Cylinder Aspect Ratio Research Articles

Estimation of heat transfer performance on mixed convection in an enclosure with an inner cylinder using an artificial neural network

The effect of variations in the aspect ratio (AR) of an inner cylinder on 3D mixed convection is examined by numerical means. Three Reynolds numbers are considered, Re = 100, 500, and 1000, with a fixed Grashof number of 105 and a Prandtl number of 0.7. The flow patterns formed inside the enclosure are categorized as spanwise, streamwise, and clockwise for the given Reynolds numbers. The rate of heat transfer was enhanced by 16.8% and 20.6% at Re = 100 and 500 for AR = 4, respectively, compared with the rate of heat of transfer at AR = 1 and AR = 0.5, respectively. Similarly, the rate of heat transfer was increased by 14.0% at Re = 1000 for AR = 1 compared with the rate of heat transfer at AR = 0.25. These results demonstrate that the heat transfer characteristics of mixed convection can be predicted readily using an ANN based on the data set created from a few cases of direct numerical simulation (DNS); the same results would otherwise require days to calculate using DNS. Furthermore, the values predicted by the ANN are in good agreement with the data obtained by DNS. The heat transfer is affected by Reynolds number, aspect ratio, radius of the cylinder, and dimensionality of the simulation, in that order.

Double Diffusive Convection of Power Law Fluids Through Taylor–Couette Flow

The proposed study focuses on the effect of double diffusive convection on the flow structures of power-law fluids between two coaxial cylinders of finite aspect ratios with end caps. We adopted a D2Q9 axisymmetric lattice Boltzmann model to predict the power-law fluid flow and D2Q4 (Lattice Boltzman method) model for azimuthal velocity, concentration, and the temperature. For ( is the solutal and thermal buoyancy forces ratio), fixed Taylor number with increasing Rayleigh number , the thermal base flow regime is more pronounced for the pseudoplastic fluid compared to the dilatant fluid. In addition, the thermal base flow in the annulus is delayed when increases. Thus, the increase of the behavior index has a stabilizing effect on the flow structure. On the other hand, when , the solute buoyancy forces in the annulus prevail over the centrifugal and thermal buoyancy forces, and the flow becomes ultimately a solute base flow for . For , the flow becomes a thermal base flow for . Thus, the mass transfer with negative values has a greater stabilizing effect on the flow structures than with positive .

Experimentally investigated vortex-induced vibration of a high aspect ratio and small mass ratio circular cylinder oscillating in low reduced velocity flows

Experimental investigations were performed for a circular cantilevered cylinder located in a circulating water channel. The cylinder of mass ratio 1.78 and aspect ratio of 17.8 was vertically fixed at one end, and its other end was free to oscillate in the stream-wise and lateral directions. The reduced velocities of the flow past the cylinder, ranging from 0.65 to 5.18, corresponded to Reynolds numbers between 6.38 ∙ 10 3 and 5.1 ∙ 10 4 . At the inlet of the water channel, a Laser doppler velocimetry system measured stream-wise and span-wise flow velocity profiles and calculated the corresponding turbulence intensities. For differing reduced velocities, cylinder accelerations, displacements, and forces acting on the cylinder in the stream-wise and lateral directions were measured. Vortex shedding frequencies were compared with cylinder response frequencies in the stream-wise and lateral directions. Strouhal numbers were obtained at different water channel depths and widths. Coefficients of forces acting in the cylinder and trajectories of the cylinder's motions and accelerations were presented and analyzed. The main objectives were to investigate the effect of relevant parameters for a cylinder oscillating in 2-DOF, such as measuring the universal wake Strouhal number. The universal Strouhal number calculations were based on the vortex shedding frequency and then compared with published data. Directly measured shedding vortex frequencies at different locations around the cylinder and at different reduced velocities were compared with the frequencies of a stationary cylinder to obtain further insight of shedding vortex frequencies. Directly measured were cylinder accelerations, cylinder oscillations, and inertial forces acting on the cylinder and their trajectories. The maximum oscillation recorded in the lateral direction of the cylinder recorded was 0.8D, where D is the cylinder diameter, at which the lock-in phenomenon was observed, and figure eight–shaped (Lissajous) trajectory patterns were captured. The universal wake Strouhal number for different reduced velocities decreased almost linearly from 0.201 at U r = 0.6 to 0.141 at U r = 5.18. At different depths and widths, the Strouhal number was almost constant for each reduced velocity considered. Another objective was to directly measure the forces coefficients. The stream-wise force coefficients were close to the coefficients for the stationary cylinder, and, at higher Reynolds numbers, they increased. Furthermore, the uncertainty methods used here were explained in details and low uncertainties of measured values substantiated the reliability of our results. The experiments provided reliable benchmark data suitable for numerical validation of vortex-induced vibration for a circular cylinder oscillating in two degrees-of-freedom. 1 Experimental investigation of Vortex induced vibration in water circulation channel. 2 Velocity profiles and Turbulence intensities of the circulation water channel using Laser Doppler velocimetry. 3 Direct measurements of forces and dynamic response in lateral and transverse directions at different reduced velocities. 4 Shedding vortex Strouhal number measurements for oscillating cylinder in two degree of freedom. 5 Fluid-structure interaction measurements uncertainty analysis.

