Narrow Results

We study the resonant behavior of a system consisting of a square array of multi-coated cylinders by calculating the effective dielectric constant of the system. The results were examined numerically using the finite element method.

This study focuses on the characteristics of flow past three side-by-side rectangular cylinders under the effect of aspect ratios (AR) and Reynolds numbers (Re) at two different gap ratios ($g$) using the lattice Boltzmann method. For this purpose, AR is varied in the range of 0.25–4, the Re values are 100, 140 and 180 and the two different values of $g$ taken into account are $g=0.5$ and 3. The results are presented in the form of vorticity contours, temporal histories of drag and lift coefficients and power spectrum of lift coefficients. Also, the variation of physical parameters like mean drag coefficient, Strouhal number and the root-mean-square values of drag and lift coefficients with Re and AR is presented for $g=0.5$ and 3. The current numerical computations yield that for both gap ratios and all Re, there exist four different flow regimes depending on AR: (a) steady flow, (b) modulated flow, (c) symmetric flow and (d) periodic flow. At narrow gap ratios, the jet flow emerging within the gaps of cylinders altered the flow structures and fluid forces abruptly. The aspect ratio is found to have more influence on the flow characteristics of cylinders as compared to the Reynolds numbers at large gap ratios.

Many precision experiments have been done in the Casimir regime and in short range gravity when the separation between the interacting bodies is in the sub-micron range. Experimental complexity is minimized when one of the bodies is a sphere and the other one is a plate, making the alignment between the two bodies ubiquitous. Our group has produced the most precise Casimir measurements, and the best limits on predicted Yukawa-like potentials by measuring a force between a $R\sim 150\mu \mathrm{\text{m}}$ sphere attached to a ${(500\mu \mathrm{\text{m}})}^2$ micro-mechanical oscillator and a planar source mass. By replacing the spherical surface with a fraction of a $500\mu \mathrm{\text{m}}$ long cylinder with $R\sim 150\mu$m, the force sensitivity can be greatly enhanced. Here, it is paramount to know the angular deviation between the long axis of the cylinder and both the axis of rotation of the oscillator and the plate. Tests between a cylinder and a structure etched into a silicon wafer show that deviations of $20\mu$rad are readily accessible. Additionally, a scaled up experiment is used to investigate if capacitance measurements can determine the orientation of the cylinder with respect to a plane with the required precision.

The aim of the present paper is to construct series of invariants of free knots (flat virtual knots, virtual knots) valued in free groups (and also free products of cyclic groups).

The near sound field generated due to a vertically mounted circular cylinder piercing a free surface in shallow water, is studied computationally using the Large Eddy Simulation (LES) approach and the sound wave equation. The flow is simulated in both the air and water phases. The interface surface is allowed to move and is simulated using the Volume of Fluid technique. The pressure distribution over the cylinder is fed back into the sound wave equation to calculate the near field. The interface surface is modeled as a zero pressure surface in the acoustic calculation and the bottom is taken as having infinite impedence modeling the case of a rigid floor. Two of acoustic calculations methods are used. In the first method, the interface surface is assumed to be fixed and the wave equation is solved in the frequency domain in a post-processing stage. In the second method, the evolution of the interface surface is taken into account and the wave equation is simulated simultaneously with the LES. Both solutions are analyzed and compared to show that the interface surface acts as a strong damper to the low frequency sound by damping the vortex Von Karman rollers as well as causing the low frequency component to be nonradiative. The variation of the near sound field with the water depth and Froude number is investigated and the propagation and damping characteristics are analyzed.

Similitude theory has been applied to design scale models in many fields of engineering. As for free vibration of cylinders, the wall thickness is too thin to manufacture scale model, which leads to similitude distortion. The aim of this study is to correct the similitude distortion and establish the distorted similitude relationship for free vibration of cylinders. First, the complete and partial similitude relationships are deduced while the similitude distortion of wall thickness is discussed. Then, with analysis of similitude for energy, an equation with prediction of modal frequencies for prototype is established and the energy similitude correction method is proposed. Finally, through numerical examples, this method is verified and the influence of wall thickness variation on the accuracy of the proposed method is analyzed.

