脉冲激励下超音速混合层涡结构的演化机理

Evolution mechanism of vortices in a supersonic mixing layer controlled by the pulsed forcing

采用大涡模拟方法对脉冲激励作用下的超音速混合层流场进行数值模拟,所得结果清晰展示了流场中涡结构的独特生长机理.基于涡核位置提取方法,对超音速混合层流场中涡结构的空间尺寸和瞬时对流速度等动态特性进行了定量计算.通过分析流场中涡结构的动态特性在不同频率脉冲激励下的变化,揭示出受脉冲激励超音速混合层流场中涡结构的演化机理:涡结构的生长不再是依靠相邻涡-涡结构之间的配对与融合,而是通过涡核外围的一串小涡旋结构被依次吸进涡核来实现,且受激励流场中各个涡结构的空间尺寸变化较小;流场中的涡结构数量与脉冲频率成正比例关系,而涡结构的空间尺寸与脉冲频率成反比例关系;涡结构的平均对流速度随脉冲频率的增大而减小.针对受脉冲激励超音速混合层,给出了能够表征涡结构特性与脉冲激励参数之间关系的方程式,即受激励流场中涡结构的平均对流速度与脉冲周期的乘积近似等于流场中涡结构的空间尺寸(涡结构平均直径).

Pulsed actuation is one of the most fundamental control types to study regularity of flow structures in supersonic mixing layers,which helps to predict the aero-optical effects caused by the supersonic mixing layer where the differentsized vortices dominate the flow field.However,the knowledge about the evolution mechanism of vortices in the supersonic mixing layer which is controlled by the pulsed forcing is limited.Based on the large eddy simulation（LES）,the visualized flow field of a supersonic mixing layer controlled by the pulsed forcing is presented and the unique growth mechanism of the vortices in such a case is revealed clearly.The method of position extraction of the vortex core in the supersonic mixing layer,which is a quantitative technique to obtain the instantaneous location of a vortex in flow field,is employed to calculate the dynamic characteristics（e.g.,instantaneous convective speed and size） of the vortices quantitatively.The pulsed forcings of different frequencies are imposed on the same supersonic mixing layer respectively,and the instantaneous convective speed and size of the vortices for each pulse frequency considered in this study are then computed.By comparing the dynamic characteristics of the vortices between cases,the evolution mechanism of the vortices in the supersonic mixing layer controlled by the pulsed forcing is revealed.as follows.1） Growth of the vortices in the supersonic mixing layer controlled by the pulsed forcing no longer depends on the pairing nor merging between adjacent vortices,which is just the growth mechanism of vortices in a free supersonic mixing layer.Actually,the size of a vortex in the controlled supersonic mixing layer is dominated by the imposed pulse frequency,so the size of each vortex in such a flow field is approximately identical.2） The number of vortices in the controlled supersonic mixing layer is proportional to the pulse frequency,whereas the size of vortex is inversely proportional to the pulse frequency.That is,the higher the pulse frequency,the bigger the number of vortices in the controlled flow field is and the smaller the size of every vortex.3） The average convective speed of vortices in the controlled supersonic mixing layer gradually decreases with pulse frequency increasing because the pulsed forcing essentially drags on the movement of vortices in flow field.Finally,an equation which describes the quantitative relationship between the dynamic characteristics of a vortex and the pulsed forcing frequency is derived,that is,the product of the average convective speed of vortices in the controlled supersonic mixing layer and the imposed pulse period is approximately equal to the mean diameter of vortices in the flow field.