HORSESHOE VORTEX FORMATION AROUND A CYLINDER

Date of Completion

January 1985

Keywords

Engineering, Mechanical

Degree

Ph.D.

Abstract

A turbulent boundary layer approaching a local obstruction, such as when annulus wall boundary layers encounter airfoils and support struts, creates a critical problem in gas turbine engines. The slower portion of the approaching boundary layer cannot negotiate the adverse pressure gradient generated by the obstruction and consequently separates from the endwall. The resulting flow field includes a horseshoe vortex that is swept downstream around the body. The separation affects both the local heat transfer coefficients and aerodynamic losses in the endwall region.^ This investigation evaluated the detailed flow processes that lead to the symmetric horseshoe vortex formation around a large-diameter cylinder. Test conditions included a freestream velocity of 30.5 m/sec, a Reynolds number based on cylinder diameter of 5.5 x 10('5), and a boundary-layer thickness equal to 13 percent of the cyclinder diameter. The final report presents endwall and cylinder surface flow visualizations, endwall and cylinder static pressure distributions, and five-hole probe measurements in the separation region.^ In contrast to some previous research, a vortex did not form in the plane of symmetry during this experiment. Measurements indicated a shallow separation streamline emerging from a single saddle point of separation in the plane of symmetry. At the same time, flow from outside the boundary layer stagnated at the endwall-cylinder junction and contributed to the formation of a single primary horseshoe vortex between the 5-deg and 25-deg planes. Losses associated with this vortex rollup were localized and offset by transport of low-loss fluid into the endwall region. The acquired data established a test case for three-dimensional computer codes. ^

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