Row Of Cylinders Research Articles

Stokes flow in model fibrous filters

The transverse flow of a viscous incompressible fluid at low Reynolds numbers in model fibrous filters – rows of parallel cylinders – is studied. Single rows of porous and composite cylinders (cylinders with porous shells) are considered as simplest models for novel filters consisting of porous fibers and fibers covered with porous layers. The obtained flow field is used to estimate the efficiency of diffusional deposition of sub-micron aerosol particles upon porous and composite cylinders as a function of Peclet number, permeability of porous material and the distance between cylinders. Also considered are three-dimensional model filters consisting of parallel cylinder rows arranged perpendicularly to the flow and to each other.

공기-횡 유동장에 놓인 유연성 실린더 관군의 유체탄성 불안정

Using wind tunnel, experimental approaches were employed to investigate fluidelastic instability of tube bundles, subjected to uniform cross flow. There are several flow-induced vibration excitation mechanisms, such as fluidelastic instability, periodic wake shedding resonance, turbulence-induced excitation and acoustic resonance, which could cause excessive vibration in shell-and tube heat exchanges. Fluidelastic is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to cross flow. The system comprised of cantilevered flexible cylinder(s) and rigid cylinders of normal square array, In order to see the characteristics of flow in tube bundles, particle image velocimetry was used. From a practical design point of view, Fluidelastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping. The threshold flow velocity for dynamic instability of cylinder rows is evaluated and the data for design guideline is proposed for the tube bundles of normal square array.

Stokes flow past periodic rows of porous cylinders

The flow of a viscous incompressible fluid at low Reynolds numbers in model filters—systems of porous permeable cylinders—is studied. A single row and hexagonal and square arrays of parallel cylinders perpendicular to the flow direction are considered. The flow field outside and inside a porous cylinder is determined by solving the Brinkman and Stokes equations. The drag force on a porous cylinder versus the cylinder permeability and the distance between the axes of neighboring cylinders is calculated. It is shown that rows of porous cylinders arranged into a square array have almost no mutual hydrodynamic effect at any array spacing. The cell model is shown to be applicable to the hexagonal array of porous cylinders over a wide range of packing densities.

Simulation of cross-flow-induced vibration of cylinder arrays by surface vorticity method

Abstract The surface vorticity method (SVM), which is a fast and practical grid-free two-dimensional (2-D) method, and a fluid–structure interaction model incorporating the effects of cylinder motions and displacements is used to simulate the vortex-induced vibration of cylinder arrays at sub-critical Reynolds number Re = 2.67 × 1 0 4 . The SVM is found to be most suitable for simulating a 2-D cylinder row with large-amplitude vibrations where the vorticity field and the fluid forces of the cylinder row change drastically, and the effect of the stream on the transverse direction vibration is very significant. The fluidelastic instability of a flexible cylinder row at small pitch ratio is also investigated, and the critical reduced velocity of the cylinder row at a reduced damping parameter S G = 1.29 is calculated, which is in good agreement with experimental and analytical results of the unsteady model. Vortex-induced vibration of a staggered cylinder array is simulated using different structural parameters. When the cylinders are relatively more flexible, the flow pattern changes dramatically and the fluid–structure interaction has a dominant impact on the flow field. Compared with grid-based methods, the grid-free SVM is a fast and practical method for the simulation of the FIV of cylinder arrays due to vortex shedding at sub-critical Reynolds numbers.

Eigenmodes of the waveguide formed in a 2D photonic crystal composed of metal cylinders

Eigenmodes of the waveguide in a photonic crystal formed from circular metal cylinders are considered. The method of compensating sources is used to solve a boundary value problem for a guiding channel in a photonic crystal formed by removing an infinite row of cylinders from the crystal. Eigenvectors for amplitudes of compensating sources are found and the dispersion relation for determining the eigenmode propagation constants is derived. The eigenvectors for amplitudes of compensating sources are used to obtain expressions for electromagnetic fields of waveguide eigenmodes. The operating frequency range of the photonic-crystal waveguide is determined. The influence of losses on attenuation of the fundamental waveguide mode is investigated. Methods of compensating sources and eigenmode expansion are used to solve the excitation problem for waveguide eigenmodes. A simple expression is derived for the power carried by the fundamental mode.

