Annular Cavity Research Articles

Physical model for acoustic resonance in annular cavity structure

A physical model for acoustic resonance in the annular cavity structure is developed to represent the typical characteristic when acoustic resonance occurs. Firstly, the measurement of sound pressure in the casing and rotor blades vibration is operated in a multistage high pressure compressor. The sharp peak frequency and discrete multi-tone occur in the frequency spectrum of sound pressure in the compressor, and the vibration of the first stage of rotor blades synchronously presents the high amplitude. The frequencies associated with rotor blades vibration can be calculated with rotating sound source theory. It is also confirmed that acoustic resonance occurs in the multistage compressor. With acoustic similarity principle, an annular cavity model is established to simulate the typical characteristics of acoustic resonance in the compressor based on Large Eddy Simulation (LES) and Lighthill acoustic analogy. The coupling relationship between cavity acoustic mode and disc vibration mode shape is expounded when acoustic resonance occurs in the model. And acoustic resonance will be locked in the certain flow rate range. All these characteristics match well with those occur in the multistage high pressure compressor.

Investigation on spatial distribution of acoustic resonance in annular cavity: Frequency and intensity

This paper studies the acoustic behavior inside the deep annular and cylindrical cavity at low Mach number. The turbulent shear layer above the cavity acts as a broadband noise source and drives resonant standing waves inside the cavity for various modes. According to previous investigation, those resonant standing waves inside the cavity play an important role in the aeroacoustic resonance of cavity noise, which gives perfect prediction of tonal frequency from the solution of wave equation. From the perspective of engineering application, it is more important to predict the spatial distribution of tonal intensity. It is needed to point out that the solution of the linear wave equation also provides the relative spatial distribution tonal intensity and the absolute value of tonal intensity can be determined from the acoustic experiments that is measured only at some locations. Based on this idea, a scheme is setup and validated to predict the amplitude spatial distribution of tonal intensity of aeroacoustic resonance. For example, an analytical model is established to provide the relative mode shape of aeroacoustic resonance in a simple geometry of cavity, which is realized by solving the wave equation with boundary conditions in a semi-closed space. This model considers the freestream velocity scaling and the depth correction factor varying with the Helmholtz number. The experimental aeroacoustic result is acquired by measuring the pressure fluctuation at some locations of cavity internal wall with the use of surface microphones. The experimental results are used to supplement and validate this analytical model. The amplitude spatial distribution at any freestream velocity (low Mach number) can be acquired by measuring the pressure fluctuation once at the leading edge or trailing edge of cavity bottom at an arbitrary Mach number, as the amplitude of most modes reaches its maximum here.

Investigation on the stability of natural convection in an annular cavity with non-isothermal walls

The onset and nature of unsteady natural convection in a tall, closed annular cavity with non-isothermal walls is investigated experimentally. In the first of a sequence of experiments, thermal boundary conditions imposed on the inner and outer cylinders promote steady bi-cellular natural convection airflow inside the cavity. As the buoyant potential of the upper convection cell is increased, steady flow inside the cavity first bifurcates into mono-periodic flow and then follows a quasi-periodic route to chaotic flow. Similarly, when the buoyant potential of the lower convection cell is increased, steady flow bifurcates into mono-periodic flow and then follows an intermittent route to chaos. All of the observed unsteady airflows inside the annular cavity were three-dimensional, with all of the periodic flows exhibiting a gentle swirling motion in the azimuthal direction.

The endgame of gas giant formation: accretion luminosity and contraction post-runaway

