Slender Cylinder Research Articles

EXPERIMENTAL PRESSURES EXERTED BY MAIZE IN SLENDER CYLINDRICAL SILO: COMPARISON WITH ISO 11697

Pilot-scale test stations make it possible to obtain reliable and comparable results applicable to full–scale systems by conforming to specified proportional limits. Therefore, in this study, normal and frictional pressures were evaluated in a pilot-scale test station composed of a slender cylinder silo using maize, a free-flowing product, as the stored product. Temporal effects were analyzed and verified during filling, static, and discharge conditions. The maximum normal and frictional pressures were also evaluated. The results were compared with ISO 11697: 1995. During filling, accommodation peaks occurred only in the α: 30° hopper. In general, normal pressures were higher for the flat bottom whereas higher frictional pressures occurred for the 30° hopper. The maximum experimental pressures (normal and frictional) were lower than those provided by ISO 11697. Therefore, it is concluded that the coefficients used in the ISO standard are sufficient to promote safety in silo projects.

A study of convective nanofluid flow over a rough slender cylinder under the influence of magnetic field and species diffusion

AbstractThe present work explores the analysis of magnetohydrodynamics nonlinear mixed conv ective nanofluid flow over a vertical slender cylinder in the presence of surface roughness. The application of the present study can be found in the process of coating wires. In fact, during such a process, thin wires in the slender cylinder need to be cooled, and also heat and mass transfer rates need to be controlled through nanofluid and liquid hydrogen to yield better results. By employing nonsimilar transformations, the partial differential equations governing the flow problem are reduced to dimensionless equations. Furthermore, the Quasilinearization technique and implicit finite difference scheme are used to solve the dimensionless governing equations. The novelty of the analysis is the impacts of surface roughness, diffusion of liquid hydrogen, and the presence of nonlinear mixed convective flow over a slender cylinder. The numerical results reveal that the energy transport strength and surface drag coefficient enhance with the roughness parameter values. The nanoparticle volume fraction profile reduces, while nanoparticle Sherwood number enhances with increasing values of velocity ratio parameter. The presence of nanoparticles in the conventional fluid diminishes the energy transfer value significantly for both smooth and rough surfaces. The velocity of the fluid enhances with increasing values of the mixed convection parameter. Due to surface roughness ( ≠ 0), sinusoidal variations of the drag are observed along the length of the cylinder in comparison to smooth surface case ( = 0). The friction between the fluid and the wall rises for a rough surface because pockets of fluid are formed in the cavities due to surface asperities.

Hydrodynamic torque on a slender cylinder rotating perpendicularly to its symmetry axis

The torque experienced by a circular cylinder rotating steadily about an axis passing trough its centroid and perpendicular to its symmetry axis is computed over a wide range of Reynolds number and aspect ratios using fully resolved simulations. In the creeping-flow regime, numerical results are shown to match predictions of an improved slender-body approximation. In strongly inertial regimes, flow symmetries and boundary layer arguments are employed to derive scaling laws for the various contributions to the torque. We finally obtain an empirical formula for the total torque, valid throughout the parameter range explored in the simulations.

Second-order monopile wave loads at linear cost

A method to compute the second-order free surface elevation, depth integrated force and mud line moment for a slender circular vertical cylinder is presented. The method is valid for unidirectional irregular waves and includes inertia loads and viscous loads.We first derive the linear transfer functions for free surface elevation, depth-integrated force and moment from the complex Fourier amplitudes of the velocity potential. Next, the second-order contributions are expressed through closed form quadratic transfer functions, which are further diagonalized through eigen decomposition. Hereby the second-order contributions can be computed as products of pseudo time series calculated by FFT, with the eigenvectors acting as transfer functions on the linear Fourier amplitudes.For a sample 3-hour sea state, we find that eight eigen vectors are sufficient to achieve an accuracy of 1% for the maximum peak value of force and moment and 1.3% for free surface elevation, relative to the standard deviation of each signal. These results are obtained 2500 faster than with the conventional approach and we demonstrate that the computational effort of the new method scales like O(NlogN), similar to linear wave loads, where N is the number of frequencies. For the eight mode approximation, the error bound of 1% for loads and 4% for free surface elevation are found to hold across various values of the normalized peak wave number from shallow to deep water. The accuracy is adjustable through the number of modes and is found to be independent of the time series length. The methods potential in practical design is discussed.