The viscous drag of three-dimensional model fibrous filters

Dependences of the viscous drag of the model filter consisting of two parallel cylinder rows arranged perpendicular to the Stokes flow and to each other, on a step and a distance between rows are found. The approximation formula for the drag force of a cylinder in contacting rows is obtained. The drag was calculated for the three-dimensional model filter where two hexagonal grids of cylinders are inserted into each other at a right angle. The drag force of a cylinder for this model is close to that for the fan model filter.

Intrinsic localized modes in photonic crystal waveguides composed of a periodic mixed array of linear and Kerr nonlinear dielectric media

A theoretical treatment is given of intrinsic localized modes in waveguides and in junctions of waveguides formed inside two-dimensional photonic crystals. Two-dimensional photonic crystals are periodic arrays of parallel-axed dielectric cylinders, and the electromagnetic modes of interest propagate in the plane perpendicular to the cylinder axes. Waveguides are formed inside the photonic crystal by replacement of a row of cylinders. The replacement cylinders are chosen so that the waveguide channels formed bind and conduct electromagnetic energy along the channel in the plane perpendicular to the cylinder axes. In the systems we study, the waveguide channels are composed of a periodic array of linear and Kerr nonlinear media in which both the linear and nonlinear dielectric materials differ from the linear dielectric materials forming the bulk photonic crystal. These waveguides are shown to support intrinsic localized modes. The intrinsic localized modes are studied by determining their scattering properties in waveguide systems. The transmission characteristics of a waveguide formed of linear dielectric media but containing a barrier that is a periodic array of linear and Kerr nonlinear dielectric is studied. Studies are also made of the transmission characteristics of waveguides of linear dielectric media that are connected at a waveguide junction that is composed of co-joined segments containing a periodic array of linear and Kerr nonlinear dielectric media. Transmission anomalies in these two types of waveguide geometries (i.e., barrier and junction geometries), associated with the development of intrinsic localized modes in the barrier or segments composing the waveguide junctions, are identified. This work is an extension of that presented in A.R. McGurn, G. Birkok. Phys. Rev. B 69 (2004) 235105, which treated the scattering characteristics of intrinsic localized modes in barriers and junctions formed solely of Kerr nonlinear dielectric media to treat other types of waveguides that can support intrinsic localized modes.

Stokes flow in periodic systems of parallel cylinders with porous permeable shells

The problem solved in this study concerns steady-state flow of a viscous incompressible liquid at low Reynolds numbers in a model filter consisting of parallel cylinders with porous permeable shells. Both a separate row and lattices (square and hexagonal) of cylinders directed perpendicularly to the flow are considered. The flow field outside and inside the porous shell is found from the solutions to the Stokes and Brinkman equations. The drag force and the filtration efficiency are determined both as functions of the ratio between the cylinder diameter and the distance between the axes of adjacent cylinders and as functions of the thickness and permeability of the shells. The cell model is shown to be applicable for describing the flow field in a hexagonal lattice of cylinders with porous shells within a wide range of packing densities.

D213 正方二面格子を構成する円柱周りの流れの可視化

The secondary flow behind circular cylinders forming a square biplane grid was visualized using a dye injection method in a water tunnel, and the surface pressure distribution of around the circular cylinders was measured using a wind tunnel. The biplane grid consisted of a row of upstream cylinders and a row of downstream cylinders perpendicular to each other. The open area ratio of the biplane grid was changed in the range of 0.25-0.74. It is consequently found that eight longitudinal vortices behind a cell of the biplane grid are produced by the secondary flows. It is expected that the structure of the longitudinal vortex is different from that behind a woven screen mesh.

Light Scattering on 2D Nanostructured Resonant Gratings

This paper studies nanostructured gratings made up by silver nanoparticles embedded in the dielectric, which are capable of maintaining quasi-static modes. The special emphasis is devoted to following specified types of gratings: row of periodical cylinders and square grating of spheres. The problem of a diffraction of a plane electromagnetic wave on such structures has been solved within the dipolar-interaction approximation. The frequency dependences of the refraction and absorption coefficients on the grating parameters have been obtained, analyzed and compared.

