Cylinder Aspect Ratio Research Articles

Experimental investigation of distributed cross-plates effects on suppressing vortex-induced vibration of a flexible cylinder

Distributed plates have been proven effective in controlling the vortex shedding and reducing the drag force of a circular cylinder, but more studies on its effect on vortex-induced vibration (VIV) are needed. This paper presents a systematic study of the effect of distributed cross-plates on the suppression of the VIV of a flexible cylinder using wind tunnel experiments. The experiments are performed by changing the length (l), width (w), and number (N) of the cross-plates, the spacing (S) between the adjacent cross-plates, and the aspect ratio (λ) of the flexible cylinder. The flexible cylinder is allowed to vibrate in the transverse direction for a reduced velocity range of 3.0–9.5, corresponding to a Reynolds number of 10,000–49,000. The results show that the optimal configuration of the distributed cross-plates for suppressing VIV is (l/D, w/D, S/D) = (2.0, 0.5, 4), where D is the cylinder diameter. The maximum amplitude decreases monotonically with increasing length and width, respectively. The optimal spacing is independent of λ of the flexible cylinder. For the optimal configuration, the vibration can be fully suppressed in many cases, and the minimum reduction of the maximum amplitude is 14% depending on N and λ, demonstrating a potential application of the distributed cross-plates.

Lattice Boltzmann Computation of Steady Cross-Flow Across a Rectangular Obstacle with Different Aspect Ratio: Effect of Blockage Ratio

This work presents a two dimensional lattice Boltzmann analysis of steady and cross-flow of New tonian fluid across a built-in rectangular cylinder. In particular, the effects of the blockage ratio and aspect ratio of rectangular cylinder (width/height) on the momentum characteristics have been explored for range of flow governing parameters such as, blockage ratio (β = 1/8, 1/12, 1/16), aspect ratio of rectangular cylinder (1 ≤ a_r ≤ 6) at constant Reynolds number of Re = 40 corresponding to the laminar range. The physical insight of system is gained by evaluation of stream-function, vorticity and pressure coefficient variation, etc. Further, the engineering gross parameter, such as drag coefficient is determined for possible use in engineering design purpose. It is observed that the increase in blockage ratio drag coefficient values decreases and drag values show proportional variation with aspect ratio. Finally, a closure relationship is developed between drag coefficient, blockage ratio and aspect ratio of rectangular cylinder for possible use in engineering/scientific practices

Impact of Aspect Ratio on Drag and Flow Structure for Cylinders With Two Free Ends

Abstract When examining the literature for flow effects on circular cylinders one can find many studies on infinite cylinders and cantilevered cylinders but minimal data related to cylinders with two free ends (Shepard, T., Law, D., Dahl, J., Reichstadt, R., and Selvamani, A. S., 2022, “Impact of Aspect Ratio on Drag and Flow Structure for Cylinders With Two Free Ends,” ASME Paper No. V001T03A031.). The limited data available shows that the cylinder aspect ratio affects the drag and frequency content of flow within the wake however these studies were done at discreet Reynolds numbers. In order to better understand the combined impact of aspect ratio and Reynolds number a series of wind tunnel tests and numerical simulations has been conducted for cylinders with two free ends having aspect ratios of 2–15. Tests were carried out in the subcritical regime with Reynolds numbers ranging 13000–105,000. Tip vortex effects, which vary with aspect ratio, are shown to impact the cylinder surface pressure, drag coefficient, and wake Strouhal numbers though Reynolds number effects are minor for the conditions studied. The results are compared against existing historical data and show the trend of drag coefficient increasing with cylinder aspect ratio (Shepard, T., Law, D., Dahl, J., Reichstadt, R., and Selvamani, A. S., 2022, “Impact of Aspect Ratio on Drag and Flow Structure for Cylinders With Two Free Ends,” ASME Paper No. V001T03A031).

