Sound Scattering and Its Reduction by a Janus Sphere Type

Copyright © 2014 Delyia Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Sound scattering by a Janus sphere type is considered. The sphere has two surface zones: a soft surface of zero acoustic impedance and a hard surface of infinite acoustic impedance. The zones are arranged such that axisymmetry of the sound field is preserved. The equivalent source method is used to compute the sound field. It is shown that, by varying the sizes of the soft and hard zones on the sphere, a significant reduction can be achieved in the scattered acoustic power and upstream directivity when the sphere is near a free surface and its soft zone faces the incoming wave and vice versa for a hard ground. In both cases the size of the sphere’s hard zone is much larger than that of its soft zone. The boundary location between the two zones coincides with the location of a zero pressure line of the incoming standing sound wave, thus masking the sphere within the sound field reflected by the free surface or the hard ground. The reduction in the scattered acoustic power diminishes when the sphere is placed in free space. Variations of the scattered acoustic power and directivity with the sound frequency are also given and discusse

Self-thermophoresis of Laser-heated spherical Janus particles

An analytical framework is presented for calculating the self-induced thermophoretic velocity of a laser-heated Janus metamaterial micro-particle, consisting of two conducting hemispheres of different thermal and electric conductivities. The spherical Janus is embedded in a quiescent fluid of infinite expanse and is exposed to a continuous light irradiation by a defocused laser beam. The analysis is carried under the electrostatic (Rayleigh) approximation (radius small compared to wave-length). The linear scheme for evaluating the temperature field in the three phases is based on employing a Fourier-Legendre approach, which renders rather simple semi-analytic expressions in terms of the relevant physical parameters of the titled symmetry-breaking problem. In addition to an explicit solution for the self-thermophoretic mobility of the heated Janus, we also provide analytic expressions for the slip-induced Joule heating streamlines and vorticity field in the surrounding fluid, for a non-uniform (surface dependent) Soret coefficient. For a ‘symmetric’ (homogeneous) spherical particle, the surface temperature gradient vanishes and thus there is no self-induced thermophoretic velocity field. The ‘inner’ temperature field in this case reduces to the well-known solution for a laser-heated spherical conducting colloid. In the case of a constant Soret phoretic mobility, the analysis is compared against numerical simulations, based on a tailored collocation method for some selected values of the physical parameters. Also presented, are some typical temperature field contours and heat-flux vectors prevailing in the two-phase Janus as well as light-induced velocity and vorticity fields in the ambient solute, and a new practical estimate for the self-propelling velocity

Investigation of improved aerodynamic performance of isolated airfoils using CIRCLE method

CC BY-NC-ND licenseThe PhD research of Moin U Ahmed is partly sponsored by Cummins Turbo Technologies Ltd and partly by Queen Mary University of London

Low Reynolds number proprotor aerodynamic performance improvement using the continuous surface curvature design approach

Low Reynolds number blade profiles of Re_C =10^5 to 2*10^5 as as based on chord length and used for small unnamed air vehicles, and near space applications are investigated for single and counter-rotating (coaxial) proprotors, i.e. acting as rotors or propellers. Such profiles are prone for early stall, significantly reducing their maximum lift to drag ratio. Two profiles previously designed by our continuous surface curvature design approach named as CIRCLE are investigated in order to improve the performance of the proprotors. The profiles are redesigns of the common symmetric NACA0012 and asymmetric E387 profiles. Using general arguments based on composite efficiency and rotor’s lift to drag ratio, the performance envelope is noticeably increased when using the redesigned profiles for high angles of attack due to stall delay. A new approach is derived to account for the distance between the rotors of a coaxial proprotor. It is coupled with a blade element method and is verified against experimental results. Single and coaxial CIRCLE-based proprotors are investigated against the corresponding non CIRCLE-based proprotors at hover and axial translation. Noticeable improvements are observed in thrust increase and power reduction at high angles of attack of the blade’s profiles, particularly for the coaxial configuration. Plots of thrust, torque, power, composite efficiency and aerodynamic efficiency distributions are given and analysed

Optimization of a horizontal axis marine current turbine via surrogate models

Flow through a scaled horizontal axis marine current turbine was numerically simulated after validation and the turbine design was optimized. The computational fluid dynamics (CFD) code Ansys-CFX 16.1 for numerical modeling, an in-house blade element momentum (BEM) code for analytical modeling and an in-house surrogate-based optimization (SBO) code were used to find an optimal turbine design. The blade-pitch angle (θ) and the number of rotor blades (NR) were taken as design variables. A single objective optimization approach was utilized in the present work. The defined objective function was the turbine’s power coefficient (CP). A 3x3 full-factorial sampling technique was used to define the sample space. This sampling technique gave different turbine designs, which were further evaluated for the objective function by solving the Reynolds-Averaged Navier–Stokes equations (RANS). Finally, the SBO technique with search algorithm produced an optimal design. It is found that the optimal design has improved the objective function by 26.5%. This article presents the solution approach, analysis of the turbine flow field and the predictability of various surrogate based techniques

