Row Of Cylinders Research Articles

Two-dimensional experimental and numerical investigations of perforated plates in oscillating flow, orbital flow and incident waves

The hydrodynamic loads on two-dimensional perforated plates are investigated experimentally and numerically. Two single perforated plate configurations, consisting of rows of either circular or square cylinders, are studied. Experiments and simulations are performed for oscillating flow, orbital flow and waves. The Keulegan–Carpenter (KC) number and period of oscillation are varied. The hydrodynamic forces and coefficients are highly KC number dependent. There is in general small dependence on the period of oscillation. An exception is in wave tests where the normalized force on the structures depend on both the wave length-to-plate diameter ratios and the submergence. Added mass and damping coefficients are presented for oscillating and orbital flow conditions. The hydrodynamic force on both configurations is dominated by damping. We find in general good agreement between the experimental and numerical results.We highlight some aspects of the influence from plate-end flow separation on the forces of the plates. A considerable reduction in the hydrodynamic forces is found in orbital motion compared to oscillating flow, increasingly so as the KC number is increased. Asymmetric flow patterns, where the vortex generation at the plate-ends occurs only on one side at a time, are observed in orbital flow, whereas the plate-end vortices from the two sides of the plate are generated simultaneously in oscillating flow. Streamline plots indicate that the relative importance of vortex generation from the plate-ends increases with increasing KC number.

Eight-channel acoustic demultiplexer based on solid-fluid phononic crystals with hollow cylinders

We design a novel eight-channel demultiplexer in a solid-fluid phononic crystal structure. The basic structure has been formed by square arrangement of holes filled by water in tungsten background. These holes are embedded in background with filling fraction of 46 %. Generally, the proposed structure consists of nine type line defects for input and output channels. These line defects consist of two types of hollow cylinders. One of them is used in input channel and the other creates output channels. Input waveguide contains an L-branch waveguide which has been made by a row of hollow cylinders of water which is internally filled with tungsten. Each of output channels contains a row of hollow cylinders of Methyl Nonafluorobutyl Ether filled with tungsten. Hollow cylinders have different inner radii in each output waveguides. Hence, each output channels can transmit diverse narrow frequency passbands. We use the finite element method to determine the eigenfrequencies of structure. This structure can separate eight acoustic waves in frequency range between 166.6 kHz and 168.1 kHz. Separation of frequencies is accomplished with high quality factor and the width of frequency spectra between outputs are about 200 Hz. Crosstalk values of the proposed structure show that the eight-channel demultiplexer has efficient performance so that, the worst case of crosstalk value is equal to -26.68 dB.

Numerical investigation of upstream cylinder flow and characterization of forming fabrics

Abstract In this work, the fundamentals of upstream flow over cylinders and forming fabrics are investigated, and measures for characterization of fabrics are proposed. Two-dimensional flow over one cylinder, two cylinders, and one and two rows of cylinders, are analysed numerically. By studying different configurations and various Reynolds numbers, the upstream flow features are characterized. It is concluded that cylinders have a short range of upstream flow impact, shortest for rows of cylinders with small spacings. For R e ∈ [ 10 , 80 ] Re\in [10,80] , the Reynolds number dependency is weak. It is shown that a downstream row positioned in tandem has negligible impact on the upstream flow, while a displaced second row influences the upstream flow if the spacing in the first row is larger than one diameter. The pressure drop required to drive the flow over the cylinders depends non-linearly on the porosity of the configuration. Flow measures of the upstream flow are proposed, which in addition to the volume flow per area are used to characterize fabric flow properties. The conclusions from the cylinder study also hold for industrial fabrics, and it can be explained how properties of the fabric influence the final paper. The wave-length of flow periodicity is studied in relation to drainage marking. This study demonstrates that simulations can greatly improve pure experimental-based fabric characterization.