The Effect of Elliptical Form of Cylinder on Free Convection in Annular Space

We try through this research to present a detailed study on free convection heat transfer inside a closed space. The work is carried out in numerical way, using the method called finite volume. The governing equations are solved for numerical simulations of steady state and laminar regime. The studied domain is an annular space consisting mainly of two cylinders positioned horizontally, the inner cylinder has an elliptical shape with different aspect ratio (E) while the outer one has a single shape which is circular. The primary conditions adopted in this research are as follow: the outer cylinder has a cold surface, while the inner cylinder has a cold surface. The space between cylinders is considered to be filled with fluid of different values of thermo-physical proprieties (Pr). The fluid in the space moves under the influence of thermal buoyancy which is controlled by Ri number. The pertinent parameters for this research are: the aspect ratio of elliptical cylinder which is E = 0.1 to 1, the Prandtl number Pr = 1 to 100 and finally the Richardson number Ri = 103, 104 and 105. The results of this work show that the elliptical form allows an increase in thermal transfer activity. Also, values of the number Pr have a limited effect on heat transfer.

Exotic liquid crystalline phases in monolayers of vertically vibrated granular particles

ABSTRACTVibrated monolayers of granular particles confined into horizontal cavities form a variety of fluid patterns with orientational order that resemble equilibrium liquid-crystal phases. In some cases, one can identify nematic and smectic patterns that can be understood in terms of classical statistical mechanics of hard bodies. Low aspect ratio cylinders project as rectangles and form uniaxial, or 2-atic, and tetratic, or 4-atic, nematic phases. Other polygonal particles may exhibit different liquid-crystal phases, in general p-atic phases, of higher symmetries. We give a brief summary of theoretical work on rectangles and triangles, and provide some experimental results on vibrated monolayers. In the case of equilateral triangles, the theory predicts an exotic triatic phase, or 6-atic phase, with six-fold symmetry and three equivalent directors. The right-angled triangles exhibit a 4-atic phase with strong octatic (8-atic) correlations. Experiments on cylinders show 4-atic textures and, even more remarkable, geometric frustration caused by confinement excites topological defects, which seem to follow the same topological rules as standard liquid crystals. Some of our findings can be understood with the help of simulations of hard particles subject to thermal equilibrium, although standard Density-Functional Theories fail to account for the correct equilibrium phases in some cases.

Numerical investigation of mixed convection from rectangular cylinders subjected to upper cross flow

Unsteady combined natural and forced convection heat transfer from a rectangular cylinder subjected to upward cross flow is investigated numerically. The working fluids are air (Pr = 0.7) and water (Pr = 7), and the flow is laminar. The governing flow and energy equations are solved using a fractional step finite-difference scheme. The effects of the cylinder aspect ratio (a = L/D), Reynolds and Richardson numbers on the flow and heat transfer characteristics are investigated. Numerical simulations were carried out for the cylinders with aspect ratios of a = 0.5, 1, and 2, while the Richardson and Reynolds numbers varied between 0 to 10, and 100 to 200, respectively. The mean drag, mean rms lift coefficients, as well as the Strouhal number and the mean Nusselt number were computed for each simulation to determine the quantitative effects on flow and heat transfer. Steady flow is observed for Pr = 0.7, Ri = 0.5, and a = 0.5 or for Re ≥ 150 and Ri ≥ 1. For Ri = 10 and Re = 200, the flow becomes unsteady in the downstream region and trigger vortex street formation for a = 0.5, 1, and 2. Decreasing the aspect ratio, as well as increasing Reynolds number, increases the heat transfer from the cylinder. The rms lift coefficient decreases sharply at Ri < 1 for Pr = 0.7, Ri < 2 for Pr = 7 due to vortex breakdown. Further increase in the Richardson leads to a slight increase in rms lift coefficient except for Re = 200 where the sharp increase is observed due to triggered vortex formation again. For air, the increase in the mean Nusselt number in Ri = 10 case with respect to the forced convection case (Ri = 0) was 54–65% and 80–98%, respectively, for a = 0.5 and 2, while these values for water were realized as 50–68% and 53–63%, respectively. Using the computed data and nonlinear regression, the mean Nusselt number and the drag coefficients were developed.

Optimising cylinder model dimensions for VARSKIN to simulate a droplet of radionuclide skin contamination using Geant4 Monte Carlo code.

