Juncture flows associated with surface-mounted obstacles can be characterized by a U-shaped tubular vortical flow known as a horseshoe vortex (HSV). Horseshoe vortices are usually detrimental to engineering applications. Therefore, numerous studies have investigated methods for reducing HSVs' strength. To design a suitable method for reducing HSVs' strength, the characteristics of HSVs influenced by external boundary geometries must first be understood. Thus, we experimentally investigated a

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... cture flow field associated with a fundamental geometry—a circular cylinder mounted perpendicular to a plane surface—with a trailing backward-facing step representing downstream effects on upstream-formed flow structures. This setup is one of the simplest nonplanar geometries that generates flow features such as an unsteady separated shear layer that may considerably affect an HSV. We used the particle image velocimetry (PIV) and PIV-based flow visualization techniques combined with a vortex-fitting algorithm to measure the juncture flow and identify the HSV and its kinematic modes at a low Reynolds number of 1100. We compared the results obtained with and without the backward-facing step and observed that the backward-facing step increased the number and complexity of kinematic modes, i.e. the unsteadiness, of the HSV, and reduced HSV's stretching, which resulted in an increased vortex diameter and increased circulation; that is, the strength of the HSV was enhanced by the backward-facing step.