ABSTRACT Giant planets are thought to form by runaway gas accretion on to solid cores. Growth must eventually stop running away, ostensibly because planets open gaps (annular cavities) in their surrounding discs. Typical models stop runaway by artificially capping the accretion rate and lowering it to zero over an arbitrarily short time-scale. In reality, post-runaway accretion persists as long as the disc remains. During this final and possibly longest phase of formation, when the planet is still emerging from the disc, its mass can more than double, and its radius contracts by orders of magnitude. By drawing from the theory of how gaps clear, we find that post-runaway accretion luminosities diverge depending on disc viscosity: luminosities fall in low-viscosity discs but continue to rise past runaway in high-viscosity discs. This divergence amounts to a factor of 102 by the time the disc disperses. Irrespective of the specifics of how planets interact with discs, the observed luminosity and age of an accreting planet can be used to calculate its instantaneous mass, radius, and accretion rate. We perform this exercise for the planet candidates embedded within the discs orbiting PDS 70, HD 163296, and MWC 758, inferring masses of 1–10 MJ, accretion rates of 0.1–10 MJ Myr−1, and radii of 1–10 RJ. Our radii are computed self-consistently from the planet’s concurrent contraction and accretion and do not necessarily equal the value of 2RJ commonly assumed; in particular, the radius depends on the envelope opacity as R ∝ κ0.5.

Large eddy simulations and global stability analyses of an annular and cylindrical rotor/stator cavity limit cycles

Although rotating cavity flows are essential components of industrial applications, their dynamics is still largely misunderstood. From computer hard-drives to turbopumps of space launchers, designed devices often produce flow oscillations that can destroy the component prematurely, or produce disturbing noise or undesired operating modes of the system. The fundamentals of encountered static and rotating flow boundary layers have evidenced, a long time ago now, the presence of specific boundary layer instabilities and structures for low Reynolds numbers. For higher Reynolds numbers and fully enclosed systems, features are, however, more complex with the apparition of multifrequency oscillations populating the entire cavity limit cycle. For these flows, Large Eddy Simulation (LES) has illustrated the capacity of reproducing features and limit cycles. However, identifying the origin and region within these flows that are responsible for mode selections remains difficult if not impossible using such computational fluid dynamics tools. The present contribution evaluates a LES and a global stability analysis framework to identify the mechanisms responsible for the observed limit-cycles of two types of rotor-stator cavities. In particular, the presence of a central body or shaft and its impact on the instability selection is of interest here, i.e., the identification of the regions of mode activation for a cylindrical as well as an annular cavity is detailed. Results issued by the conjunct use of dynamical mode decomposition and Global Linear Stability Analysis (GLSA) confirm the observed LES dynamics. Most importantly, GLSA gives access to the triggering mechanisms at the root of the limit-cycles expression as well as hints on the mode selection. In that respect, a cylindrical cavity is shown to sustain more complex features than an annular cavity because of an enhanced flow curvature near the central shaft.

Design of high shielding effectiveness magnetic shield for fiber optic gyroscope

A new type of magnetic shield with annular cavity structure is designed based on the study of the factors affecting the shielding effectiveness for fiber optic gyroscope (FOG). In order to prove the feasibility of using the finite element method (FEM) to research the magnetic shielding effectiveness problem, we design a simulation and experimental comparison verification test for the currently used magnetic shield. The optimized design parameters are obtained by discussing the effects of the number of layers, the layer spacing, the via holes and the gap width on the shielding effectiveness. The shielding effectiveness of the annular cavity structure magnetic shield under radial magnetic field is about 55 dB, which is 20 dB higher than that of the current magnetic shield. The shielding effectiveness under axial magnetic field is about 80 dB which improved by 45 dB. Meanwhile, this annular cavity structure reduces the weight of the FOG.

Dielectrophoretic buoyancy and heat transfer in a dielectric liquid contained in a cylindrical annular cavity

The effect of a thermoelectric body force on the flow of a dielectric fluid with a radial temperature gradient and an alternating electric voltage in a cylindrical annular cavity has been studied by a direct numerical simulation. The radial temperature gradient induces a vertical ascending flow near the hot surface and descending flow near the cold surface. A radial dielectrophoretic force with the electric field acting induces a thermoelectric convection in the form of columnar vortices that can transfer heat from the hot surface to the cold one. The heat transfer coefficient in the dielectric fluid significantly increases with the applied electric voltage.

Photon in the Earth-ionosphere cavity: Schumann resonances

We study a quantum analogy of Schumann resonances by solving massless and massive spin-1 particle equations derived from the Zitterbewegung model in an annular cavity background with poorly conducting walls. We also show that the massless case and the massive case in the $m_0^2 \rightarrow 0$ limit are compatible with Maxwell's electromagnetic theory. Furthermore, from the massive case, we predict an upper limit for the mass of the photon of 1.3x$10^{-50}$kg. The bound on the mass of the photon is compatible with the current limit in the literature.