Numerical study of vortex-induced vibration of a flexible cylinder with large aspect ratios in oscillatory flows

Numerical investigations on vortex-induced vibration (VIV) of a flexible cylinder with different length to diameter ratios, named the aspect ratio (L/D), ranging from 167 to 667 in an oscillatory flow have been conducted by the viv-FOAM-SJTU solver. The solver is developed based on the open source toolbox OpenFOAM and the two-dimensional (2D) strip theory. Hydrodynamic forces are calculated through the PIMPLE algorithm in each 2D fluid strip. The Finite Element Method (FEM) combined with the Newmark-β method are used to calculate vibrations of the flexible cylinder in both crossflow and inline directions. The cylinder model is pinned at both ends with top and bottom support frames forced to harmonically oscillate in the inline direction, which contributes to the generation of a relative oscillatory flow between the still water and the flexible cylinder. Mesh and fluid strip convergence studies are conducted to choose the appropriate computational model for subsequent simulations. Simulation results and experiment results are in good agreement that verifies the validity of the solver. Comparisons among simulations with different aspect ratios are analyzed detailedly. The maximum vibration amplitude, the dominant vibration mode and the non-dimensional dominant vibration frequency in both directions are enlarged apparently with the increase of the cylinder aspect ratio. The typical vibration trajectory changes from the butterfly type to the stripe type with L/D increases.

Heat transfer characteristics of nanofluids from a sinusoidal corrugated cylinder placed in a square cavity

A numerical study is performed to investigate nanofluids' flow field and heat transfer characteristics between the domain bounded by a square and a wavy cylinder. The left and right walls of the cavity are at constant low temperature while its other adjacent walls are insulated. The convective phenomena take place due to the higher temperature of the inner corrugated surface. Super elliptic functions are used to transform the governing equations of the classical rectangular enclosure into a system of equations valid for concentric cylinders. The resulting equations are solved iteratively with the implicit finite difference method. Parametric results are presented in terms of streamlines, isotherms, local and average Nusselt numbers for a wide range of scaled parameters such as nanoparticles concentration, Rayleigh number, and aspect ratio. Several correlations have been deduced at the inner and outer surface of the cylinders for the average Nusselt number, which gives a good agreement when compared against the numerical results. The strength of the streamlines increases significantly due to an increase in the aspect ratio of the inner cylinder and the Rayleigh number. As the concentration of nanoparticles increases, the average Nusselt number at the internal and external cylinders becomes stronger. In addition, the average Nusselt number for the entire Rayleigh number range gets enhanced when plotted against the volume fraction of the nanofluid.

Effect of inlet shear on turbulent flow past a wall-mounted finite-size square cylinder

The present study analyzes the effect of incoming shear flow on the wake behind a wall-mounted finite-length square cylinder of aspect ratio (AR=7) using large eddy simulation (LES). All the simulations were performed at a Reynolds number (Re) of 10000, based on cylinder width (d) and average inlet velocity (uavg), with different shear parameters, K=0.0,0.025,0.050 and 0.075. The shear-improved Smagorinsky model Lévêque et al. (2007) is used to account for the subgrid scale stress. It is found that the incoming flow shear has a profound effect on the modes of shedding. With increasing shear parameter anti-symmetric modes of shedding tends to dominate along the cylinder height, except close to the free-end where both symmetric and anti-symmetric modes of shedding co-exist. The incoming shear flow has also significant influence on the downwash flow from the free-end, the strength of which decreases with an increase in shear parameter. The resolved turbulent kinetic energy (TKE) associated with the cylinder wake is found to have maximum contribution from the transverse component of normal stress compared to the other components of normal stresses.

