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

Direct numerical simulation of sediment entrainment in turbulent channel flow

This research was supported by a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme (Grant No. PIIF-GA-2009-236457). The first author acknowledges the financial support of the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51021004), National Natural Science Foundation of China (Grant Nos. 50809047 and 51009105), and Natural Science Foundation of Tianjin (Grant No. 12JCQNJC02600)

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

An investigation on the rheodynamics of human red blood cells using high performance computations

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Studies on the haemodynamics of human circulation are clinically and scientifically important. The flow of human blood is extremely complex due to the existence of the highly deformable red blood cells (RBCs), which are able to pass through capillaries smaller than their size. To investigate the effect of deformation and aggregation in blood flow, a computational technique has been developed by coupling the interaction between the fluid and the deformable solids. The flow of 49,512 RBCs at 45% concentration and under the influence of aggregating forces was examined to improve the existing knowledge on how to simulate and study the blood flow and its structural characteristics of blood at a large scale. The simulation was carried out with full parallelization of the coupled fluid-solid code using spatial decomposition and high performance supercomputers. The large scale feature of the simulation has enabled a macroscale verification and investigation of the overall characteristics of RBC aggregations to be carried out. The results are in excellent agreement with experimental studies and, more specifically, both the experimental and the simulation results show uniform RBC distributions under high shear rates (60-100/s) whereas large aggregations were observed under a lower shear rate of 10/s. The statistical analysis of the simulation data also shows that the shear rate has significant influence on both the flow velocity profiles and the frequency distribution of the RBC orientation angles. The flow under the low shear rate also tended to have bi-phasic velocity profile which is mainly due to the formation of large scale aggregation clusters

Saltation of particles in turbulent channel flow.

This paper numerically investigates particle saltation in a turbulent channel flow having a rough bed consisting of two to three layers of densely packed spheres. The Shields function is 0.065 which is just above the sediment entrainment threshold to give a bed-load regime. The applied methodology is a combination of three technologies, i.e., the direct numerical simulation of turbulent flow; the combined finite-discrete element modeling of the deformation, movement, and collision of the particles; and the immersed boundary method for the fluid-solid interaction. It is shown that the presence of entrained particles significantly modifies the flow profiles of velocity, turbulent intensities, and shear stresses in the vicinity of a rough bed. The quasi-streamwise-aligned streaky structures are not observed in the near-wall region and the particles scatter on the rough bed owing to their large size. However, in the outer flow region, the turbulent coherent structures recover due to the weakening rough-bed effects and particle interferences. First- and second-order statistical features of particle translational and angular velocities, together with sediment concentration and volumetric flux density profiles, are presented. Several key parameters of the particle saltation trajectory are calculated and agree closely with published experimental data. Time histories of the hydrodynamic forces exerted upon a typical saltating particle, together with those of the particle's coordinates and velocities, are presented. A strong correlation is shown between the abruptly decreasing streamwise velocity and increasing vertical velocity at collision which indicates that the continuous saltation of large-grain-size particles is controlled by collision parameters such as particle incident angle, local bed packing arrangement, and particle density, etc.This work was supported by a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme (Grant No. PIIF-GA-2009-236457). The first author acknowledges the financial support of the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51321065), National Natural Science Foundation of China (Grants No. 50809047, No. 51109157, and No. 51009105), and Natural Science Foundation of Tianjin (Grants No. 12JCQNJC02600, No. 12JCQNJC04900, and No. 12JCQNJC05600

The Passing of Print

This paper argues that ephemera is a key instrument of cultural memory, marking the things intended to be forgotten. This important role means that when ephemera survives, whether accidentally or deliberately, it does so despite itself. These survivals, because they evoke all those other objects that have necessarily been forgotten, can be described as uncanny. The paper is divided into three main sections. The first situates ephemera within an uncanny economy of memory and forgetting. The second focuses on ephemera at a particular historical moment, the industrialization of print in the nineteenth century. This section considers the liminal place of newspapers and periodicals in this period, positioned as both provisional media for information as well as objects of record. The third section introduces a new configuration of technologies – scanners, computers, hard disks, monitors, the various connections between them – and considers the conditions under which born-digital ephemera can linger and return. Through this analysis, the paper concludes by considering digital technologies as an apparatus of memory, setting out what is required if we are not to be doubly haunted by the printed ephemera within the digital archive

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

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