Solvent-Specific Controlled Oxidation of Cu Nanospheres to Cu@Cu2O@Cu Nanomatryushkas

A concentric array of Cu@Cu2O core–shell nanostructures and Cu@Cu2O@Cu nanomatryushkas have been synthesized by tailoring the surface oxidation of plasmonic copper nanoparticles through manipulation of binary solvent composition. The concept is based on freezing down the rate of surface oxidation by governing the chemical equilibria directed by Le Chatelier’s principle during the evolution of these nanostructures that have been optimized by monitoring their real-time growth in solution. The formation of the nanomatryushkas could be attributed to a reverse Kirkendall mechanism. The observation of plasmon–exciton coupling across the interface as well as confinement of charge at the annular cavity in such periodic heteronanostructures has been elucidated from scattering simulation techniques. It has been revealed that the spectral response is a sensitive function of the dimensions of the core and successive shell layers of these geometrical architectures. The coupling of optical attributes at the same nanost...

Minimization of drift force on a floating cylinder by optimizing the flexural rigidity of a concentric annular plate

The deployment of suitable configurations of mutually interacting floating bodies for efficiently controlling their hydrodynamic interactions towards the reduction of the wave drift forces and, thus, of the mooring lines’ loads, has, nowadays, gained a great scientific interest. In this paper, the hydrodynamic behaviour of a floating cylinder and a concentric annular flexible plate is analysed in the frequency domain aiming at the minimization of the drift forces acting on the cylinder by optimizing the flexural rigidity of the plate. The diffraction/radiation problem is solved using a higher-order boundary element method. The analysis is implemented assuming that both floating bodies oscillate freely in heave, while for the plate, flexible modes are, additionally, considered for describing its structural deformations. The required modes shapes are determined in vacuum (“dry” mode superposition approach) through analytical expressions. The flexural rigidity of the plate, D, is optimized at a specific wave number using a real-coded genetic algorithm. Initially, results are compared with numerical results of other investigators for the case of two rigid concentric floating cylinders. Next, extended results are presented, focusing on the effect of D, including its optimum value, on various physical quantities describing the behaviour of both the cylinder and the plate. Contrary to the isolated cylinder, the presence of the plate introduces sharp peaks in the variation pattern of the drift force of the cylinder, bounded at specific wave numbers, where resonance of the seiche mode of water motion in the annular cavity or of specific flexible modes of the plate occurs. However, by reducing D to its optimum value, the cylinder’s drift force obtains practically zero values at the target wave number, due to an efficient improvement of the wave field in the annular cavity around the cylinder. Moreover, a great reduction of the drift force compared to the isolated cylinder is achieved in the subsequent high frequency range.

A high-order compact finite-difference lattice Boltzmann method for simulation of natural convection

In the present work, compact finite-difference lattice Boltzmann method (CFDLBM) is extended to simulate natural convection in square and concentric-circular-annulus cavities, in two-dimensions, for Rayleigh numbers in the range of 103 to 109. Owing to the finite-difference discretization of the method, non-uniform and curvilinear grids with a finer near-wall grid resolution could be used in the present simulations. The use of high-order methods further enabled the simulations to be run on relatively coarse meshes when compared with simulations that used classical lattice Boltzmann method. The simulation results are analyzed with the help of streamlines, isothermal contours and temperature, velocity, Nusselt number variations along the walls. The average Nusselt number on the hot wall shows less than 1% error for natural convection in the square cavity study. The circular-annular-cavity is found to have laminar flow for Rayleigh numbers in the range of 103–106 and it becomes unsteady and chaotic from 107 onwards. The instantaneous and time-average contours of isotherms and streamlines were used to study the unsteady behavior of the flow. A universal near-wall velocity profile is found to exist in all the unsteady natural convection studies in the annular cavity case. The location of the maximum velocities occurring in the annulus is analyzed for all the Rayleigh numbers. The spectra of velocity magnitude fluctuations and temperature fluctuations indicate that the unsteadiness in the cavity is dominant around 0.001 Hz. Natural convection in sine-walled and concentric star shaped cavities are simulated to demonstrate the ability of the present solver to handle complex geometries. The present results indicate that CFDLBM can be successfully used to simulate steady and unsteady chaotic natural convection flows in both rectangular and curvilinear geometries accurately.

