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[en] Complete text of publication follows. The alpha-effect is a useful tool for interpreting the generation of dynamos. The traditional alpha-effect represents the electromotive force, approximated to be instantaneous in time and local in space. However, the approximation is valid only when the magnetic Reynolds number Rm is smaller than unity and inappropriate when Rm is larger than or equal to the order of unity, which is the condition satisfied in the Earth's core or solar convection zone. We have examined the non-instantaneousness and non-localness of the mean electromotive force, the generalized alpha-effect, in G.O.Roberts (1972)'s kinematic dynamo model. We solved the fluctuating part of the induction equation explicitly and calculated the response function of the mean electromotive force as a function of Rm. The response for Rm < O(1) is instantaneous and local, which is the typical picture of the alpha-effect. When Rm is larger than or equal to O(1), the instantaneous and local alpha-effect is controlled by developed boundary layers, as suggested by previous works for high Reynolds number limit. In addition to the alpha-effect, non-instantaneousness and non-localness become significant, where the characteristic time of the memory is around the magnetic diffusion time. Above a certain value of Rm, Rm > O(1), the effect of the small-scale dynamo appears after the magnetic diffusion time, invalidating the well-known high Reynolds number limit. Our result implies that the non-local memory effect of the mean electromotive force should be important to understand geophysical and astrophysical dynamos.
[en] A spectral subggrid-scale eddy viscosity and magnetic resisitivity model based on the eddy-damped quasi-normal Markovian (EDQNM) spectral kinetic and magnetic energy transfer presented in  is used in large-eddy simulation (LES) of large kinetic and magnetic Reynold number magneto-hydrodynamic (MHD) turbulence. The proposed model is assessed via a posteri tests on three-dimensional, incompressible, isotropic, non-helical, freely-decaying MHD turbulence at asymptotically large Reynolds numbers. Using LES with an initial condition characterized by an Alfv(acute e)n ratio of kinetic to magnetic energy τA equal to unity, it is shown that at the kinetic energy spectrum EK(k) and magnetic energy spectrum EM(k) exhibit Kolmogorov -5/3 inertial subrange scalings in the LES, consistent with the EDQNM model
[en] Highlights: • The influence of elbow curvature on the flow through a 90° elbow was studied. • Flow separation occurred on inner wall with decreasing radius ratio. • The turbulence intensity grew on the inner wall with decreasing radius ratio. • Secondary flow in the elbow was modified by elbow curvature. - Abstract: The influence of elbow curvature on the flow field of a 90° elbow was studied experimentally to understand the flow and turbulence structure in the elbow using planar and stereo particle image velocimetry (PIV) measurements. Three elbows with different radius to diameter ratios (1.0, 1.2, and 1.5) were investigated in the Reynolds number range of Re = (3–10) × 104. The experimental results showed that the mean velocity decreased and turbulence intensities increased along the inner wall in the second half of the elbow, and similar observations were made in all of the elbows. However, variations in these quantities increased with decreasing radius ratio. This was caused mainly by the occurrence of flow separation on the inner wall, near the elbow outlet, for smaller radius ratios. Similarly, flow separation on the inner wall was promoted at lower Reynolds numbers. Cross-sectional velocity field measurements at the elbow outlet using stereo PIV showed the presence of a pair of counter-rotating vortices, which increased in mean velocity and turbulence energy on the inner wall with decreasing radius ratios of the elbow. This result was well reproduced in the contour of the first proper orthogonal decomposition (POD) mode. In the POD analysis, opposite signs of the high POD mode were formed on both sides of the inner wall and the peak of the POD mode increased with decreasing radius ratio.
[en] Highlights: • Direct side-by-side assessment of the Single Component Pseudopotential Lattice Boltzmann and Volume of Fluid methods. • LB roughly 10 times faster and produces 1-3 orders of magnitude lower spurious velocities than all VOF methods tested. • VOF more versatile, while LB requires further modifications to be widely applicable. • LB and VOF agree well for predicting the Reynolds number of falling droplets, while LB underpredicts droplet deformation. • Different techniques can disagree when simulating real physical problems, even upon agreeing for purely numerical benchmarks. - Abstract: While various multiphase flow simulation techniques have found acceptance as predictive tools for processes involving immiscible fluids, none of them can be considered universally applicable. Focusing on accurate simulation of liquid-liquid emulsions at the scale of droplets, we present a comparative assessment of the single-component multiphase pseudopotential lattice Boltzmann method (PP-LB, classical and modified) and the Volume of Fluid method (VOF, classical and modified), highlighting particular strengths and weaknesses of these techniques. We show that a modified LB model produces spurious velocities 1–3 orders of magnitude lower than all VOF models tested, and find that LB is roughly 10 times faster in computation time, while VOF is more versatile. Simulating falling liquid droplets, a realistic problem, we find that despite identical setups, results can vary with the technique in certain flow regimes. At lower Reynolds numbers, all methods agree reasonably well with experimental values. At higher Reynolds numbers, all methods underpredict the droplet Reynolds number, while being in good agreement with each other. Particular issues regarding LB simulations at low density ratio are emphasized. Finally, we conclude with the applicability of VOF vis-à-vis PP-LB for a general range of multiphase flow problems relevant to myriad applications.