Modelling of a wake from cylinders row

Model of intermittent near wake from a row of parallel cylinders is constructing. The aim of the investigation includes besides neutral wakes modelling the interpretation and “physical modelling” of the glow-discharge switching effect on a cylinders wake discovered by us earlier. The form of this reduced-order model of multiple wake is nonlinear-interacting von Karman streets each resulting from one cylinder of the row and each represented by one-dimensional oscillator. Here the correlation between known Landau-Stuart, Van der Pole wake models and ours is considered. Various global modes of such multiple wake are discovered within the framework of the model. Among them a two-frequency asymmetric global mode of the wake was discovered in accordance with well-known observations for relatively close cylinders arrangements. For each global mode an analytical approximation or exact expression for the wake configuration is presented that may be useful in further analysis. Matching of these approximations with the results of numerical computing within the framework of the model is considered.

Mixed Convective Transport Around Staggered Rows of Square Cylinders

The unsteady mixed convective transport around multiple bluff objects placed in a staggered configuration with respect to a uniform free stream flow is analyzed through two-dimensional numerical computation. The bluff objects are identical in shape and size with square cross-section and arranged in two different rows within an unconfined domain. A small temperature difference between the objects and the free stream results in the free convection in addition to the forced flow. Simulation is carried out using a finite volume based method considering a uniform cross flow of air (Prandtl number = 0.71) at a moderate Reynolds number (= 100). The transverse spacing between the cylinders may anticipated to influence significantly the wake dynamics, which in turn affects the thermal transport. Simultaneously, the mixed convective strength also influences the wake dynamics and vortex structure formation. An interplay between these two effects aptly dictates the resulting flow dynamics and associated thermal transport. Accordingly, the dimensionless transverse spacing is varied (= 1, 3 and 5) along with the mixed convective strength (Richardson number = 0-2). It is observed that the flow and thermal fields show chaotic nature at smaller transverse spacing. However, at larger spacing, the usual unsteady vortex dynamics persists. Very interestingly it is observed that the chaotic flow at smaller transverse spacing reduces its instability to become unsteady periodic at larger strength of the thermal buoyancy. The average heat transfer from the cylinders is found more at smaller transverse spacings and it increases with increasing mixed convective strength.

Numerical investigation of different flow regimes for multiple staggered rows

In this numerical exploration, the combined effect of Reynolds number (30 ≤ Re ≤ 150) and gap spacing (1 ≤ g ≤ 7) is studied for two dimensional cross flow across multiple staggered rows of square cylinders. Flow is simulated by using lattice Boltzmann method. Outcomes show that for the outset of vortex shedding phenomenon, the critical Re increases as the normalized g increases. At large Re and at g = 7, 6 and 5, the primary vortex shedding frequency controls the flow whereas the secondary frequency almost vanishes. The jets in the gap region have strong influence upon the wake interaction. The nature of the wakes is changed by changing the g and Re which is visualized by the change of wake size behind the cylinders. These g depending on the Re are used to split the flow regimes into chaotic, quasiperiodic-I and quasiperiodic-II flow regimes. Some physical parameters of practical importance are also analysed.

Natural convection heat transfer from a horizontal row of finite number of heated circular cylinders immersed in molten solar salt

Abstract A two-dimensional steady-state natural convection heat transfer from a single row of closely spaced isothermally heated cylinders was investigated in this study for a laminar flow regime. A few previous studies investigated the natural convection heat transfer from a single row of isothermally heated cylinders. However, no correlations or performance maps were developed for the average Nusselt number N u ¯ D for a single row of horizontal cylinders. Using the correlations proposed for natural convection heat transfer from a single cylinder results in large errors in predicting the heat transfer for multiple cylinders. Thus, numerical and statistical modeling were performed in this study to propose correlations for the average Nusselt number N u ¯ D for a single row of horizontal cylinders immersed in molten solar salt as functions of Rayleigh number R a D and dimensionless cylinder-to-cylinder spacing ( ST/D ) . The analyzed geometries are important in the solar heating and heat storage technology, nuclear reactor safety and waste heat recovery applications.

