## 1 Introduction  
Transient flow around a bluff body is a typical flow state in nature and engineering applications such as the launch of a torpedo, the take-off and landing of an aircraft, and the start-up and shutdown of pumps in nuclear reactors [1]. This paper focuses on the transient flow that is caused by sudden changes of movement on boundaries during an instantaneous start-up of a cylinder. Although the structure of the cylinder is simple, the flow phenomenon is complex, especially in the case of high Reynolds number. It involves a series of events, such as the disturbance of the boundary layer, the development of the vortex and the unsteady characteristics of the recirculation zone evolution with time. Over the past few decades, the research of transient flow caused by an instantaneously started cylinder has attracted intensive research interests. It is of great significance for solving the engineering problems that the fluid shape changes with time, due to boundary motion. Vortex shedding is known to cause an unsteady pulsating pressure on a cylinder, causing vibration and fatigue damage [2]. Studies have found that cross-sectional shape, surface roughness and proximity to other bodies are factors that affect the transient flow of incompressible viscous fluid around a bluff body [3; 4; 5]. The cross-section shapes of cylinders affect the flow around it. The separation of the boundary layer and the formation and shedding of wake vortex of flow around rectangular cylinders are more distinct than around circular cylinders [6; 7; 8; 9; 10]. Attachment on the surface of the cylinder also affects the flow around it [11; 12]. In the case of multiple cylinders, the flow of one cylinder may cause an excitation of the other cylinder, causing disturbances to the flow between each other [13; 14; 15; 16; 17; 18]. These investigations have shown the fact that attachments or proximity to other bodies can affect the development of the wake. In the past few decades, many passive control measures have been proposed to eliminate the vortex-induced vibration [19; 20; 21]. One common passive control method is to affect the generation and development of vortices by changing the roughing of the bluff body uniformly [22; 23; 24; 25; 26; 27; 28]. In recent years, vortex-induced vibrations have been enhanced to collect and utilize energy from the ocean [29]. The Marine Renewable Energy Laboratory (MRELab) has developed a device called VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) at the University of Michigan [30; 31; 32; 33; 34]. This device converts kinetic energy in the water into electrical energy by the vortex-induced vibration of the cylinder [35; 36]. Many investigations have found that selectively increasing surface roughness can enhance vortex-induced vibrations, compared to the uniform increase in surface roughness, such as the placement of strips of different shapes at different locations on the surface of the cylinder [37; 38; 39; 40; 41]. The cause of transient flow in most of the above studies is that the bluff body is immersed in the flowing fluid rather than the boundary motion. Different forms of boundary motion can affect the separation of the boundary layer, the formation and shedding of vortices, and the evolution of the flow in the recirculation zone [42; 43]. Therefore, investigating the transient flow caused by boundary motion is of great significance for many engineering applications. In the past few decades, many researchers have studied the transient flow caused by an impulsively started cylinder [44; 45; 46; 47; 48; 49; 50]. However, most studies have only proposed different calculation methods for transient flows caused by boundary motions, and few studies have been conducted on control measures for transient flows caused by boundary motions. In this present work, the finite volume method based on the dynamic mesh method is used to investigate the effects of the locations and shapes of strips on the wake behind an instantaneously started cylinder in the transient flow.