A combined visual and hot-wire anemometer investigation of boundary-layer transition.

Abstract : The results of a wind tunnel study of the structure of boundary layer transition on an ogive nose cylinder alined parallel to the flow are presented. Both natural and forced transition in both a zero and an adverse pressure gradient are studied at subsonic speeds by means of the visual (smoke) technique. In the visual technique, both still pictures and high speed (4000 frames per second) motion picture films are used. Natural transition is shown to differ from forced transition (using acoustic excitation to control the wave frequencies) in several aspects. Natural transition is intermittent, while forced transition is continuous. The vortex truss structures tend to be staggered in a thatched pattern in natural transition, but tend to be alined in rows in forced transition. In natural transition, the initially two-dimensional waves break down in sets, simultaneously, whereas in forced transition the individual waves break down successively upon reaching a fixed axial position. An adverse pressure gradient tends to produce a transition structure similar to that of forced transition. It is also shown that sound of a very low frequency can control, rather than eliminate, intermittency. Lastly, the use of a hot wire anemometer probe immersed directly in the smoke flow was shown to be a valuable research tool for the correlation of visual and hot wire anemometer data. (Author)