An experimental study of the in-line oscillations of a closely spaced row of cylinders in cross-flow

An experimental study has been carried out, to examine the vibratory motion in the streamwise direction of a closely spaced row of circular cylinders. The cylinders in the flexible row under study, free to respond in-line with the flow, are placed alternately between cylinders of a fixed row in a water channel. The transverse spacing of the cylinders of the same row was 3 cylinder diameters, yielding a cylinder spacing when the two rows were aligned equal to 1.5 diameters. The experiments were carried out at low Reynolds numbers over a range from 700 to 1200. Each programme of measurements started when the flow velocity was maximum, and the flexible row was displaced at a small distance downstream of the fixed row. As the velocity was gradually decreased and the flexible row was moved to a mean position upstream of the fixed in-line oscillations occurred, whose amplitude, which was not constant at different cycles, was dependent on both the stream velocity and the initial displacement between the two rows. For each flow velocity, the time history of the oscillation of the moving row was recorded, from which the response and the frequency of oscillations were obtained and are presented diagrammatically.

Experimental observation of resonant filtering in a two-dimensional phononic crystal waveguide

Abstract Transmission of acoustic waves through a two-dimensional composite material made of PVC cylinders surrounded by air is measured experimentally. The spectrum presents a very large absolute band gap in the audible frequency range. A waveguide created inside this phononic crystal by removing a row of cylinders can transmit very efficiently the waves falling inside the stop band. We show the existence of deaf modes in the band structure of the linear waveguide. Resonant filtering is also demonstrated experimentally by coupling the waveguide to a side branch resonator of variable length. Frequency filtering is observed in the form of narrow dips in the transmission spectrum of the waveguide. Most of these observations compare favorably with theoretical calculations of dispersion curves and transmission coefficients of model structures using the plane wave expansion and the finite difference time domain methods. Narrow dips similar to those of the guide with resonator are also observed in the transmission spectrum of a waveguide with a sharp bend.

Research of mass-transfer in fibrous sorption-active materials

The mass-transfer processes in fiberglass were studied in the course of drying the saturated glass fabric sample. Experiments were carried out using an Avance DRX Bruker spectrometer equipped with a micro tomography device. Experimental signal intensities of fluids flowing at different velocities were obtained. A one-dimensional mathematical model was applied to calculate mass-transfer coefficients from experimental data. Dependencies Nu( Re) at the Reynolds number varied between 0 and 400 were obtained at different fluid velocities. The data obtained were compared to correlation dependencies for woven Pt/Pd gauzes and for rows of parallel cylinders. Numeric experiments of gas flow filtration at low Reynolds numbers ( Re < 1) were carried out using the CFD FLUENT software package. Fibrous material was represented as porous system consisting of bunches of parallel cylinders with varied distances between the cylinders. Dependence Nu( Re) was calculated at various parameters of system.

Numerical simulation of the flow around rows of cylinders

Two-dimensional, laminar, unsteady, water flow around cylinder arrays of unequal sizes was simulated using FLUENT™ at Reynolds numbers below 150 (based on the free-stream velocity and first row cylinder diameter). The flow pattern through two rows of inline cylinders showed incomplete vortex shedding behind the first row at a separation distance less than 2 d. Karman vortices were not formed and a near-stagnant separated flow region appeared between the aligned cylinders. Cylinders in staggered arrangements shed Karman vortices regardless of the separation between the two rows. This research has shed light on the detailed flow through paper machine forming fabrics.

Enhanced diffusion from a continuous point source in shallow free-surface flow with grid turbulence

Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinder shoulders, and in the middle of the gap between cylinders. For the base case without grid turbulence, spreading rates agreed well with turbulent diffusion coefficients evaluated by others for wide channel flows. Eddy diffusivities for injections with grid turbulence were an order of magnitude greater than the base case; injections at the grid had larger diffusivities than for the injection downstream of the grid. For all experiments, the eddy diffusivity is shown to agree with Taylor’s theorem and to correlate with the size and turbulent intensity of the largest-scale coherent structures.

