Impermeable Cylinder Research Articles

Towards a rapid-distortion theory model of turbulent flow for wind noise within an impermeable windscreen

Current models for the wind noise reduction (WNR) of microphone windscreens in atmospheric turbulence generally work under the assumption of a homogeneous surface pressure averaging theorem. Under this theorem, the surface pressures should average in such a way that the noise spectrum at low wavenumbers relative to the windscreen dimension should approach that of an unscreened microphone. Contrasting this, experiments have actually observed a significant WNR at these wavenumbers. In this work, we examine a Rapid Distortion Theory model for linking turbulent distortion to the unsteady pressure received inside the windscreen. The theoretical background and underlying assumptions of Rapid Distortion Theory are examined in detail [R. Zamponi, et al., J. Fluid Mech. 915:A27 (2021)], then results for the case of an impermeable cylinder are presented. Future work on the project involving both theoretical development and experimental verification are then discussed.

The effects of axonal beading and undulation on axonal diameter estimation from diffusion MRI: Insights from simulations in human axons segmented from three-dimensional electron microscopy.

The increasing availability of high-performance gradient systems in human MRI scanners has generated great interest in diffusion microstructural imaging applications such as axonal diameter mapping. Practically, sensitivity to axon diameter in diffusion MRI is attained at strong diffusion weightings , where the deviation from the expected scaling in white matter yields a finite transverse diffusivity, which is then translated into an axon diameter estimate. While axons are usually modeled as perfectly straight, impermeable cylinders, local variations in diameter (caliber variation or beading) and direction (undulation) are known to influence axonal diameter estimates and have been observed in microscopy data of human axons. In this study, we performed Monte Carlo simulations of diffusion in axons reconstructed from three-dimensional electron microscopy of a human temporal lobe specimen using simulated sequence parameters matched to the maximal gradient strength of the next-generation Connectome 2.0 human MRI scanner ( 500 mT/m). We show that axon diameter estimation is accurate for nonbeaded, nonundulating fibers; however, in fibers with caliber variations and undulations, the axon diameter is heavily underestimated due to caliber variations, and this effect overshadows the known overestimation of the axon diameter due to undulations. This unexpected underestimation may originate from variations in the coarse-grained axial diffusivity due to caliber variations. Given that increased axonal beading and undulations have been observed in pathological tissues, such as traumatic brain injury and ischemia, the interpretation of axon diameter alterations in pathology may be significantly confounded.

Effects of MHD, Forchheimer and Heat Transfer in Annular Region between Porous and Impervious Concentric Cylinders - DTM Approach

Significant increase of numerous applications in engineering, biological and industrial purpose as metallic extrusion motivated this communication. This paper proposes unique computational procedure is Method of Differential Transforms (DTM) to get an exact solution for electrified conducting fluid over a semi-porous cylinder in an impermeable cylinder with effects of Joule heating and convection term. A key finding of study reports the different dimensionless parameters influences the variations in velocity and heat transport on the fluid flow are presented graphically. The graph reveals an interesting result of Nusselt number, Skin-friction and stream lines elucidates the flow characteristics. A qualitative agreement is found in the present paper and are well matched with earlier work.

Influence of temperature dependent heat source/sink on transient MHD free convective flow in an infinite rigid impermeable vertical cylinder with chemical reaction

It is increasingly apparent that the inclusion of mass transfer aspects, together with certain thermal conditions, in the momentum and energy equations governing MHD flows leads to a numbers of real life applications. Keeping this in view, we have attempted an exact analysis of heat and mass transfer aspects in transient hydromagnetic free convective flow of an incompressible viscous fluid through a vertical pipe under an externally applied magnetic field, assuming presence of chemical reaction and heat source/sink. The governing PDEs, which simplify to a set of 3 linear ODEs in the physical set up considered here, have been solved using Laplace transform technique, with solutions for key physical variables presented in the term of Bessel and modified Bessel functions. The influence of governing non-dimensional parameters, namely, Hartmann number, Schmidt number, source/sink parameter, Prandtl number and chemical reaction parameter, has been illustrated on the developing velocity and some concentration profiles. Some important quantities of engineering interest-surface skin friction and volumetric flow rates-have been computed too and analysed. Some notable finding worth mentioning are: (a) heat source presence causes higher fluid velocity as compared to the heat sink; (b) all important surface shear stress can be suitably controlled, among others, by chemical reaction parameter and Schmidt number. The key challenge of this study has been to obtain exact closed-form solutions of the field equations, including cumbersome Laplace inverses. This study finds innovative applications in the emerging fields such as magnetic materials processing, chemical processes, solar energy systems, etc.

