Tham khảo tài liệu 'unsteady aerodynamics, aeroacoustics and aeroelasticity of turbomachines episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 464 Introduction Transonic fbws about streamlined bodies are strongly affected particularly near the shock location by unsteady excitations. Experimental and computational studies 1 2 have shown that the unsteady pressure distribution along the surface of an airfoil or a cascade blade in unsteady transonic ft w exhibits a significant bulge near the shock location. Tijdeman and Seebass 3 reported that the unsteady pressure bulge and its phase variation resulted from nonlinear interaction between the mean and unsteady ft ws. This non-linear interaction causes a shift in the shock location which produces the observed large bulge in the unsteady pressure distribution. Studies 4 on choked flitter have shown that in unsteady transonic fl ws around a single airfoil the shock motion and thus the pressure distribution along the surface can be critical regarding to the self-exciting oscillations of the airfoil. It was also shown that the mean fl w gradients are of high importance regarding the time response of the unsteady pressure distribution on the airfoil surface. Beside numerical computations 5 pointed out that the exact location of the transition point could strongly affect the prediction of stall flitter. Further studies 6 suggested that this sharp rise in the unsteady pressure distribution was due to the near sonic condition and that the near-sonic velocity acts as a barrier they identified as acoustic blockage preventing acoustic disturbances from propagating upstream in a similar way to the shock in transonic flaws. A transonic convergent-divergent nozzle experimentally investigated by Ott et al 7 was thereafter used as a model to investigate the non-linear acoustic blockage. Analytical and numerical computations 8 9 10 11 were then carried out to analyze and quantify the upstream and downstream propagation of acoustic disturbances in the nozzle. Similarly in order to focus the present analysis on essential features the investigation has been carried out in a .
