Row Of Cylinders Research Articles

Noise attenuation by two‐dimensional sonic crystals made of rubber crumb.

It is shown that sonic crystals consisting of cylindrical rods made of rubber crumb can be used as efficient structures to attenuate noise in a wide range of frequencies. A structure made of three rows of cylinders (3‐m height) has been fabricated and its isolation properties have been characterized in a transmission chamber. Two different numerical algorithms have used to simulate the experimental characterization. A good agreement between theory and experiment is obtained and gives support to design barriers based on this technology. [Work supported by MICIIN of Spain.]

Broadband and compact ridge substrate-integrated waveguides

A novel class of substrate-integrated waveguides (SIWs) is presented, based on the concept of the classical ridge rectangular waveguide, which permits the design of broadband, compact and low-loss integrated components and interconnects. Two configurations are proposed. The former is the ridge SIW, where the structure is integrated in a dielectric substrate with top and bottom metal layers, and comprises two-sided rows of full-height metal cylinders and a central row of partial-height metal posts connected at their bottom by a metal strip. The latter is the ridge substrate-integrated slab waveguide; this structure presents a geometry similar to the ridge SIW, but includes additional air holes to further enhance the bandwidth. The design and experimental verification of two prototypes, covering the frequency band from 6.8 to 25 GHz and from 7.1 to 30.7 GHz, respectively, are also presented. The proposed structures exhibit a monomodal bandwidth three times broader than classical rectangular waveguides or SIWs, and their size is half of the SIW with the same cut-off frequency.

Experimental study and large eddy simulation of turbulent flow around tube bundles composed of wavy and circular cylinders

This paper presents the results of an investigation on the effects of wavy cylindrical tubes in a staggered heat exchanger tube bundle. Experimental measurements and large eddy simulation (LES) technique were used to study the turbulent flow characteristics in a staggered tube bundle arrangement at a sub-critical Reynolds number of Re = 7500. The aim of this investigation is to compare the flow characteristics of a new configuration of cylindrical tubes with that of a similar arrangement which comprises purely circular cylinders. The first row of circular tubes is replaced by a row of wavy cylindrical tubes. The result of the experiment shows that the mean velocity distributions and turbulence intensity, obtained by the present LES method, are in good agreement with the Laser Doppler Anemometer (LDA) measurements. The wake patterns around the second row of cylinders become more well organized upon the introduction of wavy cylinders of optimal wavelength (λ/Dm = 6). The turbulent kinetic energy behind the first row of tubes is also reduced. The power density spectra of the vortex shedding in the tube bundle are evidently suppressed. As a result, the fluctuating forces of the tube bundles are weakened. However, the drag is not noticeably reduced by such modification.

Transmission of solitary waves through slotted barriers: A laboratory study with analysis by a long wave approximation

Economic and long-term social impacts of tsunami waves, as shown in the 2004 Indian Ocean tsunami, have been devastating. In addition to the passive measures such as the tsunami early warning systems, active protective measures such as tsunami breakwaters and tsunami-resistant buildings are also necessary. Slotted barriers are low cost structures that can be very effective in reducing the transmitted energy of long waves. In this study, the transmission of tsunami waves, with the leading wave being modeled by a solitary wave, through slotted barriers in the form of a row of circular cylinders is studied experimentally. The results are also analyzed by a method based on long wave approximations. It is found that the spacing between two adjacent cylinders is one of the main factors that control the transmission of solitary waves through slotted barriers. Reflection coefficients and the reduction of drag force on coastal structures protected by slotted barriers are also discussed in the paper.

Numerical investigation of forced convection heat transfer in unsteady flow past a row of square cylinders

This paper presents the unsteady laminar forced convection heat transfer from a row of five isothermal square cylinders placed in a side-by-side arrangement at a Reynolds number of 150. The numerical simulations are performed using a finite volume code based on the PISO algorithm in a collocated grid system. Special attention is paid to investigate the effect of the spacing between the cylinders on the overall transport processes for the separation ratios (spacing to size ratio) between 0.2 and 10. No significant interaction between the wakes is observed for spacing greater than four times the diameter at this Reynolds number. However, at smaller spacing, the wakes interact in a complicated manner resulting different thermo-hydrodynamic regimes. The vortex structures and isotherm patterns obtained are systematically presented and discussed for different separation ratios. In addition, the mean and instantaneous drag and lift coefficients, mean and local Nusselt number and Strouhal number are determined and discussed for various separation ratios. A new correlation is derived for mean Nusselt number as a function of separation ratio for such flows.