Short crested wave–current interaction with a cylinder and two concentric asymmetric arc-shaped walls

The diffraction problem of the interaction of short-crested incident waves and currents with a combined structure is studied. The combined structure consists of two concentric asymmetric exterior porous arc-shaped cylinders and an impermeable interior cylinder. A variable separation and an eigenfunction expansion method are used to derive the velocity potential for the entire fluid domain. The accuracy of the current model is verified by comparing the results of this study with published calculations. Numerical results reveal the variation of wave force and run-up on the combined structure with current speed, opening angle, and the locations of the two arc exterior cylinders. Moreover, the specific parameter conditions under which the resonance of the wave and structure may occur are analyzed, and the resonance phenomenon may be enhanced owing to the presence of a uniform current. Furthermore, the hydrodynamic performance of segmented arcs in the limiting case is examined. The results show that the segmented arc is not effective in protecting the interior structure when the wavelength is short. The findings of this study are anticipated to provide theoretical direction for the design of nearshore architecture.

Analysis of Wave Force and Run-Up Acting on an Impermeable Vertical Circular Cylinder Surrounded by Multiple Thick Porous Layers

Abstract In this work, we analyze the effect of multiple thick porous layers, fitted around a rigid vertical circular cylinder, on the wave forces acting on the rigid structure. Using the eigenfunction expansion method in cylindrical coordinates, we derive the expressions for velocity potentials in the respective domains and finally calculate the wave force acting on the rigid structure by integrating the pressure term. Consideration of the multiple porous layers, each with different porosities, gives rise to a very basic question to answer: what will be the appropriate arrangement of the porous layers to reduce the wave impact? Hence, for numerical study, we consider three different arrangements of the porous layers. For such arrangements of the porous layers, we also analyze the effects of the other crucial parameters, such as the number and the thickness of the porous layers, on the wave force. The key finding of the analysis is that the wave force acting on the rigid structure can be minimized by increasing the number of porous layers with decreasing porosity from the innermost layer to the outermost layer. The wave breaking and forced oscillations for certain values of the porous impedance parameter are some of the interesting observations. The present model is also verified against an existing work in the literature which shows an excellent agreement.

Short-crested wave-current forces around a concentric multiple-cylinder structure

ABSTRACT The potential flow theory is used to develop a new analytical method to solve the diffraction problem of short-crested incident waves with uniform current acting on a concentric multiple-cylinder system. The influence of uniform current on the hydrodynamic performance of the concentric structure is discussed. The incident angle and speed of the currents have a significant influence on the short-crested wave force and run-up on the concentric structure, i.e. the wave force and wave run-up increase significantly when wave and uniform current directions are the same, while decreasing when in the opposite direction of the wave and uniform current. Additionally, the effects of parameters such as the current incidence angle, current speed, porous-effect parameters, number of perforated walls, and short-crestedness of regular waves on the hydrodynamic performance of the concentric structures are valuated by numerical experiments. It is observed that as the number of permeable walls increases, wave load on the impermeable internal cylinder gradually decreases and the wave surface around is more even. This study is expected to provide theoretical guidance for the design of nearshore architecture.

Time-dependent water wave scattering by a bottom-mounted porous compound cylinder fitted with an annular porous lid

In this work, we analyze the scattering of water waves by a fully submerged bottom-mounted concentric cylindrical system consisting of three components: an impermeable inner cylinder surrounded by a porous cylindrical wall and a porous lid covering the annular region. Consideration of a porous lid in the annular region gives rise to two different dispersion relations in two sub-regions which in turn present an additional complexity in the system in the form of evanescent modes. Locus of the propagating mode for free surface gravity wave follows an analogous path in the complex plane as the porous-effect parameter moves in its own complex plane, except for low frequency at which the path of the mode distorts significantly. The mean drift wave forces on the system are calculated numerically by using pressure integration method, and the effect of different set of physical parameters on the wave forces is also examined. The key finding of this analysis is that the wave force on the impermeable cylinder gets minimized for lower values of porous-effect parameter whereas the drift force on the outer porous wall gets minimized corresponding to higher values of porous-effect parameter. This work is also verified with an existing work in the literature and then extended to the time-dependent analysis. The plane incident waves with the Gaussian profile and the focused wave group are both considered in the time domain simulation.

