Flying hot-wire measurements in an axisymmetric sudden expansion

An apparatus was designed and fabricated so that hot-wires could be used to obtain velocity measurements in an axisymmetric sudden expansion. The recirculating flow that exists in a sudden expansion and the directional insensitivity of hot-wires, required a device that would propel the hot-wires through the flow field faster than the flow was recirculating. Such a device, termed a 'flying' hot-wire system, literally collected data 'on the fly'. The sudden expansion facility has an expansion ratio of 3:1; and all of the measurements were at a Reynolds number of 41,000 based on bulk velocity and inlet pipe diameter. Significant simultaneous multi-point cross-wire measurements were obtained using this system. From these measurements, the mean velocities and normal stresses in all three directions as well as the dominant shear stress could be calculated as a function of streamwise distance. These quantities were used to perform a kinetic energy balance between the mean convection, production, turbulent diffusion and dissipation. From this energy balance, knowledge of the spatial variation of the isotropic dissipation was obtained. From these measurements, the velocity two-point correlation tensor, a quantity difficult to obtain in a separated flow, was calculated. In this paper, the two-point correlation tensor is utilized to calculate the integrated length scales in this flow geometry at several downstream locations.