Boundary layer transition experiments with embedded streamwise vortices

Manu, K V and Dey, J and Mathew, Joseph (2018) Boundary layer transition experiments with embedded streamwise vortices. In: SADHANA-ACADEMY PROCEEDINGS IN ENGINEERING SCIENCES, 43 (10).

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Abstract

Experiments were conducted with a counter-rotating, streamwise vortex pair embedded in flat plate boundary layers, in a low-turbulence wind tunnel, to understand the role of local separation on transition. Steady, streamwise vortices were generated downstream of gaps in spanwise-uniform, smooth hills (of height h) affixed to the plate, 175 mm from its leading edge. The flow between is directed away from the plate. At the four tunnel speeds 1.8-3.5 m/s considered, the Reynolds numbers based on displacement thickness at this location varied from 248 to 346. Small, medium and large gaps of 2, 4 and 8 mm, respectively, were set up; they were about a third to twice the boundary layer thickness (2/3<b/h<8/3). With the closest vortex pairs, transition was observed at all freestream speeds considered. With larger spacing, transition occurred at the highest speed only. The vortex pair caused the flow to separate in all but the largest-gap cases. Separation was steady and reattachment unsteady in all cases. Velocity fluctuations grew slightly upstream of re-attachment in transitional cases. No evidence was found for separation or re-attachment as a direct cause for transition; transition occurred even without separation. Instead, whenever transition was observed, its origin could be traced to instability of a streak of sufficient amplitude that had been created by the vortex pair. Streak instability appeared as fluctuations growing along its sides and spreading. Anomalous behaviour was also observed with moderate spacing, where transition did not occur in spite of flow separation and streak amplitudes in excess of known thresholds for streak instability.

Item Type:	Journal Article
Publication:	SADHANA-ACADEMY PROCEEDINGS IN ENGINEERING SCIENCES
Publisher:	SPRINGER INDIA
Additional Information:	Copy right for this article belong to SPRINGER INDIA, 7TH FLOOR, VIJAYA BUILDING, 17, BARAKHAMBA ROAD, NEW DELHI, 110 001, INDIA
Keywords:	Boundary later transition; boundary layer separation; streak instability; streamwise vortex
Department/Centre:	Division of Mechanical Sciences > Aerospace Engineering(Formerly Aeronautical Engineering)
Date Deposited:	11 Sep 2018 14:57
Last Modified:	11 Sep 2018 14:57
URI:	http://eprints.iisc.ac.in/id/eprint/60639

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