VARSKIN provides a convenient way of calculating skin dose from predefined geometries but the models are limited to concentric shapes such as discs, cylinders and point sources. The aim of this article is to use the Geant4 Monte Carlo code to independently compare the cylindrical geometries available in VARSKIN to more realistic droplet models obtained from photography. It may then be possible to recommend an appropriate cylinder model that can be used to represent a droplet within acceptable accuracy. Geant4 Monte Carlo code was used to model various droplets of radioactive liquid on the skin based on photographs. The dose rates were then calculated to the sensitive basal layer 70 µm beneath the surface for three droplet volumes (10, 30 and 50 µl) and 26 radionuclides. The dose rates from the cylinder models were then compared against the dose rates from the 'true' droplet models. The table gives the optimum cylinder dimensions that best approximate a true droplet shape for each volume. The mean bias and 95% confidence interval (CI) from the true droplet model are also quoted. The evidence from the Monte Carlo data suggests that different droplet volumes require different cylinder aspect ratios to approximate the true droplet shape. Using the cylinder dimensions in the table in software packages such as VARSKIN, dose rates from radioactive skin contamination are expected to be within ± 7.4% of a 'true' droplet model at 95% CI.

End effects in low aspect ratio Taylor-Couette flow.

Taylor-Couette flow with a low aspect ratio cylinder suffers from end effects due to the finite-span of the gap between the cylinder sides and the secondary flow in the region below the inner cylinder. We experimentally explore these end effects by varying the cylinder aspect ratio between 6.67 and 40 for a range of wall gap widths and bottom gap heights. For these geometries, end effects (i.e. non-ideal Taylor-Couette flow) can be substantial due to both features of the finite-span and the bottom secondary flow. In some cases, the finite-span effects extended between 20% and 30% of the way into the Taylor-Couette flow region, and the secondary flow at the bottom accounted for nearly half of the total measured torque. By taking these effects into consideration, our high aspect ratio results agreed well with those obtained by Taylor (Taylor 1936 Proc. R. Soc. Lond. A 157, 546-564. (doi:10.1098/rspa.1936.0215)) at considerably higher aspect ratios. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

REDUCTION OF TOTAL DRAG FOR FINITE CYLINDERS IN TURBULENT FLOW WITH A HALF-C SHAPE UPSTREAM BODY

The effect of an upstream half-C shape body on the reduction of the total drag of a finite circular cylinder is investigated experimentally. Five main cylinders, each with an upstream half-C shape body placed at five upstream positions, a total of 25 different configurations, are considered during the experimentation at a Reynolds number in the range of 1.2 &amp;times; 10&lt;sup&gt;4&lt;/sup&gt; to 1.6 &amp;times; 10&lt;sup&gt;5&lt;/sup&gt;. The aspect ratios of the main cylinders are 1.25, 2.09, 3.03, 3.78, and 4.90 and the upstream bodies are taken as 90&amp;deg; peripheral cut-outs of the respective main cylinder. The total configuration drag coefficient is calculated from the time-averaged drag force on the configuration and compared with the drag coefficient of the respective main cylinder. Results suggest that at least 20&amp;#37; of the total drag of the configuration might be reduced with the upstream half-C shape body, and the reduction varies with the main cylinders' aspect ratio and the upstream body placement distance. The maximum drag reduction is about 55&amp;#37; if the half-C shape body is placed at an upstream distance of 66&amp;#37; of the main cylinder diameter with an aspect ratio of 3.03 and at a Reynolds number of 1.8 &amp;times; 10&lt;sup&gt;4&lt;/sup&gt;. A critical distance ratio is found beyond which drag cannot be reduced by the upstream half-C shape body, and the attribution might be given to a formation of intermittent swirling flow in the cavity mode.

