Transition Phenomena in a Straight Channel with a 40 to 1 Aspect Ratio with andwithout Imposed Pulsations. Part 1: Near-Wall and Central Region Profiles

摘要

A channel with a rectangular cross-section, 40 to 1 aspect ration (height is0.0127 m), and 4.27 m test section (336 channel heights) is used to study the effects of imposed pulsations on transitional flow phenomena. Periodic velocity variations are produced in the test section using a single rotating vane located in the flow downstream of the test section. Flows with Reynolds numbers (Re) ranging from 1100 to 3600 are studied at Stokes numbers of 4.08 and 5.79 and Strouhal numbers from 0.02 to 0.122 by imposing pulsations at 1 Hz and at 2 Hz. Time-averaged velocity profiles both within and outside of the Stokes layer are unaffected by the imposed pulsations at frequencies of 1 and 2 Hz for the entire range of Reynolds numbers studied. Longitudinal turbulence intensity profiles show a local maxima only at the channel centerline for Reynolds numbers from 1250 to 1550. This is evidence of a center mode of instability. Longitudinal turbulence intensity profiles show a local maxima at both the channel centerline and y/d of 0.85 to Reynolds number from 1710 to 2300. As the Reynolds number increases from 2350 to 3400 local maxima of longitudinal turbulence intensity occur only at y/d of 0.9. Normalized profiles of longitudinal turbulence intensity are the same with and without pulsations for all Reynolds number studies with the exception of Reynolds numbers 2400 and 2450. Longitudinal turbulence intensity profiles at a Reynolds number of 2400 show that magnitudes near the channel centerline are reduced at Stokes number of 5.79 (2 Hz pulsations) as compared to without pulsations. At Reynolds number of 2450 the magnitude of longitudinal turbulence intensity near the channel centerline are reduced at Stokes number of 4.08 and 5.79 (1 Hz and 2 Hz pulsation