Cylinder Aspect Ratio Research Articles

Effect of the aspect ratio of an elliptical cylinder on mixed convective heat transfer within a lid-driven enclosure

This study examines the mixed convection inside a lid-driven enclosure containing an elliptical cylinder for Reynolds numbers of 100 ≤ Re ≤ 1000. A simulation conducted to study the effects of the aspect ratio, and the temperature boundary conditions (θcyi = θC or θC/2) of the elliptical cylinder. The heat transfer performances rely on the relative magnitude of the viscous force caused by the top lid movement. Another factor is the buoyancy force that occurred due to the temperature difference between the heated bottom and the cold top walls, which also strongly depends on the aspect ratio. At Re = 500, the value of increased by about 7.6 % at θCyl = 0 and AR = 4.00 compared to the results of a circular cylinder (AR = 1.00). Moreover, a correlation has been proposed to estimate the Nusselt nuber for the geiven Reynolds number, aspect ratio, and the temperature of cylinder based on the power law model with a high goodness of curve fitting.

타원형 실린더의 종횡비와 크기 변화가 밀폐계 내부 혼합 대류 현상에 미치는 영향

본 연구에서는 2차원 lid-driven 밀폐계 내부 혼합 대류 현상에 타원형 실린더의 크기와 종횡비가 미치는 영향에 관하여 수치 해석을 진행하였다. 타원형 실린더의 종횡비는 0.5≤AR≤2.0 범위를 고려하였고, 실린더의 반지름은 0.1L≤R≤0.3L 범위를 고려하였다. Reynolds 수 및 Prandtl 수는 100 및 0.7로 각각 고정하였고, Grashof 수는 10²≤Gr≤10<SUP>5</SUP>를 고려하였다. Gr=10<SUP>5</SUP> 그리고 R=0.1L인 경우, 온도장 및 유동장은 비정상 상태에 도달하였다. 10²≤Gr≤10³ 범위에서, 시간 및 공간 평균된 밀폐계 바닥면 Nusselt 수, 〈Nu<SUB>B</SUB>〉는 실린더의 크기가 증가할수록 증가하였다. 그러나, 10<SUP>4</SUP>≤Gr≤10<SUP>5</SUP> 범위에서, 타원형 실린더의 크기 변화는 〈Nu<SUB>B</SUB>〉크기에 다양한 영향을 미쳤다. 또한, 실린더의 크기가 일정할 경우, 종횡비와 Grashof 수가 증가함에 따라 〈Nu<SUB>B</SUB>〉또한 증가하였다.

사각 실린더의 종횡비가 Lid-driven 사각 밀폐계의 혼합 대류에 미치는 영향

사각 실린더의 종횡비가 Lid-driven 사각 밀폐계의 혼합 대류에 미치는 영향

Aerodynamic study of paratrooper models used in the C-130 H/J personnel drop operations

The goal of this study is to conduct time-accurate simulations of several paratrooper models exiting the troop door of a C-130 aircraft. Solutions are calculated using HPCMP CREATETM-AV Kestrel software. Both the second-order, unstructured finite-volume solver of kCFD and the structured Cartesian finite-volume solver of SAMAir, which enables near-body/off-body calculations and adaptive mesh refinement, are used. The objective of this article is to analyze the effects of the unsteady flow field around the troop door on representative paratrooper models positioned at different locations and orientations. The predictions are presented in terms of integrated forces and moments in both time and frequency domains. Averaged forces/moments are reported as well. Vortex shedding frequency with maximum power density is predicted from forces or air velocity measurements at tap points behind the models. The contribution of primitive geometric elements (body, arms, and orientation) to the wake shedding frequencies are detailed, and results include validation of predicted vortex shedding frequency for a cylinder of aspect ratio 10 (length to diameter) with available experimental data. The results show that the adaptive mesh refinement aids in better resolution of the wake structure and better agreement with measured data. When the models exit the C-130 troop doors, the wind deflectors significantly reduce the rearward aerodynamic forces acting on the models compared with free-stream/stand-alone models. However, the flow field around the door changes the roll and yaw moments and the side force (force in the spanwise direction) as compared with the stand-alone model. Finally, the results document the aerodynamic forces and moments as functions of the static position and orientation of the paratrooper model relative to the door. The approach used and results of these simulations form a baseline for future simulation enhancements, which include calculating the exit path of the paratrooper relative to the aircraft, static line jumps, and subsequent parachute deployment.