NUMERICAL INVESTIGATION OF FORCED CONVECTION HEAT TRANSFER AROUND AND THROUGH AN ELLIPTICAL POROUS CYLINDER

Convection heat transfer from a heated elliptical porous cylinder in the incompressible and laminar cross-flow is studied numerically. The effects of Darcy number (10&lt;sup&gt;-6&lt;/sup&gt; &amp;le; Da &amp;le; 10&lt;sup&gt;-2&lt;/sup&gt;), aspect ratio (0.2 &amp;le; AR &amp;le; 5), Prandtl number (0.7 &amp;le; Pr &amp;le; 70), Reynolds number (10 &amp;le; Re &amp;le; 40), on the average Nusselt number (Nu), as well as the lift and the drag coefficients are investigated. Two types of thermal boundary conditions for porous cylinder are investigated: uniformly distributed heat source and constant temperature. The fluid flow in the porous medium is numerically simulated by the superficial velocity model. Additionally, the energy equation in the porous medium is simplified by the local thermal equilibrium (LTE) hypothesis. It is concluded that the aspect ratio of the elliptical cylinder has a great influence on the heat transfer and fluid flow characteristics of this problem. Results show that increasing the Darcy number increases the Nusselt number and decreases the drag coefficient of the porous cylinder. Moreover, values of Da smaller than 10&lt;sup&gt;-4&lt;/sup&gt; resemble the fluid flow and heat transfer characteristics of the impermeable solid cylinder.

Electron heating and radiation in high aspect ratio sub-micron plasma generated by an ultrafast Bessel pulse within a solid dielectric

When propagating inside dielectrics, an ultrafast Bessel beam creates a high aspect-ratio cylinder of plasma with nanometric diameter that extends over several tens of micrometers to centimeters. We analyze the interaction between the intense ultrafast laser pulse and the plasma rod using particle-in-cell simulations. We show that electrons are heated and accelerated up to keV energies via transit acceleration inside the resonance lobes in the vicinity of the critical surface and compute their radiation pattern.

Study of the Impacts of Different Geometry and Aspect Ratio Hot Body on Natural Convection Heat Transfer

This work deals with a two-dimensional laminar natural convection heat flow, based on the temperature difference between the cold outer corrugated cylinder and three different cross-sectional areas, i.e. circular, square, and triangle configuration at the steady-state condition. The free region between two surfaces has been filled by a fluid with (Pr=7.0), and CFD finite volume method has been employed to solve the main equations. The simulations have been performed based on the dimensionless parameters: (Ra=104-105-106), (D/Dbase, and L/Dbased), while θ=0°-90°-180° has been applied for square and triangle configuration. The results are displayed in terms of isothermal contours and stream function and the heat transfer is recognized by both local and average Nusselt number. Firstly, the ANSYS 19.2 code validation has been performed by the comparisons of the outer data with previously theoretical papers, and good agreements have been detected. It is initiated from the local and average Nusselt number that the heat transfer from the inner hot surface has been affected by the above governing parameters at a certain fixed value of Ra. In all the results of the valve of average Nu approximately constant for the same configuration when Ra&lt;106 also, flow lines become fewer symmetrical when Ra=106. It is detected that both the cylinder aspect ratio and coordinations have a significant effect on the thermal fields and flow distribution. Moreover, the results that designate the changing of D/Dbase to 0.3 cause improvement in heat transfer by 23.3% at Ra=106, and also the circular cylinder is better than the other configurations.

Modal added-mass matrix of an elongated flexible cylinder immersed in a narrow annular fluid, considering various boundary conditions. New theoretical results and numerical validation

This paper considers the fluid–structure interaction problem of two coaxial cylinders separated by a thin layer of fluid. The flexible inner cylinder is imposed a small amplitude harmonic displacement corresponding to a dry vibration mode of an Euler–Bernoulli beam, while the external cylinder is rigid. A new theoretical formulation based on the assumption of a narrow fluid annulus is derived to estimate the modal added-mass matrix of the vibrating cylinder. This formulation accounts for the finite length of the flexible cylinder, clearly highlights the effect of the aspect ratio of the vibrating cylinder on the structure of the added-mass matrix, and covers all types of classical boundary conditions in the same theory and can easily be implemented in any numerical computing environment. The diagonal coefficients of the added-mass matrix are shown to increase with the confinement, with the aspect ratio of the flexible cylinder, and are sensitive to the wave-number of the vibration mode. Also importantly, we show that the dry vibration modes generate off-diagonal coefficients that vanish for an infinitely long cylinder. Our theoretical observations are corroborated by an extensive set of CFD numerical simulations, covering all types of classical boundary conditions, different confinement configurations, and different aspect ratios of the vibrating cylinder. The results obtained are presented in graphical form, which can be directly applied in engineering applications.

