Elsevier

Atmospheric Environment

Volume 33, Issue 27, November 1999, Pages 4459-4468
Atmospheric Environment

The effect of freestream turbulence on the transport of particles in the vicinity of a blunt flow obstacle

https://doi.org/10.1016/S1352-2310(99)00201-0Get rights and content

Abstract

This paper describes the effects of intermediate and large freestream levels of turbulence (intensity and integral length scale) on the dispersion of fine particles in the near wake regions of long flat plates placed normal to the freestream. The work is particularly relevant to the transport of airborne contaminants in complex flows, especially like those encountered in many ambient and occupational atmospheric environments. An experiment was designed based on the release of fine aerosol particles upstream of the obstacle and the measurement of the concentration decay for the entrained aerosol in the near wake region after the source generation has been suddenly interrupted. Five grids were employed for generating specific levels of freestream turbulence characterized by the parameter, Λf (defined as lfk1/2f/UD, where lf and kf are the length scale and the kinetic energy of the free stream turbulence, and U and D are the freestream velocity and the plate width, respectively). The investigation covered the ranges 1500⩽Re⩽11,000 and 0.0015⩽Λf⩽0.071. The dimensionless residence time, H (defined as τU/D, where τ is the time constant for the observed decay of the aerosol concentration in the near wake cavity), appeared to be independent of the freestream turbulence for values of Λf between 0.0015 and 0.013, for which the average H-value was about 7.2±1.5. For larger freestream turbulence, corresponding to Λf greater than 0.018, H increased up to about 10. A visualization procedure was used to capture some of the qualitative features of the dispersion process for aerosols in the near wake regions of the plates and provide some support for the physical mechanisms suggested by this work. In particular, by comparing the results with those of other studies, we note the marked contrast in the effects of the freestream turbulence for aerosol flows about two- and three-dimensional bodies, respectively.

Introduction

A good understanding of the physics involved in the transport of airborne particles in the neighborhood of bluff flow obstacles is important in many areas relevant to aerosols in the atmospheric environment. In particular, these areas cover the transport of particulate air pollutants in environmental flow systems, ranging from relatively small-scale (e.g., like those inside ventilation ducts and air cleaners, around plant and machinery in workplaces, in the vicinities of air samplers, etc.) to much larger-scale (e.g., around buildings, chimneys, platforms, bridges, etc.). The identification and quantification of the important parameters governing such transport is an essential requirement in the development of models by which we might eventually predict the transport of airborne particles in the distorted air flows like those described. This paper represents a contribution towards such understanding.

The recirculation zone formed just behind a bluff body immersed in a moving air stream is characterized by the separation of the flow and the sequential shedding of vortices. If airborne particles are entrained into this near wake region, or – even more importantly – the source for particles is located there, they may persist in that region, even long after the source has been removed. In this case, the near wake region itself becomes, in effect, a secondary source. The effect of the turbulence structure of the freestream approaching the bluff body on near wake transport processes, particularly through modifications in the separated shear layers and in the dynamics of vortex shedding and re-mixing processes, will directly affect the persistence of this secondary source. In turn this will influence the temporal and spatial distributions of the particle concentration throughout the flow field downwind of the flow obstacle.

The general problem of the effect of freestream turbulence on the aerodynamics of bluff body flows is not new. But a high proportion of such work appears to have been addressed towards the effects of the turbulence on the pressure distribution and, hence, the drag on the bluff body in question (e.g., Schubauer and Dryden, 1935; Gerrard, 1966; Bearman, 1971, Bearman, 1978; Dyban et al., 1974; Lee, 1975; Nakamura et al., 1988; Laneville, 1990). There have, however, been some studies related to the consequences of freestream turbulence on the behavior of scalar entities in such flow configurations (e.g., Humphries and Vincent, 1976b; MacLennan and Vincent, 1982; Hunt, 1991; Higson et al., 1995).

In the context of the present work, the main findings from research like that cited above were that: (a) the freestream turbulence affects the flow past bluff bodies, especially the nature of the near wake region, thus influencing particle transport in that region; (b) the effect is dependent on the aspect ratio of the bluff body so that we may identify two distinct classes of flow, two-dimensional and three-dimensional; and (c) there are effects associated specifically with the turbulence intensity and scale, respectively. Here, the intensity (If) is defined locally as the ratio between the root-mean square velocity fluctuation and the mean velocity, and the scale (lf) is the characteristic dimension of the fluctuating flow structures (or eddies; more specifically, we will be looking at the dimensions of the larger eddies which correspond to the integral length scale of the freestream turbulence). In general, the magnitude of the effect of If is greater than the effect of lf. But the latter has been much less studied. Nakamura et al. (1988) found that the influence of turbulence scale is greater for two-dimensional than for three-dimensional obstacles, due to the nature of the interaction of the freestream with the near wake vortex formation process. It was postulated that such interaction is small when the turbulence scale is comparable to the thickness of the shear layers, but becomes more pronounced for scales comparable with the characteristic dimension of the obstacle or greater.

