On the flow structure within a turbulent spot

A computational technique is presented for determining the fully three-dime nsional viscid unsteady perturbation to a non-developing laminar boundary l ayer flow. The results reveal the strongly three-dimensional nature of the flow within a turbulent spot and its associated calmed region. Separation o f the flow is found to occur along the demarcation line between the spot an d the calmed region. This separation line advances downstream at the spot t railing edge velocity which is approximately half that of the freestream. I n interpreting the results it was found convenient to consider the flow as seen by an observer travelling at the spot trailing edge velocity. From thi s viewpoint the unperturbed laminar boundary layer would consist of two flu id streams. A low momentum stream close to the wall which is travelling slo wer than the observer and hence approaches from downstream and a high momen tum stream which is travelling more rapidly and hence approaches from upstr eam. The results show that once the flow is perturbed, the low momentum str eam approaching the observer from downstream is deflected away from the sur face close to the separation line and rolls up into a vortex at the rear of the spot. This vortex grows in both the streamwise and spanwise directions as more fluid is added from the low momentum stream. The high momentum str eam drops towards the surface as it approaches the observer from upstream t o fill the space vacated by the diverted low momentum stream. This results in the formation of the calmed region. The skin friction is increased withi n this region and hence some of the fluid from the high momentum stream is slowed to a velocity below that of the observer and hence moves back upstre am. However, most of the high momentum stream continues towards the separat ion line where it is deflected away from the surface as it is entrained int o the vortex. The vortex is therefore fed by both the low and the high mome ntum stream resulting in the high local transient shear rates typical of tu rbulent eddies. (C) 2001 Elsevier Science Inc. All rights reserved.