Control of the aerodynamic forces of a finite-length square cylinder with steady slot suction at its free end Hanfeng Wang a, b , Si Peng a , Ying Li a , Xuhui He a, b, * a School of Civil Engineering, Central South University, China b National Engineering Laboratory for High-speed Railway Construction, Central South University, China ABSTRACT A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces. The aspect ratio (H/d, where H and d are the height and width of the cylinder, respectively) of the tested cylinder was 5. The tested suction ratio Q, which was defined as the ratio of the suction velocity at the slot (U s ) to U ∞ , ranged from 0 to 4. It was found that the overall aerodynamic forces reduce quickly with the increase of Q from 0 to 1, then recovers slightly with Q from 1 to 2, and keep approximately constant with Q  2. The maximum reduction of the overall mean drag, fluctuating drag and fluctuating lift occurs at Q ¼ 1, which reaches 3.6%, 17.8% and 45.5%, respectively. The steady slot suction reduces the aerodynamic forces not only near the free end, but also over the entire cylinder span. At Q ¼ 1, the shear flow emanating from the free-end leading edge reattaches on the free end forming a recirculation bubble. The enhanced momentum transport between the free-end shear flow and the wake suppresses the spanwise vortex shedding and aerodynamic forces the most effectively. 1. Introduction Our understanding of the flow around a wall-mounted finite-length cylinder has been significantly deepened in the last two decades because of the continuous efforts of lots of researchers, e.g. Park and Lee (2000); Sumner et al. (2004); Adaramola et al. (2006); Wang and Zhou (2009); Bourgeois et al. (2011); Krajnovi c (2011); Kawai et al. (2012); Sumner (2013); Porteous et al. (2014) and Wang et al. (2017), etc. Generally, under the effects of finite cylinder span, wall junction and cylinder free end, the flow around a wall-mounted finite-length cylinder is highly three-dimensional (3D) and different drastically from that around a nominal two-dimensional (2D) one. Despite the horseshoe (or necklace) vortex formed at the cylinder- wall junction (Hussein and Martinuzzi, 1996; Simpson, 2001), the flow around a wall mounted finite-length cylinder is characterized by a pair of longitudinal tip vortices originates from the cylinder free end, another pair of longitudinal base vortices, and possible alternating spanwise vortices (Wang and Zhou, 2009; Zhang et al., 2017). The counter rotating tip vortices are associated with downwash flow. On the other hand, the base vortices induce an upwash flow from the wall into the near wake. The strength of base vortices and associated upwash flow depend on the boundary layer conditions on the wall where the cylinder mounted. Wang et al. (2006) suggested that a thicker boundary layer results in stronger base vortices and upwash flow. The spanwise vortices depend largely on the cylinder aspect ratio H/d, where H and d are the height and width of the cylinder, respectively. When H/d falls below a critical value, the spanwise vortex changes from staggered arranged to symmetrically arranged (Sakamoto and Arie, 1983; Okamoto and Sunabashiri, 1992; Pattenden et al., 2005). Sakamoto and Arie (1983) suggested that this critical value of H/d is 3.0 for circular cylinder and 2.5 for square one. Interestingly, this critical H/d was found depending on many factors, such as the boundary layer thickness and oncoming flow turbulence in- tensity, etc (Sakamoto and Arie, 1983; Kawamura et al., 1984). Tanaka and Murata (1999) calculated the vortex line from the mean velocity field in the near wake of finite-length circular cylinders with H/d ranging from 1.25 to 10. They suggested that the vortices shed from both sides of the cylinder are connected with each other near the cylinder free end, forming an arch-type structure. This conclusion was validated later by both numerical (Frohlich and Rodi, 2004; Krajnovi c, 2011) and experimental investigations (Sumner et al., 2004). Similar observation was also found for finite-length square cylinder (Wang and Zhou, 2009; Bourgeois et al., 2011). More recently, Kawai et al. (2012) investigated the near wake of a wall-mounted square prism with H/d ¼ 2.7 using 3D stereoscopic PIV and conditional sampling techniques. They confirmed that an arch-type vortex is formed behind the prism throughout the cycle of Karman vortex shedding. Insight into the physics of the flow around a finite-length cylinder should make it possible to effectively control its near wake and aero- dynamic forces. However, relevant papers about the flow control for finite-length cylinders are far less than those for 2D bluff bodies. The * Corresponding author. School of Civil Engineering, Central South University, China. E-mail address: xuhuihe@csu.edu.cn (X. He). Contents lists available at ScienceDirect Journal of Wind Engineering & Industrial Aerodynamics journal homepage: www.elsevier.com/locate/jweia https://doi.org/10.1016/j.jweia.2018.06.016 Received 21 December 2017; Received in revised form 16 April 2018; Accepted 22 June 2018 0167-6105/© 2018 Published by Elsevier Ltd. Journal of Wind Engineering & Industrial Aerodynamics 179 (2018) 438–448