Instability of streaky structure in a Blasius boundary layer

An experimental study was performed to analyze the stability of localized streaky structure in a Blasius boundary layer. An artificial streaky structure was created by using suction or blowing through a thin spanwise slot at the wall. The velocity gradient generated by the suction or blowing was controlled by a damper. The Reynolds number based on the displacement thickness d1 was Red1 =280 at the slot. The behavior of the artificial streaky structure was scrutinized by damping the velocity gradient. It was found that the local streamwise and spanwise velocity gradients play a significant role in the formation of different types of instability. Artificial Tollmien–Schlichting (T–S) wave packets were created by a loudspeaker to elucidate the interaction of the streaky structure with the T–S wave packets. The T–S wave packets imposed on the streaky structure become unstable when the frequency of the T– S wave packets exceeds a certain critical frequency. The development of the T–S wave packets was investigated on the basis of the neutral stability curve. Abbreviations f Frequency Red1 Reynolds number, U¥ d1/m t Time U¥ Freestream velocity u, v, w Velocity components in the x, y, z directions, respectively Vd Damping volume x, y, z Spatial coordinates in the streamwise, wallnormal and spanwise directions, respectively d Boundary layer thickness d1 Displacement thickness d2 Momentum thickness H Shape factor, d1/d2 m Kinematic viscosity Superscripts ¢ velocity fluctuations Subscripts rms Root-mean-square max Maximum min Minimum