Flow-induced rotation and wake characteristics of polygonal prisms subjected to laminar flow

This work presents numerical investigations of flow-induced rotation of three different polygonal prisms, including triangular, square, and pentagonal ones. Taking into account the torsional stiffness and damping, the rotation response is examined in a reduced velocity range of Ur = 3-15 at Re = 100. Four distinct oscillating regimes and associated vortex shedding modes are identified: 2S mode in the bifurcated-steady oscillating regime (3 ≤ Ur < 5 and 10 < Ur ≤ 15 for the triangular prism, 7 < Ur ≤ 11 for the square prism), 4S mode in the bifurcated-unsteady oscillating regime (12 ≤ Ur ≤ 15 for the square prism), 6S mode in the non-bifurcated oscillating regime (5 ≤ Ur ≤ 10 for the triangular prism), and 2S mode in the quasi-static regime (3 ≤ Ur < 8 for the square prism, the whole Ur range for the pentagonal prism). Moreover, the competitions between the pressure-induced torque and shear-induced torque vary in these four oscillating regimes. Specifically, the dominant component of the flow-induced torque is the pressure-induced torque in bifurcated-steady oscillating and bifurcated-unsteady oscillating regimes. By contrast, these two kinds of torques owning the nearly equal proportions possess the same and opposite sign in non-bifurcated oscillating and quasi-static regimes, respectively. The root-mean-squared rotary angle (θrms) depends on the cross-sectional shape and reduced velocity. The θrms of the triangular prism suddenly drops to the level of freely rotating case before two rising stages. The square prism starts to rotate violently when it enters the bifurcated-unsteady oscillating regime. For the pentagonal prism, a vortex-induced vibration-like rotation response is observed. Due to the different oscillating regimes, the general θrms of the triangular prism is the largest group, followed by the square prism and pentagonal prism in sequence.