A boundary element method (BEM) is presented to study 3D wave scattering by cracks in a cylinder. Green's functions needed in the kernel of boundary integral equations in BEM are derived with the help of guided wave functions. Guided wave modes in the cylinder are obtained by a semi-analytical finite element (SAFE) method. Green's functions are constructed numerically by superposition of guided wave modes. In this method, the cylinder is discretized in the radial direction into several coaxial circular cylinders (sub-cylinders) and the radial dependence of the displacement in each sub-cylinder is approximated by quadratic interpolation polynomials. A numerical procedure is used here to accurately calculate the Cauchy's principal value (CPV) and weakly singular integrals. The multi-domain technique is employed here to model the crack surface. Numerical results are presented to show the effectiveness of the proposed solution.

The vortex shedding flow past a piggyback pipeline close to a plane boundary is investigated numerically. The piggyback pipeline is comprised of a large pipeline and a small pipeline. The aim of the study is focused on the effects of the position angle of the small pipeline on the vortex shedding and the hydrodynamic forces of the pipeline bundle. The two-dimensional Reynolds-averaged Navier–Stokes equations are solved using the finite element method. The equations are enclosed by the k–ω turbulent model. The ratio of the small pipeline diameter (d) to the large one (D) is 0.2. The Reynolds number based on the free stream velocity (U₀) and the large pipeline diameter (D) is Re = 2 ×10⁴. Simulations are carried out for the positional angles (α) of the smaller pipeline relative to the larger one ranging from 0 to 180° with an interval of 15°, and the gaps (G) between the large pipeline and the plane boundary ranging from 0.2 to 0.6D. It is found that the critical gap ratio, below which the vortex shedding is suppressed, is a function of the positional angle of the small pipeline. The effects of the position angle of the small pipeline on the hydrodynamic forces on the pipeline system are investigated numerically.

To determine strength allowables for cylinders–such as aircraft fuselages–made of fiber-matrix composites, practical analytical tools are needed to determine damage extent and residual strength around cutouts. A higher-order shell theory is used to analyze compressive and tensile loads on a graphite/epoxy laminated cylinder containing a square cutout. Failure in the fiber, matrix, or lamination is determinated by the Hashin failure criterion and modeled by reducing the appropriate stiffness terms. This results in a redistribution of stress, leading to further failure. Under fatigue loads, graphite/epoxy will lose stiffness as cycles increase. To model this effect of cyclic loading, the stiffness matrix is further reduced at the beginning of each cycle. Therefore, the static analysis can be used to predict damage under fatigue loading by representing several thousand fatigue cycles in a single computational cycle. In compression loading, the reduction in stiffness ultimately leads to geometric instability, or collapse. In tension, the reduction is stiffness–to account for fatigue–increases the displacements, but not the stress. Therefore, a strain-based criterion is added to induce macromechanical failure from the cyclic loading. The material damage propagates until failure of the curved panel occurs. The objective is not to model the micromechanical failure in the structure, but rather to determine the damage extent, stress redistribution, and residual strength of composite cylinders under fatigue loading.

In this paper, a closed-form solution to determine location of yield in rotating thick-walled cylindrical shells made of functionally graded materials (FGMs) using both Tresca's and von Mises yield criteria is presented. The cylindrical shell is subjected to uniform internal and external pressure. The material properties are assumed to vary according to power law functions and the Poisson's ratio is assumed to be constant. Moreover, the plane strain condition is used to drive expressions for distributions of stresses and radial displacement. In the present work, rotation, internal and external pressure and variation of material properties are considered simultaneously. To the best of authors' knowledge, in previous researches in which onset of yield is investigated, variation of material density is ignored, while density is not constant in FGM rotating thick cylindrical shells. Our results show that the density variation has a significant effect on the stress distributions. Our research has also tried to explain the effect of different Poisson's ratio on the value of the critical material parameter.

The spectral element method, a direct numerical technique, is used to study the behavior of flow past two cylinders in tandem array. A control cylinder is employed in front of the main cylinder to study the drag reduction performance and flow patterns. Three major flow patterns are found, including single cylinder vortex shedding, two cylinders vortex shedding and suppression. The flow patterns are affected by the distance between two cylinders, Reynolds number and the diameter ratios of cylinders. In a bistable regime, when there is a critical distance between cylinders, drag is reduced dramatically.