[en] It is shown that a particular higher-order Reynolds stress arising from a term in the third-order gyrokinetic Hamiltonian is smaller than it nominally appears to be. However, it does not follow that all third-order terms are unimportant. The discussion is relevant to the ongoing debate about the importance of higher-order terms in the gyrokinetic theory of momentum transport
[en] We present an unsteady blade element theory (BET) model to estimate the aerodynamic forces produced by a freely flying beetle and a beetle-mimicking flapping wing system. Added mass and rotational forces are included to accommodate the unsteady force. In addition to the aerodynamic forces needed to accurately estimate the time history of the forces, the inertial forces of the wings are also calculated. All of the force components are considered based on the full three-dimensional (3D) motion of the wing. The result obtained by the present BET model is validated with the data which were presented in a reference paper. The difference between the averages of the estimated forces (lift and drag) and the measured forces in the reference is about 5.7%. The BET model is also used to estimate the force produced by a freely flying beetle and a beetle-mimicking flapping wing system. The wing kinematics used in the BET calculation of a real beetle and the flapping wing system are captured using high-speed cameras. The results show that the average estimated vertical force of the beetle is reasonably close to the weight of the beetle, and the average estimated thrust of the beetle-mimicking flapping wing system is in good agreement with the measured value. Our results show that the unsteady lift and drag coefficients measured by Dickinson et al are still useful for relatively higher Reynolds number cases, and the proposed BET can be a good way to estimate the force produced by a flapping wing system.
[en] The turbulent wake of screen-shrouded cylinders were experimentally investigated using flow visualization. Screen cylinders made from screen mesh of various porosity (37%, 49%, 61% and 67%) were used as the shrouds. The main purpose of the study is to examine the effect of screen porosity, β and screen diameter ratio, dw/D (wire diameter to cylinder diameter ratio) on the vortex development behind the shrouded cylinders, particularly in supressing the vortex shedding from a circular cylinder. The diameter ratio between the screen shroud and the plain cylinder, D/d was 2.0. The flow Reynolds number based on the shroud diameter, ReD was about 1000. Results showed that the inclusion of the screen shrouds has significant impact on the wake of the circular cylinder. With larger value of the non-dimensional parameter βdw/D, vortex was impaired and the formation length was longer in the shrouded cylinder wake. The vortex generation mechanism was also discussed. (paper)
[en] The size characteristics of atmospheric aerosol over the tropical region of Lagos, Southern Nigeria were investigated using two years of continuous spectral aerosol optical depth measurements via the AERONET station for four major bands i.e. blue, green, red and infrared. Lagos lies within the latitude of 6.465°N and longitude of 3.406°E. Few systems of dispersion model was derived upon specified conditions to solve challenges on aerosols size distribution within the Stokes regime. The dispersion model was adopted to derive an aerosol size distribution (ASD) model which is in perfect agreement with existing model. The parametric nature of the formulated ASD model shows the independence of each band to determine the ASD over an area. The turbulence flow of particulates over the area was analyzed using the unified number (Un). A comparative study via the aid of the Davis automatic weather station was carried out on the Reynolds number, Knudsen number and the Unified number. The Reynolds and Unified number were more accurate to describe the atmospheric fields of the location. The aerosols loading trend in January to March (JFM) and August to October (ASO) shows a yearly 15% retention of aerosols in the atmosphere. The effect of the yearly aerosol retention can be seen to partly influence the aerosol loadings between October and February. - Highlights: • Determining the aerosol size distribution via enhanced dispersion model. • Satellite exploration of aerosol optical properties using AERONET. • Comparative simulation between Reynolds and Unified number. • Extensive application of the aerosol optical depth. • Comparative analysis of AOD models. - Determining the aerosol size distribution using an enhanced dispersion model.
[en] An outlook on the recently proposed quasi-steady quasi-homogeneous (QSQH) theory of the effect of large-scale structures on the near-wall turbulence is provided. The paper focuses on the selection of the filter, which defines the large-scale structures. It gives a brief overview of the QSQH theory, discusses the filter needed to distinguish between large and small scales, and the related issues of the accuracy of the QSQH theory, describes the probe needed for using the QSQH theory, and outlines the procedure of extrapolating the characteristics of near-wall turbulence from medium to high Reynolds numbers. (paper)