Application of a New Hybrid Immersed Boundary Method for the Simulation of Viscous Compressible Flows

In this paper, we present a hybrid immersed boundary method (HIBM) for the simulation of two- and three-dimensional compressible viscous flows around stationary and moving obstacles. The proposed approach combines the compressible boundary condition-enforced IBM proposed by Qiu et al. and the Brinkman penalization methods. The boundary condition-enforced IBM uses a fractional step approach alongside a Dirac delta function to satisfy the boundary conditions on the body surface. Although this method works properly in subsonic regimes, it cannot correctly simulate the wake region for a supersonic flow. On the other hand, the penalization method considers the body as a porous media with a low permeability that forces the velocity and energy inside the body to converge to the body velocity and energy. However, unlike the boundary condition-enforced IBM, the streamlines penetrate the body. In the present approach, the positive features of the above-mentioned methods were included and their drawbacks were excluded. The proposed approach was applied using the finite volume method and an E-CUSP scheme. The performance of the proposed method was numerically evaluated in simulating compressible fluid flow around both stationary and moving boundaries, showing a close agreement with other numerical and experimental data available in the literature. Further, the effects of geometry, Reynolds, and Mach numbers were investigated on the supersonic flow field around elliptical cylinders of various aspect ratios. The results revealed that increasing the aspect ratio led to an increase in the shock standoff distance, recirculation zone, and drag coefficient.

Experimental study on the pressure distribution of a low aspect-ratio circular cylinder and interference effects of a square cylinder in downburst flows

Wind loads on a surface-mounted finite-height circular cylinder are strongly influenced by the flow field structure. Based on the stationary and moving downburst flows simulated by an impinging jet device, this paper studies the wind pressure distribution in a circular cylinder with a low aspect ratio of AR = 2 at different radial positions and discusses the interference effects of other cylinders. The results indicate that the Reynolds number has a significant effect on the mean wind pressure coefficient on the crosswind side. The maximum absolute value of the mean coefficients on the leeward and crosswind sides appears at r/Djet = 1.0. In general, the interference effect of the square cylinder closely spaced on the fluctuating coefficients of each side is the largest when it is in the upstream position in tandem. The pressure distribution on the crosswind side is more easily affected by the interference cylinder. Under the action of a moving downburst, the instantaneous pressure coefficients on all sides of the cylinder have both positive and negative peaks, and the maximum overshooting variation is close to three. The overshooting effect lasts in a shorter time and has a greater impact on the structure when the downburst moving speed is faster.

Flow structure and loads over inclined cylindrical rodlike particles and fibers

The flow past a fixed finite-length circular cylinder, the axis of which makes a nonzero angle with the incoming stream, is studied through fully-resolved simulations, from creeping-flow conditions to strongly inertial regimes. The investigation focuses on the way the body aspect ratio $\chi$ (defined as as the length-to-diameter ratio), the inclination angle $\theta$ with respect to the incoming flow and the Reynolds number $\text{Re}$ (based on the cylinder diameter) affect the flow structure past the body and therefore the hydrodynamic loads acting on it. The configuration $\theta=0^\circ$ (where the cylinder is aligned with the flow) is considered first, from creeping-flow conditions up to $\text{Re}=400$, with aspect ratios up to $20$ ($10$) for $\text{Re}\leq10$ ($\text{Re}\geq10$). In the low-to-moderate Reynolds number regime ($\text{Re}\lesssim5$), influence or the aspect ratio, inclination (from $0^{\circ}$ to $30^{\circ}$), and inertial effects is examined by comparing numerical results for the axial and transverse force components and the spanwise torque with theoretical predictions based on the slender-body approximation, possibly incorporating finite-Reynolds-number corrections. Semiempirical models based on these predictions and incorporating finite-length and inertial corrections extracted from the numerical data are derived. For large enough Reynolds numbers ($\text{Re}\gtrsim10^2$), separation takes place along the upstream part of the lateral surface of the cylinder, deeply influencing the surface stress distribution. Numerical results are used to build empirical models for the force components and the torque, valid for moderately inclined cylinders ($|\theta|\lesssim30^\circ$) of arbitrary aspect ratio up to $\text{Re}\approx300$ and matching those obtained at low-to-moderate Reynolds number.