Phonon transport and waveguiding in a phononic crystal made up of cylindrical dots on a thin homogeneous plate

We present a theoretical analysis of the phonon transport and guiding of acoustic waves in a phononic crystal made up of a square array of cylindrical dots, which is deposited on a thin homogeneous plate. With appropriate choice of the geometrical parameters, this structure can display several gaps, one of them being well below the Bragg gap. With the help of the finite difference time domain method, we calculate the transmission coefficient vs the frequency and demonstrate a good agreement with the dispersion curves. We show the possibility of guided modes inside an extended linear defect created either by removing one row of cylinders or by changing the height or the materials constituting the dots in a row. The wavelengths of the waves transmitted in the low-frequency gap are about ten times larger than the width of the waveguide. We discuss the transmittivity of each confined mode appearing in the band gap as well as the conversion in the polarization of the transmitted waves which can occur more or less significantly.

On the effect of Reynolds number for flow around a row of square cylinders

Flow across a row of identical square cylinders placed side-by-side has been found to show interesting flow patterns which have complex characteristics depending upon the spacing (s/d) between the cylinders and the Reynolds number (Re). The combined effects of cylinder spacing and Reynolds number on the flow across a row of cylinders are numerically studied for 30≤Re≤140 and 1.0≤s/d≤4.0, where s is the surface-to-surface distance between two cylinders and d is the size of cylinder. It is found that the critical Reynolds number for the onset of vortex shedding increases with increase in gap ratio. The Reynolds number is found to have a strong effect on the flow especially at s/d=3.0,4.0. Secondary frequency in the signal for lift and drag coefficients significantly contributes to the forces experienced by the cylinders. It is observed that at s/d=3.0,4.0 the secondary frequency disappears at larger Reynolds number and the primary frequency dominates the flow. This means that the interaction of the wakes behind the cylinders at these gap ratios weakens with an increase in the Reynolds number. It is proposed that wake interaction is strongly influenced by the jets in the gap region, the nature of which alters with spacing and Reynolds number. This is confirmed by computing the average wake size as a function of Reynolds number. Based on this, two critical gap ratios, 2.0 and 4.0 for the range of Reynolds number under consideration are proposed. These gap ratios separate synchronous, quasiperiodic-I and quasiperiodic-II flow regimes depending on the Reynolds number. The mechanism of wake interaction has been studied to bring out these critical gap ratios.

Numerical simulation of flow past row of square cylinders for various separation ratios

A systematic study for the flow around a row of five square cylinders placed in a side-by-side arrangement and normal to the oncoming flow at a Reynolds number of 150 is carried out through the numerical solution of the two-dimensional unsteady incompressible Navier–Stokes equations. Special attention is paid to investigate the effect of the spacing between the five cylinders on the wake structure and vortex shedding mechanism. The simulations are performed for the separation ratios (spacing to size ratio) of 1.2, 2, 3 and 4. Depending on the separation ratio the following flow patterns are observed: a flip-flopping pattern, in-phase and anti-phase synchronized pattern and non-synchronized pattern. These flow patterns are supposed to be a consequence of the interaction between two types of frequencies viz. the vortex shedding (primary) and the cylinder interaction (secondary) frequencies. At small separation ratio the flow is predominantly characterized by the jet in the gaps between successive cylinders and the secondary frequencies play a role in the resulting chaotic flow. On the contrary, at higher separation ratio the secondary frequencies almost disappear and the resulting flow becomes more synchronized dominated by the primary frequency.

Experimental investigation of flow through a bank of cylinders of varying geometry

An experimental investigation of a model forming fabric was conducted. The forming fabric was simplified and represented by a bank of two rows of cylinders. Particle image velocimetry (PIV), pressure drop, and hydrogen bubble visualization were used to evaluate the upstream and gross flow patterns for various diameter ratios and cylinder spacings at a Reynolds number of 65. It was found that for a row separation of 0.75 times the upstream cylinder diameter, the upstream flow matched that for a single row of cylinders. Further, it was found that when the pressure drop through the bank of cylinders was equal to the sum of the individual rows’ pressure drops, the upstream flow converged to that of a single row.

