Effects of large spanwise wavelength on the wake of a sinusoidal wavy cylinder

The wake of a sinusoidal wavy cylinder with a large spanwise wavelength lambda/D-m (=3.79-7.57) and a constant wave amplitude a/D-m=0.152, where D-m is the mean diameter of the cylinder, is investigated using three dimensional (3D) large eddy simulation (LES) at a subcritical Reynolds number Re=3 x 10(3), based on incoming free-stream velocity (U-infinity) and D-m. Attention is paid to assimilating the effects of lambda/D-m on the cylinder wake, including vortex shedding frequency, spanwise vortex formation length, streamwise velocity distribution, flow separation angle, 3D vortex structure, and turbulent kinetic energy (TKE) distribution. Based on the predominant role of lambda/D-m in the near wake modification, three regimes are identified, i.e., regime I at lambda/D-m < 6.0, regime II at lambda/D-m approximate to 6.0 and regime III at lambda/D-m > 6.0. A dramatic decrease in fluid forces is observed at lambda/D-m = 6.06, about 16% and 93% reduction in time-averaged drag and fluctuating lift, respectively, compared to those of a smooth cylinder. We identified, for the first time, an optimum lambda/D-m (= 6.06) for the wavy cylinder with relatively large lambda/D-m (> 3.5) in the subcritical flow regime. The underlying mechanisms of force reduction are discussed, including the flow characteristics at the three lambda/D-m regimes. A comparison is also made between the results of lambda/D-m effects on the near wakes of a circular and a square cylinder. (C) 2016 Elsevier Ltd. All rights reserved.