Circular Cylinders In Tandem Arrangement Research Articles

Effect of spacing on a pair of naturally oscillating circular cylinders in tandem arrangements employing IB-LB methods: Crossflow-induced vibrations

An efficient velocity-correction based immersed boundary-lattice Boltzmann method (IB-LBM) is employed and coupled with structural equations to investigate the vortex-induced vibration characteristics of two identical circular cylinders in various tandem arrangements at a low Reynolds number of 100 with 1.0 < Sx/D ≤ 12.0 and frequency ratio Fr=1.0; here, Sx is the streamwise displacement between the cylinder centers, D is the cylinder diameter, and Fr is the ratio of the vortex shedding frequency to the natural frequency of the oscillator. The cylinders are set to oscillate naturally in the transverse direction only, and a mass-spring system characterizes their motion. Considering a similar flow and structural parameters, a single cylinder is also studied for reference and comparison. Furthermore, a fixed cylinder and two stationary cylinders in tandem arrangements with same spacing ratios are also examined. Four distinct regimes of cylinder arrangements have been identified based on the cylinder maximum, gradual decrease to a minimum, persistence, and single cylinder response. At Sx/D=1.5, the cylinders vibrate violently with overall maximum amplitude. A critical spacing at (Sx/D)c=3.5 is observed both in the stationary and oscillated cylinders, where the forces and amplitude have the minimum values. The response amplitude and force coefficients remain constant over a range of spacing, i.e., Sx/D=4.0−8.0. The cylinders behave as single cylinders when placed more than eight-diameters apart as the interaction between the fluid and structure are decoupled. The interference and wake effects are diminished in this regime. The hydrodynamic force coefficients and flow physics of the oscillating tandem cylinders are also discussed. The results reveal that the flow in the gap between the cylinders and in the wake significantly affects the forces and hence, the dynamic response of the tandem structure.

Numerical Investigation on Vortex-Induced Vibration Energy Extraction Efficiency of Double Circular Cylinders In Tandem Arrangement at Low Reynolds Number

Vortex shedding from a bluff body results in fluctuating forces acting on the bluff body, which may induce vibration of the bluff body when the bluff body is elastically mounted or deformable. Researchers put forward an idea that we can ex-tract energy from the water flow based on VIV at low flow velocity. Although plenty of researches on parameters of VIV are already presented, however, the improvement of energy extraction efficiency still needs further study. According to the previous research, this essay has simulated flow-induced vibration of tandem double circular cylinders when Reynolds number is 100. Working condition has been considered as the fixed upstream cylinder and the free vibration of the downstream cylinder. The influence of the mass coefficient and the two cylinders spacing ratio on the downstream cylinder’s energy obtained from the fluid is studied. Analysis results show that, the maximum value of the energy extraction efficiency is before the frequency locked range. In the case of large spacing ratio (L/D=7~9), the phenomenon of "beat vibration" appears on the downstream cylinder. The results of this work could provide reference for the improvement of energy extraction efficiency and the design of VIV converter.

Wake-Induced Vibrations of Tandem Circular Cylinders of Different Diameters

Wake-induced vibrations (WIV) of two circular cylinders in tandem arrangement of different diameters (larger downstream cylinder) has been analyzed in the present work. The ratio of the diameter of first cylinder to that of the second cylinder (D1/D2) is kept constant at 0.8. The cylinder centers are separated by a distance S (= 5D1). The computations were carried out at Reynolds number (Re) = 100 over a range of reduced speed (U*). The fluid was assumed to be incompressible and flow to be two-dimensional. The flow equations were discretized using stabilized finite-element method and solved iteratively by Generalized Minimal RESidual (GMRES) technique. In the present work, the upstream cylinder is kept stationary while the downstream cylinder is allowed to traverse only across the flow. The observations focus the response, aerodynamic coefficients and flow features.

Wake-Induced-Vibrations of Circular Cylinders in Tandem Arrangement

Stabilized space-time finite element technique is utilized to perform numerical simulation on the wake-induced-vibrations of two identical tandem circular cylinders placed in the flow at Reynolds number 100. Cylinders are separated by a center to center distance of 5 times the diameter of the cylinder. Results are computed for various values of reduced velocities (1 to 8). The downstream cylinder, of mass ratio 10, is allowed to execute oscillations in both in-line and transverse directions, whereas the upstream cylinder is stationary. Response curve of downstream cylinder is observed to be composed of initial and lower branches. Phenomenon of lock-in is identified at a reduced velocity of 5. Nature of flow exhibits periodicity for the entire range of initial branch with single dominant lift frequency. For reduced velocity near the end of lock-in, flow and body motion shows quasi-periodic behavior with complicated non-overlapping loops in the cylinder trajectory as well as in the phase plots of drag and lift coefficients.

