Effects of 1 Hz imposed bulk flow unsteadiness on laminar/turbulent transition in a straight channel

Abstract

Laminar/turbulent transition is studied in a straight channel with 1 Hz imposed bulk flow unsteadiness for Reynolds numbers of 1103 to 2715 and Strouhal numbers of 0.06045 to 0.02455. Channel aspect ratio is 40 to 1 with 12.7 cm height, 50.8 cm width and 4.27 cm in length. Observations, videos and photographs of smoke patterns show different subcritical transition events including Tollmien-Schlicting waves, vortex-array type motion evidenced by smoke swirls and ribbon like patterns, turbulent spots and fully turbulent flow. Fluctuating intensity magnitudes relative to velocities from imposed unsteadiness are determined form phase-averaged velocity traces. Of particular interest, is whether the imposed unsteadiness advances or delays transition events. With the conditions that were investigated, transition events begin to occur at lower Reynolds numbers and extend over a wider range of Reynolds numbers compared to flows with no imposed unsteadiness.