Rectangular Cavity Research Articles

Flow visualization in cavity with dielectric barrier discharge actuator control

The results of hydrodynamic pressure fluctuations control by means of a dielectric barrier discharge plasma actuator in a rectangular cavity with sharp edges are presented. The study was carried out at the free flow velocity V=37 m/s. The discharge was organized near the leading edge of the cavity. The average energy of the discharge pulse was 0.03 J, its duration was 1 ms, and frequency modulation was implemented at the natural cavity resonant frequencies of 500, 817, 1317 Hz and at a high frequency of 2160 Hz which did not correspond to the natural resonant pressure peak. Pressure pulsations were measured with a Kulite pressure transducer. It is established that the discharge regime affects the pressure fluctuations in the cavity. The pressure amplitude at the downstream wall of the cavity can be either increased from 117 dB to 128 dB or decreased to 110 dB if, respectively, if either the dominant or higher modes are pumped by the discharge. The PIV visualization was organized in the phase-locked mode. The pressure spectrum corresponds to the magnitude of coherent structures in the shear layer of the cavity. Keywords: plasma actuator, cavity, active control, DBD, boundary layer, mixing layer, closed loop.

Solution of Navier-Stokes equations for fluids with magnetorheological compensation used in structures with energy dissipaters

The fixed-wall rectangular cavity flow problem is a classic problem that has been studied since the beginning of computational fluid mechanics. The present work aims to provide a numerical and computational solution of the Navier-Stokes equations using the finite difference method, applied to model the problem of a magnetorheological fluid in a rectangular cavity with a fixed wall in shock absorbers devices, used in civil structures that use energy dissipators.

Double Diffusive Convection Heat and Moisture Transfer Inside a Planted Roof Building Under Hot Humid Climate: Case of Lomé City in West Africa

Planted roofs have been investigated as a passive cooling technical for the energy efficiency purpose in buildings. More quantitative data on this topic are required to solve a lack of information for many specific regions. This study is focused to a numerical investigation of the thermal comfort inside a green roof rectangular ventilated cavity in a hot and humid climate as the one of Lomé in west Africa. The left vertical is heated and partly saturated with water to provide humidification of the indoor air. Transfer dimensionless equations are solved using an implicit finite difference scheme, Thomas algorithm and the Gauss Seidel iterative method. We analyze the effects of inlet airflow on the thermal process inside the ventilated and planted enclosure have been investigated. The comfort temperature range deduced from data is: 25°C< Tc < 27°, that of the indoor air humidity is: 49% < Hr <60%. The different ranges obtained are significant and lead to improve inside thermal comfort. The solar flux 350 W.m-2, the average value in case of Lomé city, was used to establish a heat transfer correlation to predict heat transfer through the roof with a relative error not exceeding 4%. This model can be very useful for engineers in the design and optimization stage of a green roof in practical buildings.

Characterizing surface-gap effects on boundary-layer transition dominated by Tollmien–Schlichting instability

Effects of gaps (rectangular surface cavities) on boundary-layer transition are investigated using a combination of linear stability theory and experiments, for boundary layers where the smooth-surface transition results from Tollmien–Schlichting (TS) instability. Results are presented for a wide range of gap characteristics, with the associated transition locations ranging from the smooth-surface location all the way forward to the gap location. The transition movement is well described by a variable $N$ -factor, which links the gap characteristics to the level of instability amplification $e^N$ leading to transition. The gap effects on TS-wave transition are characterized by two limiting behaviours. For shallow gaps $d/w &lt; 0.017$ , the reduction in $N$ -factor is a function of the gap depth $d$ and is independent of the gap width $w$ . For deep gaps $d/w &gt; 0.028$ , the reduction in $N$ -factor is a function of the gap width and is independent of the gap depth. When both the gap width and depth are sufficiently large relative to the displacement thickness $\delta ^*$ , the TS-wave transition is bypassed, resulting in transition at the gap location. These behaviours are mapped out in terms of ( $w/ \delta ^*$ , $d/ \delta ^*$ ), providing a predictive model for gap effects on transition.

Substrate integrated waveguide antenna with dual rectangular slots for material characterization for ISM band applications

An antenna with small height and width with high input impedance filled with different slots is designed for ISM band applications. The substrate integrated waveguide antenna is designed with multiple different slots which produces ISM band applications. The rectangular cavity slots are introduced here. This will radiate a lower resonant frequency. In spite of using one rectangular slot two similar rectangular equally sized slots are introduced here for 2.4 GHz ISM band applications with a return loss of-13.20803 dB.The inset type of micro strip feed line is fitted to this backed slot antenna. The FR-4 substrate has been introduced for this back slotted antenna. The low profile compact antenna dimensions are 30 x30x1.6 mm3.The simulated antenna has been further designed with four different substrate for material characterization such as Polymide, Polyester, Teflon, Cotton. The antenna simulation is created using the CST Microwave Studio programme.

