Understanding the secondary separation from an inclined square cylinder with sharp and rounded trailing edges

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The steady separated flow past a square cylinder at 45° incidence and having a sharp/rounded base is a unique kind of separated flow around a symmetric bluff obstacle. At low Reynolds numbers (Re ≈ 6–8), this flow displays diverse features that are not known to be displayed by common symmetric bluff bodies, such as circular/square cylinders at zero incidence. The uniqueness of this flow was first highlighted by Kumar et al. [“Steady separation of flow from an inclined square cylinder with sharp and rounded base,” Comput. Fluids 171, 29–40 (2018)] in their recent numerical investigation concerning the onset of initial separation. They reported detailed flow kinematics resulting from separation of the laminar boundary layer via secondary mode, primary mode, and their combination. In this work, based on stabilized finite-element computations, the fluid dynamic aspects of primary and secondary separation are delved. The parameters considered are the normalized corner radius at the cylinder base and Re. Exploration of the influence of base radius on the occurrence/absence of the secondary separation is the main objective. The occurrence of secondary separation is a function of base radius. It occurs only with small or no rounding at the base, i.e., when the pressure gradient close to the base is favorable or mildly adverse. Irrespective of the base radius, the onset of secondary separation occurs at a constant Re value of 7.3. The drag decays as Re−0.62; it becomes invariant when Re is held constant. A new nondimensional parameter η accommodating the effects of pressure recovery and radius of curvature is introduced. The drag exhibits no sensitivity to η.