Analysis of MHD mixed convection in a Ag-TiO2 hybrid nanofluid flow past a slender cylinder

The concept of MHD mixed convection with Ag-TiO2 / H2O hybrid nanofluid flow over a slender cylinder find its several applications in the field of science and engineering. Especially, in designing and manufacturing of coating of wires, fibre sheets, optical fibres, photo electric devices, solar cells etc. A uniform magnetic field is applied to analyse its impact on transport characteristics of the flow. The different nanoparticles shape factor is considered in order to study the behaviour of thermal efficiency. The study of such flow problems involving physical aspects such as mixed convection, thermo-physical behaviours of hybrid nanofluid and uniform applied magnetic field is an innovative approach. The mathematical model describing the fluid flow has been formulated by nonlinear coupled partial differential equations with boundary constraints. Further, the governing equations have been transformed into dimensionless form by utilizing appropriate non-similar transformations and are attempted for the solution in combination with Quasilinearization technique and implicit finite difference approximation. The simulation comprises the various physical parameters, such as velocity ratio parameter, mixed convection parameter (Richardson number) Ri, magnetic field parameter M, Eckert number Ec, nanofluid volume fraction φ1, hybrid nanofluid volume fraction φ2 and nanoparticles shape factor s are demonstrated through graphs. The results unveil that inclusion of the hybrid nanoparticles in the base fluid result in high heat transfer than that of nanofluid and base fluid. Further, the velocity of the hybrid nanofluid is enhanced for the enhancing values of mixed convection parameter and velocity ratio parameter. The hybrid nanofluid temperature rises with raising values of magnetic parameter and volume fraction of Ag-TiO2, significantly.

Dynamics and stability of imperfect flexible cylinders in axial flow

This paper aims to study theoretically the effects of geometric and support imperfections on the static equilibrium and linear dynamical behaviour of a flexible cylinder in confined axial flow. The support imperfection is modelled via translational and rotational springs at the upstream end of the cylinder while the geometric imperfection is represented by an initially inclined configuration with respect to the fluid flow. The static equilibrium equation is obtained from the principle of virtual work while the equation describing the dynamics of the system around the equilibrium position is obtained using the extended Hamilton’s principle. A central finite difference method and the Galerkin scheme are employed to spatially discretize the static equilibrium equation and dynamic equation of transverse motions, respectively. Numerical results are obtained to show the effects of support and geometric imperfections on the critical flow velocities of the system with various parameters. It is shown that, in general, cylinders with the upstream support imperfection may lose stability at a lower flow velocity compared to the perfectly supported cylinders. It is also shown that the geometric imperfection may have a considerable effect on the dynamic stability of very slender cylinders and those terminated with a perfectly streamlined end-piece.

Natural circulation in a short-enclosed rod bundle

Large Eddy Simulation (LES) is used to study natural circulation flow in a sealed rod bundle with a Rayleigh number (based on height) of approximately 1011 and a fluid Prandtl number of 0.74. Natural convection boundary layer development is observed on both the heating rods and cooling containment surfaces. The development of the boundary layers is key to the flow and heat transfer characteristics of the system. As the flow descends on the containment surface, the boundary layer gradually develops and transitions from laminar to turbulent flow, with transition occurring close to the bottom-end. Laminar flow at the containment is well represented by a flat plate similarity solution for buoyant flow and the Nusselt number correlation is akin to that for a flat isolated vertical surface. This indicates the boundary layer development on this surface is similar to an isolated vertical surface. On the rods boundary layer development occurs much earlier and is affected by turbulence convected from the containment surface. Laminar flow here compares well against a slender cylinder similarity solution. In the developed region, the Nusselt number correlation has a Rayleigh number dependency similar to that for a rectangular cavity.