Time-resolved wake dynamics of finite wall-mounted circular cylinders submerged in a turbulent boundary layer

Abstract

Influence of various DEM shape representation methods on packing and shearing of granular assemblies

Realistic yet efficient representation of particle shape is a major challenge for the Discrete Element Method. This paper uses angle-of-repose and direct-shear test simulations to describe the influence of several shape representation methods, and their parameters, on the bulk response of granular assemblies. Three rolling resistance models, with varying coefficient of rolling friction, are considered for spherical particles. For non-spherical particles, superquadrics with varying blockiness and multi-spheres with varying bumpiness are used to model cuboids and cylinders of several aspect ratios. We present extensive quantitative results showing how the various ways used to represent shape affect the bulk response, allowing comparisons between different approaches. Simulations of angle-of-repose tests show that all three rolling friction models can model the avalanching characteristics of cube/cuboid and cylindrical particles. Simulations of direct-shear tests suggest that both the shear strength and the dilative response of the considered non-spherical particles (but not their porosity) can only be predicted by the elasto-plastic rolling resistance model. The quantitative nature of the results allows identifying values of the shape-description parameters that can be used to obtain similar results when using alternative shape representation methods.Graphical abstract

Insights into low Reynolds flow past finite curved cylinders

Spectral proper orthogonal decomposition (SPOD) is applied to experimental digital visualizations to scrutinize the properties of the wake flow behind curved cylinders. This technique has been applied to the image data of Shang et al. [“Flow past finite cylinders of constant curvature,” J. Fluid Mech. 837, 896–915 (2018)] to emphasize the main features of the flow and to extract the most energetically and dynamically relevant space-time coherent structures. The study considers different Reynolds numbers and angles of curvature for two cylinder aspect ratios. It is first shown that the SPOD structures reproduce the base flow topology of the digital visualizations, confirming the presence of a single oblique shedding regime for a low aspect ratio and both normal and oblique regimes for a high aspect ratio. The appearance of the instabilities typical of straight cylinders, usually referred to as type-A and -B in literature, is identified as well. As principal results obtained for curved cylinders, increasing the Reynolds number the analysis of SPOD spectra has revealed two reductions in the oscillation frequency of the leading coherent structures, attributed to the occurrence of type-A instability when the wake is still laminar, and to the development of a one-sided vortex dislocation in turbulent regime. The study has also highlighted the gradual transfer of energy between flow structures during the transition from type-A to type-B instabilities, accompanied by high-frequency scales excitement. Some peculiar aspects of SPOD field reconstruction are outlined here, which are suggested by the physical characteristics of the flow.

Numerical studies of laminar flow over two tandem elliptical cylinders using Ramanujan approximation

This paper studies a laminar flow over tandem elliptical cylinders at Re = 200. While the aspect ratio (AR) of the upstream cylinder varies from AR = $$a_{1}$$ / $$b_{1}$$ = 0.25 to 2.00, the aspect ratio of the downstream cylinder is kept constant at AR = 1.00 (e.g., $$a_{2}$$ / $$b_{2}$$ = 1). This range of AR covers the most important practical cross sections of elliptical cylinders, including normal elliptic cylinder (with 90° incidence) and parallel elliptic cylinders (with 0° incidence). Although the spacing ratio between the centers of the cylinders is kept constant at $$L^{*}$$ = 4 $$D_{2}$$ , the gap ratio (G* = G/ $$D_{2}$$ ) between the surfaces of the cylinders is varied due to the alteration of AR of the upstream cylinder. Unlike the previously published studies, which estimated the hydraulic diameter of the elliptical cylinder, in this paper, the precise hydraulic diameter is evaluated and used to analyze the wake instabilities and the variation of the imposed pressure as well as forces coefficients on the cylinders. The results reveal that with the estimation of the hydraulic diameter of the elliptic cylinder, the maximum error of 178% has arisen, which significantly affects forces (lift and drag) coefficients. It was found that the phase lag between the sinusoidal lift coefficients of the cylinders varies and it reaches a minimum at AR = 1.5 and it slightly increases once AR = 2.0. Besides, the amplitude of the fluctuating drag coefficient is larger for the upstream cylinder as compared with that of the downstream cylinder. Besides, it is found that the minimum St = 0.065 occurs at AR = 0.25, and it is regularly increased to the maximum value of St = 0.211 at AR = 1.75. A parabolic equation is deduced with high accuracy and a reasonable error of less than 1.8% to show the relationship between St and AR.

