Journal of Fluids and Structures 25 (2009) 1021–1028 Brief Communication Passive control of wake flow by two small control cylinders at Reynolds number 80 C.-H. Kuo à , C.-C. Chen Department of Mechanical Engineering, National Chung Hsing University, No. 250 Kuo-Kuang Road, Taichung 40227, Taiwan Received 6 April 2008; accepted 15 May 2009 Available online 8 July 2009 Abstract Passive control of the wake behind a circular cylinder in uniform flow is studied by numerical simulation at Re D ¼ 80. Two small control cylinders are placed symmetrically along the separating shear layers at various stream locations. In the present study, the detailed flow mechanisms that lead to a significant reduction in the fluctuating lift but maintain the shedding vortex street are clearly revealed. When the stream locations lie within 0.8rX C /Dr3.0, the alternate shedding vortex street remains behind the control cylinders. In this case, the symmetric standing eddies immediately behind the main cylinder and the downstream delay of the shedding vortex street are the two primary mechanisms that lead to a 70–80% reduction of the fluctuating lift on the main cylinder. Furthermore, the total drag of all the cylinders still has a maximum 5% reduction. This benefit is primarily attributed to the significant reduction of the pressure drag on the main cylinder. Within X C /D43.0, the symmetry of the standing eddy breaks down and the staggered vortex street is similar to that behind a single cylinder at the same Reynolds number. In the latter case, the mean pressure drag and the fluctuating lift coefficients on the main cylinder will recover to the values of a single cylinder. r 2009 Elsevier Ltd. All rights reserved. Keywords: Passive wake control; Lift and drag reduction; Vortex shedding 1. Introduction Because of the diversified flow phenomena for a wide range of Reynolds numbers, the wake flows behind bluff bodies in uniform flow have attracted the interest of a large number of investigators using either numerical or experimental methods. The wake flow patterns are strong functions of the Reynolds number, turbulence intensity, aspect ratio, blockage ratio, end effects and wall proximity. Details of the circular cylinders in uniform flow can be found in a book by Zdravkovich (1997, 2003). There are active and passive methods to manipulate the wake flows behind circular cylinders (Kuo et al., 2007). The main objectives to manipulate the wake flows are: (i) to reduce the form drag (Tokumaru and Dimotakis, 1991), (ii) to suppress vortex shedding (Strykowski and Sreenivasan, 1990; Lecordier et al., 1991) and (iii) to change the heat transfer characteristics (Igarashi, 1998; Lange et al., 1998). Fig. 5 of Strykowski and Sreenivasan (1990) showed that if the small control cylinder is placed within a defined region in the near-wake of the main cylinder, the wake behind the main cylinder could be suppressed effectively at a Reynolds ARTICLE IN PRESS www.elsevier.com/locate/jfs 0889-9746/$ - see front matter r 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfluidstructs.2009.05.007 à Corresponding author. Tel.: +886 422840433x314; fax: +886 422877170. E-mail address: chkuo@dragon.nchu.edu.tw (C.-H. Kuo).