Unsteady Measurements near Leading Edge of Separated Flow

抄録

A separation and reattaching flow formed by the boundary layer separation at the edge of a blunt circular cylinder at Reynolds number of 1.0×10⁵ based on the main flow velocity U_∞ and the diameter of the cylinder D is studied. The time-mean and root-mean-square values of fluctuating surface pressure on the front surface and along the side face are presented. The reattachment length is 1.73D and is constant along the circumference. From the mean pressure distribution on the front surface, the thickness of the laminar boundary layer at separation is estimated to be 1.86×10^-4D. The low-frequency motion nL_r/U=0.011 at the leading edge, which is regarded as the flapping of the separated shear layer, was also found in the initial boundary layer upstream of separation. This pressure fluctuation inside the initial boundary layer, which is clearly identified without other disturbance generated by the shear layer rolling up, recirculating or feedback motion from the reattachment zone, is caused by upstream propagation of the fluctuating motion of the separated shear layer. In addition to this low-frequency flapping motion, the high-frequency velocity fluctuations of the shear layer next to separation, which demonstrate the Kelvin-Helmoltz instability and the evolution of the shear layer, are important for understanding and control of separation.