Flow induced vibrations of a sharp edge square cylinder in the wake of a circular cylinder

The response of an oscillating circular cylinder at the wake of an upstream fixed circular cylinder was classified by different researchers as galloping, wake induced galloping or wake induced vibration. Furthermore it is already known that a sharp edge square cylinder would undergo galloping if it is subjected to uniform flow. In this study the influence of the wake of a fixed circular cylinder on the response of a downstream square cylinder at different spacing ratios (S/D=4, 8, 11) is experimentally investigated. The subject appears not to have received previous attention. The lateral displacements, lift forces and the pressure data from gauges mounted in the wake of the oscillating cylinder are recorded and analyzed. The single degree of freedom vibrating system has a low mass-damping parameter and the Reynolds number ranges from 7.7×102 to 3.7×104.In contrast to that for two circular cylinders in tandem arrangement, the freely mounted downstream square cylinder displays a VIV type of response at all spacing ratios tested. There is no sign of galloping or wake induced galloping with the square cylinder. With increase at the spacing ratio the cross-flow oscillations decrease. It is shown that the vortices arriving from the upstream fixed circular cylinder play a major role on the shedding mechanism behind the downstream square cylinder and cause the square cylinder to shed vortices with frequencies above Strouhal frequency of the fixed square cylinder (St=0.13). The VIV type of oscillations in the downstream square cylinder is most probably caused by the vortices newly generated behind the square cylinder.

Numerical investigation of mixed convection heat transfer from two isothermal circular cylinders in tandem arrangement: buoyancy, spacing ratio, and confinement effects

This paper presents a two-dimensional numerical study for mixed convection in a laminar cross-flow with a pair of stationary equal-sized isothermal cylinders in tandem arrangement confined in a channel. The governing equations are solved using the control volume method on a nonuniform orthogonal Cartesian grid, and the immersed boundary method is employed to identify the cylinders placed in the flow field. The numerical scheme is first validated against standard cases of symmetrically confined isothermal circular cylinders in plane channels, and grid convergence tests were also examined. The objective of the present study was to investigate the influence of buoyancy and the blockage ratio constraint on the flow and heat transfer characteristics of the immersed cylinder array. Using a fixed Reynolds number based on cylinder diameter of \(Re_{D} = 200\), a fixed value of the Prandtl number of \(Pr = 7\), and a blockage ratio of \(D/H = 0.2\), all possible flow regimes are considered by setting the longitudinal spacing ratio (\(\sigma = L/D\)) between the cylinder axes to 2, 3, and 5 for values of the buoyancy parameter (Richardson number) in the range \(-1\le Ri\le 4\). The interference effects and complex flow features are presented in the form of mean and instantaneous velocity, vorticity, and temperature distributions. The results demonstrate how the buoyancy, spacing ratio, and wall confinement affect the wake structure and vortex dynamics. In addition, local and average heat transfer characteristics of both cylinders are comprehensively presented for a wide range in the parametric space.

Numerical study on phase change of water flowing across two heated rotating circular cylinders in tandem arrangement

The problems of fluid flow and heat transfer phenomena over an array of cylinders are quite prominent in fluid dynamics and industry applications. The current work focuses on fluid flow and heat transfer analysis over two heated rotating cylinders arranged in tandem. The flow of water over heated cylinders faces a phenomenon of phase change from liquid (water) to vapor phase (steam). The mechanism of this phase change is studied through a numerical simulation supplemented with verification of the code and validation. The problem is simulated when flows from two cylinders in a tandem arrangement become interacting and non-interacting. The Eulerian model is used during simulation to comprehend the multiphase phenomena. The volume fractions of both the phases such as water and vapor and heat transfer coefficients of both the cylinders have been computed and presented as findings of the problem. The mass and heat transfer mechanism is unidirectional from one phase to the other phase. The vapor fraction of each phase is to be observed and compared when three different rotations are given to the two cylinders immersed in a turbulent flow of water.

Numerical Study on Phase Change of Water Flowing Across Two Heated Circular Cylinders in Tandem Arrangement

AbstractThis paper focuses on fluid flow and heat transfer analysis over two heated cylinders arranged in tandem. The flow of water over heated cylinders faces a phenomenon of phase change from liquid (water) to vapor phase (steam). The mechanism of this phase change is studied through a numerical simulation supplemented with verification and validation of the code. The problem is simulated when flows from two cylinders in a tandem arrangement become interacting and noninteracting. The Eulerian model is used during simulation to comprehend the multiphase phenomena. The effect of spacing between these two cylinders and the Reynolds number effect are studied in this paper. The volume fractions of both the water and vapor phases and the heat transfer coefficients of both the cylinders have been computed and presented as findings of the problem.