Improving Induction Crucible Furnaces

The main deficiencies of existing induction crucible furnaces are associated with elevated wall thickness of the lined crucible, the tubular copper single-layer inductor (with internal cooling by conditioned water), and the absence or presence of surrounding vertical I-shaped composite metal rods, forming a discrete ferromagnetic screen. The first deficiency is that much of the working electromagnetic flux Φwo is wasted, since it passes along the crucible lining, which is not electrically conducting, rather than along the furnace charge. Only 38.5–57.0% of Φwo is utilized. The second deficiency is associated with the high cost and laboriousness of manufacturing the inductor turns from special copper pipe, which vibrates at double frequency. That creates noise and weakens the furnace structure. Such inductors are characterized by low electrical efficiency and high costs of preparation and cooling of the conditioned water in systems of area several times greater than the furnace itself. The consequence of the third deficiency is that much of the electromagnetic scattering flux Φsc is not involved in heating the batch and the melt but instead heats the electrically conducting elements of the furnace, including the magnetic rods that surround the inductor. Because of the poor utilization of the total flux Φ generated by the inductor, the efficiency is decreased practically to 19–30%, while the power factor falls to 0.03–0.10, with increase in the energy consumption. To minimize or eliminate the deficiencies, three modifications are proposed: decrease in the sleeve’s wall thickness, with simultaneous strengthening by installing a cylindrical shell between the crucible and the inductor; introduction of a magnetic ring around the inductor; and the use of a single-wire or multicore inductor, instead of a tubular inductor. The combination of a cylindrical shell, a magnetic ring, and upper and lower housing plates permits the formation of a closed annular cavity where the inductor may be placed, with cooling of the inductor and the magnetic ring by circulating fluid. On the basis of research, a new design of an induction crucible furnace with a wire inductor and a composite magnetic ring (magnetic core) has been developed at Polzunov Altai State Technical University, tested, and patented. In experiments, the efficiency of the proposed structural elements is confirmed, indicating that the design merits further study.

Numerical simulation of flow control by a heat pulse under aerodynamic hysteresis in supersonic flow over an axisymmetric body with annular cavity

Numerical simulation of flow control by a heat pulse under aerodynamic hysteresis in supersonic flow over an axisymmetric body with annular cavity

Numerical investigation of buoyancy balance effect on thermosolutal convection in a horizontal annular porous cavity

The effect of cooperating and opposite buoyancy forces on the flow structure and the heat and mass transfer rates is numerically investigated in a horizontal annular space of radius ratio R = 2, filled with a porous medium saturated by a binary fluid. Uniform and constant temperatures and concentrations are imposed along the active walls. The steady-state solutions have been obtained using the discretization of the governing equations with the Centered Finite Difference method based on the ADI scheme. The influence of the dimensionless thermosolutal parameters, namely Darcy-Rayleigh numbe, Ra, Lewis number, Le, and buoyancy ratio, N , is investigated. The study is focused on the effect of Ra and Le on the steady-state solution under the cooperating (N = 2) and opposite (N = -2) buoyancy forces cases. The increase in Rayleigh number in the opposite case results in a full development of the convection and gives rise to multicellular flow structures. The critical Rayleigh number values corresponding to the onset of this flow pattern are determined for a large range of Lewis number values by using two initial conditions types. On the other hand, the unicellular flow dominates the cooperating case whatever the Darcy-Rayleigh and Lewis numbers values. The heat and solutal transfer behaviors are also considered.

