Turbulence around two obstacles in tandem: Effects of obstacle height and separation

High-resolution simulations have been extensively utilized to analyze the turbulent structures developing around wall-mounted square cylinders immersed in a turbulent boundary layer. While previous studies have demonstrated that parameters, such as the turbulence intensity of the incoming flow and the cylinder aspect ratio, significantly influence flow structures around isolated obstacles, the interaction between multiple obstacles introduces additional complexity. To systematically investigate the physics of this interaction, high-resolution Large-eddy simulations are carried out for two wall-mounted, square cylinders with different heights h 1 and h 2 , and the same width d, in a tandem configuration. The inflow in all cases is a canonical zero-pressure-gradient turbulent boundary layer at a friction Reynolds number ≈ 180 upstream the leading obstacle. Three configurations are distinguished by increasing obstacle separation, namely, “skimming flow,” “wake interfence,” and “isolated roughness” regimes, in analogy to the flow classification of a building array. While previous studies suggest that these regimes may also qualitatively describe the flow around two identical cylinders, the present paper shows that for cylinders with different heights, the combined effect of the obstacle separation G, and the aspect ratio of the rear cylinder is also critically important. In addition to the mean velocity fields, we examined the vortical motions and the turbulent kinetic energy budget to further reveal how the changes in the vortex dynamics induced by both h 2 and the obstacle separation affect the energy exchange from the fluctuation field to the mean flow.