Effect of Reynolds number on phase change of water flowing across two heated circular cylinders in tandem arrangement

The situations of fluid flow and heat transfer across an array of cylinders have been quite common in fluid dynamics and, particularly, industry applications. One such situation is flow of water over heated cylinders in a tandem arrangement. The flow of water over heated cylinders faces a phenomenon of phase change from liquid (water) to vapor phase (steam).The mechanism of this phase change is studied through a numerical simulation in this project. The Eulerian model is used in the present simulation method to comprehend the multiphase phenomenon. The effect of Reynolds number on the phase change is studied. The phase change of water in an unsteady flow across cylinders has been prominently affected by Reynolds number. Only critical cases have been discussed in this paper. The volume fraction of water and steam is plotted against the position of flow from inlet to exit of the flow domain that is a channel for a particular flow conditions to demonstrate the phenomenon of heat and mass transfer during the flow of water.

A Numerical Study of the Forces on Two Tandem Cylinders Exerted by Internal Solitary Waves

A three-dimensional numerical wave flume is employed to investigate the forces exerted by internal solitary waves (ISWs) on a pair of circular cylinders in tandem arrangement, using large-eddy simulation (LES) model. The effect of the centre-to-centre distance (L) ranging from 1.5 to 5 diameters (D) is studied for various ISWs amplitudes (ηa) in the two-layer fluid system. Vertical-averaged vorticity distribution and vertical-averaged pressure gradient distribution in each layer are presented to investigate the different hydrodynamic interference between cylinders and the ISWs forces on each cylinder at variousL. Furthermore, the force behaviors of the two cylinders are also compared with that of an isolated cylinder in the same environment. The interaction between the two piles occurs in both layers, and it is found that, for1.5≤L/D&lt;3.5, strong mutual interference appears between two cylinders; for3.5≤L/D&lt;5, the two cylinders continue to influence each other in a weak-interference state; forL/D≥5, the interaction gradually decreases into a noninteracting state. This paper tries to provide some references to structural arrangement of double-cylinder structure and grouped-cylinder structure in stratified flow environment.

Numerical simulation of flow past twin near-wall circular cylinders in tandem arrangement at low Reynolds number

Fluid flow past twin circular cylinders in a tandem arrangement placed near a plane wall was investigated by means of numerical simulations. The two-dimensional Navier-Stokes equations were solved with a three-step finite element method at a relatively low Reynolds number of Re = 200 for various dimensionless ratios of 0.25 ≤ G/D ≤ 2.0 and 1.0 ≤ L/D ≤ 4.0, where D is the cylinder diameter, L is the center-to-center distance between the two cylinders, and G is the gap between the lowest surface of the twin cylinders and the plane wall. The influences of G/D and L/D on the hydrodynamic force coefficients, Strouhal numbers, and vortex shedding modes were examined. Three different vortex shedding modes of the near wake were identified according to the numerical results. It was found that the hydrodynamic force coefficients and vortex shedding modes are quite different with respect to various combinations of G/D and L/D. For very small values of G/D, the vortex shedding is completely suppressed, resulting in the root mean square (RMS) values of drag and lift coefficients of both cylinders and the Strouhal number for the downstream cylinder being almost zero. The mean drag coefficient of the upstream cylinder is larger than that of the downstream cylinder for the same combination of G/D and L/D. It is also observed that change in the vortex shedding modes leads to a significant increase in the RMS values of drag and lift coefficients.

Fluid-Structure Coupling between Two Tandem Elastic Cylinders

The paper presents an experimental investigation on characteristics of flow-induced vibrations of two circular cylinders in tandem arrangement. Both cylinders are allowed to vibrate in the cross-flow direction only. The spacing ratio L/D (where L is the spacing between the two cylinder centers and D is the diameter) is varied as 1.2, 1.5, 2.0, 2.5, 3.0, 4.0 and 6.0. Four regimes are identified. In regime I (L/D ≤ 1.5), the two cylinders experience vortex excitation (VE) combined with ‘galloping. In regime II (1.5 <L/D < 2.5), again both cylinders undergo VE followed by galloping, but the VE and galloping are separated. Regimes III (2.5 ≤ L/D ≤ 3.0) and IV (L/D > 3.0) are characterized by separated VE and galloping for the downstream cylinder and only VE for the upstream cylinder. Compared to that of the downstream cylinder, in regime I the vibration amplitude of the upstream cylinder is larger, and in regime II it is larger at a smaller reduced velocity Ur and smaller at a larger Ur. On the other hand, regimes III and IV correspond to a larger vibration amplitude for the downstream cylinder and very smaller amplitude for the upstream cylinder. Vibration of the downstream cylinder is however violent in the former regime and weaker in the latter. The wake structure of vibrating cylinders is demonstrated by PIV measurements.