Experimental two-dimensional field mapping of total internal reflection lateral beam shift in a self-collimated photonic crystal

A lateral beam shift is demonstrated both theoretically and in microwave experiments when total internal reflection takes place at the boundary of a self-collimating two-dimensional photonic crystal consisting of an array of high index dielectric cylinders. We further show the dependence of this shift on the cut of the last row of cylinders that defines the crystal interface.

2 상 횡 유동장에 놓인 관군의 유체탄성불안정성

Experiments have been performed to investigate fluid-elastic instability of tube bundles, subjected to twophase cross flow. Fluid-elastic is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to the cross flow. The test section consists of cantilevered flexible cylinder(s) and rigid cylinders of normal square array. From a practical design point of view, fluid-elastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping parameter. For dynamic instability of cylinder rows, added mass, damping and the threshold flow velocity are evaluated. The Fluid-elastic instability coefficient is calculated and then compared to existing results given for tube bundles in normal square array.

Directional coupler with high directivity using metallic cylinders on microstrip line

A directional coupler with high directivity performance using metallic cylinders on the microstrip line is presented. By using two rows of periodic metallic cylinders on a microstrip coupler, the odd-mode phase velocity is slowed down, and thus the directivity can be greatly improved for equal odd- and even-mode phase velocities. The parameters of metalised cylinders, which have an effect on the coupler performance, are investigated. A 12 dB coupler at 2.41 GHz has been designed and measured, providing excellent directivity. Theoretical and experimental results are presented.

First results of operating and monitoring an innovative design of a permeable reactive barrier for the remediation of chromate contaminated groundwater

Abstract An innovative setup of a permeable reactive barrier (PRB) was installed in Willisau, Switzerland to remediate chromate contaminated groundwater. Instead of a conventional continuous barrier, this PRB consists of cylinders installed in rows: a single row for lower expected Cr VI -concentrations and an offset double row for higher expected Cr VI -concentrations. The cylinders are filled with reactive grey cast-Fe shavings mixed with gravel to prevent extensive precipitation of secondary phases in the pore space. The treatment of the contaminants takes place both within the cylinders and in the dissolved Fe II plume generated downstream of the barrier. Monitoring of the contamination situation over a period of 3 a provided evidence of the mobilization, transport and behavior of the contaminants in the aquifer. Groundwater and reactive material were sampled upstream, within and downstream of the barrier by a Multi-Port Sampling System (MPSS) that revealed the geochemical processes as a function of time and space. Comprehensive chemical analyses included sensitive parameters such as Cr VI , Fe II /Fe III , redox potential, dissolved O 2 and pH. Several campaigns using multiple optical tracers revealed a rather complex hydrological regime at different scales, thereby complicating the barrier performance. Results from the large 3D hydrogeochemical dataset show that the double row of cylinders successfully treated the chromate contamination. Remediation by the single row was not effective enough due to insufficient lateral overlap of the cylinders and their Fe II -plumes. The low amount of precipitated secondary phases observed in the pore space of the reactive material reduced the risk of clogging the system and suggested a favorable longevity of the barrier. Limiting factors for the long-term operation are inferred to be the availability and accessibility of Fe II within the cylinders and the concentration within the generated Fe II -plume.

Simulation of flow around a row of square cylinders

In this paper, the low-Reynolds number (Re= 80) flow around a row of nine square cylinders placed normal to the oncoming flow is investigated using the lattice-Boltzmann method. The effects of the cylinder spacing on the flow are studied for spacing to diameter ratios of 0.3 to 12. No significant interaction between the wakes is observed with spacings greater than six times the diameter. At smaller spacings, the flow regimes as revealed by vorticity field and drag coefficient signal are: synchronized, quasi-periodic and chaotic. These regimes are shown to result from the interaction between primary (vortex shedding) and secondary (cylinder interaction) frequencies; the strength of the latter frequency in turn depends on the cylinder spacing. The secondary frequency is also related to transition between narrow and wide wakes behind a cylinder.The mean drag coefficient and Strouhal number are found to increase rapidly with a decrease in spacing; correlations of these parameters with spacing are proposed. The Strouhal number based on gap velocity becomes approximately constant for a large range of spacings, highlighting the significance of gap velocity for this class of flows. It is also possible to analyse the vortex pattern in the synchronized and quasi-periodic regimes with the help of vorticity dynamics. These results, most of which have been obtained for the first time, are of fundamental significance.

