ABSTRAKT

A turbulent rectangular duct flow with an asymmetric flow separation over a backward-facing step has been measured by using a newly developed digital particle tracking velocimetry (PTV). The center-line velocity in the upstream entry region of the duct of 40 mm in height is 13.40 m/s, and the duct expansion ratio is 1.5 for a step-height Reynolds number of 17,800. The flow is visualized by the plastic micro-balloons of 36 kg/m³ in density. Their motions are illuminated by a double-pulsed strobe light sheet whose time interval is adjusted to 100−150 μs. A high-performance parallel image processing technique developed here has made it possible for all procedures from image acquisition to data analysis to be done digitally, hence achieving substantial improvements in speed and accuracy of the measurement. The results of the mean velocities and the Reynolds stresses obtained in the entry region compare well to previous data. It is shown that the method for noise correction proposed here works properly. In the separation region, the streamwise variations of the mean velocities and the Reynolds normal and shear stresses are obtained. They agree well with the previous results measured in the similar flow geometries. Other important characteristics including the reattachment length and the maximum flow reversal ratio are also presented.