Wall Curvature and Spanwise Rotation Effects on Instability of Boundary Layer Flow. Boundary Layer Transition on Concave Wall.

Abstract

The behavior of the boundary layer flows in rotating channels such as the flow in impeller passages of turbomachinery is dominantly influenced by the centrifugal and Coriolis forces due to the wall curvature and system rotation, respectively, which predominate the boundary layer development by suppressing or promoting the turbulent motion. Though many studies have been perfomed about the turbulent boundary layer over a concave or convex surface in the stationary state, the resultant effects due to the Coriolis force in the rotational frame of reference have been scarcely researched. In the present study the velocity and turbulent intensity are measured near the concave surface in two different channels whose radii of center line curvature are 1000 mm and 2000 mm, respectively, on the rotating system. When the Coriolis force acts toward the concave wall due to the channel rotation, the transition of the boundary layer to the turbulent state is promoted because the Gortler instability is enhanced. For the prediction of the boundary layer transition, Gortler number can be an appropriate parameter because the transition occurs at the point where Gortler number exceeds 7, irrespective of the axial Reynolds number, rotation rate, or curvature radius of the wall.