Solid-flow interactions of a large aspect ratio cylinder in a shallow open channel

The complex wake created by an emergent slender circular cylinder in a shallow open channel flow is studied at a cylinder Reynolds number (ReD) of 3300 and at a subcritical Froude number of 0.58 in water. Methodical laser Doppler velocimetry measurements were taken in a very fine grid in three horizontal planes at near bed, mid-depth, and near free surface both upstream and downstream of the cylinder. Demarcation of the entire wake region starting from the first point of the onset of disturbed flow upstream to undisturbed flow downstream entirely based on experiments successfully added a novel contribution to the research on wake flow over a single cylinder. The proximity of the bed and free surface has a significant effect on the structure of the wake compared to the mid-depth. The size, shape, and development of the recirculation region behind the cylinder vary in the vertical direction from the bed to free surface. The longest wake closure point was found in the near bed region and the shortest was found near the free surface. The new model developed pertinent to longitudinal velocity deficit can be used as a velocity deficit scale for the entire far wake region for shallow wakes at mid-depth and near free surface. Urms/Us profiles along the transverse direction initially show a double peak behavior for all three levels, and well away from the cylinder downstream, the double peak becomes broader and less distinct, which is attributed to the effect of the separating shear layers. It is also noted that the wake characteristics of slender cylinders are significantly different from those of wakes generated by cylinders with small aspect ratios.

Transverse displacement phase analysis on 2DOF FIV of rigid cylinders in tandem configurations

With the development of offshore oil production and the resulting application of riser pipes, it is necessary to get a better understanding of the flow-induced vibrations (FIV) phenomenon involving multiple cylindrical structures. In this scenario, little is available concerning the phase difference between the transverse displacements of two long rigid cylinders (aspect ratio of 30) in tandem and free to vibrate due to the flow incidence. Aiming to fill this gap, experiments were carried out at the towing tank of the Technological Research Institute (IPT) for two similar rigid cylinders in a tandem arrangement. Both cylinders were elastically supported with two degrees of freedom (2DOF), the in-line and transverse direction. Different tandem distances were tested ( $$T=2D$$ , 2.5D, 3D, 4D and 6D) throughout the complete excitation of the lock-in range for the upstream cylinder, i.e., reduced velocities ranged from 3 to 13, which corresponds to Reynolds numbers from $$3\times 10^3$$ to $$3\times 10^4$$ . This work found the upstream cylinder having a qualitatively similar response compared with the isolated one. The downstream cylinder exhibited large response amplitudes compared with the upstream one in the lower branch of vibration. Lastly, the phase difference between transverse displacements of both cylinders was classified according to five phase patterns, two of them apparently related to the initial and upper branches of vibration in all cases.

Effects of geometry on resistance in elliptical pipe flows

This paper considers the significant role of cross-sectional geometry on resistance in co-axial pipe flows. We consider an axially flowing viscous fluid in between two long and thin elliptical coaxial cylinders, one inside the other. The outer cylinder is stationary, while the inner cylinder (rod) is free to move. The rod poses a resistance to the axial flow, while the viscous fluid poses a resistance to any motion of the rod. We show that the equations for flow in the axial direction – driven by a prescribed flux – and for flow within the cross-section of the domain – driven by the motion of the rod – decouple in the asymptotic limit of small cylinder aspect ratio into axial Poiseuille flow and transverse Stokes flow, respectively. The objective of this paper is to calculate numerically the axial and cross-sectional resistances and to determine their dependence on cross-sectional geometry – i.e. rod position and the ellipticities of the rod and bounding cylinder. We characterise axial resistance, first for three reduced parameter spaces that have not been fully analysed in the literature: (i) a circle in an ellipse, (ii) an ellipse in a circle and (iii) an ellipse in an ellipse of equal eccentricity and orientation, before extending our geometric parameter space to determine the overall optimal geometry to minimise axial flow resistance for fixed cross-sectional area. Cross-sectional resistance is characterised via coefficients in a Stokes resistance matrix and we highlight the interdependent effects of cross-sectional ellipticity and boundary interactions.

