Experiments on a two-dimensional laminar separation bubble

A two-dimensional separation bubble on a flat plate is studied experimental ly by means of hot-wire anemometry and flow visualization. Separation of th e laminar boundary layer on the plate is caused by an adverse pressure grad ient imposed by a curved wall opposite to the plate. The instability of, an d transition process in, the separation bubble are focused on. The bubble i s found to be highly susceptible to high-frequency two-dimensional instabil ity waves, which are studied under both natural and forced conditions. A si milar development of these instability waves in the separation bubble is fo und in both cases. The exponential growth of the two-dimensional disturbanc es dominates the flow except for in the reattachment region, where large-sc ale three-dimensional structures appear. Some difficulties associated with experimental investigations of boundary-layer separation-bubble flows are d iscussed.