Experimental Study on the Transition Process to the Oscillatory Thermocapillary Convections in a Floating Half Zone

The transition process of the thermocapillary convection from a steady and axisymmetric mode to the oscillatory mode in a liquid bridge with a fixed aspect ratio and varied volume ratio was studied experimentally. To ensure the surface tension to play an important role in the ground-based experiment, the geometrical configuration of the liquid bridge was so designed that the associated dynamic Bond number Bd ≈ 1. The velocity fields were measured by Particle Image Velocimetry (PIV) technique to effectively distinguish the different flow modes during the transition period in the experiments. Our experiments showed that as the temperature difference increased the slender and fat bridges presented quite different features on the evolution in their flow feature: for the former the thermocapillary convection transformed from a steady and axisymmetric pattern directly into an oscillatory one; but for the latter a transition flow status, characterized by an axial asymmetric steady convection, appeared before reaching the oscillatory mode. Experimental observations agree with the results of numerical simulations and it is obvious that the volume of liquid bridge is a sensitive geometric parameter. In addition, at the initial stage of the oscillation, for the former a rotating oscillatory convection with azimuthal wave number m = 1 was observed while for the latter a pulsating oscillatory pattern with azimuthal wave number m = 2 emerged, and then with further increase of the temperature difference, the pulsating oscillatory convection with azimuthal wave number m = 2 evolved into a rotating oscillatory pattern with azimuthal wave number m = 2.

疲劳寿命预计方法的探计

<正> 一、引言过去,承受交变载荷作用的零件或结构,人们一直是采用静强度理论来进行计算和设计的,有些部门直至今天仍在沿用这种传统的方法。但是静强度理论不能充分反映材料在变应力作用下的特性,因此,这种按不变载荷公式设计而实际是在变载荷作用下工作的零构件,在使用过翟中往往就会发生突如其来的破坏。起初,人们对这种破坏感到莫明其妙,零件构件的设计都已充分满足了静强度的要求,为什么还会突然破坏呢?后来逐渐了解到,这种突然性的破坏绝大多数都是疲劳引起的,因为疲劳破坏与静力破坏有着本质的差别。