Reflection of sound from a forest: Effective tree spacing for a scattering model with a single line of cylinders

A scattering model for the reflection of sound at forest rims is presented by the authors in another contribution to this conference. The forest is thereby modeled by a single row of cylinders with the effective spacing as single free parameter. In the present article an engineering scheme for this effective tree spacing is presented. The scheme is based on results of numerical calculations with the Euler model for sound propagation in a model forest consisting of cylinders. The numerical results show that forests are widely transparent for incoming as well as reflected sound waves. Multiple reflections can be neglected. Reflections from trees deep inside the forest experience additional propagation attenuation and are therefore weakened relative to contributions from trees in the first row. Apart from geometrical spreading foliage attenuation is the most prominent effect. The penetration depth of the forest reflection is proportional to the wavelength. The frequency dependent penetration depth is derived by means of a statistical analysis for numerous situations. Assuming the effective tree spacing to be inversely proportional to the penetration depth, the engineering scheme for the effective tree spacing is derived. Comparisons of the frequency response with measurements show good agreement.

Reflection of sound from a forest: comparison of experimental data to scattering theory

Reflection of sound by a forest has been investigated by means of measurements near Thun in Switzerland. Sound pulses were generated at 50 and 150 m from the forest, and reflected sound was recorded at distances of 100, 200, and 300 m from the forest. By comparison with numerical calculations of reflection of sound from a hard wall, using a parabolic‐equation model for sound propagation in an atmosphere with wind and temperature as recorded during the measurements, the attenuation of sound waves upon reflection at the forest was determined. It was found that the reflection attenuation decreases from about 20 dB at low frequency to about 10 dB at high frequency. The experimental results for the reflection attenuation are compared with results of a theoretical model for scattering of spherical sound waves by a single row of cylinders. The model employs an effective spacing between the cylinders, to account for trees that are not at the edge of the forest, i.e., inside the forest. The model results are in good agreement with the experimental results. The comparison indicates that the effective spacing decreases with decreasing frequency, as low‐frequency waves penetrate deeper into a forest than high‐frequency waves do.

616 ロックイン状態の二次元円柱列に作用する流体力(GS3-3 流体工学(3),一般セッション:3 流体工学)

616 ロックイン状態の二次元円柱列に作用する流体力(GS3-3 流体工学(3),一般セッション:3 流体工学)

Fluid Force Acting on Two-Dimensional Circular Cylinder Row in Lock-in Condition

Fluid Force Acting on Two-Dimensional Circular Cylinder Row in Lock-in Condition

Finite-Difference Time-Domain Study of Guided Modes in Nano-Plasmonic Waveguides

A conformal dispersive finite-difference time-domain (FDTD) method is developed for the study of one-dimensional (1-D) plasmonic waveguides formed by an array of periodic infinite-long silver cylinders at optical frequencies. The curved surfaces of circular and elliptical inclusions are modelled in orthogonal FDTD grid using effective permittivities (EPs) and the material frequency dispersion is taken into account using an auxiliary differential equation (ADE) method. The proposed FDTD method does not introduce numerical instability but it requires a fourth-order discretisation procedure. To the authors' knowledge, it is the first time that the modelling of curved structures using a conformal scheme is combined with the dispersive FDTD method. The dispersion diagrams obtained using EPs and staircase approximations are compared with those from the frequency domain embedding method. It is shown that the dispersion diagram can be modified by adding additional elements or changing geometry of inclusions. Numerical simulations of plasmonic waveguides formed by seven elements show that row(s) of silver nanoscale cylinders can guide the propagation of light due to the coupling of surface plasmons.

Flow characteristics around a row of circular and wavy cylinders

The near-wake and flow interference around a row of five circular cylinders, staggered wavy cylinders and nonstaggered wavy cylinders are investigated experimentally by using PIV and LIF techniques. The effects and characteristics of employing wavy cylinders instead of circular cylinders are discussed. The cylinders were arranged atT/d=1.5 withRe ranging from 125 to 40000. Results showed the staggered wavy cylinders give rise to a more stable flow pattern with less fluctuation and longer wake vortex closure length.