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[en] The orientation of a cone settling in a vertical duct has been numerically investigated by using the direct-forcing fictitious domain method. Our results indicate that with the characteristic Reynolds number Re being 30, the cone settles with apex pointing upward if the cone apex angle is smaller than 48°, irrespective of the initial orientation. For the apex angle larger than 48°, the cone also turns upward as long as the releasing angle is smaller than a critical one, otherwise it either turns to an inclined angle if the apex angle is smaller than (or equal to) 52°, or turns downward for the apex angle larger than 52°. The fluid inertial effect helps to turn the cone downward and both the critical apex angle for the unanimous upward-turning and the releasing angle for the turning direction at a fixed apex angle become smaller for a higher Reynolds number, although the effects of the Reynolds number on the critical angles are not enormous. The hairpin-like vortex structures are observed in the wake of the settling cone at Re = 400, accompanied by the persistent oscillation of the orientation angle. Both stable tilted and apex-upward orientations for the cone with the apex angle of 50°, depending on the releasing angle, are confirmed in our experiments. (paper)
[en] We propose and experimentally validate a wavelength diversity technique combined with a passive depolarizer in order to improve the performance of Brillouin optical time domain reflectometry (BOTDR). The wavelength diversity technique enables the maximization of the launch pump power and suppresses the nonlinear effects, the latter of which limits the conventional BOTDR performance. As a result, the signal-to-noise ratio increases, thus improving the measurement accuracy for strain and temperature. In addition, considering the complexity and expensive methods required for polarization noise suppression in BOTDR system, a simple, low-cost passive depolarizer is employed to reduce the polarization noise. The experimental results show that the signal-to-noise ratio is improved by 4.85 dB, which corresponds to 174% improvement compared to a conventional BOTDR system. (paper)
[en] By following the Kazantsev theory and taking into account both microscopic and turbulent diffusion of magnetic fields, we develop a unified treatment of the kinematic and nonlinear stages of a turbulent dynamo process, and we study the dynamo process for a full range of magnetic Prandtl number P m and ionization fractions. We find a striking similarity between the dependence of dynamo behavior on P m in a conducting fluid and (a function of ionization fraction) in a partially ionized gas. In a weakly ionized medium, the kinematic stage is largely extended, including not only exponential growth but a new regime of dynamo characterized by a linear-in-time growth of magnetic field strength, and the resulting magnetic energy is much higher than the kinetic energy carried by viscous-scale eddies. Unlike the kinematic stage, the subsequent nonlinear stage is unaffected by microscopic diffusion processes and has a universal linear-in-time growth of magnetic energy with the growth rate as a constant fraction 3/38 of the turbulent energy transfer rate, showing good agreement with earlier numerical results. Applying the analysis to the first stars and galaxies, we find that the kinematic stage is able to generate a field strength only an order of magnitude smaller than the final saturation value. But the generation of large-scale magnetic fields can only be accounted for by the relatively inefficient nonlinear stage and requires longer time than the free-fall time. It suggests that magnetic fields may not have played a dynamically important role during the formation of the first stars.
[en] A CFD strategy is proposed that combines delayed detached-eddy simulation (DDES) with an improved RANS-LES hybrid model aimed at wall modelling in LES (WMLES). The system ensures a different response depending on whether the simulation does or does not have inflow turbulent content. In the first case, it reduces to WMLES: most of the turbulence is resolved except near the wall. Empirical improvements to this model relative to the pure DES equations provide a great increase of the resolved turbulence activity near the wall and adjust the resolved logarithmic layer to the modelled one, thus resolving the issue of 'log layer mismatch' which is common in DES and other WMLES methods. An essential new element here is a definition of the subgrid length-scale which depends not only on the grid spacings, but also on the wall distance. In the case without inflow turbulent content, the proposed model performs as DDES, i.e., it gives a pure RANS solution for attached flows and a DES-like solution for massively separated flows. The coordination of the two branches is carried out by a blending function. The promise of the model is supported by its satisfactory performance in all the three modes it was designed for, namely, in pure WMLES applications (channel flow in a wide Reynolds-number range and flow over a hydrofoil with trailing-edge separation), in a natural DDES application (an airfoil in deep stall), and in a flow where both branches of the model are active in different flow regions (a backward-facing-step flow)
