Dimensional Cavity Research Articles

Non-homogeneous model for a side heated square cavity filled with a nanofluid

A side heated two dimensional square cavity filled with a nanofluid is here studied. The side heating condition is obtained by imposing two different uniform temperatures at the vertical boundary walls. The horizontal walls are assumed to be adiabatic and all boundaries are assumed to be impermeable to the base fluid and to the nanoparticles. In order to study the behavior of the nanofluid, a non-homogeneous model is taken into account. The thermophysical properties of the nanofluid are assumed to be functions of the average volume fraction of nanoparticles dispersed inside the cavity. The definitions of the nondimensional governing parameters (Rayleigh number, Prandtl number and Lewis number) are exactly the same as for the clear fluids. The distribution of the nanoparticles shows a particular sensitivity to the low Rayleigh numbers. The average Nusselt number at the vertical walls is sensitive to the average volume fraction of the nanoparticles dispersed inside the cavity and it is also sensitive to the definition of the thermophysical properties of the nanofluid. Highly viscous base fluids lead to a critical behavior of the model when the simulation is performed in pure conduction regime. The solution of the problem is obtained numerically by means of a Galerkin finite element method.

Polariton lasing of quasi-whispering gallery modes in a ZnO microwire

We report the experimental observation of the quasi-whispering-gallery mode (quasi-WGM) polariton lasing in a one dimensional ZnO microrod cavity at room temperature. Under nonresonant optical pulsed excitation, we measure the pumping power dependence of the emission intensity and the blueshift of the polariton modes, and a fairly low lasing threshold for the quasi-WG mode is determined. We also measure the lasing threshold versus the photon-exciton detuning by dispersion simulations. These experiment results show that both of the WGMs with higher Q factors and the quasi-WGMs with lower Q factors can lase when the detuning between the optical modes and the exciton energies is optimum.

Electromagnetic Casimir effect on the boundary of a D -dimensional cavity and the high temperature asymptotics

We consider the finite temperature Casimir stress acting on the boundary of a D ⩾ 3 dimensional cavity due to the vacuum fluctuations of electromagnetic fields. Both perfectly conducting and infinitely permeable boundary conditions are considered, and it is proved that they correspond mathematically to the relative and absolute boundary conditions. The divergence terms of the Casimir free energy are related to the heat kernel coefficients of the Laplace operator. It is shown that the Casimir stress is free of divergence if and only if D is exactly three. The high temperature asymptotics of the regularized Casimir free energy are also found to depend on the heat kernel coefficients. When D > 3, renormalization is required to remove terms of order higher than or equal to T2.

English

The entropy generation for natural convection heat and mass transfer in a two dimensional porous cavity subjected to a magnetic field is selected for numerical investigation. The Darcy model is used in the mathematical formulation of the fluid flow in porous media. The mathematical model is derived in dimensionless form and the governing equations are solved using the finite volume method. The governing parameters arise in the mathematical model are the Rayleigh number, Lewis number, buoyancy ratio and Hartmann number. The results are presented as average Nusselt number (), Sherwood numbers () and dimensionless form of local entropy generation rate (Ns) for different values of the governing parameters.  The numerical results show that increasing the magnetic field parameter (Hartmann number) leads to deterioration of the flow circulation strength in the cavity and this leads to a decrease in the rates of the heat and mass transfer as well as the rate of entropy generation. The results show a stagnate fluid everywhere in the cavity when the buoyancy forces generated due to temperature and concentration differences are in the same order and opposite directions. In this case, the values of ,  and Ns are the minimum. Increasing or decreasing the value of the buoyancy ratio parameter leads to enhance the fluid circulation and hence increase the values of ,  and Ns. The average Sherwood number can be increased with increasing Lewis number. It is observed that the strength of the fluid circulation in the cavity is reduced by increasing the Lewis number. This leads to the decrease in the average Nusselt number and the entropy generation by increasing Lewis number. Key words: Natural convection, mass transfer, entropy generation, magnetic field, porous media.

