vortex ring falls below the vapor pressure of the ambient liquid. By generating a vapor bubble in a rigid tube, a toroidal cavity can be produced outside the tube. The pulsation and propagation behaviors of vortex-ring cavitation are studied using a high-speed video camera and a hydrophone. The experimental results show that the cavity continues to oscillate with a period that depends heavily on the maximal cross-section radius of the cavity and circulation of the vortex flow, under the influence of the surrounding vortex flow field. It is also shown that the cross-radial oscillation of the toroidal cavity can be measured both by a high-speed camera and hydrophone. Moreover, three different methods for estimating the circulation are compared to propose an accurate model of toroidal cavity oscillation. The phenomenon of a toroidal cavity impinging on a fixed wall is also investigated.

Three-Dimensional Flow Problems in a Lid-Driven Cubical Cavity with a Circular Cylinder

Abstract Three-dimensional numerical simulations were conducted for lid-driven cavity phenomena around an inner circular cylinder positioned in the center of a cubic enclosure in the Reynolds number range of (100≤Re≤2000) at the Prandtl number of Pr=0.71. The numerical method is based on the finite volume method (FVM) and multigrid acceleration. In this study, the transition of the flow regime from steady state to the unsteady state and consequent three-dimensionality in the system induced by the increase of Reynolds number to (Re=1798) were investigated. By increasing further Reynolds number over the critical value, the flow in the cavity exhibits a complex behavior. Typical distributions of the transverse velocity contours and kinetic energy fields at (Re=2000) have been obtained.

Effects of gas velocity and break size on steam penetration depth using gas jet into water similarity experiments

A method is given to predict the steam penetration depth under an incidental Steam Generator Tube Rupture (SGTR) accident. Several similar experiments were performed by injecting gas into water to simulate the steam jetting into liquid Lead Bismuth Eutectic (LBE). The steam penetration behaviors including flow regimes and cavity phenomena were captured by visualization method and a semi-empirical correlation was modified for the penetration depth based on dimensional analysis and experimental data. The results showed that the penetration depth was proportional to the density ratio of gas jet to liquid coolant, gas velocity, and break size of tube. Further predictions with similarity theory have been conducted. The penetration depth was about 0.7 m during a large break (a double-ended break) of one SG cooling tubes under a postulated SGTR accident of lead-based reactor.

Body parameters with application of optimal differential variation method in super cavity model

When a slender body moves very fast through water at sufficient speed the cavity phenomena is happened. In a cavity model the body’s lengths and cavitator diameter are determined by the differential variationoptimal method so that the velocity of body is maximum.

Finite Element Analysis of Structural Performance of Anti-Freezing Layer via the Korea Pavement Research Program

PURPOSES : Nowadays, cavity phenomena occur increasingly in pavement layers of downtown areas. This leads to an increment in the number of potholes, sinkholes, and other failure on the road. A loss of earth and sand from the pavement plays a key role in the occurrence of cavities, and, hence, a structural-performance evaluation of the pavement is essential. METHODS: The structural performance was evaluated via finite-element analysis using KPRP and KICTPAVE. KPRP was developed in order to formulate a Korean pavement design guide, which is based on a mechanical-empirical pavement design guide (M-EPDG). RESULTS: Installation of the anti-freezing layer yielded a fatigue crack, permanent deformation, and international roughness index (IRI) of 13%, 0.7 cm, and 3.0 m/km, respectively, as determined from the performance analysis conducted via KPRP. These values satisfy the design standards (fatigue crack: 20%, permanent deformation: 1.3 cm, IRI: 3.5 m/km). The results of FEM, using KICTPAVE, are shown in Figures 8~12 and Tables 3~5. CONCLUSIONS: The results of the performance analysis (conducted via KPRP) satisfy the design standards, even if the thickness of the anti-freezing layer is not considered. The corresponding values (i.e., 13%, 0.7 cm, and 3.0 m/km) are obtained for all conditions under which this layer is applied. Furthermore, the stress and strain on the interlayer between the sub-grade and the anti-freezing layer decrease gradually with increasing thickness of the anti-freezing layer. In contrast, the strain on the interlayer between the sub-base and the anti-freezing layer increases gradually with this increase in thickness.

Polariton Resonances for Ultrastrong Coupling Cavity Optomechanics in GaAs/AlAs Multiple Quantum Wells.

