Excitation of instabilies in a Blasius boundary layer by surface vibration

Abstract

Here we have demonstrated that small amplitude vibration can artificially excite both two dimensional (2D) and three dimensional (3D) instability modes. The 2D modes were typical of Tollmien–Schlichting (TS) waves provided that the frequency of excitation lies within the unstable region of the neutral stability predicted by modal linear stability theory. However, even if the frequency of the mechanically forced mode was within the stable bounds of the neutral curve the harmonics generated by the non-linear response of the flow could develop as instability modes. Further analysis of the streamwise and spanwise evolution of the instability modes identified from the temporal Fourier transform confirmed the presence of 3D modes excited due to the nature of the mode shape deflection of the vibrating panel which was not uniform in the spanwise direction. The effect of spanwise non-uniformity could be increased by activating the motors along the spanwise direction. However, due to the forcing from a combination of both streamwise and spanwise motors, strong interaction with the 3D mode led to a reduction in the growth rate of the TS wave in the far-field region despite higher initial perturbation generated by a larger number of motors.