Effect of boundary layer state on the wake of a cantilevered square cylinder of aspect ratio 4

We examine the effect of an incoming boundary layer state (laminar vs. turbulent) on the near wake of a surface-mounted square cylinder with a height-to-width aspect ratio of 4 at a Reynolds number ~${10}^{4}$. The mean wake structure for both cases is a dipole structure consisting of a counter-rotating pair of streamwise vortices extending from the recirculation region. For the laminar boundary layer case, the wake contains an additional vortex pair. We use oil-film flow visualizations over the base plate, reconstructed 3D phase-averaged velocity fields, and energy transfer between coherent and incoherent fields, to elucidate the influence of the boundary layer state.

A novel model of pressure drop for airflow penetrating randomly packed cylindrical particles

Pressure drop is the key factor related to performance and energy consumption of fixed bed. However, the packing of cylindrical particles produces anisotropic gaps and an extremely complex airflow field and makes the formation mechanism of flow resistance undefined. This study conducted a comprehensive investigation on how such factors as the void fraction, diameter, and aspect ratio of cylinders on the flow resistance, in terms of experimental and numerical methods. With the DEM-CFD simulation, the laminar and transitional flow pattern was acquired, and the critical Reynolds number was determined. The expression of void fraction in the inertia term of the original Ergun equation was confirmed not applicable to cylindrical particles, which was proved to be the reason why previous predictions cannot accurately give the flow resistance of packed cylindrical particles. Finally, a new prediction model for the pressure drop of airflow penetrating the packed cylindrical particles was developed.

Numerical study on flow-induced vibrations of elastically mounted rectangular cylinders without and with barrier walls

This work presents a two-dimensional (2D) numerical study on the flow-induced vibrations (FIV) of elastically mounted rectangular cylinders without and with barrier walls in a wide range of reduced velocities U*=1–200 and Scruton numbers Sc=0–64 at the Reynolds number of 1000. It reveals that a slender rectangular cylinder of aspect ratio 5:1 without barrier walls may experience vortex-induced vibration (VIV), while the one with barrier walls can have both VIV and soft galloping motions. The VIV motions of both cylinders occur at relatively small reduced velocities U*≤20, and their maximum amplitudes are gradually reduced with the increase in the Scruton number. The galloping motions of the cylinder with barrier walls take place at higher reduced velocities U*&amp;gt;30 with smaller frequencies and larger oscillation amplitudes. Quantitative analysis on the amplitude, displacement, and frequency of oscillation for both cylinders is carried out. Two phase diagrams of the vortex-shedding flow patterns are presented to illustrate the flow characteristics in VIV and galloping modes. Meanwhile, the dynamic mode decomposition analysis indicates the difference between the dominant mode of the dynamic flow field in the VIV and galloping motions. With the quasi-steady theory, it further shows that the occurrence of the soft galloping motion of the cylinder with barrier walls is caused by the negative slope of the lift coefficient at the angle of attack zero. These results may shed new light onto deeper understanding of the FIV phenomenon and provide some inspirations to engineering applications in the design of civil and offshore structures.

Particle Scale Investigation of Influencing Factors on Heat Transfer in Nonspherical Particle–Fluid System

Abstract Heat transfer characteristics of random suspensions of 0.25 aspect ratio (AR) cylinders are investigated for Reynolds numbers (Re) between 10 and 300 and solid fraction (φ) ranging from 0.1 to 0.3 using particle resolved simulations. The effect of particle inclination with respect to flow and particle clustering on heat transfer is investigated. The Nusselt number decreases with an increase in inclination angle and the dependence becomes stronger as φ and Re increase. On the other hand, while prolate ellipsoid suspensions of AR 2.5 follow the same trend, the Nusselt number increases with inclination angle as AR increases to 5 and 10 and as φ increases. Local particle clustering nominally decreases the Nusselt number because of the dominance of thermal wakes. At low φ, this effect is felt only at low Re, but as φ increases, the effect spreads to higher Re. Similar but weaker trends are also found in suspensions of prolate ellipsoids of AR 2.5, 5, and 10. High AR, low Re prolate ellipsoids exhibit the greatest dependence of Nusselt number on local solid fraction. Implementation of two independent definitions of reference length, i.e., volume equivalent sphere diameter deq for ellipsoids and diameter dp of the cylindrical particle in the correlation of Tavassoli et al. (2015, “Direct Numerical Simulation of Fluid-Particle Heat Transfer in Fixed Random Arrays of Non-Spherical Particles,” Chem. Eng. Sci., 129, pp. 42–48) provides good estimates of the respective suspension mean Nusselt numbers.