Characteristics of the free-end mean pressure distribution for a surface-mounted finite-height cylinder

Wind tunnel experiments at a Reynolds number of Re = 6.5 × 104 were used to study the effect of aspect ratio and boundary layer thickness on the mean static pressure distribution on the free end of a surface-mounted finite-height cylinder. The cylinder's aspect ratio was changed in small increments from AR = 0.5 to AR = 11. Two different boundary layer thicknesses (relative to the cylinder diameter) were employed, δ/D = 0.6 and δ/D = 1.9. From analysis of the mean pressure contour plots, it was found that the sizes and locations of regions of lower pressure, adverse pressure gradient, and higher pressure, and the appearance of “eye-like spots”, are sensitive to both AR and δ/D. The adverse pressure gradient occurs just ahead of the mean reattachment line while the eye-like spots are related to termination points of the legs of the arch vortex within the free-end mean recirculation zone. The total normal force coefficient experienced by the cylinder is strongly influenced by the contribution of the wall shear stress on the sides of the cylinder, with a change in direction of the net vertical shear stress contribution occurring between AR = 7 and AR = 8 for both boundary layers.

Numerical simulation of hydrothermal features of Cu–H2O nanofluid natural convection within a porous annulus considering diverse configurations of heater

The purpose of the current study is to numerically investigate the effects of shape factors of nanoparticles on natural convection in a fluid-saturated porous annulus developed between the elliptical cylinder and square enclosure. A numerical method called the control volume-based finite element method is implemented for solving the governing equations. The modified flow and thermal structures and corresponding heat transfer features are investigated. Numerical outcomes reveal very good grid independency and excellent agreement with the existing studies. The obtained results convey that at a certain aspect ratio, an increment in Rayleigh and Darcy numbers significantly augments the heat transfer and average Nusselt number. Further, enhancement of Rayleigh number increases the velocity of nanofluid, while that of aspect ratio of the elliptical cylinder shows the opposite trend.

Impact of a rotating cone on forced convection of Ag–MgO/water hybrid nanofluid in a 3D multiple vented T-shaped cavity considering magnetic field effects

Forced convection of hybrid Ag–MgO/water nanofluid in a three-dimensional T-shaped vented cavity with multiple ports under the effects of a inner rotating cone and magnetic field is numerically studied with finite volume method. The simulation is performed for various values of parameters such as: Reynolds number (between 100 and 1000), Hartmann number (between 0 and 60), angular velocity of the rotating cone (between − 200 rad/s and 0), aspect ratio of the circular cylinders of the base of the cone (between 0.5 and 2) and nanoparticle solid volume fraction of the hybrid nanofluid ( $$\phi _1$$ between 0 and 0.01, $$\phi _2$$ between 0 and 0.01). It was observed that the average heat transfer rate rises with higher values of Reynolds number, Hartmann number above a specified value, angular rotational speed of the cone, aspect ratio of the cone for values above 1 and solid nanoparticle volume fractions of the hybrid particles. In total, 61% of average heat transfer enhancement for left horizontal upper surface is achieved with the imposed magnetic field. The enhancement in the average Nusselt numbers is 25.6% for the rotating cone at the highest angular velocity as compared to a motionless one. The average heat transfer increases almost linearly with hybrid solid nanoparticle volume fraction, while 8.96% and 15.52% enhancements are obtained for varying the solid volume fraction of the particles with the lower and higher thermal conductivity up to 0.01.