Numerical investigation of vortex shedding mechanism for staggered rows of cylinders

This numerical investigation deals with study of flow past seven square cylinders arranged in staggered configuration. Simulations to the underlying problem are carried out by using the single relaxation time lattice Boltzmann method. The gap spacing between the staggered rows is varied in streamwise direction from 0.25 to 2 and for three transverse spacing i.e. T* = 1, 1.5 and 2. The complex vortex shedding mechanism is visualized for different streamwise and transverse gap spacings. Furthermore, the vortex shedding mechanisms in the wake of downstream row of cylinders are debated by using vorticity snapshots, time-trace plots of drag and lift coefficients and power spectra visualization of lift coefficient. It is also observed that the secondary cylinder interaction frequency contributes significantly to the hydrodynamic forces experienced by the cylinders. The physical parameters, such as Strouhal number, the drag and lift coefficients and their root-mean-square values have also been discussed in detail.

Water wave trapping in a long array of bottomless circular cylinders

This study tries to identify wave trapping situations by engaging and properly combining two well established phenomena: (i) the trapped modes induced by arrays of cylinders and (ii) the pumping trapped modes which are known to occur in moonpools. To this end, the fundamental hydrodynamic boundary value problem for arrays of bottomless cylinders was solved using standard domain decomposition. The method employed expansions of the solutions for the velocity potentials in polar harmonics combined with the eigenfunction expansions technique. The solution sought for the velocity potentials is achieved using the “direct” method of approach which accordingly requires the employment of a sophisticated matrix manipulation process.The elaboration of the concerned concept was motivated by three basic tasks: (i) to identify whether arrays of truncated and bottomless cylinders indeed preserve the occurrence of Neumann, Dirichlet and near trapped modes, extensively investigated for bottom-seated cylinders; (ii) to examine whether the expected pumping modes in moonpools modify the characteristics of the hydrodynamic resonance regimes (trapped modes) in the open liquid space between the cylinders and vice versa and (iii) to explore the possibility to suggest relevant configurations as parts of integrated mechanisms for practical applications, focusing a fortiori to clusters of hydrodynamically interacting Oscillating Water Columns (OWCs).The method developed is generic and can be employed for arbitrary configurations of multi-body arrays accommodating bottomless cylinders with uneven geometrical characteristics. Trapped modes are identified numerically as peaks in loading and this fact has been explicitly demonstrated in rows of cylinders. Therefore, the numerical results shown and discussed in the present are based on a specific in-line array that has been investigated in the past for bottom-seated cylinders. The investigated subject, i.e. whether the combined wave trapping induced by the examined configuration could be conceived as an efficient water wave power extraction mechanism is approached and discussed through dedicated computations of the free-surface displacements in the moonpools.

Effect of aspect ratios on flow past a row of rectangular cylinders for various gap spacings

Numerical simulations are carried out to study the effect of aspect ratio (AR) and gap spacing (g) for flow past a row of rectangular cylinders using lattice Boltzmann method (LBM). The Reynolds number (Re) is fixed at 150, gap spacing is taken in the range from 0.25 to 4 while the aspect ratio varies from 0.5 to 2. Depending on AR and g, the flow is classified in to three major classes; (i) modulated (ii) symmetric and (iii) synchronized. In each class different flow patterns are observed. In modulated class, the flow behind cylinders shows merging and distortion while the force coefficients are modulated. The observed flow patterns for this class are:nearly-bistable, deflected, flip-flopping and chaotic. The flow behind middle cylinder in symmetric class is either steady or nearly steady. In this class symmetric and nearly-symmetric flow patterns are observed. In synchronized class, the flow behind each cylinder is fully developed with alternate negative and positive vortices. The flow pattern observed for this class are: quasi-synchronized, synchronized and non-synchronized. Fluid forces acting on each cylinder, flow structure mechanism, time signal analysis of drag and lift coefficients and vortex shedding frequencies are investigated systematically for different AR and g. The results show that at small g and AR, flow is complex and is more affected by AR as compared to g. At large g and AR the flow is synchronized throughout the computational domain. The results further show that the mean drag coefficients of all cylinders increase with corresponding decrease in g for fixed AR. While at fixed values of g the mean drag coefficients show variation for all AR. At large g and fixed AR the mean drag coefficient of all cylinders approaches to single cylinder and all cylinders experience same drag. It has been observed that the smaller spacing values have more influence on Strouhal number as compared to higher spacing values.

