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In this paper, the spatiotemporal evolution and the instability of the turbulence mixing layer in a supersonic ejector are deeply studied based on large eddy simulation. Time-averaged distributions and transient results are discussed based on mixed fluids. In particular, the general flow characteristics are identified through the examination of the spatiotemporal evolution of temperature, entropy, Mach number, and density gradient fields. A thin shear layer is gradually formed, and several ring vortices are effectively captured at the lip of the nozzle. Surprisingly, the annular vortex gradually evolves into a V-shaped vortex with time evolution, which reveals that the transition of turbulence occurs in the first half of the mixing chamber. It is found that the supersonic mixing layer exhibits slow growth initially, and a significant increase in velocity is effectively captured at the throat position based on time averaged velocity profiles. A similar trend is also evident in the transition region of the supersonic ejector throat. The mixing of motive flow and suction flow is effectively enhanced by a shock train generated within the diffusion chamber. A recirculation region is created, which leads to a narrowing of the potential core of the jet, and the symmetry of the flow is broken due to the shock location. It is further found that the instability of the main jet is the dominant factor in the fluid mixing process by studying the power spectrum of the pressure signal and characteristic frequency inside the mixing chamber. It further provides important physical insights into understanding the spatiotemporal evolution of internal turbulence and the mixing mechanism in the supersonic steam ejector. [ABSTRACT FROM AUTHOR]

Diarylethene (DAE) is one of the most widely used functional units for electrochromic or photochromic materials. To better understand the molecular modification effects on the electrochromic and photochromic properties of DAE, two modification strategies, substitution with functional groups or heteroatoms, were investigated theoretically by density functional theory calculations. It is found that red-shifted absorption spectra caused by a decreased highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap and S0 → S1 transition energy during the ring-closing reaction become more significant by adding different functional substituents. In addition, for two isomers, the energy gap and S0 → S1 transition energy decreased by heteroatom substitution of S atoms with O or NH, while they increased by replacing two S atoms with CH2. For intramolecular isomerization, one-electron excitation is the most effective way to trigger the closed-ring (O → C) reaction, while the open-ring (C → O) reaction occurs most readily in the presence of one-electron reduction. Moreover, it is confirmed that substitution with strong electron donating groups (–OCH3/–NH2) or with one O/two CH2 heteroatoms leads to a more favorable closed-ring (O → C) reaction. Functionalized with strong electron-withdrawing groups (–NO2 and –COOH) or one/two NH heteroatom substitutions, the open-ring (C → O) reaction is easier. Our results confirmed that the photochromic and electrochromic properties of DAE can be tuned effectively by molecular modifications, which provides theoretical guidance for the design of new DAE-based photochromic/electrochromic materials. [ABSTRACT FROM AUTHOR]

Tunable complementary split ring resonators (CSRRs) based on monolayer graphene are presented in terahertz regime. By applying different gate voltage, the Fermi level and optical conductivity of monolayer graphene pattern can be changed. Here, we employ a numerical simulation to study the interaction of light with graphene CSRRs. The results indicate that the extinction in transmission becomes stronger, and the resonance frequency presents blue shift with higher Fermi level of the graphene pattern. Three pronounced resonant peaks appear which can be modulated dynamically in the range of 1-2THz and 3-7THz, and realizing dynamic broadband terahertz modulation, the modulation depth exceeds 85% at all three resonant peaks, the highest modulation depth reaches 98.8% at 7.47THz. [ABSTRACT FROM AUTHOR]