Evolution of the wake of three inline square prisms

A study of the flow around three tandem square prisms may provide us a better understanding of complicated flow physics related to multiple closely spaced structures. In this paper, a numerical investigation on the flow around three tandem square prisms at a Reynolds number Re(=U∞W/ν)=150 is conducted for L/W=1.2-10.0, where U∞ is the freestream velocity, W is the prism width, L is the prism center-to-center spacing, and ν is the kinematic viscosity of the fluid. Extensive analyses are done of flow structures, Strouhal numbers St, fluid forces, and vorticity, velocity, and pressure fields. Four distinct flow regimes and their ranges are identified, viz., single bluff-body flow (Regime I, 1.2≤L/W<3.0), alternating reattachment flow (Regime II, 3.0<L/W<4.3), synchronous coshedding flow (Regime III, 4.3<L/W<7.3) and asynchronous coshedding flow (Regime IV, 7.3<L/W≤10.0). Regime III is further subdivided into two regimes: single St flow (Regime IIIA, 4.3<L/W<5.1) and dual St flow (Regime IIIB, 5.1 < L/W < 7.3). The dependence on L/W of fluctuating and time-mean fluid forces and St of the three prisms in each regime is studied in detail and connected to the flow structures. A secondary vortex street following the primary vortex street is observed for Regimes IIIB and IV. The detailed physics of the evolution of the primary vortex street to the secondary is imparted. The inherent frequency associated with the secondary vortex street is smaller than that with the primary. The evolution process of the primary vortex street to the secondary leads to a tertiary frequency. Dynamic mode decomposition analysis is proposed for the first time as a useful and quantitative tool to identify and quantify the secondary vortex street and its onset position.