Secondary flow of the second kind in rectangular ducts with one rough wall

Abstract

A study was made to clarify experimentally the influences of a rough wall in a rectangular duct on secondary flow of the second kind and to obtain fundamental data on flow characteristics. Measurements were conducted on turbulent air flows through a square duct and a rectangular (aspect ratio 2:1) duct having one rough wall using hot-wire anemometers. The secondary flow patterrn in the roughened ducts was remarkably different from that in the smooth ducts. In the roughened ducts, only one large longitudinal vortex appeared near the smooth wall on each side, and the secondary currents in the core region proceeded downward from the top smooth wall to the bottom rough wall along the midplate of the duct. The secondary flow was considerably intensified by the existence of the rough wall. The vorticity balance was examined by evaluating the production and convection terms in the vorticity transport equation using the measured turbulent stresses. The two terms almost balanced near the corner bisector of the smooth ducts. However, in the rough ducts, they did not necessarily balance near the corner region formed by the smooth and rough walls.