Three-dimensional numerical simulation on flow past three circular cylinders in an equilateral-triangular arrangement

Flow past three circular cylinders in an equilateral-triangular configuration is simulated for Reynolds numbers 200 ≤ Re ≤ 3900 and spacing ratios 1.25 ≤ L/D ≤ 4.0, where L is the center-to-center distance between each two cylinders and D is the diameter of the cylinder. Five different flow regimes are identified: proximity with deflected flow (P-D), proximity with flip-flop flow (P–F), dual vortex shedding (DV), transition between dual vortex shedding and triple vortex shedding (DV-TV), triple vortex shedding (TV). The boundary spacing ratios between the five regimes are affected by the Reynolds number. With the increase of the spacing ratio or the Reynolds number, the three-dimensionality of the wake flow becomes stronger. Mode B flow pattern with streamwise vortices is found at L/D = 1.25, 3.5 and 4.0 for Re = 1500 and L/D = 1.25 for 3900. The pressure coefficient on the surface of the downstream cylinder C3 is significantly different from that on a single cylinder, leading to significant reduction of the mean drag coefficient on C3 for all the studied spacing ratios and Reynolds numbers. The Strouhal numbers of C1 and C3 are the same and smaller than that of C2 in DV and TV regimes.