Flow visualization studies in (1) a curved rectangular channel with 40 to 1 aspect ratio and (2) a straight channel with imposed bulk flow unsteadiness.
Longest, Jerry Mark
Ligrani, Phillip M.
Subramanian, Chelakara S.
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Flow visualization results are presented which were obtained in a curved channel with mild curvature and 40 to 1 aspect ratio. Inside channel dimensions are 1.27 cm x 50.80 cm. For Dean numbers from 60 to 200 and angular positions from 85° to 175° measured from the start of curvature, video movies and photographic sequences of patterns in spanwise/radial planes show unsteady Dean vortex pair behavior. In particular, information is provided on mechanisms by which vortex pairs appear and disappear. Videos and still photographs of visualized flow in a straight channel with 40 to 1 aspect ratio and imposed bulk flow unsteadiness show different stages of transition, including: (1) three-dimensional Tollmien -Schlichting waves, (2) Lambda waves, (3) Lambda vortices, (4) vortex type motion, (5) turbulent spots and (6) fully turbulent flow. Instantaneous velocity traces from hot-wire probes are presented for Reynolds numbers from 1400 to 8400 and Strouhal numbers from 0. 004 to 0. 047. These data, without unsteadiness, show that transition occurs at a Reynolds number of approximately 3000. With imposed sinusoidal unsteadiness at a Strouhal number of 0. 028 and 7V. peak to peak amplitude (relative to the mean velocity), transition occurs at a Reynolds number of approximately 2300.
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