Assessment of three-dimensional effects on slamming load predictions using OpenFoam

Three-dimensional effects on slamming loads predictions are investigated numerically using the unsteady incompressible Reynolds-Average Navier-Stokes (RANS) equations and volume of fluid (VOF) method, which are implemented in the open-source library OpenFoam. Three cases of free-falling water entry problems are considered. The numerical solutions are validated through the comparisons of slamming loads and motions between the CFD simulations and the available experimental values. The total slamming force and slamming pressures on the 3D wedge cylinder, ship section and slender circular cylinder are compared with the results on the 2D sections, respectively. The results show that the 3D predictions on the maximum slamming force coefficient are around 21%∼23% lower for the wedge with a deadrise angle of 30º and are around 16% lower for the slender circular cylinder, relative to the 2D calculations.

DIFFUSE TERMITINAE NESTS SHED LIGHT ON THE AFFINITIES OF LAETOLICHNUS KWEKAI (KRAUSICHNIDAE)

ABSTRACT The insect trace fossil Laetolichnus kwekai, which is composed of a small chamber extending to slender cylinders at each end, was tentatively included in the ichnofamily Krausichnidae as termite nests. New evidence presented here provides information to validate these inferences. A more complex structure formed by interconnected Laetolichnus was recently found in the same Pliocene deposits (Laetoli, Tanzania) as the isolated specimens reported previously. Our study confirms inclusion of Laetolichnus in Krausichnidae and supports the inference that it represents a nest of a social insect. Neoichnological field studies in the coastal dunes of Buenos Aires Province, Argentina, were undertaken to refine further the nature of these affinities. Survey of the dune surface revealed abundant loose fragments of termite nests of a size and shape comparable to that of L. kwekai. The fragile nests constructed by Onkotermes brevicorniger, which are described here in detail for the first time, enable us to interpret the fossil structures. They consist of connected chambers similar to the interconnected Laetolichnus. These were frequently exposed and broken by wind action resulting in loose fragments similar to the isolated Laetolichnus. The Celliforma ichnofacies represented at Laetoli, which contains L. kwekai, indicates arid or semiarid shrublands and woodlands. The distribution of O. brevicorniger also corresponds to arid and semiarid shrublands and dry woodlands of Argentina. Although the African termite producer of L. kwekai and the South American Onkotermes would be phylogenetically unrelated, the analogous structures probably reflect convergent nesting behaviors as an adaptation to similar arid to semiarid environmental conditions.

Effect of heat radiating and generating second-grade mixed convection flow over a vertical slender cylinder with variable physical properties

The boundary-layer flow and heat transfer characteristics of an incompressible thermally radiating second-grade fluid on a linearly stretching permeable vertical slender hollow cylinder subject to viscous dissipation and convection heat exchange at the wall with the surroundings in conjunction with augmented far-field condition is studied. The free-stream or ambient velocity is considered to be linearly stretching while the prescribed surface temperature of the cylinde varies nonlinearly as the axial distance in the presence of elastic deformation and volumetric heat generation. The model incorporates the temperature dependent fluid properties as per dynamic viscosity and thermal conductivity while the viscoelastic or second-grade parameters are temperature-free. In virtue of appropriate transformation, the energy and momentum equations are reduced to nonlinear ordinary differential equations, and then solved numerically using forty-fifth order Runge-Kutta-Fehlberg (RKF45) integration scheme in tandem with shooting method. Analysis is carried out through simulated tables and codes generated graphs. This study reveals that the wall shear stress factor increases with the velocity ratio, mixed convection, curvature and suction parameters but reduces with the stretching parameter. Also the local heat transfer rate enhances with the Prandtl number, suction, curvature, velocity ratio and stretching parameters in contrast to that due to viscoelastic parameter.

