Laminar Separation, Reattachment, and Transition of the Flow Over a Downstream-Facing Step

Abstract

Results of an experimental investigation of the laminar flow of air over a downstream-facing step are presented. The experiments include visual observations of smoke filaments (in the viscous layer), qualitative velocity fluctuation measurements, and mean velocity profiles. Results are reported over a range of 0.36 – 1.02 cm in step height, 0.61 – 2.44 m/sec in free stream velocity at the step, and 0.16 – 0.51 cm in boundary layer displacement thickness at the step. Laminar flow to reattachment of a free shear layer is observed for subsonic flow and two criteria for which transition to turbulence at reattachment exists are presented. The laminar reattachment length is not a constant number of step heights as for turbulent flow, but varies with Reynolds number and boundary layer thickness at the step. The shape of the velocity profile at reattachment is found to be similar to the shape of a laminar boundary layer profile at separation and the boundary layer profiles downstream of reattachment are similar to those in a laminar boundary layer developing toward separation except that they are traversed in the reverse sense.

Validation study of vortex methods

“…al. [19] experimentally observe an approximately linear relationship between normalized reattachment…”

“…Within the oscillating recirculation bubble, the mean flow speed is one order of magnitude less than the mean main flow speed [26]. The large eddies move downstream and cause local flow reversal, as observed in [3,14,15,19,26]. Durst and Tropea [14] report that in the transitional regime, streamlines are wavy right after the step, and break into "vortex rolls" after 3-4 step heights downstream; as R .…”

“…In the laminar regime, the flow structure is characterized by a single, elliptically shaped recirculation zone [3,13,19,25,32,37]. The flow is essentially two-dimensional; Armaly et.…”

“…Laminar flow separates from the step and becomes unsteady [3,7,13,14,19,25]. These eddies are threedimensional in that their cross-sections vary in the spanwise direction [3].…”

Effect of partial blockages in simulated LMFBR fuel assemblies

“…Considerable information is available on flow separation caused by obstructions such as cylinders,* disks, 7 steps in a surface of a plate,*' 10 and walls perpendicular to the flow path.…”