Interactions for regular patterns of turbulent spots in a laminar boundary layer

A laminar boundary layer at constant pressure is tripped by using a rotating camshaft to displace small pins momentarily into the flow near the leading edge. The disturbance frequency is variable up to about 80 Hz. The main instrumentation is a rake of 24 hot wires placed across the flow at a constant distance from the surface. The main variable observed is intermittency. Data for a single spot confirm the conical growth property and include the pressure signature at the wall in the plane of symmetry. Data for interleaved patterns of spots show that the interaction is strong and tends to limit severely the lateral and longitudinal growth rate for adjacent spots. In some cases the interaction amounts to a replacement of the original spots by others which form and grow in their near wake. This phenomenon is given the name eddy transposition.