Influence Of Porous Structure On The Transition Of Hypersonic Boundary Layer Along Wedge Surfaces

The boundary layer transition has always been the focus of hypersonic aerodynamic research.Predicting the transition location and controlling the transition process are of great significance in the design of hypersonic aircraft.This paper focuses on the problem that porous structure affects the transition of hypersonic boundary layer.The effect of porous structure with different scales and characteristics on the boundary-layer transition and instabilities are simulated numerically.This study can provide a guidance for the flow control method of transition/aerodynamic thermal using surface porous characteristics.The main research conclusions are as follows:(1)?-Re?t model is adopted to simulate macroscopic porous wedge.The results show that the transition is triggered in advance by the porous structure with D=2 mm and large aspect ratio L/D=2.Meanwhile,the surface St number increases to the turbulent level in advance,and the transition position is advanced about 40%.When the pore size is reduced to 1mm,the transition onset is delayed to the downstream of porous structure,but it is still earlier than the smooth wedge,and the effect of early transition is enhanced with increasing the number of pores.The results show that different location of transition is due to the sudden change of velocity profile and consequently a sharp increase of local Rev/Re? of the flow,which determines the forward movement of the transition onset.(2)LES model was used to explore the effect of macroscopic porous structure on the Mack second-mode instability,and a comparative analysis with the boundary-layer instability characteristics of a smooth wedge found that the porous structure effectively suppressed the Mack second mode perturbation and promotes the amplitude of Mack second mode increase after the next synchronization point.When the position of the pore is placed closer to the next synchronization point,the peak value of wall pressure fluctuation is obviously smaller than that downstream of porous1(streamwise position is not backward).Compared with porous1,porous2(streamwise position is backward)does not promote the instability amplification of mode S.(3)When the scale of porous structure is mesoscopic,the calculation results of LES show that mesoscopic pores have a strong inhibitory effect on Mack second mode.After that,DMD method is introduced to analyze the main dynamic information of the flow field downstream of mesoscopic porous structure and macroscale porous 1(streamwise position is not backward).It is found that the first DMD mode flow field downstream of mesoscopic porous structure captures a higher percentage of energy than macroscale porous 1,while the energy amplitude percentage of the second,third and fourth DMD modes are all lower than macroscale porous 1,indicating that overall development of the flow field downstream of the mesoscopic porous structure is more stable,which reflects the inhibition of mesoscopic pores on the instability of downstream boundary layer.