Three-dimensional Open Cavity Research Articles

ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones with broadband circular dichroism in the mid-infrared wavelength.

This paper proposed ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones with broadband CD in the mid-infrared band. The experimental results show that when the deposition angle θ = 45°, the first ITO deposition of ta = 100 nm, the second Si deposition of tb = 200 nm with the azimuth angle unchanged, and the third ITO deposition of tc = 200 nm after rotating the azimuth angle of 60°, the prepared chiral structure has a broadband CD response in the mid-infrared band of 2.5-4 µm. The broadband CD effect is produced by the internal resonance of the three-dimensional open cavity. The cone structure can be regarded as a plurality of planar open resonant rings with different diameters, and these rings resonate at different wavelengths. The experimental results also show that the proposed chiral ITO structure exhibits a better broadband CD response than that of the structure composed of traditional metal Ag. Such a chiral structure provides a new method for the design of CD devices in the mid-infrared band.

Analysis of Flow Oscillation Due to Sidewall of Three-Dimensional Supersonic Open Cavity Flow

Unsteady turbulent flow simulations were performed based on the Reynolds-averaged Navier–Stokes (RANS) equations to investigate flow oscillation due to three-dimensional (3D) configuration of a Mach 1.5 supersonic open cavity flow with a length-to-depth ratio of 3. Two-dimensional (2D) and 3D unsteady simulation results were analyzed and compared with experimental data and Rossiter’s empirical prediction data. The three-dimensional cavity width-to-depth ratio (W/D) was 1, 3.8 and 7.6. Computational results indicated that pressure oscillation in the 2D flow was generated by a single-flow structure, whereas a multiple-flow structure generated multiple oscillation peaks in the 3D flow. The flow structure in the 3D cavity was investigated. For the 2D flow case, the cavity internal pressure wave was directly synchronized with the free shear layer. In the 3D flow case, an unstable spanwise flow due to the sidewall was observed. This spanwise fluctuation produced additional pressure oscillations coupled with the streamwise internal pressure wave. The numerical results indicate that the spanwise flow reduces the propagation speed of the internal pressure waves and the intensity of the corresponding pressure fluctuation.

Successive bifurcations in a fully three-dimensional open cavity flow

The transition to unsteadiness of a three-dimensional open cavity flow is investigated using the joint application of direct numerical simulations and fully three-dimensional linear stability analyses, providing a clear understanding of the first two bifurcations occurring in the flow. The first bifurcation is characterized by the emergence of Taylor–Görtler-like vortices resulting from a centrifugal instability of the primary vortex core. Further increasing the Reynolds number eventually triggers self-sustained periodic oscillations of the flow in the vicinity of the spanwise end walls of the cavity. This secondary instability causes the emergence of a new set of Taylor–Görtler vortices experiencing a spanwise drift directed toward the spanwise end walls of the cavity. While a two-dimensional stability analysis would fail to capture this secondary instability due to the neglect of the lateral walls, it is the first time to our knowledge that this drifting of the vortices can be entirely characterized by a three-dimensional linear stability analysis of the flow. Good agreements with experimental observations and measurements strongly support our claim that the initial stages of the transition to turbulence of three-dimensional open cavity flows are solely governed by modal instabilities.

Hydroacoustic analysis of open cavity subsonic flow based on multiple parameter numerical models

The prediction of the flow-induced noise level is a key issue in the fluid–dynamic acoustics. In the hydroacoustics field, the complicated feedback induced by the flow past open cavities can amplify the convection instability in the shear layer which further leads to important noise radiations. The noise consists of intense narrowband and broadband components. In this paper, the level of the noise radiated by a subsonic cavity flow is calculated by using numerical flow computations based on the large eddy simulation (LES) and by solving the Ffowcs Williams-Hawkings equation. A series of three-dimensional open cavity models with overset grids and appropriate boundary conditions are developed for the hydroacoustic numerical computation. The self-sustained oscillation characteristics of the cavity flow are investigated, together with the mechanisms of the cavity noise generation. The distinguishing features of the flow-induced noise of the underwater structure cavities are studied with respect to the parameters of the cavity models, such as the free stream velocity, the dimensions of the cavity mouth, the angle of the cavity neck, the horizontal and vertical porous cavity models and the actual submarine open cavity model with an incoming flow attack angle. It is shown that it may be feasible to reduce the flow-induced noise by appropriate optimal parameters of the underwater structure cavities.

Analysis of Transient Electromagnetic Scattering from a Three-Dimensional Open Cavity

This paper is concerned with the mathematical analysis of the time-domain Maxwell equations in a three-dimensional open cavity. An exact transparent boundary condition is developed to reformulate the open cavity scattering problem in an unbounded domain, equivalently, into an initial-boundary value problem in a bounded domain. The well-posedness and stability are studied for the reduced problem. Moreover, an a priori estimate is established for the electric field with a minimum regularity requirement for the data. (The PDF has been changed.)

Efficient modeling and experimental validation of acoustic resonances in three-dimensional rectangular open cavities

The goal of this paper is to investigate the acoustic resonances of a three-dimensional open cavity by a fast and efficient method in the time domain. This method models the time response in any point as the convolution of the source waveform with the impulse response of the cavity, which, in turn, is obtained as a sequence of attenuated and delayed impulses coming, the first from the real, and the subsequent from the mirror imaged sources (Image Source Model). This method, which main advantages with respect to others that work in the frequency domain are that the results for all frequencies can be calculated at once, the time domain data is recovered directly, and the computational cost does not increase with frequency, can easily provide the frequency response at each point of the cavity by Fourier transform. From these frequency responses, the relevant resonances of the cavity can simply be obtained. Illustration and experimental validation of the proposed method is presented by its application to a rectangular open cavity.