Variation of bed microtopography with time around an isolated surface-mounted cylindrical roughness element and its influence on wind flow

Abstract Bed microtopography around roughness element in the arid environment is often altered by airflow and sediment transport. Understanding the interaction among the roughness element, microtopography and airflow is important to reveal the mechanism of wind erosion around an isolated surface-mounted roughness element. The wind tunnel experiments studied the influence of cylindrical roughness element with larger aspect ratios on bed microtopography evolution, and the influence of microtopography on the wind flow around the isolated cylinder was analyzed by numerical simulation method. A deep well or scour surrounds the cylinder and a symmetrical V-shaped deposition area appears in the lee. Both the erosion and deposition areas of the wider cylinder with aspect ratio of 2 are larger than that of the taller cylinder (aspect ratio of 8). With the increase of blowing time, both the erosion and deposition areas around the wider cylinder increase, while for the taller cylinder, the sand deposition area increases first and then decreases, and the erosion area shows an increasing trend. After the local surface microtopography is adjusted, the streamwise wind velocity in the lee of the cylinder at half of cylinder height is less than the initial wind velocity on the unadjusted flat bed, while the wind speed near the ground in the lee is accelerated obviously. Moreover, the vortex in the recirculation zone behind the cylinder moves upward, leading to the increase of surface shear stress in the lee and surface erosion enhancement.

On the origins of steady streaming in precessing fluids

Abstract

Pressure center determination for regularly shaped non-spherical particles at intermediate Reynolds number range

The present contribution aims to determine the dependence of the location of the non-dimensional center of pressure (λ) with orientation (β) for non-spherical particles of regular shape. Prolate and oblate ellipsoids as well as cylinders of various aspect ratios (AR) at several Reynolds numbers (Rep) have been considered. The required flow coefficients (drag, lift and pitching torque) were determined through DNS and validated with recently published results. The strategy for determining the center of pressure consisted in the evaluation of the pitching torque acting on the non-spherical particles resulting from the fluid-dynamic forces (i.e. drag and lift) and its comparison with the torque directly determined from DNS. The performed analysis did not only allow determining the shape of the curve λ(β), but also revealed that the location of the center of pressure depends additionally on particle aspect ratio and Reynolds number. It is found that for all the particles considered, the position of the center of pressure at a fixed incidence angle displaces upstream of the geometrical center with increasing Reynolds number; additionally, it exhibits a non-monotonic behavior in dependence of the aspect ratio.

Experimental and numerical investigation of turbulent wake flow around wall-mounted square cylinder of aspect ratio 2

The turbulent wake velocity fields around a short wall-mounted square cylinder of aspect ratio 2 fully immersing into a thick turbulent boundary layer are obtained experimentally with PIV and numerically with LES. Comparison between the two sets of data is made on the time-averaged mean velocities, turbulent fluctuating velocities, and the detailed characteristics of energetic large-scale coherent structures. It is noted that a good comparison of the low-order statistics of velocity fields does not guarantee a satisfactory reproduction of the coherent structures. These flow structures show great similarities in the spatial patterns and dynamic performance between PIV and LES, but apparent differences in turbulent kinetic energy contribution. On the horizontal plane parallel to the wall, the dominant antisymmetric spatial mode pair illustrates the evolution process of Karman vortex shedding with a clear spectral peak around the characteristic frequency while three extra frequency peaks are found in the symmetric spatial modes. Further investigation on the velocity field on the central vertical plane reveals the connection between the unsteadiness of shear layer flow at the free end and the spanwise symmetric vortex shedding activities. It is confirmed that the flapping motion and vortex convection mechanisms observed for the present cylinder could modulate the symmetric wake dynamics. The three-dimensional extreme wake patterns, available from LES results, are related to the most energetic Karman vortex shedding demonstrate upward flow in the vortex weakening side. Meanwhile, the downwash flow at the extreme time instants is found to become stronger always for this kind of short cylinder with a dipole wake pattern.