Polariton-mediated light-sound interaction is investigated through resonant Brillouin scattering experiments in GaAs/AlAs multiple-quantum wells. Photoelastic coupling enhancement at exciton-polariton resonance reaches 10(5) at 30 K as compared to a typical bulk solid room temperature transparency value. When applied to GaAs based cavity optomechanical nanodevices, this result opens the path to huge displacement sensitivities and to ultrastrong coupling regimes in cavity optomechanics with couplings g(0) in the range of 100 GHz.

Numerical modeling of the cavity phenomenon and its elimination way in rectangular radial moving bed reactor

A complete three-dimensional (3D) reactor model based on the Eulerian–Eulerian two-fluid approach was developed to assist in the understanding of the cavity phenomenon in rectangular radial flow moving bed reactors (RFMBRs). The effectiveness of proposed model was firstly validated by the experimental data. The simulation results showed that the cavity phenomenon appears on the top of catalyst bed and the cavity scale increases with the increase of the superficial gas velocity. The effects of bed voidage and solid-seal height on the formation of cavity were thoroughly investigated. It was demonstrated that an appropriate high bed voidage and solid-seal height can effectively reduce the occurrence possibility of the cavity. Moreover, it was found that a trapezoidal structure is helpful to alleviate the cavity phenomenon in RFMBRs. It is thus concluded that a numerical modeling technique provides a promising approach to eliminate the unfavorable two-phase flow as well as improve the flexibility and efficiency in RFMBRs.

Structural-acoustic coupling study of tyre-cavity resonance

The tyre cavity resonance induced cabin noise has been a major unsolved customer complaint issue for a long time. A study of coupled tyre-cavity structural-acoustic system using impedance compact mobility matrix is presented in this paper. This method offers a better physical interpretation and numerical prediction about a coupled tyre-cavity system than previous models. It can calculate the sound pressure inside the tyre cavity as well as the tread wall vibration velocity at the same time. The analytical results have been verified by the results of the VAOne vibro-acoustic model. The VAOne software was also validated by the results from a case study of a rectangular box-cavity system in previous literature. From the analytical calculation, the tyre cavity coupling modal frequency is found to be split into two close resonance frequencies if the tyre structural natural frequency is close to the fluid cavity resonance frequency. The geometric coupling coefficient has been calculated using Matlab code. This study provides a better insight into the resonance coupling phenomenon of the tyre cavity to the tyre structure and its root causes. It demonstrates more clear physical meaning of the tyre cavity coupling model and its inherent characteristics than that from ‘black box’ finite element software. This develops a better understanding of the problem and its root causes facilitates effective solutions for the problem. Even though the study was conducted in a tyre-cavity system, the solution and methodology are applicable to other toroidal shape structural-acoustic systems.

New parametric approach for the general Lorenz system

We are concerned with the study of the system of coupled of motion for a system of two-level atoms interacting with a single-mode field in the laser cavity on resonance. The passage from the equation of motion to the Lorenz equations is established. Some theoretical aspects of the slowly time-varying phenomena in cavities are discussed and the linear stability analysis is presented. A new approach to solve these nonlinear coupled differential equations is explored and its generalization to the case of variable coefficients is performed.

A Two-Phase CFD Modeling Approach to Investigate the Flow Characteristics in Radial Flow Moving-Bed Reactors

Abstract Based on a complete CFD Eulerian–Eulerian two-fluid approach, a comprehensive three-dimensional (3D) two-phase reactor model was suggested to describe the flow behavior in radial flow moving-bed reactors (RFMBRs). A porous media model was incorporated into the reactor model in order to describe the flow resistance provided by the porous walls of the center and annular pipes. Compared with these previous reactor models, the reactor model considers the solid-phase movement instead of immobilization, which benefits for predicting the formation of cavity practically. The simulation results are agreement with the published experimental data. By employing the verified model, the flow field parameters in the reactors such as pressure drop and flow velocity were obtained. Besides, the simulations were then carried out to investigate the effect of the bed voidage on the flow behavior and to understand the phenomenon of cavity in the RFMBRs. The simulation results showed that both the centripetal and the centrifugal flow configurations have the inhomogeneous flow distribution and the phenomenon of cavity. Furthermore, the inhomogeneous distribution increases with the increase of the bed voidage, whereas the phenomenon of cavity is more obvious with the increase of gas inlet velocity. As a whole, this work provided a realistic modeling and a useful approach for the understanding of RFMBRs.