Analysis of thermally radiative magnetized viscoelastic (second-grade) fluid by impermeable vertical cylinder

We made an attempt to disclose the phenomenon of dual stratification in magnetized viscoelastic second-grade fluid model generated by the movement of vertical cylinder. Mass and heat transportation is accounted under the aspects of Joule and Ohmic dissipations. The flow is coupled due to the consideration of nonlinear convected terms. Boundary-layer concept is implemented for the simplification of mathematical system. This system is made into the non-dimensional forms by reporting the similarity variables. The system of non-dimensional equations is executed with the use of homotopic criteria. The convergent region of solutions is reported and discussed. The convergent results are shown by graphical illustrations by considering distinct emerging constraint values. An increment in Darcy and inertia coefficient parameters corresponds to a decay in the flow. Temperature distribution is an increasing function of thermal Biot number and radiation parameter while it diminishes when thermal stratification parameter increases. Concentration is reduced for larger Schmidt number along with solutal stratification parameter. Besides the heat–mass transportation rates are increasing functions of thermo-solutal Biot numbers. The considered problem finds relevance in functional smart electro conductive enrobing coating processes in manufacturing. This is a principal application of the model studied where external axisymmetric boundary layers occur with combined magnetic, thermal and rheological effects.

Wave interaction with a concentric bottom-mounted cylinder system with dual porous ring plates

The interaction between linear surface waves and a concentric bottom-mounted cylinder system has been investigated using the eigenfunction expansion approach. This system contains an outer porous cylinder and an inner impermeable cylinder, which are connected by dual porous ring plates. Both cylinders are surface-piercing and rigidly installed on the flat bottom of the ocean, while the porous ring plates are fixed below the free water surface. The analytical solution of the velocity potentials can be obtained by matching the boundary conditions. After obtaining the velocity potentials, the wave force and free surface elevation are computed. The numerical results obtained for limiting cases agreed well with the published results. The results of the study show that reducing the draft, spacing, and permeability of the dual plates all contribute to reduce the horizontal force of the inner cylinder. However, the significance of the lower plate is mainly to cope with the condition where the water surface is lower than the upper plate.

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<sup>-6</sup> ≤ Da ≤ 10<sup>-2</sup>), aspect ratio (0.2 ≤ AR ≤ 5), Prandtl number (0.7 ≤ Pr ≤ 70), Reynolds number (10 ≤ Re ≤ 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<sup>-4</sup> resemble the fluid flow and heat transfer characteristics of the impermeable solid cylinder.

An analytical study of scattering of water waves by a surface-piercing bottom-mounted compound porous cylinder placed on a porous sea-bed

The scattering problem of a bottom-mounted surface-piercing compound porous cylinder located on a porous sea-bed is theoretically investigated under the assumption of small amplitude wave theory. The compound cylinder is comprised of an impermeable upper cylinder and a porous lower cylinder beneath it. The boundary conditions on the porous boundaries follow Darcy’s law by assuming fine pores in the porous structure. The whole fluid region is split into three bounded and an unbounded sub-regions, within which the individual velocity potentials are found by using eigenfunction expansion technique. Further, utilization of the matching conditions along the boundaries of individual successive regions leads to a semi-analytical solution of the proposed problem. The impact of the non-dimensional porous-effect parameter of the cylindrical wall, the draft ratio, radius ratio and the sea-bed porosity on waveloads and wave run-ups are studied. The results show that suitable consideration of porosity and structure parameters enhance the efficiency of the proposed compound cylinder in mitigating wave impact. Furthermore, the hydrodynamic waveload acting on the lower as well as upper cylinder can be reduced by the suitable positioning of the annular spacing of the system, which will provide explicit information for the purpose of engineering design in the coastal area.

