Numerical simulation and flow visualization using soap film of the self-organized vortex structure in the wake of an array of cylinders

Wakes behind blunt bodies, such as a cylinder or square, are classical problems in fluid mechanics (e.g., Oertel 1990; Williamson 1996). The wake structures past the pillars of a bridge, or the wake behind a propelling ship are some real life examples. In this study, numerical simulations and flow visualizations have been performed to elucidate vortex interactions in the wake of an array of cylinders. The computational results are obtained by using both the two-dimensional unsteady laminar viscous model, and the Large-Eddy-Simulation (LES) model, with the Fluent® Software for Reynolds numbers corresponding to both laminar and turbulent flows, respectively. The spacing between neighboring cylinders is S =w/D = 4.0, where D is the diameter of the cylinders (1.8 mm) and w is the distance between two cylinders' center. The computational domain (5.2 cm x 4.5 cm) contains 340,654 cells and the computational time step for the simulations equals dt = 0.0050 sec. A uniform flow is imposed at the inlet boundary and a pressure outlet boundary for the flow outlet. The cylinders and side walls are modeled under no-slip boundary conditions (Huang et al. 2006). The CFD results are presented in Fig. 1 showing the vorticity contours, which illustrate in detail the vortex dynamics of the cylinders' wake. For comparison, the wake structure present for the same configuration and approximately the same flow conditions (excluding the low Reynolds number laminar condition due to the limitation of the experimental techniques) is visualized using the soap-film method (Zhang et al. 2000) and photo results are presented in Fig. 2. A two-dimensional soap film tunnel is used, which consists of a test section of 1 m by 10 cm capable of producing mean flows between 2.70 m/s to 6.50 m/s. The kinematic viscosity ν, of the soap film at room temperature of 20 °C is 1.004×10-6 m 2 /s. Once the array of obstacles is placed in the soap film, we make use the optical properties of the soap film which are visible in the presence of a low pressure sodium lamp (590 nm). To track and capture the wake dynamics of the flow motion, a high-speed pco.1200hs camera is used recording at a frame rate of 800 fps. The use of the present lighting system clearly highlights the interference patterns of the soap film, and illustrates how the wakes evolve in the flow. For the low Reynolds number laminar regime considered in …