Conjugate Heat and Mass Transfer on Steady MHD Mixed Convection Flow along a Vertical Slender Hollow Cylinder with Heat Generation and Chemical Reaction Effects

This paper studies the effects of heat generation and chemical reaction on the coupled conjugate heat and mass transfer by MHD laminar mixed convective flow along a vertical slender hollow cylinder. The governing boundary layer equations along with the boundary conditions are first cast into a dimensionless form by a non similar transformation and the resulting equations are then solved by the finite difference method using Matlab@ following the code bvp4c. Numerical results of the velocity, temperature and concentration for different values of the conjugate heat transfer parameter p, the magnetic parameter M, the heat generation Q, and the chemical reaction K are studied. The local skin friction, Nusselt number and Sherwood number are also analyzed and presented graphically. In the numerical ranges of the main parameters, it is found mainly that working with strong conjugate heat transfer or/and all others parameters affects negatively the Nusselt and Sherwood numbers. The same trend is revealed for the skin friction factor.

Experimental investigation of free connective heat transfer augmentation using transformer oil-Al2O3 nanofluid

There is a definite need to improve the heat transfer capability of the transformer oil as it plays a vital role in the life span of transformers. As the thermal conductivity of the transformer oil is very less compared to the conventional liquids and those liquids cannot be used as dielectric liquids as they do not possess electrical insulation properties. It is very much essential to improve the thermal conductivity of the transformer oil in any manner in order to augment the “heat transfer or thermal management performance” of the transformer oil. Dispersion of metallic or metallic oxide nanoparticles into the base fluid in a systematic way is a proven method to enhance the thermal properties for various practical heat transfer applications like forced convection, boiling, refrigeration, solar and electronic cooling. In this study, alumina nanoparticles are dispersed in transformer oil to synthesize stable, homogenous transformer oil – Al2O3 nanofluid and to investigate the free convection “coefficient of heat transfer performance” along a vertical slender cylinder submerged in the liquid. Experiments are conducted using the transformer oil - Al2O3 nanofluid at different particle loadings or fractions starting from 0.05 % to 0.6 %. It is learnt that the highest “coefficient of heat transfer performance” occurs at 0.1 Vol% among all the volume fractions. It has been acknowledged that the “coefficient of heat transfer” is got better up to 0.1 % particle loading and then decreased with additional accumulation of nanoparticles.

Analysis of Random Wave Interaction with Cylinders using Extremal Statistical Methods

Long slender cylindrical structures are widely used in offshore structural designs. Their interactions with random waves are very complicated and it is extremely difficult to model their motions in a direct analytical approach. Accurate characterization of the observed extreme values in their response behavior is of vital importance for the safety of the designed structure. In this study a general statistical methodology based on generalized extreme value (GEV) family of distributions is formulated for the characterization of both laboratory and field data associated with the wave-cylinder interactions in random waves. An iterative process is developed to determine the most appropriate block size and the corresponding statistical model for the block maxima constructed from the time series of the target variable. The model's capability to fit the data is assessed using the Anderson-Darling (AD) test criterion, visually with quantile plots and histograms. Special attention is paid to the quality of fit in the upper tail of the distribution, which corresponds to the data points of the extreme statistical events. Case studies are performed that investigate the performance of the statistical methodology in characterizing the measured data from two industrial scale model basin test programs. The first case study addresses data on the in-line interactions of closely spaced deep-water cylinder arrays for two- and three-cylinder configurations and the second considers the VIV response amplitudes of a slender horizontal cylinder. In both test programs the random seaways were generated using JONSWAP wave amplitude spectrum. The effectiveness of this statistical methodology in characterizing the response behavior observed in the model basin experimental data is discussed in detail using both tabular and graphical interpretations.

