Visualization and Measurement of Flow over a Cylindrical Surface with a Bump

Abstract

The complex flowfield around a cylindrical bump with a hemispherical free end mounted on a cylindrical surface has been studied by means of liquid-crystal flow visualization, surface-pressure measurements, and cross-hot-wire anemometry. This setup constitutes a prototype of a typical wall-mounted sensor on an aircraft. The slenderness parameter of the bump is one and the curvature of the hemispherical ending starts at the half-height of the bump. The radii of both the cylindrical and hemispherical parts of the bump are equal. The thickness of the turbulent boundary layer over the cylindrical surface, which controls the location of the separation over the bump's surface, is smaller than the bump height at the position at which the bump is located. The flow over the hemispherical free end separates further downstream than does the flow over the cylindrical part of the bump. The size of the recirculation region downstream of the bump is larger than the recirculating region of a finite cylinder with a flat end. A Reynolds number dependence of the reattachment point behind the bump is observed. Large turbulence fluctuations exist because of the interaction between the downwash and the turbulent boundary layer developing over the ground surface. The upstream disturbance of the bump, retardation due to an adverse pressure gradient, and push up of the flow due to the finite length of the bump are also captured by the velocity measurements. Effects due to the curvature of the surface plane are not detectable far downstream of the bump.