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.

Transport in the near aerodynamic wakes of flat plates

“…Insofar as this could effect the shedding of vorticity, then this could influence the residence time. However, it is reasonable to expect that any such variation would not necessarily follow the simple diffusion model we have proposed for the situation where If is substantially less than D. Previous measurements of H and fl by Humphries & Vincent (1976b) were carried out only for conditions where li was substantially smaller than D. I n the present paper we present results for a wider range of conditions, including some where lr > D.…”

Transport of airborne particles within a room

“…In this paper, we test an approach for analyzing the transport of aerosols from the wake behind a flat disk (Humphries and Vincent, 1976a,b, hereafter referred to as HV) for studying aerosol transport in a furnished room. That approach is described in detail in the section of this paper.…”

The effects of turbulence on the mean flow past square rods

“…Bearman (1971) reported that the mean base pressure of square and circular plates decreased as (u'/U) (L,/h)2 increased, where L, is the longitudinal scale of turbulence and h is the size of a plate. On the other hand, Humphries & Vincent (1976) claimed that it was not controlled by (u'/U) (L,/h)2 but by a different parameter (u'/D) (L,/h). A correct similarity parameter controlling the plate flow has not yet been established.…”