Wake behind a discontinuous cylinder: Unveiling the role of the large scales in wake growth and entrainment

Identifier:  TDX:3864

Handle: https://hdl.handle.net/20.500.11797/TDX3864

Authors:  Mandava, Venkata Subba Rao

Abstract:
A study of the existing literature indicates that there is a lack of agreement regarding the mechanism of entrainment in turbulent/non-turbulent interface (TNTI) in free flows, with various studies finding differences in the relative importance of the nibbling and engulfment mechanisms. This suggests a significant flow dependence and that by controlling the development of large-scale structures, one can control the extent to which each of these mechanisms contribute to the overall entrainment rate. The hypothesis motivating this study is that enhancing vortices that are aligned with the mean shear will increase the rate of entrainment based on observations of flow structure and entrainment in the self-preserving regions of far wakes. We tested this hypothesis with the discontinuous cylinder (DC) configuration, consists of cylinder segments 5D long (with D being the diameter of the cylinder) separated by gaps of width 2.5D. Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA) measurements were used to analyze the flow at two Reynolds numbers, Re=4,000 and 10,000, for x/D<180. Large eddy simulations (LES) for both the DC and the infinite continuous cylinder (CC) wakes were also carried out at Re=10,000. The DC configuration was devised to trigger the shedding of horseshoe vortices (HSV) in the very near wake region with the intent of illustrating the role that these three–dimensional HSVs, previously identified in the far wake region of CC, play in the entrainment process in turbulent wakes. The DC wake was found to grow and spread in the transverse direction with a much faster rate than for the CC wake, up until about x/D ≈ 50. Prior to this location, the enhanced growth rate caused by the shear-aligned HSV led to a wake width of about 3 times that of the CC wake, with a maximum mean velocity deficit that was about half.