Analytic expression to derive the image source point between two symmetrical sloping planes

The sound field in open cavities with sloping walls can be represented by image sources and edge diffraction components. Sloping walls will generate a systematic structure to the positions of the image source, and explicit expressions for those positions are presented here. Using such explicit expressions, rather than the general image source method, makes it straightforward to study arbitrarily high orders. When two opposite walls form parts of an infinite wedge, then their image sources will be positioned on a cylindrical surface. The addition of a floor is easy, while adding more walls requires an iterative procedure, with explicit visibility tests. First-order diffraction components can then be generated for each image source. Simulation results are compared with experimental measurements in a three dimensional open cavity with sloping walls.

Chaos and beyond in a water filled ultrasonic resonance system

Finite amplitude ultrasonic wave resonances in a one dimensional liquid-filled cavity are reported. The resonances are observed to include not only the expected harmonic and subharmonic signals but chaotic signals as well. The nonlinear features of this system were recently investigated and are the focus of this presentation. An ultrasonic interferometer having optical precision was constructed. The transducers having the frequency range 1-10 MHz, driven by a high power amplifier. Both an optical diffraction system and a receiving transducer were used to assess the generated resonance response in the cavity. Five regions of excitation are identified: 1. Linear region: at low intensity of the ultrasonic wave the diffraction pattern of a light beam is symmetric. 2. Nonlinear region: with increased sound amplitude the diffraction pattern becomes asymmetrical. 3. Subharmonic region: further increase of the amplitude above a threshold value leads to the generation of subharmonics. 4. Chaos: increasing the drive amplitude to a second threshold level the diffraction pattern is smeared out indicating a time-chaotic region. 5. Beyond chaos: further increase of the amplitude results again a stable diffraction pattern. A first-principle-based explanation is presented. This work is supported by the Aircraft Aging Program, at NASA Langley Research Center.

Supersonic flow over three dimensional cavities

AbstractThis work presents a study of acoustic oscillations generated and the wave structure associated with supersonic flow past wall mounted 3D open cavities of varying length-to-width (L/W) ratio. Experiments were conducted to investigate the acoustic signature generated by the cavities at freestream Mach number of 1·7. The effect of L/W ratio of the cavity on the dominant modes of the acoustic signature registered on different walls of the cavities is investigated for an L/W range of 0·83-4. Shift in the dominant acoustic mode is observed as L/W ratio changes from 3 to 4. Statistical analysis of pressure data showed existence of acoustic waves and spreading of acoustic energy over different modes with change in cavity width. Time averaged schlieren visualisation indicated variation of shock and shear layer structure in the mainstream for the different cavities. Acoustic waves generated by the presence of the cavity and the dynamic behaviour of the shear layer were observed during instantaneous shadowgraph visualisation. Numerical simulation was done to make a prior assessment of the flow structure and the results are in good agreement with those from experiments. Ratio of mass exchange between cavity and mainflow and the cavity volume was observed to have profound effect on the magnitude of pressure oscillations generated by the cavities.

Proton beam writing of three-dimensional microcavities

Optical micro cavities exhibit high quality factors due to the circulation of resonant optical fields within the cavity. Polymers are good materials for the fabrication of micro cavities for practical applications due to the availability of various refractive indices and their low cost. Polymer micro cavities generally yield low Q-factors compared to semiconductor materials because of inherent material absorption losses, and their Q-factors are limited by the low index contrast between the polymer and the substrate material. In the present work, three dimensional micro cavities were fabricated in SU-8 using proton beam writing to enhance the index contrast by isolating the cavities from the substrate.

Acoustic and velocity field over 3D cavities

Effects of wall mounted cavity on a Mach 1.7 freestream flow over it are investigated experimentally and numerically. Three different three dimensional (3D) cavity configurations have been used in the study. The results are compared with those of a two dimensional (2D) cavity. Flow field over the cavity is observed to depend intensely on the cavity width and on the allied aerodynamic flow structure in the vicinity of the cavity. Pressure oscillations generated by presence of wall mounted cavity in supersonic freestream was also observed to affect the fluid motion over cavities.

