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In this article, two-dimensional numerical simulations are performed to investigate the effectiveness of a hinged splitter plate for manipulating the unsteady laminar wake regime of tandem circular cylinders (TCCs) at a pitch ratio of G/D = 5 and Reynolds number of Re = UD/υ = 100, where G is the distance between the centers of the cylinders having diameter D, U is the free-stream velocity, and υ is the kinematic viscosity of the fluid. These simulations are conducted using the in-house developed flexible forcing immersed boundary-one-step simplified lattice Boltzmann method (FFIB-OSLBM) solver. The splitter plate is hinged to the upstream cylinder's rear base point (HSPU) that pitches at amplitudes θ m (10°–20°) and non-dimensional frequencies S t f (0.1–0.4). The plate length L f / D is varied between 0 and 1. These pitching parameters substantially influence the wake topologies, vortex-interaction modes, pressure distribution, and flow-induced forces on the cylinders. Moreover, the TCC-HSPU combination exhibits four different wake patterns. In Type-I, regular vortex shedding occurs, with the upstream cylinder vortex (UCV) dominating the plate vortex (PV) in the cylinder gap region. The stronger and bigger PV in Type-II forms parallel vortex streets. In Type-III, the PV becomes strong enough to prevent the shedding of UCV. Finally, in Type-IV, the PV attains its maximum strength, and its interaction with UCV forms a new vortex that dominates the cylinder gap region. Among them, Type-II and Type-III regimes are associated with a lower range of drag force on TCC-HSPU configurations. In the considered parametric space, the TCC-HSPU arrangement achieves a maximum drag reduction of 43% compared to the TCC when L f / D = 1.00, S t f = 0.20, and θ m = 15°. [ABSTRACT FROM AUTHOR]

This article explores the aerodynamic performance of a two-dimensional elliptical airfoil undergoing sinusoidal heaving and asymmetric pitching motions in forward-flight conditions for the Strouhal number (St) range of 0.1–0.6. The study employs numerical simulations and water tunnel experiments to investigate the effects of non-zero pitch angular offset angles (θoffset) while maintaining a fixed Reynolds number of 5000 and an effective angle of attack amplitude of 15° at the pivot location. The θoffset is varied from −15° to +15° at 5° intervals. The present research shows that these parameters significantly impact the leading-edge effective angle of attack, flow velocity, and the formation of high-pressure regions, which are crucial factors in thrust and lift generation throughout the flapping cycle. Moreover, the pitch angle determines whether the resultant force favors thrust or lift. It is observed that the cyclic time-averaged lift consistently increases with θoffset, surpassing symmetric cases (θoffset = 0°). Conversely, the cyclic time-averaged thrust is lower for non-zero θoffset values. Increasing St enhances both cyclic time-averaged thrust and lift up to the respective critical Sts, after which their performance declines. Notably, the critical St of cyclic time-averaged lift exceeds that of cyclic time-averaged thrust; interestingly, their values are invariant with θoffset. Moreover, in the conditions where thrust efficiency maximizes, lift efficiency attains a minimum value and vice versa. So, depending upon the application, one needs to suitably select the pitch angular offset and flapping frequency to maximize thrust or lift performance. [ABSTRACT FROM AUTHOR]

Previous investigations of flapping hydrofoils for the purpose of thrust production have been limited to one or two in tandem. Tandem foils were found to have superior performance because the performance of the aft foil was augmented by the vortices shed from the fore foil. It is however not clear if increasing the number of foils will continue to have increased performance or if there exist an optimal number after which the overall performance either stagnates or reduces. A 2D computational study was conducted to investigate the effect of increasing the number of hydrofoils to four at a Reynolds number of 8000 flapping in-phase and out-of-phase. Optimal and sub-optimal conditions found previously with tandem hydrofoils were found also be applicable to three and four hydrofoils.

This study investigates the Taylor–Couette flow (TCF) with a longitudinal corrugated surface on a stationary outer cylinder and a rotating smooth inner cylinder using large eddy simulation for three values of amplitude to wavelength ratios (A*) (0.1875, 0.2149, and 0.25) to explore the influence of the corrugated surface on the flow structures and the variation of torque for a wider range of Reynolds numbers (Re) (60–650). From the results, four flow regimes are observed. At Re = 60, initially, a pair of secondary vortices appears at the inner wall of the minimum gap region and it evolves to a pair of axisymmetric stationary wall induced vortices (ASSWIVs) in the maximum gap region. As Re increases to 80, 85, and 103 for the three values of A* (0.1875, 0.2149, and 0.25), respectively, another pair of axisymmetric stationary secondary vortices is seen at the minimum gap region of the inner wall. A further increase in Re (Re > 125, 130, and 138 for the three values of A*, respectively) results in the appearance of axisymmetric periodic secondary axial flow. Increasing Re further (Re > 225, 240, and 260 for A* = 0.25, 0.2149, and 0.1875, respectively) leads to the emergence of non-axisymmetric and non-periodic secondary axial flow (NANPSAF) with an azimuthal wave. Generally, the torque in TCF with the corrugated surface is found to be lower than TCF with a smooth surface except for the occurrence of the ASSWIV flow regime and weak axial secondary flow in the NANPSAF regime. [ABSTRACT FROM AUTHOR]