Моделирование процесса неизотермического отверждения термопластичного шликера BeO в кольцевом канале установки литья

Моделирование процесса неизотермического отверждения термопластичного шликера BeO в кольцевом канале установки литья

Radiative transfer in a semi-transparent medium enclosed in a spherical annulus with specularly reflecting surfaces

The behaviour of the radiative field inside an emitting-absorbing gray semi-transparent medium at radiative equilibrium, enclosed in a spherical annular cavity bounded by specularly reflecting hot surfaces, is described in an exact analytical way. In this approach, the incident radiation and the radiative flux are expressed as a linear combination of the internal temperatures power 4 at each point of the medium, whatever the reflection on the boundary surfaces is, the coefficients of the combination being the weighting coefficients determined from an analytical way. Some examples show that the proposed method gives perfectly smooth results of high accuracy for both the temperature and flux fields inside the medium.

Visualization experimental study for venturi tube unsteady cavitating flows

A visualization experiment of a venturi tube unsteady cavitating flows was conducted, by combining high speed visualization and dynamic pressure measurement technology, on the base of simulating the dynamical boundary conditions of the engine feed system. The detailed interior structure of cavitating flows and the pressure fluctuation data were captured. The variation of cavity shapes and the unsteady characteristics were analyzed. The experiment results indicate that as the cavitation number decreases, the venturi tube cavitating flow is changed from inception cavitation to film cavitation, and to cloud cavitation. At cloud cavitation stage, the venturi tube cavitating flows are revealed as some very complex dynamical processes. The cloud cavitating zone can be divided as attached annular cavity film and detached cavity cloud. The attached cavity film is relatively stable as a whole, and its inner pressure fluctuation is really little. It is found that large part of cavity cloud is breaking and shedding from the total detached cavity cloud, and is going to collapse. These processes induce evident pressure fluctuation pulses, which have wide frequency range but not fixable frequency peak in the frequency region. At the aft end of the cavity cloud, the local high velocity re-entrant flow is discovered, which drives the axial length of attached cavity film shortening and extending, and induces the film length obviously unequal along the circle. The self-oscillation of 290--400 Hz also takes place at the downstream system of venturi tube, at the cloud cavitation stage.

Compact acoustic lens composed of annular cavities covered by a membrane

A thin planar acoustic lens composed of concentric annular slits on a circular steel plate covered by a membrane made up of aluminum foil is numerically designed, and its performance in focusing low-frequency sound in air is experimentally demonstrated. Resonant fundamental modes of the membrane-backed cavities are calculated via the Finite-Element Method. By carefully adjusting the radii of the concentric cavities, the phase discontinuity across the cavities can be employed to achieve constructive interference of sound waves at a focal length of 200 mm, which is close to the acoustic wavelength, at 2190 Hz. The designed lens has an overall radius which is approximately one fourth of the radius of a conventional acoustic Fresnel lens employing a wavelength path length difference between adjacent cavities.

Computational and experimental study of solar thermal energy store for low-temperature application

Abstract This paper evaluates solar thermal energy storage (STES) systems for a low-temperature application. The STES used in this study was an annular cavity storage unit filled with heat storage material. Two latent heat storage materials (benzoic acid and stearic acid), and one sensible heat storage material (palm olein) were selected for this study. Transient thermal analysis was carried out on the STES models using ANSYS 16.0. The performances of the STES systems were evaluated by installing them in solar cookers to heat water. The heating power, sensible heat efficiency (SHE), the amount of energy and exergy stored were estimated. The temperature distribution, total heat flux and directional heat flux of the STES systems were observed to be affected by the type of heat storage materials used. The heating power, the energy stored and exergy stored reduced, while the SHE increased with an increase in the amount of water heated. In the absence of solar radiation, the STES systems sustained the temperature of 1 and 1.5 kg of water above 60 and 50 °C respectively for 200 min; this shows the suitability of the annular cavity storage unit for low-temperature applications.

Standard map-like models for single and multiple walkers in an annular cavity

Recent experiments on walking droplets in an annular cavity showed the existence of complex dynamics including chaotically changing velocity. This article presents models, influenced by the kicked rotator/standard map, for both single and multiple droplets. The models are shown to achieve both qualitative and quantitative agreement with the experiments, and make predictions about heretofore unobserved behavior. Using dynamical systems techniques and bifurcation theory, the single droplet model is analyzed to prove dynamics suggested by the numerical simulations.