Closure of a laminar separation bubble by natural and wake-induced transition
The concentrated use of one-dimensional hot-wire anemometry has shown leading edge boundary layer disturbances induced under each passing wake, which grow steadily via by-pass and natural transition methods into turbulent strips that convect with the flow. These disturbances are of such strength that the separated region is resisted and effectively swept away by the passing turbulence, momentarily giving rise to a wholly attached laminar boundary layer across the entire flat plate surface.;Propagation rates have shown leading edge speeds in excess of freestream values, a combination of boundary layer destabilisation and the negative jet effect of each wake. Trailing edge values are of typically 50% freestream.;Controlling the chordwise proximity of neighbouring wakes allowed for the investigation of the effect and extent of the calmed region behind each induced turbulent strip. Measurements have shown that although there is no slowing of the advancing turbulence by the calmed flow, a strong suppression of velocity fluctuations is seen, related to the proximity of the turbulent strips. Turbulence level reductions of up to 40% have been demonstrated as wake spacing is reduced.;The use of microphones to measure surface pressure fluctuations revealed the amplification of instabilities in the separated shear layer. These have been shown to be viscous Tollmien-Schlichting vortices, originating from fluctuations in the attached laminar boundary layer, and are responsible for the natural development of turbulent flow between wakes.