Correcting for tube curvature effects on condensation in the presence of a noncondensable gas in turbulent free convection

Condensation in the presence of noncondensable gasses (NCG) has many practical applications, and hence researchers continue to study its many facets. In the particular case when the vapor-NCG mixture is driven by free convection, the use of slender cylinders as condenser surfaces is very frequent, and one needs to estimate the effect of tube curvature in order to translate condensation rates from curved surfaces to flat ones, and vice-versa.In our previous investigation [1], we solved the coupled gas and condensate boundary layer equations to deduce the curvature correction factor for laminar free convection over a vertical tube. In this work, we aim at addressing the same issue for turbulent regimes which are much more prevalent. To this effect, we model the transport of the vapor-NCG mixture as an ideal gas, and solve the flow field within a CFD-based approach that employs the k-ε model of turbulence and fully resolves the gas boundary layer. Sink terms in the continuity, momentum, species and energy equations are used to account for the removal of vapor at the condenser wall. In contrast to most CFD approaches to date, condensate film resistance is taken into account to allow a general use of the model up to very high steam fractions.We first validate the model against experimental heat transfer databases for representative tube diameters of 12 mm and 38 mm. For these conditions, the Grashof number based on tube length varies between 4.2.109 and 3.5.1012, which denotes a fully turbulent regime. The model agreement with the data is very good, with discrepancies comparable to the experimental error. We also show that including film resistance is very important even at moderate steam mass fractions of the order of 0.5. The error for not modeling film resistance soars to 50% for steam fractions of order 0.9.We thereafter conduct an exhaustive set of 112 parametric CFD simulations that span tube diameters from 8 mm to 100 mm, steam mass fractions from 0.2 to 0.95, pressures from 1 bars to 5 bars, and wall temperature subcooling from 5 K to 80 K. The results of these simulations are subsequently presented in a compact correlation for the curvature correction factor. The proposed correlation reflects the simulation results within a ± 5% band.The outcomes of this investigation show qualitatively the expected trends. All other things held constant, the curvature correction factor decreases with tube diameter, steam fraction and Grashof number, and is insensitive to tube length. We show finally that the use of correlations developed for laminar regimes underestimate the effects of curvature in turbulent flows. Turbulence hence unequivocally promotes the heat transfer enhancements due to curvature.

Erratum: “Wave-induced vortex generation around a slender vertical cylinder” [Phys. Fluids 32, 042105 (2020)

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Thermophoresis and Brownian Model of Pseudo-Plastic Nanofluid Flow over a Vertical Slender Cylinder

This study focuses on the industrial and engineering interest quantities, such as drag force and rate of transmission of heat, for pseudo-plastic nanofluid flow. The attributes of natural convection of the pseudo-plastic nanofluid flow model over a vertical slender cylinder are explored. The pseudo-plastic flow is studied under the influence of concentration of nanoparticles, rate of heat transmission, and drag force. For the first time, the pseudo-plastic nanofluid flow model has been implemented over a vertical slender cylinder which is not yet investigated. The acquired model is based on thermophoresis and Brownian motion mechanisms. The governing equations of pseudo-plastic nanofluid in cylindrical coordinates are modelled. The developed system of nonlinear equations is tackled by boundary layer assumptions and similarity transformations. Moreover, the solution of the acquired system exhibited by using a new powerful numerical technique. A comprehensive debate on drag force and transmission of heat under the influence of various emerging parameters is illustrated in the table. Furthermore, the effects of dimensionless parameters over the velocity profile, temperature profile, and concentration of nanoparticle profile have been exhibited graphically.

