Momentum entrainment of turbulent/turbulent interface in a cylinder wake

This study explores the spatial evolution of the momentum entrainment of the

turbulent/turbulent interface of a cylinder wake, in the context of the

diminishing of the coherent vortices from the near to the far field. The wake

was marked with a high-Schmidt-number fluorescent dye, and measurements combined

planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV)

were carried out from x/d = 5 to 40, where x is the streamwise distance

from the cylinder center and d is the cylinder diameter. The cylinder is

placed at various downstream distances from various grids which are used to

generate the background turbulence, such that the the parameter space of

turbulence intensity and integral length scale was explored as widely as

possible. It is found that the momentum flux across the interfaces decays

substantially beyond x/d = 10. This is highly likely associated with the

rapidly decay of the coherent vortices when x/d >10, since the

strong coherent vortices in the near field (x/d < 10) can induce

intense engulfment. The scaling of the momentum entrainment while the wake

evolves downstream is also explored, the effect of the coherent motions on the

scaling will be reported in the meeting presentation.