Study Of The Effect Of Surface Roughness On The Hypersonic Boundary Layer Stability

Hypersonic boundary layer transition is of great importance in the design of hypersonic vehicles,the success of transition prediction relies on a fundamental understanding of the transition mechanism.In a weak-disturbance environment,the path to transition consists of three stages: receptivity,linear eigenmode growth,and nonlinear breakdown to turbulence.As the initial transition stage,the boundary layer receptivity and stability which dominate the subsequent development of the boundary transition,more and more receive attentions.In this paper,the high-order WCNS method is adopted to numerically study the effect of the surface roughness on the hypersonic boundary layer stability,and the physical mechanism of the surface roughness induced boundary layer transition is analyzed.In order to study the flat-plate boundary layer stability,efficient time-marching methods are compared and the effects of several parameters on the second mode instability are analyzed.First,a steady laminar flowfield is computed by solving the two-dimensional compressible NS equations.Then,unsteady simulation is performed by introducing a local periodic suction-blowing upstream on the flat-plate surface,and the second mode instability is well captured.On the premise of the rational numerical solution,the second-order implicit dual-time-step method and the explicit RK method are compared to improve computational efficiency.The convergence criterion of the dual-time-step method is obtained.The effects of suction-blowing frequency and flat-plate wall temperature on the boundary layer stability are also conducted,the starting location of the second mode instability moves upstream with the increase of the frequency.The wall cooling unstabilizes the second mode instability.The two-dimensional wavy wall and elliptical roughness element boundary layer stability are respectively investigated.Compared with the flat-plate boundary layer,the wavy wall effectively suppresses the relatively high-frequency second mode instability,thus delays the boundary layer transition,while amplifies the relatively low-frequency second mode instability.Both the increase of the wavy-wall depth and the downstream movement of the wavy wall can reinforce the suppression of the second mode instability.As for the elliptical roughness element,the second mode instability is damped when the roughness element is located downstream of the synchronization point.The effect of double roughness elements on the boundary layer stability is essentially relative to the roughness location.The second mode instability is amplified or suppressed artificially with rational combination of different roughness location.In addition,in consideration of ablation and collision in real flight environments,the implicit LES method is used to simulate the three-dimensional combined roughness/pit model.In the beginning,transition mechanisms of four different shape roughness elements are compared.The conclusion shows that the cube roughness element induces the boundary layer transition earliest,the diamond roughness element has the widest wake region,while the hemisphere roughness element trips transition most inefficiently.Then a cylinder roughness element is chosen to induce the boundary layer transition,and a cylinder concave pit with the same size is placed downstream of the cylinder roughness element to study its effect on the transitional boundary layer.The variations of diameter,depth and streamwise location of the concave pit are simulated in detail.Overall,the concave pit helps to induce the boundary layer transition,which has great guiding significance for the understanding of the vechile surface flow in real flight environment.