Experiments to investigate transport processes in the near wakes of disks in turbulent air flow

Abstract

Experiments reported previously determined the detention time of airborne smoke particles momentarily trapped in the wake bubble behind a flat disk normal to smooth air flow. The dimensionless groupH, the product of the detention timetdand mainstream air velocityUdivided by the disk diameterD, was found to be 7.44 for all combinations ofU, Dand the Reynolds number, a result that was consistent with a suggested physical model for particle transport across the bubble boundary. The work is now extended into the regime of turbulent free-stream flow, whereHis seen to decrease with an increasing level of turbulence while the base pressure coefficient becomes more negative. At the same time, the length of the bubble decreases, as does the bubble shape factor (the ratio of bubble volume to surface area, non-dimensionalized with respect toD). A simple theoretical relationship betweenHand the base pressure coefficient is argued, and is found to be in good agreement with experiment.An important conclusion from this work is that the free-stream turbulence parameter$\Lambda \equiv l_fk^{\frac{1}{2}}_f/DU $(wherelfandkfare the length scale and the kinetic energy of the free-stream turbulence respectively) controls the properties of the flow about the disk.This work has potential applications in several areas of topical technological interest.