Structural performance of a submerged bottom-mounted compound porous cylinder on the water wave interaction in the presence of a porous sea-bed

Scattering problem of a submerged bottom-mounted compound porous cylinder located on a porous sea-bed is theoretically investigated under the assumption of linear potential flow theory. The compound cylinder is comprised of an impermeable inner cylinder and a porous outer cylinder. The boundary conditions on the porous boundaries follow Darcy's law by assuming fine pores in the porous structure. The whole fluid region is split into three bounded and unbounded sub-regions, within which the individual velocity potentials are found by using the eigenfunction expansion technique. Furthermore, utilization of the matching conditions along the boundaries of individual successive regions leads to a semi-analytical solution of the proposed problem. The impact of the non-dimensional porous-effect parameter of the cylindrical wall, the draft ratio, radius ratio, and the sea-bed porosity on wave loads and free-surface elevation are studied. In addition, the wave power dissipation for the system is calculated by integrating the power absorbed by the outer cylinder porous wall via direct method. Also, the far-field scattering coefficients are obtained with the help of asymptotic forms of Hankel functions in the plane wave representation form. Numerical results for the far-field scattering coefficient and power dissipation are investigated for various parameters. The theoretical model is verified by comparing it with the results of the conventional analytical work and experimental work. The results show that suitable consideration of porosity and structure parameters enhances the efficiency of the proposed compound cylinder in mitigating wave impact. Furthermore, the hydrodynamic wave load acting on the inner and outer cylinders can be reduced by the suitable positioning of the annular spacing of the system, which will provide explicit information for the purpose of engineering design in offshore and coastal regions.

Interaction of solitary wave with a concentric structure with multiple porous outer walls

This paper gives an analytical solution for a plane solitary wave interaction with a concentric structure with coaxial multiple-layered porous walls. An analytical solution is obtained based on shallow water wave theory and eigenfunction expression matching. Furthermore, a series of simultaneous equations are used to determine potential coefficients. The accuracy of the present model is verified by comparing its output with published results. Meanwhile, the impact of various important parameters (i.e., the number of porous walls, annular spacing and porosity of walls) with respect to wave forces and relative wave height is examined. It is sufficient to safeguard the inner cylinder by using two-three porous walls. For multiple walls with different porous-effect parameters, the arrangement of the selected porous effect parameters has a marginal influence on the force of the impermeable cylinder. However, the minimum wave run-up on it can be observed when the coefficient of the porous walls increases from the outside to the inside. In addition, an impermeable cylinder with two concentric porous walls is investigated owing to its higher application prospects.

Reproducibility of the Standard Model of diffusion in white matter on clinical MRI systems

Estimating intra- and extra-axonal microstructure parameters, such as volume fractions and diffusivities, has been one of the major efforts in brain microstructure imaging with MRI. The Standard Model (SM) of diffusion in white matter has unified various modeling approaches based on impermeable narrow cylinders embedded in locally anisotropic extra-axonal space. However, estimating the SM parameters from a set of conventional diffusion MRI (dMRI) measurements is ill-conditioned. Multidimensional dMRI helps resolve the estimation degeneracies, but there remains a need for clinically feasible acquisitions that yield robust parameter maps. Here we find optimal multidimensional protocols by minimizing the mean-squared error of machine learning-based SM parameter estimates for two 3T scanners with corresponding gradient strengths of 40and80mT/m. We assess intra-scanner and inter-scanner repeatability for 15-minute optimal protocols by scanning 20 healthy volunteers twice on both scanners. The coefficients of variation all SM parameters except free water fraction are ≲10% voxelwise and 1−4% for their region-averaged values. As the achieved SM reproducibility outcomes are similar to those of conventional diffusion tensor imaging, our results enable robust in vivo mapping of white matter microstructure in neuroscience research and in the clinic.

Numerical study of the flow through an annular gap with filtration by a rotating porous cylinder

A numerical simulation approach is substantiated and verified for predicting the Taylor-Couette flow with an impermeable outer stationary cylinder and porous rotating inner one. An imposed throughflow is considered, supplied via one end of the annular gap and leaving the domain through another gap end (retentate) and from inside of the rotating cylinder (permeate). The flow is typical for dynamic filtration by rotating cylindrical filters. In contrast to known publications, the rotating porous cylinder is included into the computational domain and the liquid flow inside of it is fully resolved. The influence of the permeate-retentate ratio and permeability of the rotating inner cylinder onto the centrifugal stability boundary and supercritical flow details is discussed. It is found that filtration velocity becomes distributed not uniformly along the porous cylinder when its hydraulic resistance is less than a definite value. After some critical threshold, the flow structure drastically changes, and spiral vortices appears, which crosses the porous rotating cylinder back and forth several times, providing multiple flow recirculation through the porous cylinder. The results obtained create the basis for the development of dynamic rotational filters for various applications.