Magnetoconvection around an elliptic cylinder placed in a lid‐driven square enclosure subjected to internal heat generation or absorption

AbstractThe impacts of MHD and heat generation/absorption on lid‐driven convective fluid flow occasioned by a lid‐driven square enclosure housing an elliptic cylinder have been investigated numerically. The elliptic cylinder and the horizontal enclosure boundaries were insulated and the left vertical lid‐driven wall was experienced at a fixed hot temperature, and the right wall was exposed to a fixed cold temperature. COMSOL Multiphysics 5.6 software was used to resolve the nondimensional equations governing flow physics. A set of parameters, such as Hartmann number (), Reynolds number (), Grashof number (), heat generation‐absorption parameter (), and elliptical cylinder aspect ratio (AR) () have been investigated. The current study discovered that for low Reynolds number, the adiabatic cylinder AR of 2.0 provided the optimum heat transfer enhancement for the model investigated, also the impact of cylinder size diminishes beyond Gr = 104. But for high Reynolds (Re = 1000), the size of the cylinder with AR = 3.0 offered the highest heat transfer augmentation. The clockwise flow circulation reduces because of an increase in AR, which hinders the flow circulation. In addition, heat absorption supports heat transfer augmentation while heat generation can suppress heat transfer improvement.

On mean Green operator and Eshelby tensor of hollow cylinders with length from infinite tubes to flat rings in isotropic elastic media

Using the (RT-IRT) Radon transform and inversion formula method, the author has previously reported the exact formal expressions for the mean shape function (mSF) of a finite solid cylinder of general aspect ratio in any infinite medium and for the related mean Green operator (mGO) and mean Eshelby tensor (mET) in cases of isotropic elastic-like (including dielectric-like) linear properties. Nearly exact solutions were obtained provided an accurate analytical approximation of an elliptic integral. Using the same RT-IRT method, the present work addresses the case of hollow cylinders with finite length in same isotropic property contexts. From examining the general formal mSF expression (and comparing with the limits of infinitely long cylinders and infinitely flat platelets) a simple accurate approximation is provided for the mSF, mGO and mET of "thin" hollow cylinders, from tubes to rings what covers a wide range of practical situations. The same expression being shown to hold on the thick side limit, the conjecture that it may be relevant over the whole thickness range is questioned.

Non-Newtonian fluid flow over a rotating elliptic cylinder in laminar flow regime

In this paper, a numerical investigation is done to understand the two-dimensional flow behaviorof power-law fluids past a rotating elliptic cylinder in unconfined domain. The flow field around the cylinder is calculated using ANSYS (FLUENT 15) The sliding mesh method (SMM) is used to treat moving solid–fluid boundary. The engineering parameters like drag coefficient, lift coefficient and complete kinematic variables such as stream function and vorticity have been calculated for the range of dimensionless parameters namely aspect ratio of the cylinder e=0.1 and 0.7, rotational speed (1≤α≤4), Reynolds number (5≤Re≤40) and power-law index (0.4≤n≤1.8). The emphasis has been given to see the effect of the power-law index (n) and Reynolds number (Re), rotational speed α and aspect ratio of cylinder e on flow characteristics around the cylinder in laminar flow regime. Before presenting the results, a qualitative as well as a quantitative comparison has been done between the present result and experimental and numerical result presented in the literature. Further, a detailed streamline and vorticity patterns in the vicinity of the cylinder is shown to demonstrate the wake formation phenomenon around the cylinder. The increase in rotational speed and power-law index exhibit a similar effect on wake formation phenomena. Due to rotational motion of the cylinder drag and lift coefficient vary in periodic manner with time/orientation of cylinder. The average value of drag is seen to decrease with Re and α and a complete trend is displayed with power-law index, n. The negative lift coefficient is observed due to anticlockwise motion of the cylinder for all parameters considered here. The results of this work clearly show the strong dependency of flow phenomenon around the rotating elliptic cylinder on power-law index, rotational speed, aspect ratio, and Reynolds number.