Journal of Fluids and Structures 24 (2008) 750–755 Brief communication Flow field of self-excited rotationally oscillating equilateral triangular cylinder S. Srigrarom à , A.K.G. Koh Nanyang Technological University, School of Mechanical and Aerospace Engineering, 50, Nanyang Avenue, Singapore 639798, Singapore Received 10 January 2007; accepted 30 October 2007 Available online 25 January 2008 Abstract This paper studies the flow field of a particular fluid-structure interaction phenomenon—the continuous angular oscillation of a centrally pivoted equilateral triangular cylinder (prism), under uniform two-dimensional incompressible flow. Dye flow visualization of a 30 cm long and 10 cm wide cylinder in a two-dimensional water tunnel was conducted. Under a uniform incoming flow of 7.5 cm/s, the cylinder oscillated continuously after an initial perturbation. On the windward side of the cylinder, a vortex was formed at the sharp edges of the cylinder during the initial phase, whereas on the leeward side, the flow stayed attached. The phase-averaged particle image velocimetry (PIV) measurements are also presented. PIV results show the interchange of flow patterns from that over a flat plate to flow past a sharp edge and vice versa. r 2007 Elsevier Ltd. All rights reserved. Keywords: Self-excited rotational oscillation; Equilateral triangular cylinder (prism) 1. Introduction Oscillating flaps or duck-fins are commonly used to suppress the ocean surface waves approaching the shore. The flaps or duck-fins have the drawback that they work only on the water surface. Recent studies (Nagashima and Hirose, 1982) revealed that if an isosceles triangular wedge is placed in an otherwise uniform flow, it can be induced to oscillate incessantly. The equilateral triangular wedge is more effective than the flap or duck-fin, since it can also function when submerged in the water. One can also extract energy from the oscillating and/or spinning motion of the wedge when it is under the influence of incoming flow or wave. The detailed motion of the wedge is, however, not fully understood. This is due to the unsteady surrounding flow, as well as continuously moving boundary (the wedge either oscillates or rotates). This coupled fluid-structure interaction was first noticed by Nagashima and Hirose (1982) and it has not yet been well studied. Previous research only dealt with the stationary wedge at a different position or oscillation in translational mode (Luo et al., 1993), or with other geometries (Naudascher and Wang, 1993; Nakamura and Nakashima, 1986; Sakamoto et al., 2001; Hu et al., 2002). ARTICLE IN PRESS www.elsevier.com/locate/jfs 0889-9746/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfluidstructs.2007.10.015 à Corresponding author. Tel.: +65 6790 5952; fax: +65 6792 4062. E-mail address: mssrigrarom@ntu.edu.sg (S. Srigrarom).