Numerical Investigation on Oscillation Behavior of a Non-isothermal Self-excited Jet in a Cavity: The Effects of Reynolds Number and Temperature Differences

A self-excited oscillating jet can be naturally produced by discharging a plane jet into a rectangular cavity due to pressure effects and without a need for external aid. In recent years, the self-oscillatory jet in non-isothermal conditions has attracted research interests because of its wide range of industrial applications. Therefore, the current study aimed to compare the oscillatory behavior of downward vertical self-excited jet with Reynolds number (Re) 1000 and 3000 under various temperature differences (0, 100, and 300 K) between inletflow and cavity’s wall. Computational solutions were obtained using unsteady Reynolds averaged Navier-Stokes (URANS) and energy equations for an incompressible flow. The numerical simulation was carried out by the finite-volume based tool OpenFOAM code. The results showed that depending on the value of temperature difference, oscillatory and non-oscillatory flows were observed. Also, at Re=3000, the temperature differences can change oscillation frequency up to 10% compared to isothermal conditions. This value reaches 58% at Re=1000. The results indicated that where the Archimedes number is less than 0.1, the effects of temperature differences between jet and cavity walls on the oscillating behavior are negligible.

Influence of Anisotropy on Thermosolutal Convection in Porous Media with Magnetic Field Effect

In the present study, both anisotropy and magnetic field effects on bi-diffusive natural convection in a rectangular cavity filled with a porous medium saturated by a binary fluid are investigated analytically for fully developed flow regime. The cavity is heated isothermally by the sides and its horizontal walls are thermally insulated or conducted. The porous medium is anisotropic in permeability whose principal axes are oriented in a direction that is arbitrary to the gravity field. On the basis of the generalized Brinkman-extended Darcy model of newtonian fluids on steady flow through porous media, analytical expressions were obtained for the flow and thermal fields, the concentration of speaces, the average Nusselt and Sherwood numbers in terms of the Darcy number, the anisotropic permeability ratio, the orientation angle of the principal axes and the Hartmann number. The limiting case corresponding to pure porous media (<i>Da</i>→0) and pure fluid media (<i>Da</i>→∞) for the thermal conditions mentioned on the cavity completed these results in order to compare them to those obtained in the literature. It is found that, Nusselt and Sherwood numbers increase by increasing anisotropic parameters of the porous medium while increasing magnetic field magnitude greatly reduces the intensity of the flow and thus affects significantly heat and mass transfer.

Nonlinear regimes of binary mixture convection in a two-layer porous medium of different configurations

The results of modeling the convection of a binary mixture of liquid hydrocarbons in a two-layer porous medium are presented. The simulation area is a horizontally elongated rectangular cavity divided into two layers. In one of the considered cases, these sublayers are of equal height, and in the other case, the interface between the layers has the shape of a convex downward circular arc, which imitates a synclinal geological fold. The layers have different permeability. In the cavity, there is a geothermal temperature gradient with an average temperature characteristic of a depth of 2000 m, which corresponds to the average oil depth. The composition of the mixture, the thermal conditions and the geometry used make up the reservoir model of the hydrocarbon deposit. The problem is solved in terms of the Darcy–Boussinesq model with allowance for the effect of thermal diffusion. The emergence and establishment of nonlinear convection regimes, as well as the distribution of the concentration of mixture components for different values of the permeability of the layers and their dependence on which of these layers is more permeable, are considered in simulations. It is found that in the case of a small difference in the permeabilities of the layers, a stationary regime is established in the cavity of any of the considered configurations. With a significant difference in permeability of the layers in the cavity, either quasi-periodic oscillations of a complex shape or irregular oscillations can be observed. A "local" occurrence of convection is shown for a large permeability difference, followed by a weak penetration of the flow into a less permeable layer, as well as a "large-scale" occurrence of convection in the case of an insignificant permeability difference.

