Structure of large-scale vorticity in a turbulent far wake

Abstract

Simultaneous measurements have been made of large-scale approximations of the three components of the vorticity vector in the far wake of a slightly heated cylinder. Analysis of the results is directed towards the properties of large-scale spanwise vortices and shear-aligned double-roller vortices and the relationship between them. The relationship between vorticity and temperature is also studied. The probability density function of the inclination of the vorticity vector in the (x, y)-plane exhibits a maximum at about 45° in the central part of the wake. The p.d.f. of the inclination of the vorticity vector in the (x, z)-plane supports the existence of a concentration of spanwise vorticity near the outer part of the wake. In this region, there is significant correlation between vorticity and temperature. Conditional averages of velocity and temperature fluctuations and vorticity components are presented with respect to two types of detection, the first focusing on peaks in the ωx and ωy signals, and the second focusing on aspects of the large spanwise structures. Results from the two procedures are consistent, although different in emphasis, and are interpreted in terms of the downstream development of the wake. The upstream ends of large structures stretch and shear-align smaller, probably older, vortices while transporting heat and momentum defect outwards. Near the centreplane, the downstream ends of large structures transport colder, high-speed, spanwise-vortical fluid towards the centreplane, but ωx and ωy are not correlated with either temperature or the large structure. In (x, z)-planes, cuts through the shear-aligned vortices would appear to show double rollers.