In this paper, transient response of a simply supported finite length hollow cylinder made of functionally graded piezoelectric material (FGPM) subjected to coupled hygrothermal loading was investigated. The coupled equations of heat conduction and moisture diffusion as well as motion equations and the electrostatic equation of FGPM were solved employing the Fourier series expansion method through the longitudinal direction, the differential quadrature method (DQM) along the radius and Newmark method for time domain. Finally, the distribution of temperature, humidity, electric potential, stresses and displacements was achieved. The effect of coupled and uncoupled hygrothermal loading, grading index and hygrothermal loading was illustrated in the numerical examples. The results show that using the coupled model is vital for analysis of transient response of the cylinder subjected to hygrothermal loading.

In this paper, we give multiple situations when having one or two geometrically distinct closed geodesics on a complete Riemannian cylinder, a complete Möbius band or a complete Riemannian plane leads to having infinitely many geometrically distinct closed geodesics. In particular, we prove that any complete cylinder with isolated closed geodesics has zero, one or infinitely many homologically visible closed geodesics; this answers a question of Alberto Abbondandolo.

The creep behavior of a thick walled hollow circular cylinder, made of aluminum silicon carbide particulate composite material has been investigated in the present study and creep behavior in this case is assumed to follow Sherby's constitutive model. The stress and strain rate distributions of cylinder rotating about its own axis, have been obtained using von Mises and Tresca yield criteria. The effect of pressure on the stresses and strain rates in the cylinder has been investigated and it is observed that with the increase of the internal pressure in the cylinder the strain rate increases. The introduction of external pressure along with the internal pressure causes the strain rates to decrease. It is also observed that the values of effective stress and strain rate obtained using Tresca criterion are higher than those obtained using Mises criterion. Thus it is suggested to use Tresca criterion for the analysis while designing the cylinder.

The wave impact on marine structures is concerning in ocean and coastal engineering. Cylinders are important components of various marine structures such as piers of sea-crossing bridge, columns of oil and gas platforms and subsea pipelines. In this study, the interaction of solitary wave with a submerged horizontal cylinder and a surface-piercing vertical cylinder are numerically studied by the Smoothed Particle Hydrodynamics (SPH) code SPHinXsys. SPHinXsys is an open-source multi-physics library based on the weakly compressible SPH and invokes the low-dissipation Riemann solver for alleviating numerical noises in the simulation of fluid dynamics. The capability of SPHinXsys in reproducing the fluid fields of solitary wave propagating through cylinders is demonstrated by comparing with the experimental data. With the validation in hand, the features of the wave–structure process are examined.

An analytical-numerical method is proposed to analyze the surface waves in multi-layered piezoelectric cylinders. In this present method, the cylinder is divided into layer elements with three-nodal-lines along the wall thickness. The Hamilton principle is used to develop approximate dynamic equilibrium equations. The dispersion relationship for multi-layered cylinder is defined and deduced from the Rayleigh quotient and the orthogonal condition of the eigenvalues. The method of Fourier transform combined with the modal analysis is used to determine the displacement response and electronic potential to mechanical and electromechanical loads. Numerical examples are presented for calculating the frequency and group velocity dispersion behaviors, as well as electronic potential distribution in multi-layered piezoelectric cylinder.

This chapter deals with the calibration of a new simplified experimental method to evaluate absolute roughness of vegetated channels. The method is based on boundary layer measurements in a short channel rather than on uniform flow measurements, as usual. The proposed method can be applied to any kind of rough bed, but it is particularly useful in vegetated beds where long channels are difficult to prepare. In this paper a calibration coefficient is experimentally obtained. In order to perform suitable comparisons with literature data relationships between ε absolute roughness and Manning's n coefficient are deepened. The results are successfully compared with literature experimental data with a very good fit. Finally, a particular dependence of ε values on the vegetation density are explained through further experiences. In conclusion it is possible to state that the proposed method, once calibrated, can provide reliable prediction of absolute roughness in vegetated channels.