Effects of 2 Hz imposed bulk flow unsteadiness on laminar/turbulent transition in a straight channel.

Abstract

Laminar turbulent transition is studied in a straight channel with 2 Hz imposed bulk flow unsteadiness for Reynolds umbers of 1 100 to 3800 and Strouhal numbers of 0.121 to 0.035. Channel aspect ratio is 40 to 1 with 1.27 cm height. 50.8 n width and 4.27 m length. Observations, videos, and photographs of smoke patterns show different subcritical transition 'ents including Tollmien-Schlichting waves, vortex-array type motion evidenced by smoke swirls, ribbon-like patterns, trbulentspots. and fully turbulent flow. Fluctuating intensity magnitudes relative to velocities from imposed unsteadiness e determined from phase-aveiaged velocity traces. With 2 Hz unsteadiness, transition events begin to occur at lower eynolds numbers and extend over a wider range of Reynolds numbers compared to flows with no imposed unsteadiness.