Combined effects of fluid elasticity and plasticity on flow past a rotating circular cylinder

The flow past a rotating cylinder is a classic benchmark problem in the domain of fluid dynamics, extensively studied in the past through experiments and numerical simulations. However, most prior research has focused on simple Newtonian fluids. In contrast, many fluids encountered in practical applications display complex non-Newtonian characteristics, such as elasticity and plasticity. This study presents a numerical investigation into the influence of these non-Newtonian properties on the flow dynamics around a rotating circular cylinder within the laminar vortex-shedding regime. Simulations are performed at a fixed Reynolds number (Re=100), exploring a wide range of Weissenberg numbers (0 <= Wi <= 5), Bingham numbers (0 <= Bn <= 1), and cylinder rotational speeds (0 <=Omega <= 1). The Saramito constitutive model is employed in this study to capture the combined effects of fluid elasticity and plasticity. Results indicate that both the fluid's non-Newtonian behavior and cylinder rotation significantly alter the flow characteristics. For instance, the vortex-shedding frequency downstream of the cylinder decreases with increasing Wi and Bn, showing minimal change beyond critical values in the case of a rotating cylinder. In contrast, for a fixed cylinder, the vortex-shedding frequency initially decreases and subsequently increases at higher Wi. Additionally, the time-averaged drag coefficient increases with both Wi and Bn, while the root mean square lift coefficient decreases both for fixed and rotating cylinders. However, the variation in lift coefficient with Bn is minimal for the rotating cylinder. The flow structures downstream of the cylinder also exhibit distinct patterns. For instance, at high Wi, the streamlines become highly distorted and broken for a fixed cylinder but remain more ordered for a rotating cylinder. Most importantly, the present study finds a transition of the flow field from regular periodic behavior to irregular aperiodic one at higher Wi, observed for both fixed and rotating cylinders in elastoviscoplastic (EVP) fluids. This transition highlights the increased complexity of flow dynamics in EVP fluids compared to Newtonian fluids under identical flow conditions. This needs to be understood thoroughly, particularly in applications involving non-Newtonian fluids, where accurate predictions of flow behavior are essential.