Flow patterns and turbulence effects in large cylinder arrays

Studies on flow-induced vibrations in large tube bundles are usually focused solely on frequency analysis, without considering the flow patterns which are responsible for the fluid forces. Furthermore, investigations which involve variations in the spacing ratios do not separate transversal and longitudinal proximity effects. The purpose of this article is to separately analyze the influence of the transversal (T/D) and longitudinal (L/D) spacing ratios of a confined in-line cylinder array with five rows on the flow characteristics and to identify flow patterns. The laser Doppler anemometry technique was employed to acquire the mean velocity and its fluctuations in the transversal and longitudinal directions between the cylinder rows. Strouhal numbers and regimes reported in the literature were identified in the experiments. The same regime did not always persist along all cylinder rows for a given spacing ratio, as a result of the combined longitudinal and transversal proximity effects and also of the generation of turbulence by the array. For the smallest T/D ratio, a quasi-steady behavior associated with the biased flow pattern was noted in the experimental set-up and flip-flopping was observed in one case. Additionally, the flow characteristics in these arrays diverged from tube bundle classifications described in the literature. The behavior of the fluid forces and susceptibility to vibrations in the array were predicted based on the turbulence intensity of the incident flow of the cylinders. The results reinforced the need to extend flow pattern investigations to arrays with more cylinder rows and to consider both transversal and longitudinal proximity effects, when studying flow-induced vibrations.

Nanoscale beam splitters based on gradient metasurfaces.

Beam splitters are essential components in various optical and photonic applications, for example, interferometers, multiplexers, and so on. Present beam splitters based on cubes or plates are normally bulky. Realizing beam splitters in nanoscales is useful to reduce the total size of photonic devices. We demonstrate here a beam splitter with nanoscale thickness based on a gradient metasurface comprising lithium niobate cylinder arrays. Since one unit cell of metasurface comprising two cylinder rows shows two opposite phase gradients, the incident light is split into different directions according to the generalized Snell's law. The split ratio is proven to be effectively tunable.

Utilization of rice husks in a water-permeable material for passive evaporative cooling

This research aims to use the byproducts of rice processing: the rice husk, coarse rice bran and rice polish, to experimentally develop a water-permeable material for evaporative cooling. The starting materials consisted of laterite, fine sand, Portland cement, rice byproducts and water. Mixtures with different material proportions were molded into interlocking blocked and cylindrical shapes, dried and cured in air indoors. In interlocking blocks connected into a column, the proportion of laterite, fine sand, Portland cement and coarse rice bran with a ratio of 4:2:1:3 absorbed the most water and yielded a maximum water height of 42 cm. In the cylindrical porous material, the maximum water height of 54 cm was found in the mixture with volumetric proportions of 3:2:1:4. The pore sizes of the porous cylinder were in the range of 0.03–3 μm. The investigation of the thermal performance of the developed material was conducted in an experimental chamber with an air temperature of 32–37 °C and relative humidity of 54–76% flowing through four rows of water soaking porous cylinders. The distance between two rows of water soaking porous cylinders was also varied from 11.2 cm to 14.0 cm and 16.8 cm. The air temperature was found to be reduced by 0.5-2.7 °C. The highest temperature reduction was found in the porous cylinders that had a distance between the rows of 16.8 cm and operated under high air temperature. The water consumption of 31.6 mL/h was lowest for the porous cylinder with a distance between two rows 16.8 cm apart. The developed product was appropriate for evaporative cooling in agricultural houses and outdoor spaces.

Flow and heat transfer over a row of multiple semi-circular cylinders: selection of optimum number of cylinders and effects of gap ratios

The present work aims at studying the laminar flow and heat transfer characteristics of a Newtonian fluid over a row of semi-circular cylinders placed in uniform cross-flow configuration. The effects of spacing to diameter (gap) ratio, varied from 1 to 10, on the flow and heat transfer patterns have been studied at a Reynolds number of 100 for air as the working fluid. There has been no significant interaction in the flow at a spacing ratio greater than 4 and each semi-circular cylinder behaved more or less like the situation of a single cylinder. At lower gap ratios, the flow becomes more complicated due to the shear layer interactions that exist in the flow. The streamline and isotherm contours have been presented and discussed in detail for various gap ratios. The variation in flow behavior is explored further by studying the drag coefficient and lift coefficient signals of various semi-circular cylinders. The presence of a secondary frequency at low gap ratios that has adverse effect on the drag coefficient has been confirmed. The variation of measured global quantities, namely the drag coefficient, the Strouhal number and the Nusselt number have been studied and discussed in detail.