Sound Scattering by an Elastic Spherical Shell and its Cancellation using a Multi-pole Approach

The scattering and transmission of sound by an elastic spherical shell is considered when it is subject to an incoming monochromatic planar wave. It is aimed to cancel the sound scattering using combinations of multi-pole sources located at the centre of a shell filled with compressible fluid. Assuming linear acoustics and structural dynamics, exact solutions are derived for total elimination of the sound scattering for three cases: a free-space, near a hard ground or near a free-surface, where in the last two cases it is assumed that the incoming wave propagates normal to the interface to maximize sound reflection back unto the source of the incoming wave. An elastic spherical shell of 1 m radius embedded in water and filled with air or oil is analysed to show the dominance of low-mode numbers for frequencies of less than 10 kHz and thus demonstrate the ability of this approach to damp acoustic scattering by means of low-order multi-poles inside the shell. Contour and mode distribution plots are also given and analysed

Computational methods for investigation of surface curvature effects on airfoil boundary layer behavior

This article presents computational algorithms for the design, analysis, and optimization of airfoil aerodynamic performance. The prescribed surface curvature distribution blade design (CIRCLE) method is applied to a symmetrical airfoil NACA0012 and a non-symmetrical airfoil E387 to remove their surface curvature and slope-of-curvature discontinuities. Computational fluid dynamics analysis is used to investigate the effects of curvature distribution on aerodynamic performance of the original and modified airfoils. An inviscid–viscid interaction scheme is introduced to predict the positions of laminar separation bubbles. The results are compared with experimental data obtained from tests on the original airfoil geometry. The computed aerodynamic advantages of the modified airfoils are analyzed in different operating conditions. The leading edge singularity of NACA0012 is removed and it is shown that the surface curvature discontinuity affects aerodynamic performance near the stalling angle of attack. The discontinuous slope-of-curvature distribution of E387 results in a larger laminar separation bubble at lower angles of attack and lower Reynolds numbers. It also affects the inherent performance of the airfoil at higher Reynolds numbers. It is shown that at relatively high angles of attack, a continuous slope-of-curvature distribution reduces the skin friction by suppressing both laminar and turbulent separation, and by delaying laminar-turbulent transition. It is concluded that the surface curvature distribution has significant effects on the boundary layer behavior and consequently an improved curvature distribution will lead to higher aerodynamic efficiencyThe author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The PhD research of Xiang Shen is funded by China Scholarship Council (CSC)/Queen Mary Joint PhD scholarship

A numerical study on the influence of curvature ratio and vegetation density on a partially vegetated U-bend channel flow

Aquatic vegetation dramatically shifts the main flow, secondary flow and turbulent structures in a meandering channel. In this study, hydrodynamics in a bending channel with a vegetation patch (VP) has been numerically studied under the variation of curvature ratios (CRs=0.5, 1.0, 1.5, 2.0) and the vegetation density i.e. Solid Volume Fractions (SVF=1.13%, 4.86%). Both effects on vegetation shear flow, helical flow, bed shear stress and bulk drag coefficients are studied in twelve cases by using Ansys Fluent package. Unsteady Reynolds Averaging Navier-Stokes (URANS) framework coupled with the Reynolds Stress turbulence Model (RSM) and Volume Of Fluid (VOF) approach is successfully applied to predict the entire flow field including multi-circulation cells as well as the free surface. The conclusions are summarized as three points. Firstly, an increase of CR moves the main circulation cell and thalweg's location towards the outer bank, while decreasing the drag coefficients in streamwise and spanwise. However, the CR weakly affects the normalised shear flow velocity profiles and dominant eddy frequencies downstream of the VP. Secondly, the trend of the dominant shedding frequency to fall with the increase of SVF that has been known only for SVF<3.4% is extended up to 10.4%. Furthermore, an opposite trend is found between the frequency and SVF for 10.4%<SVF<20%. Thirdly, a newly proposed patch dimensionless frequency number, , links Stp and SVF, where N is the number of stems in the patch. This number stays almost constant for each case series regardless of the variation of SVF (for SVF<10.4%). We also conclude that is strongly determined by the patch shape factor, mildly influenced by the patch Reynolds number, but it excludes the influence of the SVF and N. The insights from the present study unveil the complicated eco-hydro-morphic interactions among the bio-mass density, turbulent flow and channel meanders’ variation. It provides a better understanding of natural bending river systems’ development and fundamentals for the recovery of urban channel ecosystems by vegetated re-meandering