Polarimetric Bistatic Scattering From Multiple Parallel Cylinders

Electromagnetic scattering from multiple cylinders has been an active research topic in many research fields, and the special case of parallel cylinders has seen important applications ranging from remote sensing to biology. Yet, the rigorous treatments have been mostly focusing on the 2-D cases, whereas for the more realistic 3-D cases, approximated approaches are often adopted. This article proposes a more rigorous treatment of 3-D polarimetric multiple scattering from parallel cylinders by extending our previously proposed virtual partition method (VPM) for single-cylinder case to multiple-cylinder case. The appeal of the method includes the appreciably reduced longitudinal dimension for subcylinders and its corresponding encircling sphere so as to effectively address the otherwise thorny issue of violation of the required mutual exclusion of encircling spheres of large aspect ratio cylinders by the conventional multiple scatterers' equation. The proposed VPM demonstrates the capability of capturing very well the polarimetric bistatic scattering amplitudes and phases, and of meeting physical requirements of energy conservation and reciprocity theorem. A systematic examination of the coupling effect is carried out against a number of factors, including average interneighbor-cylinder (intercylinder for short) distance, cylinder size, dielectric constant, and cylinder number, with the numerical results clearly revealing the complicated pattern of coupling effect. The studied cases suggest that the coupling effect may still be felt for a large average intercylinder distance. For electromagnetic wave propagation in the parallel cylinders, the coupling effect is visible, yet it tends to be mitigated by the average process.

Turbulent Convection of Liquid Sodium in an Inclined Cylinder of Unit Aspect Ratio

Turbulent convection of liquid sodium (Prandtl number Pr = 0.0093) in a cylinder of unit aspect ratio, heated at one end face and cooled at the other, is studied numerically. The flow regimes with inclination angles β = 0°, 20°, 40°, 70° with respect to the vertical are considered. The Rayleigh number is 1.5 × 107 . Three-dimensional nonstationary simulations allow one to get instant and average characteristics of the process and to study temperature pulsation fields. A mathematical model is based on the Boussinesq equations for thermogravitational convection with use of the LES (large-eddy simulations) approach for small-scale turbulence modeling. Simulations were carried out with a nonuniform numerical grid consisting of 2.9 × 106 nodes. It is shown that the flow structure strongly depends on β. The large-scale circulation (LSC) exists in the cylinder at any β. Under moderate inclination (β = 20°), the strong oscillations of the LSC orientation angle with dominant frequency are observed. Increasing the inclination up to 40° leads to stabilization of the large-scale flow and there is no dominant frequency of oscillations in this case. It is shown that more intensive temperature pulsations occur at small cylinder inclinations. At any β the regions with intensive pulsations are concentrated in the areas along low and upper cylinder faces. The maximum values of pulsations occur in the area close to lateral walls, where hot and cold fluid flows collide. The intensity of temperature pulsations decreases with increasing distance from the lateral walls. The Reynolds number which characterizes the total energy of the flow reaches its maximum value at β = 20° and then decreases with increasing β. The mean flow has maximum intensity at β = 40°. Turbulent velocity pulsation energy decreases monotonically with increasing inclination angle. It is shown that the inclination leads to an increase in heat transfer along the cylinder axis. The Nusselt number at β = 40° is 26% higher than that in the vertical cylinder.

Effects of Reynolds number on vortex structure behind a surface-mounted finite square cylinder with AR = 7

This paper presents the numerical solutions of flow around a surface-mounted square cylinder of aspect ratio h/d = 7 at Reynolds numbers of 652 and 13 041. The aim is to investigate the effect of the Reynolds number, between its medium-to-high range, on the flow and vortex structure around such a cylinder. The present simulations have successfully reproduced the primary flow, as well as the three-dimensional large-scale vortex structure in the wake of the finite wall-mounted body. The observation of base vortices, tip vortices, and a horseshoe vortex is consistent with previous experimental studies. A dipole wake is captured at the higher Reynolds number, while a quadrupole wake is captured for the lower, indicating that the Reynolds number strongly influences the wake structure. In the near-wake region, by plotting the isosurface of instantaneous second invariant of the velocity gradient, the full-loop structure is observed for the quadrupole wake, while the half-loop structure is observed for the dipole wake. In the far-wake region, a braided vortex structure formed by asymmetric hairpin vortices is observed at both Reynolds numbers and a new wake topology is proposed for flows with a similar geometry.

