Effects of 2HZ Imposed Bulk Flow Unsteadiness on Laminar/Turbulent Transition in a Straight Channel.

摘要

Laminar/turbulent transition is studied in a straight channel with 2 Hz imposed bulk flow unsteadiness for Reynolds numbers of 1100 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 cm width and 4.27 m length. Observations, videos, and photographs of smoke patterns show different subcritical transition events including Tollmien-Schlichting waves, vortex-array type motion evidenced by smoke swirls, ribbon-like patterns, turbulent spots, and fully turbulent flow. Fluctuating intensity magnitudes relative to velocities from imposed unsteadiness are determined from phase-averaged velocity traces. With 2 Hz unsteadiness, 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.