Browsing by Author "Yang, J."
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Item Numerical Study on Uniform-Shear Flow over a Circular Disk at Low Reynolds Numbers(American Institute of Physics Inc.) Yang, J.; Liu, M.; Wang, C.; Zhu, Xiaowei; Zhang, A.; Zhu, XiaoweiUniform shear flows over a circular disk of aspect ratio 10 (thickness/diameter) at low Reynolds numbers are numerically investigated with the main focus on the effect of inlet shear on the wake evolutions. The Reynolds numbers considered are Re = 140, 160, and 180 based on the inlet center velocity uc and disk diameter d. The non-dimensional shear rate k(= |V̅u|d/u_c) is varied from 0 to 0.09. The bifurcations leading to unsteady states with hairpin vortex shedding occur much earlier in uniform shear. In most cases, wake evolutions occurring as the shear rate increases in uniform shear are similar to those as the Reynolds number increases in uniform flow. A new wake mode termed as dragonfly-wings (DW) mode is captured at Re = 180 and k = 0.01 and 0.03. At DW mode, hairpin vortex structures are shed from diametrically opposite orientations, but with irregularity in strength and shape, i.e., three different vortex loops are observed in the wake, and produce three peaks at low frequencies in the frequency spectrum of the drag. The planar-symmetry plane for standing-wave and zig-zig modes is determined by both the initial conditions and the direction of the uniform shear. It is found that with increasing inlet shear rate, the non-dimensional shedding frequency remains nearly constant for the low shear rates (k < 0.1). Time-averaged drag and lift coefficients slightly increase with increasing inlet shear rate. Finally, the hysteretic property of the DW mode transition is examined and further investigated using the Landau mode, indicating that DW mode transition is non-hysteretic (supercritical). ©2018 Author(s).Item Propagation Characteristics of Plasmaspheric Hiss: Van Allen Probe Observations and Global Empirical Models(Amer Geophysical Union, 2017-03-13) Yu, J.; Li, L. Y.; Cao, J. B.; Chen, Lunjin; Wang, J.; Yang, J.; 0000-0003-2489-3571 (Chen, L); Chen, LunjinBased on the Van Allen Probe A observations from 1 October 2012 to 31 December 2014, we develop two empirical models to respectively describe the hiss wave normal angle (WNA) and amplitude variations in the Earth's plasmasphere for different substorm activities. The long-term observations indicate that the plasmaspheric hiss amplitudes on the dayside increase when substorm activity is enhanced (AE index increases), and the dayside hiss amplitudes are greater than the nightside. However, the propagation angles (WNAs) of hiss waves in most regions do not depend strongly on substorm activity, except for the intense substorm-induced increase in WNAs in the nightside low L-region. The propagation angles of plasmaspheric hiss increase with increasing magnetic latitude or decreasing radial distance (L-value). The global hiss WNAs (the power-weighted averages in each grid) and amplitudes (medians) can be well reproduced by our empirical models.Item Tuning the Structure of Bifunctional Pt/SmMn₂O₅ Interfaces for Promoted Low-Temperature CO Oxidation Activity(Royal Society of Chemistry, 2019-01-30) Liu, X.; Yang, J.; Shen, G.; Shen, M.; Zhao, Y.; Cho, Kyeongjae; Shan, Bin; Chen, R.; 0000-0003-2698-7774 (Cho, K); 0000-0001-7800-0762 (Shan, B); Cho, Kyeongjae; Shan, BinThe interfacial structure of metal-oxide composite catalysts plays a vital role in heterogeneous catalysis, which is crucial to the adsorption and activation of reactants. Herein, the interfacial effects of bare and Fe/Co/Ni doped Pt/SmMn₂O₅ mullite oxide supported Pt clusters on CO oxidation have been investigated by first-principles based microkinetics analysis. A robust formation of Pt/Mn₂ trimer structures is demonstrated at the bifunctional interfaces irrespective of the Pt_{n} cluster's size, which can provide spatially separated sites for CO adsorption and O₂ dissociation. The binding strength of CO at the interfacial Pt sites is in the optimal range due to the charge transfer from Pt clusters to oxide, while the strong polarization of Mn₂ dimers induced by Pt clusters with stable three-dimensional morphologies can lower the energy barrier of O₂ dissociation. Based on the microkinetics analysis, the O₂ dissociation is the rate-determining step in the full CO oxidation cycle, and the introduction of Mn-Fe hetero-dimers at the interface is predicted to further enhance the low temperature CO oxidation activity of Pt/SmMn₂O₅ catalysts. © The Royal Society of Chemistry 2019.