MHD mixed convection in a partitioned rectangular enclosure

A numerical study is carried out to explore the influence of external magnetic field on mixed convective heat transfer in a partitioned rectangular cavity with on side moving wall. The vertical walls are isothermally heated while the horizontal walls are thermally insulated. The left vertical wall is moving in + y direction and remaining walls are maintained no-slip condition. A magnetic field of uniform strength is imposed transverse to the temperature gradient. The governing equations are solved utilizing the finite element method for several physical parameters including Richardson number, Hartmann number and Prandtl number. The numerical results are presented graphically using streamlines, isotherms, local and average Nusselt numbers. It is observed that the flow filed is affected significantly for moving wall and the variation in Hartmann and Richardson numbers. The velocity field is found more effective in natural convection regime than forced convection. The results demonstrated that maximum amount of heat transfer is obtained in natural convection domination and higher values of Prandtl number. The enhancement of heat transfer rate is found 14.25% more at higher Prandtl number (Pr =2.56) than lower (Pr =0.71) and it reduction is found 3.03% more at Ha = 50 compared to Ha = 0.

LATTICE BOLTZMANN METHOD INVESTIGATIONS OF NATURAL CONVECTION WITHIN A CAVITY EQUIPPED WITH A HEAT SOURCE: A PARAMETRIC STUDY

In the present work, a numerical study is carried out to investigate, with the aid of the lattice Bolkmann method (LBM), the natural convection air (Pr = 0.71) cooling of an electronic component presented as a heat source attached to the bottom wall of a rectangular cavity. The top wall is isothermal and other walls are adiabatic. In order to optimize the ability of the heat sink to reject more heat from the heat source and ensure an effective cooling of the electronic component, a series of numerical simulations are conducted by varying the cold wall position (top - right), inclination angle (0° ≤ γ ≤ 90°), the aspect ratio of the cavity (0.25 ≤ A ≤ 1.5), and the heat source position (ε = 0.25, ε = 0.35, and ε = 0.5), while the Rayleigh number is kept constant, Ra = 106. Results show that the cavity structure has a deep impact on the cooling of the electronic component. The aspect ratio A = 1.5 ensures a relatively excellent heat transfer rate when the cavity is cooled by the top wall. The heat source position within the cavity has, also, a noticeable impact on dynamic and thermal fields. Indeed, it was found that the local Nusselt number of the cold wall and heat source indicate that, by varying the position of the heat source, fluid flow and natural convection rate are more important in the right cavity compared to the left one.

Monolithic edge-cladding process for the elliptical disk of N31-type Nd-doped high-power laser glass

Abstract This paper investigates the monolithic edge-cladding process for the elliptical disk of N31-type Nd-doped phosphate laser glass, which will be utilized under liquid cooling conditions for high-power laser systems. The thermal stress, interface bubbles and residual reflectivity, which are due to high-temperature casting and bonding during the monolithic edge-cladding process, are simulated and determined. The applied mould is optimized to a rectangular cavity mould, and the casting temperature is optimized to 1000°C. The resulting lower bubble density makes the mean residual reflectivity as low as 6.75 × 10−5, which is enough to suppress the amplified spontaneous emission generated in the Nd-glass disk, and the resulting maximum optical retardation is converged to 10.2–13.3 nm/cm, which is a favourable base for fine annealing to achieve the stress specification of less than or equal to 5 nm/cm. After fine annealing at the optimized 520°C, the maximum optical retardation is as low as 4.8 nm/cm, and the minimum transmitted wavefront peak-to-valley value is 0.222 wavelength (632.8 nm). An N31 elliptical disk with the size of 194 mm × 102 mm × 40 mm can be successfully cladded by the optimized monolithic edge-cladding process, whose edge-cladded disk with the size of 200 mm × 108 mm × 40 mm can achieve laser gain one-third higher than that of an N21-type disk of the same size.

Local heat/mass transfer distributions on the bottom surface of a cavity exposed to an approaching turbulent boundary layer

Heat/mass transfer on the bottom surface of a rectangular cavity in an incompressible turbulent boundary layer flow is investigated, with emphasis on the effects of cavity width (W) to depth (d) ratio, for a cavity with aspect ratio L/d (length L to depth) of 6. For a wide cavity, this value of L/d is known to establish a flow structure in which the streamwise flow does not reattach on the bottom surface of the cavity, and instead shears past the entire open surface of the cavity, impinging on the downstream wall. A naphthalene sublimation mass transfer technique is used to evaluate local (mass) Stanton numbers at the cavity bottom surface for W/d ranging from 0.51 to 10, and two Reynolds numbers of 8100 and 12,800 defined using the cavity depth and freestream velocity. For W/d>2, the area-averaged mass transfer Stanton number asymptotes to a constant value. Despite the nominal two-dimensional behavior implied by the area-averaged Stanton number for W/d > 2, the details of the local mass transfer distributions show the effects of a system of multiple vortices that remain influential beyond W/d>2 and cause strong spatial variations in transport on the bottom surface. For W/d<2, the area-averaged Stanton number decreases as width is reduced, scaling as a power law with W/d, and appears to be independent of Reynolds number. Computations using the SST k−ω RANS model are able to predict the measured heat/mass transfer at the bottom surface well for W/d>3. For 0.8<W/d<3, the computations are able to capture the trends only qualitatively. For W/d<0.8, computations fail to predict the observed distributions and significantly under-perform.