Heat transfer and fluid flow characteristics of laminar flow past an open cavity with heating from below

Laminar mixed convective flow over a three-dimensional open cavity with heating from below at constant temperature was numerically simulated using direct numerical simulation and the most hydrodynamic and thermal aspects of the flow are presented. The effects over the velocity and temperature distribution of the buoyancy forces acting perpendicular to the mainstream flow are studied for a range of Reynolds numbers between 100 and 1500, and Richardson numbers from 0.001 to 10 to obtain a phenomenological description of the convective air flowing through the channel and the cavity. At low Reynolds and Richardson numbers the flow becomes steady and the heat diffusion is predominant, whereas at high Richardson number the heat transfer by convection becomes important and the flow experiences unsteady behavior with a significant interaction between the external flow and the recirculating flow inside the cavity, and exhibits a three-dimensional nature.

Analysis of the flow and heat transfer characteristics for assisting incompressible laminar flow past an open cavity

A numerical study has been carried out to analyze the effects of mixed convective assisting flow past three-dimensional open cavity over a wide range of Reynolds (100–1000) and Richardson (0.001–10) numbers. The vertical walls in the inflow and outflow sides are isothermal while all other walls are adiabatic. The cavity is assumed to be cubic in geometry and the flow is laminar. A direct numerical simulation is undertaken to investigate the flow structure, the heat transfer characteristics and the complex interaction between the induced stream flow at ambient temperature and the buoyancy-induced flow from the heated wall. It is found that the flow becomes stable at moderate Grashof number and exhibits a three-dimensional structure, while for high Richardson number the mixed convection effects come into play and push the recirculating zone further upstream and the flow may becomes unstable.

Large-eddy simulation of a compressible flow in a three-dimensional open cavity at high Reynolds number

Large-eddy simulations of a subsonic three-dimensional cavity flow with self-sustaining oscillations are carried out for a Reynolds number based on the length of the cavity equal to . Meticulous comparisons with available experimental data corresponding to the same configuration demonstrate a high level of accuracy. Special attention is paid to the mixing layer that develops over the cavity and two different zones are identified. The first one is dominated by Kelvin–Helmholtz instability, and the linear as well as quadratic energy transfers leading to the filling of velocity spectra are described. The Kelvin–Helmholtz instability also appears to be forced near the origin of the layer, and it is postulated that the small recirculation bubble located in this area is responsible for the forcing. Downstream of the first zone and up to the vicinity of the aft wall, the layer behaves very similarly to a free mixing layer by exhibiting a linear spreading. An influence of the recirculating flow inside the cavity upon the growth of the layer is nevertheless observed at downstream stations. Analysis of the pressure on the floor of the cavity reveals that the self-sustaining oscillation-related pressure modes (Rossiter modes) are independent of their spanwise location inside the cavity. On the contrary, Rossiter modes exhibit streamwise modulations and it is demonstrated that a very simple two-wave model is able to reproduce the spatial shape of the modes. Nonlinear interactions between Rossiter modes are encountered, as well as nonlinear interactions with low-frequency components. A joint time–frequency analysis shows a temporal modulation of the Rossiter mode levels at similar low frequencies, resulting in a special form of intermittency with competitive energy exchanges between modes.

Electromagnetic density of modes for a finite-size three-dimensional structure.

The concept of the density of modes has been lacking a precise mathematical definition for a finite-size structure. With the explosive growth in the fabrication of photonic crystals and nanostructures, which are inherently finite in size, a workable definition is imperative. We give a simple and physically intuitive definition of the electromagnetic density of modes based on the Green's function for a generic three-dimensional open cavity filled with a linear, isotropic, dielectric material.

Numerical Prediction of Fluid-Resonant Oscillation at Low Mach Number

A method (governing equation set and numerical procedure) suited to the numerical simulation of fluid-resonant oscillation at low Mach numbers is constructed. The new equation set has been derived under the assumption that the compressibility effect is weak. Because the derived equations are essentially the same as the incompressible Navier-Stokes equations, except for an additional term, we can apply almost the same numerical procedure developed for incompressible flow equations without difficulty. With application of a pressure-based method that treats the continuity equation as a constraint equation for pressure, the stiffness problem that arises in solving the usual compressible flow equations under low Mach number conditions has been alleviated. To verify the present method, we apply it to the flows over a three-dimensional open cavity

Numerical Prediction of Fluid-Resonant Oscillations at Low Mach Number.

Our objective is to predict the Fluid-Resonant Oscillation at low Mach number numerically. Because the compressible flow equations become stiff in low-Mach-number flows, we derive a new equation set that is suited for the simulation of such flows under assumption that the compressibility effect is weak. The derived equations are essentially the same as the incompressible Navier-Stokes equations except for an additional term. So we can apply almost the same numerical procedure as developed for the incompressible flow equations. Using the present method, a numerical analysis of flows over a three-dimensional open cavity is performed. The results show that strong pressure fluctuations occur at specific flow velocities. Also shown is that the frequency of the pressure fluctuations is locked-in at the Helmholtz resonant frequency of the cavity. Thus, it is confirmed that the present method is able to predict the Fluid-Resonant Oscillation in low-Mach-number flows.