Flow over rectangular cylinder: Effects of cylinder aspect ratio and Reynolds number

Numerical simulations of two- and three-dimensional unconfined flows over rectangular cylinders are conducted at Reynolds number Re = 30 – 200. The cylinder cross-sectional aspect ratio AR (length-to-width) is varied as 0.25, 0.5, 1.0, 2.0, 3.0 and 4.0. The focus is given on how AR and Re influence the flow structure and associated aerodynamic parameters. The first critical Reynolds number (Recr1) associated with the transition from the steady flow to the two-dimensional unsteady flow is determined for each AR and found to linearly increase with increasing AR. The same observation is made for the second critical Reynolds number (Recr2) where the two-dimensional unsteady flow metamorphoses into the three-dimensional unsteady flow. A larger Re is required for a larger AR to have the onset of vortex shedding or the transition from two-dimensional flow to the three-dimensional. Three distinct scenarios of the flow separation and reattachment are identified in the ranges of Re and AR examined. The physical insight into the flow dependence on AR and Re is provided. Both AR and Re play a role in the formation of separation bubbles on the cylinder side surfaces. Two distinct mechanisms of the separation bubble formation are imparted. The detailed flow structures are linked to the mean and fluctuating forces and Strouhal numbers. The vortex strength beefs up with increasing Re but diminishes with AR, as do the normal and shear stresses and fluctuating forces.

Enhancement of natural convection heat transfer in concentric annular space using inclined elliptical cylinder

The governing equations of continuity, momentum and energy are numerically solved to study the laminar natural convection heat transfer of Newtonian fluid confined within two concentric cylinders. The inner cylinder is elliptical cross-section with different aspect ratio E = 0.1 to 0.5 and it is considered to be hot, whereas the outer cylinder is circular and it is supposed to be cold. The annular spacing between the cylinders is defined based on radii ratio (RR = 2.5). Also, the inner cylinder is inclined with an inclination angle (θ = 0 to 90). The main purpose of this study is to determine the effects of inclination angle (θ = 0° to 90°), aspect ratio of inner cylinder (E = 0.1 to 0.5), Prandtl number (Pr = 0.71 and 7.01) and Rayleigh number (Ra = 103 to 105) on fluid flow and heat transfer rate. The flow patterns and temperature distributions are potted in terms of streamlines and isotherms respectively. The obtained results showed that increase in inclination angle enhances the heat transfer rate of inner cylinder for all values of aspect ratio. Also, for the inclination angle (θ = 90°), the decrease in aspect ratio (E) improves the heat transfer rate of inner cylinder.

Aerodynamic noise of high-speed train pantographs: Comparisons between field measurements and an updated component-based prediction model

Aerodynamic noise from pantographs becomes an important source of noise from trains at high speeds. Previous studies have mostly been based on numerical predictions using computational aeroacoustic methods, which require large computing resources, or measurements conducted in a wind tunnel which cannot take all the real conditions into account. A component-based model relying on empirical constants obtained from the literature has been shown to predict aerodynamic noise from pantographs that agrees well with wind tunnel measurements. This model is extended in this paper by making use of simulation results on individual cylinders to refine the model constants and the Reynolds number dependence. In addition, allowance for the effect of incoming turbulence and cylinder aspect ratio is also extended. The updated model shows improved agreement with wind tunnel measurements, particularly at low frequencies. This model is then used to predict pantograph noise in more realistic conditions during train pass-by. The incoming flow conditions in terms of the incident flow speed, the turbulence intensity and the turbulence length scale are estimated from the literature considering the development of the boundary layer along the train roof. The sensitivity of the model to these assumptions is assessed using Monte Carlo simulations. The predicted results are compared with field measurements obtained using microphone array techniques for pantograph on different operational trains. Good agreement is obtained between the predictions and the measurements in terms of the far-field noise spectra and the dependence of noise level on speed. Differences are noted between measured levels for different orientations of the pantograph which according to the model are mainly related to the distance of the pantograph from the front of the train.