Use of 2-D and 3-D unsteady RANS in the computation of wall bounded buoyant flows

The present study numerically investigates the performance of a range of RANS turbulence closures with different near-wall treatments in the prediction of natural convection flows. Comparisons are performed of both 2-D and 3-D steady and time-dependent computations in differentially heated cavities, which involve very simple geometries, but give rise to complex flow physics of relevance to cooling applications, including safety features of nuclear reactors. The turbulence closures assessed include eddy-viscosity and second-moment closures and involve low-Reynolds-number versions (LRN) which fully resolve the viscous sublayer, as well as high-Reynolds-number versions (HRN) which adopt wall function treatments to prescribe boundary conditions for the wall shear stress and heat flux.In the 2-D computations of the tall rectangular cavity, there are only small deviations in the predictions of different models, and these are mainly confined to the local Nusselt number comparisons. Among the LRN models, the second-moment EBRSM is in good agreement with the data, while the LRN eddy-viscosity models under-estimate the Nusselt number levels. The HRN models, when used with the standard log-law-based wall function, over-estimate the Nusselt number. During the course of the present investigation, a new, more general, version of the Analytical Wall Function (AWF) has been developed and introduced to the open-source CFD code utilised. Here, this more general numerical wall treatment brings the HRN k-ε predictions of the Nusselt number to very close agreement with the experimental data.In the 2-D square cavity computations, the simulations focus on a high Rayleigh number (Ra) of 1011, for which DNS data has recently become available. At such high Ra value all the models are challenged due to the unsteadiness of the flow and the presence of a largely stationary and laminar core, and thin turbulent boundary layers. Assessment of different 3-D configurations of the same square cavity demonstrates that the effect of the turbulent mixing in the third (z) direction is significant, but can be taken into account by introducing a cavity depth of D=0.15H, matching that used in the DNS study. The deviations in the predictions of the different models are stronger than those observed in the corresponding rectangular cavity comparisons, especially for the local Nusselt number distribution. As is also the case in the rectangular cavity, the eddy-viscosity models under-estimate the Nusselt number, while the EBRSM with the more elaborate turbulent heat flux model once more is closest to the DNS data. All HRN models with the log-law wall function severely under-estimate the Nusselt number levels. Introduction of the buoyancy-adapted AWF leads to substantial predictive improvements. The HRN AWF approach is thus shown to be a reliable and cost-effective modelling strategy for natural convection flows.

Dynamically tunable plasmon-induced transparency effect based on graphene metasurfaces

Plasmon-induced transparency (PIT) is theoretically explored for a graphene metamaterial using finite-difference time-domain numerical simulations and coupled-mode-theory theoretical analysis. In this work, the proposed structure consists of one rectangular cavity and three strips to generate the PIT phenomenon. The PIT window can be regulated dynamically by adjusting the Fermi level of the graphene. Importantly, the modulation depth of the amplitude can reach 90.4%. The refractive index sensitivity of the PIT window is also investigated, and the simulation results show that a sensitivity of 1.335 THz RIU−1 is achieved. Additionally, when the polarization angle of the incident light is changed gradually from 0° to 90°, the performance of the structure is greatly affected. Finally, the proposed structure is particularly enlightening for the design of dynamically tuned terahertz devices.