Dynamics of Cage Floating Breakwater

Floating breakwaters have potential applications in protecting minor ports and harbors such as fisheries and recreational harbors, where-in stringent tranquillity requirements are not warranted. In field applications of the existing floating breakwaters, limitations are imposed due to their large relative width (ratio between breakwater width and wave length) requirements to achieve desirable tranquillity level. This relative width requirement is greater than 0.3 for the existing floating breakwaters. To overcome the above drawback associated with the existing system a new configuration for a floating breakwater is derived, which could yield the desired performance with minimum relative width requirement. The floating breakwater comprises of two pontoons rigidly connected together and each of the pontoons having a row of cylinders attached beneath, for improved performance characteristics. The laboratory tests were conducted in both regular and random wave flumes to study the dynamic behavior of the breakwater. Transmission and reflection coefficients, water surface elevations and velocities inside the cage like area provided in between the pontoons, rigid body motions floating breakwater and mooring forces were studied under regular and random waves and under the regular waves followed by a uniform current. The results proved the suitability of the floating breakwater to the field conditions even for large wave periods. In addition the variations in water particle kinematics, rigid body motion and mooring forces show nominal magnitudes when compared to the existing systems indicating the rigidness of the breakwater.

Experiments With Three-Dimensional Passive Flow Control Devices on Low-Pressure Turbine Airfoils

The effectiveness of three-dimensional passive devices for flow control on low pressure turbine airfoils was investigated experimentally. A row of small cylinders was placed at the pressure minimum on the suction side of a typical airfoil. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered under low freestream turbulence conditions. Streamwise pressure profiles and velocity profiles near the trailing edge were documented. Without flow control a separation bubble was present, and at the lower Reynolds numbers the bubble did not close. Cylinders with two different heights and a wide range of spanwise spacings were considered. Reattachment moved upstream as the cylinder height was increased or the spacing was decreased. If the spanwise spacing was sufficiently small, the flow at the trailing edge was essentially uniform across the span. The cylinder size and spacing could be optimized to minimize losses at a given Reynolds number, but cylinders optimized for low Reynolds number conditions caused increased losses at high Reynolds numbers. The effectiveness of two-dimensional bars had been studied previously under the same flow conditions. The cylinders were not as effective for maintaining low losses over a range of Reynolds numbers as the bars.

Transmission of acoustic waves through waveguide structures in two‐dimensional phononic crystals

Transmission of acoustic waves through waveguide structures in a two-dimensional phononic crystal made of PVC cylinders arranged on a square array in air is studied. We investigate particularly the transmission through a perfect rectilinear waveguide and waveguides coupled with a side branch resonator. A rectilinear wave guide obtained by removing one row of cylinders in the perfect phononic crystal can support one or several modes falling in the complete band gap of the phononic crystal. The effect of a side branch resonator is to induce zeros of transmission in the spectrum of the perfect guide that appear as narrow dips with frequencies depending upon the shape of the resonator. Most of these theoretical predictions obtained by using the plane wave expansion (PWE) and the finite difference time domain (FDTD) methods are compared to experimental measurements made in the audible frequency range. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunable filtering and demultiplexing in phononic crystals with hollow cylinders.

Acoustic band gap (ABG) materials constituted of steel hollow cylinders immersed in water can exhibit a tunable narrow pass band (NPB) located inside their gap. We theoretically investigate, using the finite difference time domain (FDTD) method, the properties of waveguides composed of a row of hollow cylinders in a two-dimensional (2D) phononic crystal made of filled steel cylinders. These waveguides exhibit NPB's at frequencies slightly higher than their infinite periodic ABG counterpart. The frequency of the waveguide's NPB can be selected by adjusting the inner radius of the hollow cylinders or by changing the nature of the fluid that fills them. We show that a waveguide constituted of a row of hollow cylinders with different inner radii can transport waves at two different frequencies. By selectively filling the cylinders with water or mercury we have created an active device that permits the transmission of waves at one, both, or neither of these frequencies. Finally, we examine the multiplexing and demultiplexing capabilities of Y shaped waveguides constituted of hollow cylinders.

Surface elastic waves in solid composites of two-dimensional periodicity

We have studied the elastic surface waves in a truncated two-dimensional solid composite. The periodic structure is constituted by a square array of parallel tungsten (W) cylinders embedded in silicon (Si). The surface plane that terminates the crystal cuts perpendicularly the plane of periodicity along the (10) direction. Thus the surface plane is parallel to the axes of a row of cylinders. We found that surface waves can propagate with wave vector laying along the one dimensional surface periodicity that results from the truncation. Both mixed and transverse polarizations were considered, but only surface waves of mixed polarization exist when the truncation leaves complete cells at the surface. We have also studied the guidance of elastic waves by a homogeneous layer of Si grown at the surface of the crystal. The wave confinement within this adlayer is possible due to the scattering effects suffered by the field of displacement inside the periodic structure at frequencies in the band gaps.