Supersonic Boundary Layer Bleed Flow Field Modelling And Its Application

Boundary layer bleed is a traditional and effective flow control method,which has been widely used in supersonic inlet design.With the development of hypersonic vehicle technology in recent years,the requirements for the design level and technology of bleed system are constantly improved.In this paper,the key issues is studied by the methods of theoretical analisys and numerical simulations,when boundary layer bleed is applied in hypersonic inlet.There are complex phenomenon including shock boundary-layer interactions,shock-shock interactions and buzz induced by shock oscillations in hypersonic inlet,which can easily induce the inlet unstart.It is an effective method to improve the performance of hypersonic inlet by the layout of multi bleed holes to discharge the boundary layer of low-energy gas at different positions.Focused on the design of the bleed system of the hypersonic inlet,the investigation are carried out about the supersonic bleed model,the scale effects of bleed,the layout of the multi bleed holes and the curvature boundary layer bleed.The bleed system is a combination of porous bleed holes,but the single hole is extracted from the multi holes so as to to obtain the clear physical mechanism of bleed flow thenomenon.Beasuse the three-dimensional effects of circular bleed hole can introduce complex issues,the two-dimensional inviscid bleed is extracted from the single hole.According to the local flow characteristics of the bleed hole and the assumption that the pressure difference is the driving force of bleed,the inviscid two-dimensional bleed model is obtained.In supersonic condition,the disturbance can only be confined in the Mach cone and cannot propagate upstream,therefore,the bleed mass flow rate with the supersonic leading edge of the bleed hole is independent of the swept angle.Therefore,based on the two-dimensional bleed model and the relationship between polygons and circular hole geometry,the inviscid three-dimensional circular bleed model is established by gradually approaching with polygons.Furthermore,considering the viscous effect,the influence of boundary layer on bleed is equivalented to the influence of displacement thickness on bleed hole size,and a three-dimensional thin boundary layer bleed model is established finally.The size of bleed hole is a key parameter in the design of bleed system.For the depthless bleed hole,by adjusting the displacement thickness δ* of the boundary layer at the leading edge of the bleed hole,the numerical results show that the small change of boundary layer parameters leads to the significant change of bleed mass flow rate when the displacement thickness is approximately equal to the diameter of the bleed hole,which indicates that bleed system has the ablility of adaptive adjustment.Future,the evolution and development process of bleed flow coefficient are obtained under different ratios of boundary layer thickness to diameter δ/D and adepth to diameter L/D.The result of numerical simulation verified the theoretical analysis that lateral flow occurs at the subsonic leading edge of the the circular,thus revealed the reason of the lateral flow phenomenon observed in the experiments and numerical simulations.The interactions of both the flow direction and lateral direction of multi bleed holes and the layout of multi bleed holes are important factors that affect the performance of the hypersonic bleed system.And these factors have an influence on the capture coefficients and structural strength in the finite space of inlet.Firstly,the evolution of flow coefficients are studied with the interactions of multi-hole in the fow direction.Then,aimed at the bleed of sepration bubble,the relationship is clarified between the bleed mass flow rate and the relative position of the bleed hole from the shock incident point.Finally,the flow coefficients of the multi hole layout of diamond,rectangle and triangle are compared and shows a small difference in the case of thin boundary layer,which indicates that the bleed mass flow rate mainly depends on the bleed area.This can provide reference for the engineering application of hypersonic inlet bleed system.Finally,the bleed performance with cuvature boundary layer is studied.Aimed at the development of curvature surface compression of hypersonic inlet,the 2-D streamwise curvature boundary layer bleed is studied.The influence of cuvature wall effecs on pressure ratio of bleed is revealed.The process is clarifed that when concave and convex curvature wall turn the same angle corner,the increase of bleed mass flow rate of concave wall is greater than the decrease of convex wall.