STRUCTURE OF TURBULENT CHEMICAL PLUMES

School of Civil Environmental Engineering, Georgia Institute of Technology 

Trace Chemical Sensing of Explosives, Edited by Ronald L. Woodfin Copyright 2007 John Wiley Sons, Inc. 

Turbulent mixing is significantly more effective than molecular diffusion alone. Consider the mixing process in a coffee cup. The time for molecular diffusion to mix a small patch of milk throughout the cup is several days, at which time the coffee would be also cold and unpleasant. However, by stirring with a spoon, and hence creating a turbulent-like flow, the mixing process is complete within seconds. 

Diffusion of momentum of the velocity fluctuations (or dissipation of turbulent kinetic energy) occurs at the Kolmogorov scale, which is estimated as 

The length scales for the turbulent concentration field range from the plume width to the scale at which molecular diffusion acts to homogenize the distribution (or dissipate the variance of the scalar fluctuations). The smallest length scale is referred to as the Batchelor scale and is estimated as 

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Several laboratory studies have contributed to our understanding of turbulent chemical plumes and the effects of various flow configurations. Fackrell and Robins [25] released an isokinetic neutrally buoyant plume in a wind tunnel at elevated and bed-level locations. Bara et al. [26], Yee et al. [27], Crimaldi and Koseff [28], and Crimaldi et al. [29] studied plumes released in water channels from bed-level and elevated positions. Airborne plumes in atmospheric boundary layers also have been studied in the field by Murlis and Jones [30], Jones [31], Murlis [32], Hanna and Insley [33], Mylne [34, 35], and Yee et al. [36, 37], In addition, aqueous plumes in coastal environments have been studied by Stacey et al. [38] and Fong and Stacey [39], The combined information of these and other studies reveals that the plume structure is influenced by several factors including the bulk velocity, fluid environment, release conditions, bed conditions, flow meander, and surface waves. 

Taken together, research presented in this chapter has made clear that (1) the physical environment can promote or retard distance chemical signaling by affecting the structure of turbulent plumes (2) organisms have developed strategies to obtain information, but these strategies do not work equally well under all conditions ... 

The Reynolds number is the ratio of inertial to viscous forces and depends on the fluid properties, bulk velocity, and boundary layer thickness. Turbulence characteristics vary with Reynolds number in boundary layers [40], Thus, variation in the contributing factors for the Reynolds number ultimately influences the turbulent mixing and plume structure. Further, the fluid environment, air or water, affects both the Reynolds number and the molecular diffusivity of the chemical compounds. 

Based on the discussed example parameters that influence the turbulent mixing process, it is clear that plume structure can be described in general terms, but the specific characteristics are likely to be case dependent. Nevertheless, certain characteristics, such as those employed by the odor-gated rheotaxis with bilateral comparison strategy, may be similar enough to allow animals and engineered systems to track chemical plumes for a wide range of flow conditions. 

The movement of a chemical substance within the vapor phase occurs by the combined driving forces of flow and diffusion. An illustration of these effects can be visualized by considering a smokestack plume in the absence of wind, the plume will rise vertically in a more or less uniform column until it reaches an elevation where density considerations result in its spreading out into a relatively broad and flat mantle. When wind is factored into the equation, the plume may move in a more nearly horizontal direction, more or less parallel to the surface of the ground, and at certain wind speeds the plume structure can break up into loops or bends due to turbulent aerodynamic effects such as eddy formation. In addition, small eddies can result in the breakdown of the coherent plume structure, with the formation of... 

The sample data presented in this chapter were collected for fairly simple flow conditions. The flow was a unidirectional open-channel flow without large-scale flow meander, and the release condition was isokinetic in the direction of the bulk flow. Thus, chemical filaments were advected by the bulk flow in the stream-wise direction, while turbulent mixing acted to expand the plume size and dilute the chemical concentration. Changes in the flow and release conditions lead to significant variation in the plume characteristics and structure. 

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