Polymeric organic nanogels: structural studies and correlation between morphology and catalytic efficiency

The development of novel synthetic systems with tailored catalytic activity is a high priority area with interesting applications. The use of the templating approach with nanomaterials allows the creation of three dimensional cavities in polymeric matrices with specific molecular recognition properties that can be tailored by the structure of the template used. We report an in-depth study of the effects of solvents on the catalytic activity of templated and non-templated nanogels in the Kemp elimination, a reaction for which there is no enzyme. A full morphology characterisation of the nanoparticles, using small angle neutron scattering techniques and transmission electronic microscopy, has allowed the identification of significant differences in shape, size and, more significantly, in the internal structure of templated and non-templated nanogels. Moreover the data clearly indicate a very different behaviour of the nanoparticles, as a result of variations in acetonitrile content. The data provide for the first time structural evidence of templated cavities and identify important differences in swelling behaviour, aggregation, colloidal stability between templated and non-templated nanogels, which not only explain the differences in kinetic data but also identify important characteristics required for the development of novel catalysts with improved efficiency.

Fully coupled numerical simulation of fire in tunnels: From fire scenario to structural response

In this paper we present an efficient tool for simulation of a fire scenario in a tunnel. The strategy adopted is based on a 3D-2D coupling technique between the fluid domain and the solid one. So, the thermally driven CFD part is solved in a three dimensional cavity i.e. the tunnel, and the concrete part is solved on 2D sections normal to the tunnel axis, at appropriate intervals. The heat flux and temperature values, which serve as coupling terms between the fluid and the structural problem, are interpolated between the sections. Between the solid and the fluid domain an interface layer is created for the calculation of the heat flux exchange based on a wall law. In the analysis of the concrete structures, concrete is treated as a multiphase porous material. Some examples of application of this fully coupled tool will be shown.

Simulation of Heat and Mass Transport in a Square Lid-Driven Cavity with Proper Generalized Decomposition (PGD)

The aim of this study is to apply proper generalized decomposition (PGD) to solve mixed-convection problems with and without mass transport in a two dimensional lid-driven cavity. PGD is an iterative reduced order model approach which consists of solving a partial differential equation while seeking the solution in separated form. Comparisons with results in the literature and with results from a standard solver are make. For the case of a mixed-convection problem without mass transfer, three Richardson numbers are considered, Ri = 0.1, Ri = 1, and Ri = 10. In this case, PGD is seven times faster than the standard solver with Ri = 10 with a similar accuracy. For the case with mass transfer, simulations are done with different Lewis numbers, Le = 5, Le = 25, and Le = 50, and with different value of the ratio N between the solutal and the thermal Grashoff numbers. In this case, too, PGD is ten times faster than the standard solver.

Advanced Algorithms for Etching Simulation of 3D MEMS-Tunable Lasers

 Abstract—The integrated circuits (ICs) industry uses a number of technology computer aided design (TCAD) software tools to simulate the manufacturing and the operation of many ICs at different levels. At very low level, the simulation tools are used to simulate the device fabrication and design. These simulation tools are based on solving mathematical equations that describe the physics of doping diffusion, silicon oxidation, etching, deposition, lithography, implantation, and metallization. The simulation of physical etching solves etching equations to calculate the etching rate. And this rate is used to move the geometry of the device. The simulation of non-physical (geometrical) etching is based on geometrical Boolean operations. In this paper, we are proposing new and advanced geometrical etching algorithms for the process simulation of three dimensional (3D) micro electro mechanical systems (MEMS) and MEMS-tunable vertical cavity semiconductor optical amplifiers (VCSOAs). These algorithms are based on advanced domain decomposition methods, Delaunay meshing algorithms, and surface re-meshing and smoothing techniques. These algorithms are simple, robust, and significantly reduce the overall run time of the process simulation of 3D MEMS and MEMS-tunable laser devices. The description of the proposed etching algorithms will be presented. Numerical simulation results showing the performances of these algorithms will be given and analyzed for realistic 3D MEMS and MEMS-tunable laser devices.

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρl/ρv,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρl/ρv,clip=43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.

Inducing an Incipient Terahertz Finite Plasmonic Crystal in Coupled Two Dimensional Plasmonic Cavities

We measured a change in the current transport of an antenna-coupled, multigate, GaAs/AlGaAs field-effect transistor when terahertz electromagnetic waves irradiated the transistor and attribute the change to bolometric heating of the electrons in the two dimensional electron channel. The observed terahertz absorption spectrum indicates coherence between plasmons excited under adjacent biased device gates. The experimental results agree quantitatively with a theoretical model we developed that is based on a generalized plasmonic transmission line formalism and describes an evolution of the plasmonic spectrum with increasing electron density modulation from homogeneous to the crystal limit. These results demonstrate an electronically induced and dynamically tunable plasmonic band structure.