RETRACTED: Aqueous aluminium–copper hybrid nanofluid flow past a sinusoidal cylinder considering three-dimensional magnetic field and slip boundary condition

We analyzed the problem of the steady general three-dimensional magnetohydrodynamics stagnation-point boundary layer flow past an impermeable wavy circular cylinder considering aluminium–copper/water hybrid nanofluid as the working fluid and velocity slip as well as temperature jump boundary conditions. The induced magnetic field effect was also taken into account. The analytical procedure is based on the model that implements the nanoparticles and base fluid masses to formulate the equivalent volume fraction, equivalent density, and equivalent specific heat at constant pressure which is then substituted in the chosen single-phase thermophysical properties. Then, the foregoing relations were used in basic governing PDEs (partial differential equations), according to Tiwari–Das nanofluid scheme. It is worth mentioning that the bvp4c code from MATLAB software that is a famous finite-difference method has been exploited for solving the final similarity ODEs (ordinary differential equations). Results demonstrate that the developed mass-based model can be successfully employed with great confidence to study the flow and heat transfer of hybrid nanofluid in other similar problems. Moreover, it is proved that the nodal stagnation points possess higher values of skin friction coefficients and local Nusselt numbers relative to those for the saddle stagnation points. Besides, enhancing the second nanoparticle's mass leads to increase in all parameters of engineering interest including skin friction coefficients along x and y directions as well as local Nusselt number.

Analytical Solution for Wave Diffraction by a Concentric Three-Cylinder System near a Vertical Wall

In this study, a semi-analytical model was developed to study wave diffraction around a concentric three-cylinder system near a wall based on linear potential theory. As a critical element, the target problem is transformed into bidirectional incident wave diffraction around two concentric structures based on the image principle and an analytical solution is obtained through eigenfunction expansion combined with a matching technique and Graf’s addition theorem. The validity of the proposed model was verified by comparing its results to known values. Parametric studies on porosity, annular spacing, incident angle, space between the structure and wall, and water depth were performed. The hydrodynamic loads and free-surface elevations in the system were calculated and compared to those reported in existing works on impermeable and permeable cylinders near a wall. The results indicate that the wave loads and run-ups on the exterior cylinder increase significantly based on the existence of the wall. However, based on the presence of an exterior porous protective structure, a significantly reduced influence of the wall on the interior cylinder can be observed. Considering the widespread use of concentric circular structures in ocean engineering, it is essential to conduct study on the hydrodynamic performance of concentric systems near walls, which can provide useful information for the design of marine structures.

Wave diffraction from a concentric truncated cylinder system with a porous ring plate fixed inside

The diffraction problem of a concentric truncated cylinder system with a porous ring plate fixed inside is studied in the framework of linear potential theory. The system consists of a porous exterior cylinder and an impermeable interior cylinder, which are connected by an impermeable top and bottom plate, and a porous ring plate arranged below the free surface. Darcy’s law is applied to the porous boundaries under the assumption of fine pores. The velocity potential of the whole fluid domain is analytically derived by the method of variable separation and eigen-function expansion. The hydrodynamic loads of the system are obtained by integrating the pressure on the wet surface. The calculation results of the present paper are compared with the previous works with similar models to verify the correctness of the model. The effect of the dimensionless porous effect parameter of the ring plate and of the exterior cylinder, the draft–depth ratio of the ring plate, the draft–depth ratio of the system, the ratio of the interior and exterior radii are discussed. The results show that appropriate permeability and structural scale parameters can improve the hydrodynamic performance of the structure, which will provide directive guidance for engineering design.

Flow and heat transfer in eccentric annulus

Eccentric annular regions formed by two impermeable cylinders are considered. Flow of Newtonian fluid in the annular region in the presence of heat transfer is studied. The eccentric annulus is mapped to concentric annular region through conformal mapping. Analytical solution for both temperature and velocity is obtained and graphically depicted. An increase in eccentricity results in an increase in velocity but a decrease in temperature.