Analysis of Fluid Flow Patterns in Cylindrical Vessels of Anaerobic Digester using CFD

The production of energy from biomass and waste material is gaining popularity worldwide due to the expected depletion of fossil fuels shortly. A device commonly used is anaerobic digester in which microorganisms react with biomass/biodegradable in a vessel and produce useful gases. The performance of anaerobic digester depends on mixing or proper contact of bio-matter with microorganisms. In this paper, flow behaviour is studied in cylindrical vessels of anaerobic digester using Computational Fluid Dynamics (CFD) technique. The study includes simulations for various cylinder dimensions obtained by changing the height and diameter of the digester vessel and locations for fluid inlet and outlet. The results are shown in qualitative terms using velocity profiles and quantitatively in terms of volume of the stagnant zone. The comparison of several geometries at a constant velocity indicates the considerable effect of cylinder aspect ratio (height to diameter ratio) and positions and numbers of inlet and outlet ports on mixing performance. The influence of inlet velocity/Reynolds number is also examined for a few cases. The digester which has an inlet and two outlets on the curved surface (both on the same side) is found to be most suitable. The volume of the dead zone in this configuration at various Reynolds number is less than 30% (based on 0.5% criterion).

Kinetics of freezing and melting of encapsulated phase change materials with effective convection: Experiments and simulations

Storage of thermal energy in packed beds of encapsulated phase change materials (PCM) is becoming nowadays a common strategy, but its modeling requires deep knowledge of the physical processes of heat transfer. In this work, the kinetics of melting and freezing of a well-studied PCM (water) has been modeled by simulations with an effective description for convection, and tested against experiments. In our setup, a single spherical nodule is placed in a bath, and cooled or heated by natural or forced convection. In the model, one parameter per fluid phase needs to be fixed, which we determine by fitting with experiments (for the bath fluid and PCM in the fluid phase). These two parameters are then used to study the kinetics of melting at different temperatures, kinetics of freezing, and the freezing and melting in cylinders of different aspect ratios. In all cases, the results from the model are compared with experiments. The agreement is quantitative, with accurate predictions of melting and freezing times (discrepancies below 15%), and temperature plateaux, both in natural and forced convection. Notably, this simulation method does not use the fluid velocity, speeding up the modeling and making it suitable for large scale simulations of thermal storage tanks.

Mixed convection of non-Newtonian nanofluid in an H-shaped cavity with cooler and heater cylinders filled by a porous material: Two phase approach

In the present problem, two-phase mixed convection of a non-Newtonian nanofluid in a porous H-shaped cavity is studied. Inside the enclosure there are four rotating cylinders, using the Boussinesq approximation, mixed convection is created. Nanofluid includes H2O + 0.5% CMC and copper oxide nanoparticles. The mixture model was used to model physical phenomena. Different aspect ratios were used in order to achieve the best heat transfer rate. The Darcy and Richardson numbers ranges are 10−4 ≤ Da ≤ 10−2 and 1 ≤ Ri ≤ 100 respectively. Also, the aspect ratio and dimensionless angular velocities of cylinders ranges are 1.4 ≤ AR ≤ 1.6 and −10 ≤ Ω ≤ 10 respectively. Streamlines and isotherm-lines contours have been obtained for the variation of Darcy and Richardson numbers, aspect ratio and angular velocity. The heat transfer rates have been obtained for various aspect ratios, Darcy and Richardson numbers, and the direction of the cylinder's rotation, and are compared with each other. The results show that the direction of cylinders rotation influences the strength and extent of the generation vortices. Also, the use of porous material in high permeability can be a good alternative to lowering the angular velocity of the cylinders and ultimately reducing the need for less energy.

RETRACTED: Natural convection heat transfer in a circle-square annulus using lattice Boltzmann method- Treatment of curved boundary conditions

Abstract The lattice Boltzmann simulation of natural convection fluid flow and heat transfer in a square-circle annulus is carried out. A comprehensive discussion regarding the treatment of curved boundary condition for temperature and velocity is presented. The annulus is filled with CuO-water nanofluid. The dynamic viscosity of nanofluid is estimated using KKL (Koo-Kleinstreuer-Li) model. The second law analysis is added to the investigation order to show the effect of different parameters on the irreversibility. The Rayleigh number (103≤Ra≤106), nanoparticle concentration (0 ≤ ϕ ≤ 0.04) and the aspect ratio of inner square cylinder are the governing parameters. The influence of the mentioned parameters on the nanofluid flow, temperature field, local and total entropy generation, Bejan number and average Nusselt number are presented, comprehensively.