Reduction of sound transmission across plenum windows by incorporating an array of rigid cylinders

The potential improvement of plenum window noise reduction by installing rigid circular cylinder arrays into the window cavity is investigated numerically using the finite-element method in this study. A two-dimensional approach is adopted. The sound transmission characteristics and propagation within the plenum window are also examined in detail. Results show that the installation of the cylinders in general gives rise to broadband improvement of noise reduction across a plenum window regardless of the direction of sound incidence. Such acoustical performance becomes better when more cylinder columns are installed, but it is suggested that the number of cylinder rows should not exceed two. Results also show that the cylinder positions relative to the nodal/anti-nodal planes of the acoustic modes are crucial in the noise reduction enhancement mechanisms. Noise reduction can further be enhanced by staggering the cylinder rows, such that each cylinder row supports the development of a different acoustic mode. For the simple cylinder arrangements considered in this study, the traffic noise reduction enhancement observed in this study can be as high as 4–5 dB, which is already comparable to or higher than the maximum achieved by installing sound absorption into a plenum window.

Transitions in the flow patterns and aerodynamic characteristics of the flow around staggered rows of cylinders

A two-dimensional numerical study of flow across rows of identical square cylinders arranged in staggered fashion is carried out. This study will unreveal complex flow physics depending upon the Reynolds number (Re) and gap spacing (g) between the cylinders. The combined effect of Reynolds number and gap spacing on the flow physics around staggered rows of cylinders are numerically studied for 20 ≤ Re ≤ 140 and 1 ≤ g ≤ 6. We use the lattice Boltzmann method for numerical computations. It is found that with increase in gap spacing between the cylinders the critical Reynolds number for the onset of vortex shedding also increases. We observed a strong effect of Reynolds number at g = 2 and 4. Secondary cylinder interaction frequency disappears for large Reynolds number at g = 6 and 5 and the flow around cylinders are fully dominated by the primary vortex shedding frequency. This ensures that at large gap spacing with an increase in the Reynolds number the wakes interaction between and behind the cylinders is weaken. Furthermore, it also ensures that the wake interaction behind the cylinders is strongly influenced by the jets in the gap spacing between the cylinders. We also found that g = 2 is the critical gap spacing for flow across rows of staggered square cylinders for the considered range of Reynolds number. Depending on the Reynolds number we observed; synchronous, quasi-periodic-I, quasi-periodic-II, and chaotic flow patterns. In synchronous flow pattern, an in-phase and anti-phase characteristics of consecutive cylinders has been observed. The important physical parameters are also analyzed and discussed in detail.

Improvement of torque and power characteristics of V-type diesel engine applying new design of Variable geometry turbocharger (VGT)

Based on the new design concept in VGT the commercial turbocharger with vaneless turbine volute and two gas inlet casings was converted to an adjustable one. Modification assumed the processing of turbine housing and installation of the specially designed mechanism of adjustment. Prototype of variable geometry turbocharger with two gas inlet casings can be installed on V-type engine in the way when each row of cylinders will deliver exhaust gases to one of the gas inlet casting. In the new design concept the elements of adjustment was profiled in a special way to ensure the adjustment with maximum effectiveness. Adjustable turbocharger was tested on a motorless test bench and characteristics of variable geometry turbine were analyzed in a wide range of the gas flow rates. Only small fluctuations in turbine efficiency during adjustment were detected that is evidence of high gas dynamics performance of VGT. Based on experimental data the possibility to improve engine indices that are most impacted when VGT is used was made. Improvement of torque and power characteristics of diesel engine when turbocharger adjustment with vaneless turbine volute is used also presented.

Scattering of a Gaussian beam from a row of cylinders with rectangular cross section.

