Experimental Investigation On The Evolution Of Disturbances In A Flat Plate Laminar Boundary Layer

The experimental investigation on the evolution of disturbances in a flat plate laminar boudary layer was executed.The experiment was carried out on a flat plexiglass plate with elliptic leading edge in the environment of wind tunnel freestream.The flow was two-dimensional and parallel.The freestream disturbances were multi-frequency,spatial mode and normal mode.Streamwise velocity measurement was made using single-wire hot-wire probe in the boundary layer region,which is from 140mm to 440mm in the streamwise direction.The velocity were measured on the normal direction at every streamwise location.The mean velocity profiles were obtained from the velocity signal in the first step.Because the probe cannot acquire the velocity signal in the boundary layer close to the wall,the velocity were fitted by the polynomial least-squares method,and then compared with the Blasius velocity profile.Most of the flow field were verified to be laminar by the contrast to the Blasius velocity profile.It's indicated that the disturbances were reduced and evolved in the boundary layer for the quasi-periodic oscillations in the velocity signals.The initial disturbances were induced by the receptivity of the flat plate's leading edge to the freestream disturbances.The disturbances in the flow field were illustrated in the frequency-domain by the Fourier transform of velocity signals.There were 9 waves in the flow field from 140mm to 286mm in the streamwise direction,of which the lowest frequency was 292.97Hz,the biggest frequency was 1464.84Hz and the frequency interval was 146.48Hz. These waves had the similar evolutions in both the streamwise and normal direction.Higher and lower amplitude alternately dominated in the boundary layer in the normal direction. The magnitude of amplitude grew with the streamwise direction.