In this paper, an experimental study on the stability of hypersonic plate boundary layer is carried out using a spanwise plasma actuation array. The characteristics and evolution of different kinds of unstable waves in the hypersonic plate boundary layer are analyzed based on the results of linear stability theory and high-frequency pulsation sensors. The typical morphological characteristics of the boundary layer and the macro-control effect of the plasma actuation array are explored through a high-speed schlieren method. Finally, based on grayscale mode extraction and proper orthogonal decomposition (POD), the influences of three different actuation frequencies on the instability waves and characteristic structure of the boundary layer are studied, including the dominant frequency of the first mode wave, the half-frequency of the dominant frequency of the first mode wave, and the dominant frequency of the second mode wave, the change of characteristic structures under the regulation of plasma actuation is further discussed. The corresponding regulation rules and mechanisms are summarized. The results show that the plasma actuation array can advance the starting position of laminar discontinuities and that the induced coherent structure can excite instabilities at an earlier flow-direction position. The actuation can be used to control the stability of the boundary layer by acting on the first mode wave to break the original unstable wave spectrum characteristics. This verifies the ability of extensional array plasma actuation to regulate the stability of the hypersonic plate boundary layer and suggests it has great potential in the promotion of hypersonic boundary layer transition.
The dc field dependencies of the ac susceptibility χ″ have been studied in polycrystalline samples of YBa2+xCu3O7‐y with x‐values ranging from 0 to 0.2. Data on χ″ versus T show the expected intergranular coupling peak at the temperature Tp. For a specific dc field Tp is found to increase with x varying from 0 to 0.10, and to decrease sharply at x > 0.10. Moreover, the transport critical current density Jct of samples increases with x up to 0.10. According to this evidence it can be concluded that a certain excess of barium in YBa2 + xCu3O7‐y improves the weak links at grain boundaries.
The effect of pressure on the electronic and optical properties of SrAl2O4 up to 25 GPa was studied by means of the pseudo-potential plane waves method within the generalized gradient approximation for exchange and correlation. The calculated lattice parameters are consistent with available experimental and theoretical data. By analyzing the electronic and optical properties, the pressure dependences of the electronic structures and optical constants were investigated. The band structures show an indirect band gap for this compound and the calculated band gaps expend with increasing pressure. Meanwhile, the optical properties including the dielectric spectra, absorption coefficient spectra, reflectivity, and the real part of the refractive index spectra in the low energy range have a blue shift. Given this, the optical properties of SrAl2O4 could be tuned by changing pressure to some degree, which is beneficial to the optical applications.
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