Wake-induced vibrations of an elastically mounted cylinder located downstream of a stationary larger cylinder at low Reynolds numbers

Two-dimensional numerical simulations of flow past two unequal-sized circular cylinders in tandem arrangement are performed at low Reynolds numbers (Re). The upstream larger cylinder is stationary, while the downstream cylinder has both one (transverse-only) and two (transverse and in-line) degrees of freedom (1-dof and 2-dof, respectively). The Re, based on the free stream velocity U∞ and the downstream cylinder diameter d, varies between 50 and 200 with a wide range of reduced velocities Ur. The diameter of the upstream cylinder is twice that of the downstream cylinder, and the center-to-center spacing is 5.5d. In general, for the 1-dof case, the calculations show that the wake-induced vibrations (WIV) of the downstream cylinder are greatly amplified when compared to the case of a single cylinder or two equal-sized cylinders. The transverse amplitudes build up to a significantly higher level within and beyond the lock-in region, and the Ur associated with the peak amplitude shifts toward a higher value. The dominant wake pattern is 2S mode for Re=50 and 100, while with the increase of Re to 150 and 200, the P+S mode can be clearly observed at some lower Ur. For the 2-dof vibrations, the transverse response characteristics are similar to those presented in the corresponding 1-dof case. The in-line responses are generally much smaller, except for several significant vibrations resulting from in-line resonance. The obvious in-line vibration may induce a C (chaotic) vortex shedding mode for higher Re (Re=200). With regard to the 2-dof motion trajectories, besides the typical figure-eight pattern, several odd patterns such as figure-double eight and single-looped trajectories are also obtained due to the wake interference effect.

Experimental study on the flow around two tandem cylinders with unequal diameters

In this paper, flow around two circular cylinders in tandem arrangement with unequal diameters has been investigated using the particle image velocimetry technique (PIV) in a water channel. The upstream to downstream diameter ratio was kept constant at d/D = 2/3, the centre-to-centre distance was varied from 1.2D to 5D and the Reynolds number was varied from 1200 to 4800. The flow characteristics were analyzed through ensemble-averaged patterns of velocity, vorticity, normalized Reynolds stress contours and streamlines. Based on ensemble-averaged and instantaneous flow fields, different flow patterns, including single-wake-shedding at small spacing ratio, bi-stable flow behavior (alternating behavior of reattachment and vortex shedding) at intermediate spacing ratio and co-shedding pattern at large spacing ratio were observed. The effects of Reynolds number and the centre-to-centre spacing ratio on flow patterns and turbulent characteristics were also investigated. It was found that the diameter ratio appears to have a certain effect on the flow patterns at intermediate spacing ratios, where the reattachment of shear layer depends on the lateral width of the wake flow in the lee of the upstream cylinder. Extensive discussion on the distributions of Reynolds stress and turbulent kinetic energy was presented.

Re-examination of laminar flow over twin circular cylinders in tandem arrangement

Viscous fluid flow past two identical circular cylinders in a tandem arrangement is numerically investigated at a typical Reynolds number of 200. By considering a large span of spacing ratio (0.1 ⩽ G/D ⩽ 6.0) with a fine interval of 0.1 or less, the dependences on the spacing ratio of the drag force, lift force, lift fluctuation frequency, torque on the cylinder pair and phase difference between the lift fluctuations of the two cylinders are investigated in detail, where D is the diameter of the cylinder and G the surface-to-surface distance between the cylinders. The phase difference between the lift fluctuations of the two cylinders is addressed based on correlation analysis together with the phase diagram, which has received scarce attention before. The phase difference provides further understanding to the dependence of the wake evolutions behind the twin circular cylinders. The numerical investigations show that diverse regimes can be identified according to the dependence of the hydrodynamics on the spacing ratio. The hydrodynamic discontinuities at G/D = 0.9, which have previously been ignored, are reported in this work. The physical correlations between the hydrodynamic discontinuities and the wake patterns are presented.

