The first-order liquid-liquid phase transition in supercooled Si is revisited by long-time first-principle molecular dynamics simulations. As the focus of the present paper, its nature is revealed by analyzing the inherent structures of low-density liquid (LDL) and high-density liquid (HDL). Our results show that it is a transition between a sp(3)-hybridization LDL and a white-tin-like HDL. This uncovers the origin of the semimetal-metal transition accompanying it and also proves that HDL is the metastable extension of high temperature equilibrium liquid into the supercooled regime. The pressure-temperature diagram of supercooled Si thus can be regarded in some respects as shifted reflection of its crystalline phase diagram.

To investigate the flow and heat transfer characteristics of a synthetic jet driven by piezoelectric actuator, experimental investigation utilizing particle image velocimetry, hot-wire anemometer and infrared camera was carried out. The results show that: (1) At the jet orifice exit, pairs of vortexes are generated, broken down and merged together periodically, forming a steady jet within a several slot width from distance near the orifice exit. And during the development, the synthetic jet spreads rapidly along the minor axis direction of the orifice. While along the major axis direction, the synthetic jet contracts firstly and then spreads slowly. (2) Excitation frequency forced on the actuator has a great effect on the synthetic jet flow field. There are two resonance frequencies at which the mean velocity and vorticity of the synthetic jet are maximized, especially at the higher resonance frequency. The resonance frequency values obtained by the experiment are lower than the theoretical values. (3) Similarly to the common jet impingement, the convective heat transfer coefficients at the target surface impinged by the synthetic jet also take on up-down tendency varying with the jet-to-surface spacing increment. But the jet-to-surface spacing ratio for optimum cooling achievement is greater and the cooling action region is wider than the former, indicating that the synthetic jet introduces a stronger entrainment and more vigorous penetration in the surrounding fluid.synthetic jet, impinging cooling, flow, heat transferThe synthetic jet is a jet-like mean fluid motion formed by time-periodic, alternate suction and ejection of fluid through an orifice bounding a small cavity, by the time periodic motion of a diaphragm that is built into one of the walls of the cavity (Figure 1). The diaphragm is forced to oscillate, with fluid being expelled through the orifice as the diaphragm moves upwards. The flow separates at the edge of the orifice, inducing a vortex ring that moves outwards under its own

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