Flow Visualization Studies in (1) A Curved Rectangular Channel with 40 to 1 Aspect Ratio and (2) A Straight Channel with Bulk Flow Unsteadiness.

摘要

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 degs. to 175 degs. 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 7% peak to peak amplitude (relative to mean velocity), transition occurs at a Reynolds number of approximately 2300. Theses. (aw)