Heating Characteristics of Prototype Knee Rehabilitation System Using Rectangular Resonant Cavity Applicator

Heating Characteristics of Prototype Knee Rehabilitation System Using Rectangular Resonant Cavity Applicator

Broadband Controllable Asymmetric Accelerating Beam via Bilayer Binary Acoustic Metasurfaces

AbstractIn this work, asymmetric accelerating beam is achieved numerically and experimentally by taking advantage of a bilayer binary acoustic metasurface (BAM) composed of the rectangular cavity (bit ‘0’) and the waveguide with seven Helmholtz resonators (bit ‘1’). The fundamental physical mechanism is based on the different phase modulations induced from both sides of the structure because the left and right layers can be regarded as an angle‐selective refractor and an accelerating beam generator, respectively. The presented structure can work in a wide range of 5000–6000 Hz and switch from unidirectional to bidirectional transmission conveniently by altering the air gap between the two composing layers. Furthermore, asymmetric self‐focusing in a broadband is able to be realized as well by utilizing two symmetrical bilayer BAMs, which enriches the functionality of the design. Compared with previous asymmetric transmission devices, the authors' approach has the advantages of simple design, broadband multifunctionality, and tunable property, which may have promising applications in numerous acoustic fields including biomedical engineering, nondestructive testing, and acoustic communication.

Study on oil starvation at the oil seal edge of the oil film of hydrostatic bearing with double rectangular cavity

PurposeWhen the clearance oil film of hydrostatic bearing friction pair is in critical lubrication state, the phenomenon of zero flow of local lubricating oil will aggravate the oil film temperature rise, which needs to be solved.Design/methodology/approachIn this paper, the critical lubrication parameter equation and the oil film temperature rise mathematical model are derived for the new type q1-205 double rectangular cavity hydrostatic bearing. Based on a combination of theoretical analysis, simulation and experimental verification, this paper analyzes the flow characteristics and temperature rise characteristics of the lubricating oil when the hydrostatic bearing is in a critical lubrication state under different operating conditions and finally obtains the critical lubrication state of the oil film.FindingsThis study found that the numerical simulations and the derived formulas agree with the results. When the oil film is in critical lubrication, the cross-section side flow of the oil side is almost zero. The heat cannot be taken away in time, resulting in the local temperature rise of the oil film, which causes serious heat accumulation.Originality/valueIt is concluded that the operating condition parameters corresponding to the critical lubrication state provide a theoretical basis for the selection of actual hydrostatic bearing operating conditions, which is of great scientific significance.

Influence of thermal gravitational-capillary convection on temperature fields in a thin wall

The development of convective flow in a layer of ethyl alcohol when heating one of the vertical walls of a rectangular cavity was investigated experimentally. Thermal films were obtained, whose processing allowed plotting in time the distribution of temperature and temperature gradients on the free surface of the liquid layer and the opposite thin vertical wall of the cavity after the flow of heated liquid on it.

Aero-acoustic feedback mechanisms in supersonic cavity flow with subcavity

An open rectangular cavity with a length to depth ratio of 2 (L/D = 2) and inlet Mach number 1.71 was considered as a baseline configuration for an unsteady computational analysis. Fast Fourier transform using power spectral density, correlation, and overall sound pressure level (OASPL) was carried out on the unsteady pressure data, to analyze the flow dynamics. Many flow visualization techniques were employed such as density gradient-based numerical schlieren, vorticity contour, and streamlines for revealing major features associated with the flow. Vortex shedding visualization was carried out in terms of the Lambda 2 (λ2) criterion, where the vortex core can be observed moving downstream in the shear layer. In the acoustic pressure contours and OASPL contours, the characteristics of the acoustic waves can be distinctly observed traversing within the cavity. The analysis was extended for different flow conditions and configurations. The front wall subcavity with a ratio of subcavity length (l) to cavity length (L) l/L = 0.20 has already been established as a passive control device experimentally whereas the aft wall subcavity with l/L = 0.20 was found to behave like a passive resonator with distinct harmonic fluid-resonant modes. A more detailed investigation of these configurations with different dimensions and combinations resulted in a comparative and better understanding of the flow features, mode frequencies, and fluid-resonant oscillations in a supersonic cavity flow. Further, a method for designing front wall subcavity for passive control device has been applied to a variety of test cases and has been proven effective in suppressing the oscillations for cavity configurations with longitudinal oscillation mechanisms.

Conjugate heat transfer in the mode of thermal gravitational-capillary convection in the model of a fuel tank, after a sudden heating of the sidewall

The evolution of unsteady gravitational-capillary convection in a layer of ethyl alcohol with a free surface after sudden electric heating of one of the vertical walls of a rectangular cavity was investigated numerically. The effect of the incoming flow of hot liquid on the time evolution of the temperature field on the opposite thin metal wall of the cavity was investigated. The calculations were carried out by the finite element method in the conjugate two-dimensional formulation with the Prandtl number Pr = 16, and the range of Grashof numbers determined by the heat flux density, 33·103 ≤ Gr ≤ 28·106. It is shown that the maximum local temperature gradients occur on the wall near the liquid-gas interface.