According to Humphries and Vincent (1976a), in their investigation of the dispersion of fine particles in the vicinity of a flat disk placed normally to the freestream, it is useful to define a characteristic dimensionless residence time parameter (H) for particles in the near wake region, whereH=τUD,in which τ is the time constant for the exponential decay of the particle concentration in the near wake region after the source for these particles is suddenly removed, U is the freestream air velocity, and D is the characteristic dimension of the obstacle. In a subsequent paper, Humphries and Vincent (1976b) defined a dimensionless parameter Λf for characterizing the freestream turbulence and its influence on H and other near wake properties. They showed that Λf should include the effects of both the freestream turbulence intensity and scale, thusΛf=lfk1/2fUDlfIfD,where kf is the freestream turbulence kinetic energy which defines the intensity of the turbulence. Here, it is seen that both the numerator and denominator of Eq. (2) have dimensions of (m2s−1), in which case Λf is, in effect, a dimensionless turbulent diffusion coefficient. Experimentally, Humphries and Vincent (1976b) found for the decay of the concentration in the near wake region behind a disk that H decreased from around 7.4±0.5 for smooth flow f≈0) to about 6 for a turbulent freestream with Λf equal to about 0.05. Their results were independent of Reynold's number (Re=DUρ/η, where ρ and η are the density and absolute viscosity of air, respectively) for the range 4000<Re<25,000. By contrast, in their experiments with two-dimensional, long flat plates, MacLennan and Vincent (1982) could find no significant dependence of H on the freestream turbulence for that particular geometry. In their experiments, however, H was seen to vary unsystematically between about 5 and 6.

The present work is part of a larger body of work to further our understanding of such effects. In a previous paper, we described our investigations of the residence time of fine, inertialess particles in the near wake regions of long, two-dimensional flat plates placed normal to a freestream with a very low levels of freestream turbulence (Gomes et al., 1997). Here that work is extended to report new studies aimed at identifying and quantifying the nature of the effects of freestream turbulence for the same type of flow geometry. The study comprised both: (a) new experiments to investigate quantitatively the dependence of residence time, and hence H, on freestream turbulence, and (b) flow visualizations to obtain a supporting qualitative understanding of the nature of the changes which take place resulting from changes in the intensity and scale of the freestream turbulence.

Section snippets

Experimental

The methodology employed in this investigation involved the measurement of the residence time of fine particles in the near wakes of long, two-dimensional flat plates after an upstream source of fine particles has been suddenly interrupted. The measurements were conducted in a small wind tunnel at the University of Minnesota. The test section of the tunnel was 0.3 m×0.3 m in cross-section and 1.5 m in length. A purpose-built optical detector system was employed for measuring the temporal change in

Experimental

Flow visualization was carried out in support of the quantitative measurements described above. This involved making a video-tape of the visualized aerosol flow and then extracting pictures from sequential still frames of the tape. To visual the flow, a familiar slide project with a zoom projection lens was used as the light source. This was employed to project a two-dimensional plane of light, by means of a slit prepared by mounting a pair of razor blades approximately 1 mm apart on a 35-mm

Conclusions from the research

We have described experiments to investigate of the effects of freestream turbulence on particle retention in the near wakes of two-dimensional flat plates positioned perpendicular to the flow. Five grids were used for generating defined levels of freestream turbulence intensity and length scale, providing values between 0.0015 and 0.071 for the dimensionless freestream turbulence parameter f) which incorporates the effects of both the turbulence intensity and turbulence scale in a manner

Implications of the research

This research confirms that, as reported elsewhere, the dispersion of airborne scalar entities near bluff bodies is strongly influenced not only by the shape of the body (including whether it may be regarded as two or three-dimensional) but also by the level of freestream turbulence. The latter in turn is dependent on the upstream history of the flow. Bearing in mind the importance of understanding the fate of airborne pollutants in ambient and occupational environments, the insights gained

Acknowledgements

This research was conducted with support from CAPES-BRASIL and the University of Minnesota School of Public Health.

References (20)

  • A. Laneville

    Turbulence and blockage effects on two dimensional rectangular cylinders

    Journal of Wind Engineering and Industrial Aerodynamics

    (1990)
  • Y. Nakamura et al.

    The effects of turbulence on bluff-body mean flow

    Journal of Wind Engineering and Industrial Aerodynamics

    (1988)
  • W.D. Baines et al.

    An investigation of flows through screens

    Transactions of the ASME

    (1951)
  • P.W. Bearman

    An investigation of the forces on flat plates normal to a turbulent flow

    Journal of Fluid Mechanics

    (1971)
  • Bearman, P.W., 1978. Turbulence effects on bluff body mean flow. Personal communication of conference notes to James H....
  • Cowdrey, C.F., 1962. A note on the use of end plates to prevent three-dimensional flow at the ends of bluff cylinders....
  • Doebelin, E.O., 1983. Measurement Systems. McGraw-Hill Book Company, New...
  • Y.P. Dyban et al.

    Effects of free-stream turbulence on the flow past a circular cylinder

    Fluid Mechanics – Soviet Research

    (1974)
  • V.O. Flachsbart

    Der Widerstand quer angeströmeter Rechteckplatten bei Reynoldsschen Zahlen 1000 bis 6000

    ZAMM-Zeitschrift für Angewandte Mathematik und Mechanik

    (1935)
  • J.H. Gerrard

    The mechanics of the formation region of vortices behind bluff bodies

    Journal of Fluid Mechanics

    (1966)
There are more references available in the full text version of this article.

Cited by (0)

1

Current address: Department of Environmental and Industrial Health, School of Public Health, University of Michigan, 109 S. Observatory, Ann Arbor, MI 48109-2029, USA.

View full text