Studies On Mechanism And Control Methods Of Aero-optical Effects For Supersonic Mixing Layers

For all kinds of precision-guided weapons,infrared imaging guidance is one of the main guidance systems to achieve accurate attack.However,the complex flow structures around the optical window(such as mixed layer,turbulent boundary layer,shock wave and expansion wave)will produce such as target image offset,blur and jitter,and this phenomenon is called aerooptical effects,which is one of the important factors affecting the guidance accuracy of the high speed weapons and it has become a research hotspot.The supersonic mixing layer is taken as the research object in this dissertation,especially focus on the flow characteristics of vortices in the supersonic mixing layer,spatial and temporal evolution of aero-optical effects as well as the control methods of the aero-optical effects.The relationship between the aero-optical effects of the supersonic mixing layer and the vortices in flow field is revealed,and the mechanism of aero-optical effects of the supersonic mixing layer is studied.Numerical and experimental methods are used to find effective control methods to reduce the aero-optical effects of the supersonic mixing layer notably.Specifically,the main research work is as follows:⑴ The instantaneous convection velocity of vortices in a mixing layer is studied.Convection velocity of a vortex is an important parameter to reflect flow characteristics of the vortex.Considering the cases of a single vortex with different sizes and two adjacent vortices in the process of pairing and merging,the instantaneous convection velocity of each case is calculated by using method for extracting position of a vortex core,and the results shown that: for the single vortex,its instantaneous convection velocity is fluctuating and the amplitude of convection velocity fluctuation increases with the size of the vortex;for the two adjacent vortices in the process of pairing and merging,the instantaneous convection velocity of each vortex exhibits a variation characteristics similar to the sine wave.In addition,influence of convection Maher number of a mixing layer,mixing angle and inflow components on the vortex evolution characteristics is also studied.⑵ The modeling and numerical verification of the spatial and temporal evolution of aero-optical effects of the supersonic mixing layer are carried out.Based on analysis of the relationship between aero-optical effects of a mixing layer and flow characteristics of vortices in the flow field,mathematical modeling of spatial and temporal evolution of aero-optical effects of a mixing layer is performed by using inflow parameters and reasonable assumptions,respectively.Then,the typical supersonic mixing layers are taken as examples to verify the validity of the built mathematical models.An empirical formula for predicting the growth rate of a supersonic mixing layer,including the convection Maher number,is proposed.Comparing with other prediction formulas,the new prediction formula more accords with physical reality and has better adaptability.⑶ The control methods for aero-optical effects of a supersonic mixing layer is studied.Considering the close connection between the vortex in flow field and the aero-optical effects of a mixing layer,control methods of the aero-optical effects are proposed from the viewpoints of modulation and suppression of a vortex respectively.Among them,the modulation of vortex is realized by the periodic control of the mixing layer and the suppression of vortex growth is realized by applying suction and blowing upstream of the source of the mixing layer,and both the modulation and suppression of vortex are studied by numerical simulations and wind tunnel experiments.A mathematical formula is summarized to describe the relationship between the control period,the vortex structure size and convection velocity of a vortex,that is,the size of a vortex in the controlled mixing layer is approximately equal to the product of control period and convection velocity of the vortex,and this formula can be used to predict the size of a vortex in the controlled mixing layer.The effect of blowing intensity on the growth of vortices downstream of the blowing slot is analyzed.It is found that a lower blowing intensity maybe achieve a better suppression effect.⑷ Numerical simulations of a supersonic mixing layer in near space are performed by using DSMC.By extracting the boundary of the mixing layer,thicknesses of a mixing layer at different altitudes are calculated to obtain the growth rate of the mixing layer,and the results shown that: growth rate of a mixing layer increases with the increasing altitude,and for a higher altitude,growth rate increased more significantly.For the same altitude,growth rate of the mixing layer decreases slowly with the increase of streamwise distance.The innovations of this dissertation are as follows:1.Convective velocity characteristics of vortices in a mixing layer and growth mechanism as well as evolution law of a vortex under periodic control are all revealed;2.Mathematical models describing relationship between the spatial and temporal evolution of aero-optics and flow characteristics of vortices are presented;3.Two ways(i.e.,modulation and suppression of a vortex)to control the aero-optical effects of the mixing layer are proposed and two test models used to blow and inhale upstream the mixing layer are designed and manufactured for controlling the aero-optical effects of the optical dome.