Cracking behavior of basalt fibre reinforced polymer‐reinforced concrete: An approach for the determination of crack spacing and crack width

AbstractThe limitation of the crack width is of central importance for the design in the serviceability limit state. For FRP‐reinforced concrete members, the crack width has to be limited for service condition in use and not due to corrosion protection. In this paper, an experimental program of pull‐out tests and slender concrete cylinders with basalt fiber‐reinforced polymer rebars under centric tension load is presented. The used basalt fiber‐reinforced polymer rebars are sand coated and have in addition a helically wrapped thread for a decent profiling. Equations to calculate crack spacing and crack width of FRP‐reinforced concrete is derived and calibrated with the obtained experimental results. Finally, the equations are verified with experimental data from other authors and a crack model is given for reinforced concrete members with FRP rebars with a helically wrapped and sand coated surface.

Dynamic characteristics of a slender flexible cylinder excited by concomitant vortex-induced vibration and time-varying axial tension

Flexible cylinder structures, such as top-tensioned risers (TTRs) and tethers of tension leg platforms (TLPs), display complicated dynamic characteristics under the combined excitation of vortex-induced vibration (VIV) and time-varying axial tension, which causes serious fatigue damage and is a major concern in ocean engineering fields. In this paper, model tests were conducted on a flexible cylinder with an aspect ratio of 350 and a mass ratio of 1.90 to study the effect of time-varying tension on the VIV response. Three tension amplitude ratios (Tv/Tc = 0.1, 0.2 and 0.3, where Tv is the amplitude of the varying tension and Tc is the constant tension) and six tension frequency ratios (fv/f1 = 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0, where fv is the frequency of the varying tension and f1 is the 1st-order natural structural frequency) were considered. The Reynolds number ranged from approximately 800 to 16,000. The displacements were reconstructed with the measured strains using a model analysis method. The effects of the tension amplitude ratio and tension frequency ratio on the dynamic characteristics were discussed from the maximum root mean square (RMS) of the displacements, power spectral density (PSD) plots, time-space-varying displacements, and motion trajectories. The tension excitation places the vibration into a higher-order mode earlier. For a large tension amplitude ratio (Tv/Tc = 0.3), the max RMS of the cross-flow (CF) displacement remarkably increases because the frequency of the varying tension is equal to the parametric resonance frequency (fv/f1 = 2.0). The effect of the tension frequency ratio is enhanced with the increase in the amplitude ratio. For the case of fv/f1 = 2.0, the sum and difference frequencies disappear, and the bandwidth of the dominant frequency widens. Figures in the shape of the number 8 are not found in the motion trajectory plots for Tv/Tc = 0.3, indicating that energy transfer is influenced by tension excitation. The in-line (IL) vibrations show an alternate conversion between the 5th-order mode vibrations and the analogous 3rd-order mode vibrations.

Analytical study on phase transition of shape memory alloy wire under uniaxial tension

This paper considers the stress-induced phase transitions of shape memory alloy slender cylinder, and analytically studies the phase transition process and the associated instability. A three-dimensional (3D) phenomenological model with an internal variable is adopted, which is simplified to a 1D system of two strains in three regions (austenite, martensite and phase transition regions). Suitable boundary conditions and interface conditions are proposed. Theoretically, it is a free boundary problem, as the position and shape of phase interfaces are unknown. We then focus on planar interfaces (which are energetically favored), and a symmetric case when phase transition occurs in the middle. For given applied stress, two-region solutions, three-region solutions and the connecting solutions between them are obtained analytically or semi-analytically, including many period-k solutions. Two-region solutions show that phase transition does not take place at one point, but simultaneously in a small region. The width of phase transition region is found analytically, revealing the roles of the material and geometrical parameters. Three-region solutions represent localized inhomogeneous deformations in experiments, and capture that the stress stays almost at the Maxwell stress during propagation of transformation front. For displacement-controlled process, the transition process is demonstrated by the stress-strain curve, which captures the stress drop/rise and the transition from homogeneous deformations to period-1 localized inhomogeneous deformations. When the radius is smaller than a critical value (given by material constants), the stress drop is very sharp due to transition of solutions in a snap-back bifurcation. These features show good agreement with experimental observations and shed light on the difficulties of numerical simulations.

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.