Interaction energies for hollow and solid cylinders: Role of aspect ratio and particle orientation

Particles of various shapes (e.g., spheres or rods), sizes (e.g., nm to microns), interiors (e.g., solid or hollow), and materials are used in many industrial and environmental applications. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory has commonly been used to calculate and predict the adhesive interaction of these particles with solid surfaces. However, DLVO theory treats these particles as equivalent spheres for simplicity ignoring shape in numerous cases. The surface element integration (SEI) approach allows DLVO theory to be extended for different particle shapes, orientations, and interiors. In this study, the SEI approach was applied to calculate the interaction energy between hollow and solid cylinders with a flat surface. The effect of aspect ratio and particle orientation on interaction energies was investigated under different solution chemistry conditions. Our study is relevant for an extensive range of particle aspect ratios, ranging from nanosized particles such as carbon nanotubes or nanowires, which have high aspect ratios, to micron-sized particles such as bacteria, which have low aspect ratios. The energy barrier tended to increase when the angle that the larger axis of the particle made with the normal to the surface changed from perpendicular (0 rad) to parallel (π/2 rad). The aspect ratio did not affect the trend of the energy barrier for solid (200–1000) and hollow (25–1000) cylinders of relatively high aspect ratios, but it produces a non-monotonic trend for solid (2–100) and hollow (2–10) cylinders of low aspect ratios. The present study advances our understanding of adhesive interactions for particles having a wide range of aspect ratios and interior properties at various orientations with the surface.

Effect of the presence of end plates and aspect ratio on the aerodynamic forces on circular cylinders in various flow regimes

The presence of end plates and the cylinder aspect ratio significantly influence the aerodynamics of circular cylinders in wind tunnel tests. Although a large aspect ratio sufficiently reduces the effects of the free end and boundary layer of the tunnel walls, smaller end plates and aspect ratios are required for practical reasons. Thus, the aerodynamic characteristics of circular cylinders with small end plates and aspect ratios in the critical Reynolds number range are worthy of investigating because of the complexity of the flow states in these regimes. The wind pressure distribution on the circular cylinder is tested in the wind tunnel for Reynolds numbers of Re = (0.47–5.07) × 105, which cover the entire range of the critical Reynolds numbers. The effects of the existence of end plates and the aspect ratio of the cylinder on the aerodynamic forces and wind pressures at the middle of the cylinder and along the length of the cylinder in different flow regimes are discussed. The phenomenon of flow reattachment in the critical Reynolds number range is focused on to reveal the effects of the cylinder aspect ratio on the aerodynamics related to dry galloping for a cylinder with small end plates. The results show the necessity of including end plates and assuming a large aspect ratio to simulate a two-dimensional flow in the subcritical range. In the critical range, the less significant influences of the end plate diameter and cylinder aspect ratio strongly depend on the development of attachment along the length of the cylinder, which creates a three-dimensional flow and mitigates the spanwise correlation.

The effect of aspect ratio on the wake structure of finite wall-mounted square cylinders

This paper presents a combined experimental and large-eddy simulation study to characterise the effect of aspect ratio on the near-wake structure of a square finite wall-mounted cylinder (FWMC). The cylinder aspect ratios (span $L$ to width $W$) investigated in the experiments were $1.4\leqslant L/W\leqslant 21.4$ and the oncoming boundary-layer thicknesses were $1.3W$ and $0.9W$ at a Reynolds number based on cylinder width of $1.4\times 10^{4}$ and $1.1\times 10^{4}$, respectively. In complementary simulations, the cylinder aspect ratios investigated were 1.4, 4.3, 10 and 18.6. The cylinder wake structure was visualised in three-dimensional space using a vortex core detection method and decomposed to its oscillation modes using the spectral proper orthogonal decomposition (SPOD) technique. A parametric diagram is proposed to predict whether the time-averaged wake structure is a dipole or a quadrupole pattern, based on oncoming boundary-layer height and aspect ratio. Cellular shedding occurs when the aspect ratio is high with up to three shedding cells occurring across the span for aspect ratios $L/W&gt;18$. Each of these cells sheds at a distinct frequency, as evidenced by the spectral content of the surface pressure measured on the side face and the near-wake velocity. Amplitude modulation is also observed in the vortex shedding, which explains the amplitude modulation of the acoustic pressure emitted by square FWMCs. SPOD is shown to be a viable method to identify the occurrence of cellular shedding in the wake.