Effect Of Roughness Element On Hypersonic Boundary Layer Receptivity Under Freestream Disturbance

The laminar-turbulent transition of hypersonic boundary layer leads to a significant increase in skin friction and surface heating of hypersonic vehicles,which may affect the propulsion efficiency of hypersonic vehicles and put forward higher requirements for the thermal protection system of hypersonic vehicles.Therefore,it is very necessary to acquire the transition mechanism of hypersonic boundary layer.The first and essential phase of hypersonic boundary layer transition is called "Receptivity".Receptivity refers to the response phase of boundary layer to external disturbances,in which perturbations in boundary layer are excited by external disturbance and propagate downstream.The study of receptivity is the basis for revealing the mechanism of boundary layer transitionReceptivity is affected by multiple factors,which can be simply divided into disturbances with freestream and wall disturbance.A large number of roughness elements,including isolated roughness element and distributed roughness elements,exist on the surface of actual hypersonic vehicles.In order to unify the definition of roughness element,the cavities which are below the smooth wall are also referred to as roughness elements,named D-type roughness elements.Correspondingly,the conventional roughness elements are called K-type roughness elements.In recent years,many scholars have shown great interest in the receptivity mechanism of boundary layer under roughness elements.Previous studies have shown that the roughness element can not only be applied to drag reduction and heat reduction of hypersonic vehicles,but also make the boundary layer more stable.However,the exact receptivity mechanism of hypersonic boundary layer under various disturbances remains unclear,which is very fatal to explain boundary layer transition.In view of this,a high-order precision finite difference method is used to simulate a hypersonic flowfield over blunt cone in order to obtain the effect of roughness element on hypersonic boundary layer On this basis,Fast Fourier transform method is used to analyze the effect of roughness element on hypersonic boundary layer receptivity.Some conclusions are obtained:New compression waves and expansion waves are formed in the flow field.The intensity of compression waves and expansion waves increases with the roughness element becoming higher and deeper and moving upstream.The flow parameters in the boundary layer are amplified in the upstream half of K-type roughness element and the downstream half of D-type rough element,and vice versa.The inhibitory effect of K-type roughness element can last for a long distance in its downstream region.Isolated roughness element can lead to the appearance of vortices in the boundary layer,and D-type rough element is more likely to lead to the appearance of vortices.Isolated roughness element can lead to the appearance of vortices in the boundary layer,and D-type rough element is more likely to lead to the appearance of vorticesContinuous freestream disturbance has a great influence on hypersonic flow field,in which acoustic wave has the greatest influence,followed by entropy wave and vortex wave.The dominant modes of disturbances in the boundary layer caused by fast waves,entropy waves and vortical waves are fundamental modes,while the dominant modes under slow waves include second-order harmonic mode.Isolated roughness element suppresses the second harmonic mode caused by slow waves,and the fundamental mode becomes the dominant mode.Isolated roughness element suppresses the second harmonic mode caused by slow waves,and the fundamental mode becomes the dominant mode.The amplitude of the disturbances in the boundary layer caused by the pulse wave is small,and the spectrum of the disturbances in the boundary layer is continuous.Isolated roughness element changes the transformation process of dominant modes in the boundary layer.The dominant modes in the boundary layer are fundamental and second-order harmonic modes,and there are two dominant mode transformations in the boundary layer under the conditions of smooth wall,D-type roughness element and shorter K-type roughness element.When the K-type roughness element reaches a certain height,the number of dominant modes in the boundary layer increases to four,and the third-order harmonic mode becomes dominant mode in a certain distance.When the K-type roughness element reaches a certain height,the number of dominant modes in the boundary layer increases to four,and the third-order harmonic mode becomes dominant mode in a certain distance.Whether K-type roughness element or D-type roughness element,the transformation of dominant modes in the boundary layer is accelerated by the increase of deformation degree of the roughness element.A roughness element in distributed roughness elements is affected by its adjacent upstream and downstream roughness elements.For distributed roughness element,the adjacent upstream roughness element inhibits the amplification of the upstream half to the freestream disturbances,while the adjacent downstream roughness element inhibits the suppression of the downstream half to the evolution of the disturbances.The response characteristics of different modes of disturbances to distributed roughness elements in boundary layer are different.