Research On Control Methods And Mechanisms Of Supersonic/Hypersonic Boundary Layer Drag Reduction Subject To Active Flow Control

The transition of hypersonic boundary layer from laminar to turbulent flow will increase the friction drag coefficient by 3-5 times,which greatly affects the flight range and aerodynamic performance of the vehicle.Aiming at delaying the onset of the boundary layer transition and reducing the friction drag in turbulence zone,this thesis carries out the research on control methods and mechanisms of supersonic/hypersonic boundary layer drag reduction subject to active flow control.Combined with direct numerical simulation(DNS),wind tunnel experiment and theory analysis,the main work of the dissertation is listed as follows:Aiming at the classical Ma2.25 supersonic boundary layer flow characteristics,LST(linear stability theory)is used at first to provide the most unstable frequency information for the inlet generation conditions of the TBL,and then DNS is carried to simulate the spatially developed supersonic TBL.Sufficient verification of turbulence statistical data confirms the reliability of the DNS method and data.Based on the instantaneous coherent structures and two-point spatial correlation,the existence of alternating distributed high-and low-speed streaks and their scale features are examined and confirmed.Combined with compressible Renard-Deck(R-D)decomposition of skin friction coefficient,the main composition and corresponding proportion are given.The spectral proper orthogonal decomposition of the skin friction coefficient obtains the spatial-temporal decoupled single-frequency mode and a more realistic flow field.These lay the foundation for the next step of study on turbulence drag reduction.The streamwise-striped wall blowing is proposed for supersonic turbulent boundary layer drag reduction,and the effects of different slit width,spacing and blowing amplitude on turbulence statistics and structures are investigated via DNS.The results show that within the range of studied parameters,the drag reduction rate(DR)is approximately proportional to the control area and the amplitude.A blowing amplitude of 0.2% can achieve a DR of 14.5% on the premise of obtaining a positive net energy saving rate.The statistic and structure characteristics of TBL before and after flow control are compared in detail,and it is found that the decrease of mean viscous shear stress is the direct cause of drag reduction,and the spatial scaling of coherent structures in control flow field is decreasing significantly.Furtherly,a novel drag reduction method of compressible TBL based on velocity-temperature coupled control is proposed,and a considerable DNS database is constructed.It is found that appropriately increasing the temperature of wall blowing can greatly increase the DR up to 20.1% while maintaining the net energy saving rate.A G-S map between gain and net energy saving rate is given,which indicates that the balance between them should be carefully considered.It is proved that there is a correspondence between DR and the slope of wall law.The more significant reduction in mean density offsets the effect of the increase in viscosity,and the reduction in mean viscous shear stress is still the direct cause of drag reduction,although turbulence amplification happens.And it is found that the significant reduction of spatially growth term of compressible R-D decomposition is an important contribution.As the Mach number increases to 6,the control ability and efficiency of proposed velocity-temperature control technique decrease,and only a DR of 10.58% can be achieved,although it is still almost the sum of wall blowing(5.27%)and wall heating(6.35%).The anisotropy invariant of Reynolds stress is investigated and the turbulence amplification can be attributed to the increased intensity of coherent structures.Based on the finite-time Lyapunov exponent method,Lagrangian coherent structures of TBL is extracted and compared,and flow topology structures in Lagrangian system are revealed.Wind tunnel experiment and DNS study on the transition promotion of hypersonic boundary layer based on self-sustaining synthetic jets are conducted.The results show that as transition occurs,wall friction drag and wall heat flux increase simultaneously,which indicates the importance of transition delay control.Using high-order numerical simulation and LST,the transition delay control of hypersonic boundary layer based on steady wall blowing and suction is studied,and the effect on unstable mode and flow structures by applying control upstream or downstream of the synchronization point are compared and analyzed.The influence law on unstable mode disturbance waves in different wave numbers and frequency ranges is obtained.The results indicate that the combined control of upstream suction and downstream blowing has better control effect.At the same time,it is found that the steady wall blowing and suction would cause the destabilization of high-frequency instability waves.Thus,a wideband-frequency disturbance suppression method combined steady blowing and suction with porous wall is proposed for transition delay.Substantive combined parameter studies reveal the control mechanism and control law under active-passive combined control.