Combination of the fictitious domain method and the sharp interface method for direct numerical simulation of particulate flows with heat transfer

In this paper, the fictitious domain (FD) method and the sharp interface (SI) method are combined for the direct numerical simulations of particulate flows with heat transfer in three dimensions. The flow field and the motion of particles are solved with the FD method. The temperature field is solved in both fluid and solid media with the SI method. The accuracy of the proposed FD/SI method is validated via two problems: the natural convection in a two- dimensional cavity with fixed solid particles, and the flow over a cold sphere. The method is then applied to the natural convection in a three dimensional cavity with a fixed sphere, the motion of a spherical particle in a non-isothermal fluid, and the rising of spherical catalyst particles in an enclosure. The effects of the thermal conductivity ratio are examined in the first and third problems, respectively, and the significant effects of the thermal expansion coefficient ratio on the particle motion are demonstrated in the second problem.

A high-resolution spectrometer based on a compact planar two dimensional photonic crystal cavity array

We demonstrate a compact spectrometer based on an array of high-quality-factor photonic crystal nanocavities, coupled via a planar two-dimensional waveguide. This architecture enables spectral analysis of incident light with resolution as high as the bandwidth of the cavity mode–0.3 nm at 840 nm for our device. The design is easily extended to the visible and deep-infrared spectral ranges. The two-dimensional cavity array can be mated to commercial two-dimensional optical detector arrays, creating a compact and high-resolution spectrometer suitable for a range of applications including materials and chemical analysis.

Three dimensional density cavities in guide field collisionless magnetic reconnection

Particle-in-cell simulations of collisionless magnetic reconnection with a guide field reveal for the first time the three dimensional features of the low density regions along the magnetic reconnection separatrices, the so-called cavities. It is found that structures with further lower density develop within the cavities. Because their appearance is similar to the rib shape, these formations are here called low density ribs. Their location remains approximately fixed in time and their density progressively decreases, as electron currents along the cavities evacuate them. They develop along the magnetic field lines and are supported by a strong perpendicular electric field that oscillates in space. In addition, bipolar parallel electric field structures form as isolated spheres between the cavities and the outflow plasma, along the direction of the low density ribs and of magnetic field lines.

Design and Development of Low Cost and Light Weight Microwave Filters by using Metallized ABS Plastic as a substitute of High Cost Substrate and Metals

The main objective to introduce the ABS (Acryonitrile Butadyne Styrene) plastic substrate in place of RTDuroid substrate for microstrip filters is to reduce the cost. The cost of plated ABS plastic substrate is substantially less (Rs. 4/sq.inch) compared to the cost of RT-Duroid (US$ 02/-/sq.inch). Two microstrip (hairpin line) band pass filters at 1537.5 MHZ and 1575.42 MHz have been developed and tested. The performance of filters have been verified over temperature range, -10 deg. C to 60 deg. C.The ABS plastic blocks have been used in place of metal blocks of brass, copper and Commercial Aluminium to fabricate three dimensional microwave cavity filters for communication systems.The specific gravity of ABS plastic is 1.03 gms/cubic cm compare to 2.7, 6.3 and 9.6 gms/cubic cm of Com. Al, Copper and Brass respectively. Hence the weight of metallized ABS cavity filters will be 1/3rd , 1/6th and 1/9th of the com. Al, copper and brass cavity filters. General Terms Acryonitrile butadiene styrene plastic, poly tetra flouroehtelene, RT-duroid, thermal conductivity, surface resistivity, heat distortion temperature, coating, plating.

High quality factor two dimensional GaN photonic crystal cavity membranes grown on silicon substrate

We report on the achievement of freestanding GaN photonic crystal L7 nanocavities with embedded InGaN/GaN quantum wells grown by metal organic vapor phase epitaxy on Si (111). GaN was patterned by e-beam lithography, using a SiO2 layer as a hard mask, and usual dry etching techniques. The membrane was released by underetching the Si (111) substrate. Micro-photoluminescence measurements performed at low temperature exhibit a quality factor as high as 5200 at ∼420 nm, a value suitable to expand cavity quantum electrodynamics to the near UV and the visible range and to develop nanophotonic platforms for biofluorescence spectroscopy.