Simultaneous Measurements of the Velocity and Vorticity Vector Fields in the Turbulent Near Wake of a Circular Cylinder

Abstract

Hot-wire measurements of all components of the instantaneous velocity and vorticity vectors in the wake of a circular cylinder are presented. The experiments were performed at x / d = 20 and 30, at Reα = 2000, using a miniature 12-sensor probe for the simultaneous velocity-vorticity measurements and a 4-sensor probe for the velocity-only measurements. A calibration and a data reduction method for multi-sensor hot-wire probes are introduced. The calibration technique is independent of the number of sensors and requires minimal information about probe geometry. The data reduction scheme involves the solution of an overdetermined nonlinear algebraic system of equations in a least-squares sense. The measurements indicate that the Karman vortices are accurately resolved. Statistical characteristics of the velocity aJ1d vorticity fields in this flow, including moments, probability distributions and one-dimensional spectra components are documented for the first time. Conditional analysis of the measurements at x/ d = 30 with respect, to the passage of the Karman vortices are presented. The vortex center and the saddle regions are identified and characterized. The instanteaneous velocity and vorticity signals are decomposed into mean, coherent and incoherent parts using the triple decomposition technique. A significant percentage of the incoherent fluctuations are observed to be phase-locked to the Karman vortices. The enstropy balances are computed for three decomposed parts. The generation of incoherent enstrophy due to incoherent vortex stretching is detected to be the most dominant term and is balanced by the viscous dissipation.