On the Three-Dimensional Structure of Turbulent Spots

A detailed measurement of turbulent spots propagating in a laminar boundary layer over a flat plate was made at a zero pressure gradient and three favorable pressure gradients. Data were recorded across the span of turbulent spots at a number of streamwise locations along the plate using a hot-wire probe and surface-mounted hot films. In this work we aim at extending the existing studies that are largely based upon measurements made in the plane of symmetry of the spots and away from the wall and investigating the three-dimensional structure of turbulent spots and its dependence on streamwise pressure gradients. The results from the present experiment reveal some interesting aspects of the overall structure of the spots and the role that regions with negative velocity perturbations play in the spanwise growth of turbulent spots. The presence of a spanwise overhang is also confirmed that is believed to be responsible for the smaller spreading angle of spots measured at the wall. This finding is expected to have important implications to the development of improved transition models that are used to predict heat transfer and skin friction for turbine blades.