Horseshoe vortex / shock wave interaction formed around circular and square cylinders

In this study, the horseshoe vortex formed around the base of circular and square cylinders by a separating boundary layer has been investigated experimentally to acquire a more fundamental understanding of the structure. The cylinders were mounted vertically on the test section floor of the wind tunnel and the height-to-cylinder-diameter ratio for circular models was varied from 1 to 3. Two conditions of freestream velocities of M = 2.2 (756 m/s) and M = 0.9 (309 m/s), corresponding to Reynolds numbers of 2.0 × 10⁵ and 8.2 × 10⁴ based on diameters (D) of cylinders, respectively, are considered to explore the geometry effects of obstacles, the height of one, two and three diameter, two cross-sectional shapes of obstacles, circular and square, with variations in number of obstacles. The number of the obstacles in tandem array is two and the spacing between obstacles is 3D. Shock wave boundary layer interactions generated by such cylinders on a flat plate were visualized with surface-streakline (proper mixture of oleic acid, paraffin, titanium - dioxide) and Schlieren techniques. The study showed that the flow contains three interacting features formed from the junction flow between the cylinder and the ground, separation from the cylinder wall and resultant turbulent wake, and the flow over the free-end of the cylinder. The location of the separation and attachment points scales with the height of the cylinder. The horseshoe vortex forming at the cylinder-bed junction affects the vortex formation length and the width of the near-wake region. This investigation evaluated the detailed flow processes that lead to the symmetric horseshoe vortex formation around a large-diameter cylinder.