[en] We investigate the structure of trapped Bose-Einstein condensates (BECs) with long-range anisotropic dipolar interactions. We find that a small perturbation in the trapping potential can lead to dramatic changes in the condensate's density profile for sufficiently large dipolar interaction strengths and trap aspect ratios. By employing perturbation theory, we relate these oscillations to a previously identified 'rotonlike' mode in dipolar BECs. The same physics is responsible for radial density oscillations in vortex states of dipolar BECs that have been predicted previously
[en] An experimental observation of spatiotemporal evolution of dust density waves (DDWs) in cogenerated dusty plasma in the presence of modified field induced by glass plate is reported. Various DDWs, such as vertical, oblique, and stationary, were detected simultaneously for the first time. Evolution of spatiotemporal complexity like bifurcation in propagating wavefronts is also observed. As dust concentration reaches extremely high value, the DDW collapses. Also, the oblique and nonpropagating mode vanishes when we increase the number of glass plates, while dust particles were trapped above each glass plates showing only vertical DDWs
[en] Highlights: • A bench top flow rig of hemodialysis is realised. • Dynamic similarity is preserved using dimensional scaling of Reynolds number. • A disturbed flow region develops downstream of the venous needle jet. • Disturbed flows represents a potential site of stenosis on the roof of the vein. • Disturbed flows can be minimized through careful application of the venous needle. - Abstract: Stereoscopic particle image velocimetry (S-PIV) was used to quantify the dynamic flow field caused by the venous needle jet (VNJ) in an idealised model of hemodialysis cannulation. Scaling based on Reynolds number ensured dynamic similarity with physiological conditions. Measurements taken along the center plane indicate the presence of a steady secondary flow region, which develops downstream of the impingement region. Upon impingement, a wall jet forms on the floor of the vein and spreads along the curvature of the vessel. Circulating flow forms due to the interaction between the jet spreading and the wall jet. This secondary flow region represents a potential site of stenosis on the roof of the vein where flow is reversed. The effects of the circulating flows can be minimized by using shallow needle angles, low needle flow rates and placement of the needle away from the walls of the vein.
[en] In this paper, we investigate the coupling of linear three-hole cavities (L3) into PC waveguides. We choose the L3 cavities owing to the high value of their quality factor (Q) to mode volume (V) ratio and their good match between cavity and waveguide field patterns, which improves the in-plane coupling efficiency. The systems are designed to increase the overlap between the evanescent cavity field and the waveguide mode, and to operate in the linear dispersion region of the waveguide. Our simulations indicate increased coupling when the cavity is tilted by 60° with respect to the waveguide axis. The transmission spectra and the field patterns are obtained by using Fullwave, a commercially available finite-difference time domain (FDTD) code. From a transmission calculation, a very high Q-factor value has been achieved at λ = 1.51 μm.
[en] In 1980s, Arak has obtained powerful inequalities for the concentration functions of sums of independent random variables. Using these results, he has solved an old problem stated by Kolmogorov. In this paper, one of Arak’s results is modified to include generalized arithmetic progressions in the statement.
[en] Highlights: • Froude number affects the critical normalized submergence depth. • Proximity to a free surface fosters/weakens VIV for low/high Fr. • Phase lag of transverse displacement behind lift jumps at some reduced velocity. • Free-surface vortex strongly interacts with cylinder-shedding ones for high-Fr VIV. • Frequency of high-amplitude VIV differs from natural structure frequency in fluids. - Abstract: Two-degree-of-freedom vortex induced vibration (VIV) of a low-mass zero-damping circular cylinder horizontally placed near a free surface at Re = 100 was numerically studied with an adaptive Cartesian cut-cell/level-set method. Two Froude numbers and various normalized submergence depths were considered. The results reveal that the Froude number affects the critical normalized submergence depth and possible physical mechanisms are proposed. The in-line vibration amplitude cannot be neglected. Proximity to a free surface strengthens and suppresses the VIV for low and high Froude numbers, respectively; increases the occurrence of amplitude modulation; and in general enhances the magnitude of the time-averaged lift coefficient, which is always negative. The phase lag of the transverse displacement behind the lift coefficient jumps at some reduced velocity, which strongly depends on the Froude number and normalized submergence depth. Regular trajectories exist only in cases with a small vibration amplitude or a large normalized submergence depth. The vortex structures in any case with large transverse amplitude basically originate from the alternative vortex shedding with the negative vortex weaker than the positive one. For the higher Froude number, an extra free surface positive vortex interacts with the vortices from the cylinder surface. The vibration frequency deviates from the natural structure frequency in fluids in the large-amplitude regime.