The transverse magnetic Gaussian beam diffraction from a finite number, equally spaced and rectangular cross section dielectric cylinder row is studied. The infinitely long cylinders' axes are perpendicular to the beam's direction of propagation. The cylinder row, with dielectric constant εc=nc2, is treated as a periodic inhomogeneous film, with period ax and thickness wy, bounded by two semi-infinite homogeneous media. With this restriction, the method is valid only for square or rectangular cross section cylinders. The supercell and the plane wave expansion methods are used to calculate the eigenfrequencies and eigenvectors supported for a one-dimensional photonic crystal. Then, these eigenfrequencies and eigenvectors are used to expand the field in the inhomogeneous film. Numerical results are presented for ax greater than λ (the incident light wavelength), wx (the cylinder width), and wg (Gaussian beam waist). Two cases are studied. In the first (second) case, the unit cell contains one cylinder (a cylinder row), which simulates the scattering from a single cylinder (an inhomogeneous thin film). The total integrated scattering in transmission (reflection) shows three well-defined minima (maxima), which are due to interference effects. Its positions can be approximately obtained with the formula λk=4ncwy/k, with k=3, 4, and 6. The total integrated scattering in transmission decreases linearly as a function of the cylinder number.

Symbolic Dynamics Applied to the Identification of Flow Patterns Inside Tube Banks

This paper presents a study of the identification of flow patterns inside a tube bank with the technique of symbolic dynamics. The experimental signals of the mean velocity and its fluctuations are measured by hot-wire anemometry in an aerodynamic channel and used as input data for the symbolic dynamics technique. The tube bank consists of 23 circular cylinders in a triangular arrangement. The pitch-to-diameter ratio chosen was 1.26 and the Reynolds numbers are in the range from 7.5 × 103 to 4.4 × 104, computed with the tube diameter, D = 25.1 mm, and the percolation velocity. In this work, a binary alphabet was chosen to convert and analyze the data. The partitioning process is performed through the mean value of the time series and via discrete wavelet reconstruction, according to a chosen reconstruction level. The flow patterns are presented for different positions inside the tube bank, where histograms and probability density functions support the statistical interpretation. The histograms with a decimal representation for the original experimental time series with partitioning performed through the mean value show that the signals do not present fast changes of velocity fluctuations. This behavior was observed in the five rows of cylinders. However, by changing the partitioning according to a wavelet reconstruction of the signal with high frequency, which means that the signals are close to the partitioning function, fast changes appear in all of the time series observed. The results indicate that the turbulence in tube banks has chaotic characteristics. Flow visualizations performed with ink injection inside the tube bank helped in the interpretation of the results.

Combined effects of bed friction and emergent cylinder drag in open channel flow

Open channel flows subjected to a longitudinal transition in roughness, from bed friction to emergent cylinder drag and vice versa, are investigated experimentally in an 18 m long laboratory flume. These are compared to uniform flows subject to (1) bed roughness only and (2) an array of emergent vertical cylinders installed on bed roughness. The nearbed region is investigated in detail for uniform flows through the cylinder array. The water column can be divided into two parts: a region of constant velocity and a boundary layer near the channel bed. In the latter region, a local increase in velocity, or velocity bulge, is observed in line of a cylinder row. The velocity bulge may be related to the disorganization of the von Karman vortex street by the bed-induced turbulence, resulting in reduced momentum loss in the cylinder wake. The boundary layer height is found to be independent of water depth and bed roughness (smooth or rough bottom). Strong oscillations of the free surface (seiching) are observed. Oscillation amplitude is dependent on the longitudinal position within the cylinder array and is found to decrease with decreasing array length. When water depth/boundary layer height ratio is close to unity, the disorganization of the von Karman vortex street throughout the water column prevents seiching from occurring. In the case of roughness transition flows, the water depth is found to vary only upstream of the change in roughness. Vertical profiles of velocity and turbulence are self-similar upstream of the transition and collapse with the uniform flow profiles. Downstream of the roughness change, velocity and turbulence vary over a distance of 35 to 50 times the water depth. Roughness transition flows show that seiching is lowered by flow non-uniformity. A 1D momentum equation integrating bed friction and drag force exerted by the cylinder array predicts accurately the water surface profile (0.9% mean relative error). The computed profiles show that, upstream of the transition, flow depth varies over a distance of about 2600 times the uniform water depth of the upstream roughness. The 1D equation is solved analytically for zero bed friction.