Numerical Study of Atmospheric Icing on Non Rotating Circular Cylinders in Tandem Arrangement

Numerical study of atmospheric ice accretion on two non-rotating circular cylinders in tandem arrangement was carried out at different operating and geometric conditions. To validate the numerical model, initially the results of ice accretion on single circular cylinder were compared with the experimental data obtained from CIGELE atmospheric icing research wind tunnel (CAIRWT) [1, 2]. A good agreement was found between experimental and numerical results. Numerical analyses of ice accretion on two circular cylinders in tandem arrangement showed that accreted ice loads decreases with the increase in distance between the cylinders and also affects the rate and shape of ice accretion. Parametric study at different droplet sizes and temperatures showed a significant change in ice accretion. This research work provides a useful base for better understanding and further investigation of atmospheric ice accretion on circular overhead power network cables in tandem arrangement, installed in the cold regions.

Ghost Cell Immersed Boundary Method for Simulating Flow over Two Circular Cylinders in Tandem Arrangement

In this work, a ghost cell immersed boundary method is applied to the numerical simulation of a uniform flows over a circular cylinder and two circular cylinders in tandem arrangement. The Navier-Stokes equations are solved using an implicit fractional step method employed on collocated arrangement variables. Immersed boundary method permit the use of structured Cartesian meshes to simulate flows involving complex boundaries. The shedding of vortices and flow interference between two circular cylinders in tandem arrangement are investigated numerically. The calculations are validated against the experimental and numerical results obtained by other researchers to prove the accuracy and effectiveness.

Correlating Flow Pattern with Force Coefficients in Air Flow past a Tandem Unit of Three Circular Cylinders

Air Flow past three circular cylinders in tandem arrangement is simulated at 20 different Reynolds numbers ranging from 20 to 300. The spacing ratio L=D is set at 2, where L is center to center distance. The drag of all three cylinders is assessed with varying Reynolds number. Accordingly, pressure and velocity distribution in the near vicinity of cylinders is closely examined. It is found that Reynolds numbers of 42, 63 and 150 are critical Reynolds numbers for pressure, viscous and total drag forces respectively. Moreover, time evolutions of force coefficients are investigated to find the transitional Reynolds number at which the flow becomes unsteady and vortices are shed from the cylinders. It was observed that transition starts at Re = 101 and vortex shedding occurs at Re = 105 for the first time. Flow pattern is illustrated at a specific moment for all steady, transitional and unsteady states at various Reynolds numbers. Also, the time evolution of flow pattern is depicted for unsteady cases. It was observed that frequency of vortex shedding and also Strouhal number increase with Re.

Convective heat transfer from two rotating circular cylinders in tandem arrangement by using lattice Boltzmann method

The numerical investigation of the two-dimensional laminar flow past two rotating circular cylinders in the tandem arrangement is conducted by the lattice Boltzmann method. The numerical strategy is used for dealing with curved and moving boundaries of the second-order accuracy for velocity and temperature fields. The effects of various rotational speed ratios and gap spacing are studied with the Reynolds number of 100 and the Prandtl number of 0.71. A varied range of rotational speed ratios are investigated for four different gap spacing, i.e., 3.0, 1.5, 0.7, and 0.2. The results show that, for the first cylinder, the lift and drag coefficients for large gap spacing are similar to those for a single cylinder; for the second cylinder, the lift coefficient descends with the increase in the angular velocity for all gap spacing, while the drag coefficient ascends except for the gap spacing of 3.0. The results of the averaged periodic Nusselt number on the surface of the cylinders show that, for small distances between the cylinders and low angular velocities, conduction is a dominant mechanism of heat transfer, but for large distances and high angular velocities, convection is the main mechanism of heat transfer.

Secondary instability in the near-wake past two tandem square cylinders

Floquet stability analysis has been carried out to detect the onset of the secondary instability in the laminar flow past two identical square cylinders in tandem. Presence of a neighbouring cylinder alters flow topology, leading to change of stability characteristics. A parametric study has been performed with the gap between the two square cylinders as the key parameter. Three distinct patterns of base flow are found depending on the gap, and distinctive modes of the secondary instability are identified for each pattern of the base flow. The modes exhibit either odd reflection-translation symmetry or even RT symmetry. The critical Reynolds number and the corresponding dominant spanwise wavelength are presented for a wide range of the gap. Temporal and spatial characteristics of the dominant Floquet modes are described in detail. A hysteresis is also noticed in a certain range of the gap. Neutral stability curves are presented for some selected values of the gap. The averaged in-plane vorticity of the dominant normalized Floquet mode is depicted as a function of the downstream distance from the rear face of the cylinder that sheds vortices. Comparison of stability characteristics is also made between the current flow and the flow past two circular cylinders in tandem arrangements.