Turbulent Diffusion in the Environment

Csanady, G. T.

doi:10.1007/978-94-010-2527-0

Cited by 763 publications

(432 citation statements)

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“…Within the range of reported measurements, the scale of meandering σ M was comparable to the relative plume width σ R , in support of the conjecture that dye dispersion was dominated by eddies comparable in size to the width of the instantaneous plume. In the range x 1 /L 20, all three widths grew approximately linearly, whereas in the following range, having 20 < x 1 /L 37, σ and σ R maintained their quasilinear growth but the growth rate of σ M decreased monotonically, possibly tending towards a zero asymptote; the latter observation agrees with the expectation that, in the far field of the plume, σ M would approach an asymptotic value of the order of L 11,1 [15].…”

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confidence: 85%

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Relative dispersion of a passive scalar plume in turbulent shear flow

Vanderwel

Tavoularis

2014

Phys. Rev. E

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Relative dispersion of a passive scalar plume was investigated in uniformly sheared, nearly homogeneous, turbulent flow with Re λ ≈ 150 using planar laser-induced fluorescence. Mean concentration maps were determined both in the laboratory frame and in a frame attached to the instantaneous center of mass of the plume cross section. The distance-neighbor function had a shape that was compatible with Richardson's expression. The mean square particle separation, two estimates of which were found to be nearly identical, had a streamwise evolution that was consistent with Richardson-Obukhov scaling with a Richardson's constant of g = 0.35. Batchelor scaling was also consistent with a wide range of the results. In these expressions, r 0 is the initial pair separation, C 2 is the Kolmogorov constant [3,4], is the mean dissipation rate, t B = (r 2 0 / ) 1/3 is the Batchelor time scale, g is the Richardson constant, and T L is the Lagrangian integral time scale. The range t t B is known as the Batchelor regime, whereas the term Richardson-Obukhov regime signifies the range t B t T L . Theoretical estimates of Richardson's constant span the wide range 0.06 < g < 3.52 [5], whereas recent experimental and numerical estimates have narrowed it down to the range 0.5 < g < 0.6 [2].Particle separations have been measured directly with the use of particle tracking methods [2,4,6-8]. Alternatively, mean particle separation can also be estimated from measurements of concentration in a diffusing cloud of particles relative to its center of mass [5,[9][10][11]. A property of interest in such studies is the distance-neighbor function q(s), defined as the probability density function (pdf) of the distance s between particle pairs within the cloud [1,5]. q(s) can be estimated as the marginal pdf of the ensemble-averaged autocorrelations of instantaneous concentration maps [12]. In the Richardson-Obukhov regime, theoretical predictions have led to the relationship q(s) ∝ e −s n ; the exponent n was given as 2/3 by Richardson and as n = 2 by Batchelor [2], but values of n from particle tracking measurements [6][7][8] and concentration maps [12] have so far been inconsistent. The accuracy of relative diffusion * stavros.tavoularis@uottawa.ca measurements in laboratory studies has improved significantly in recent years, following advances in planar laser-induced fluorescence and particle tracking techniques. Even so, the fact that most laboratory flows have been conducted at relatively low turbulence Reynolds numbers Re λ limits their assertiveness, particularly in the light of the ongoing debate concerning the existence of the Richardson-Obukhov regime at low Reynolds numbers. Bourgoin et al. [4] studied the motion of particles in a closed tank stirred by counter-rotating baffled disks and observed that the evolution of r 2 (t) was consistent with Batchelor scaling for t t B , but found no evidence of Richardson-Obukhov scaling for t t B , for values of the turbulence Reynolds number Re λ up to 815. In contrast, Ott and Mann [7] reported ...

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confidence: 85%

“…A coordinate transformation of each instantaneous map to a frame centered on the instantaneous center of mass resulted in instantaneous concentration maps in a wandering frame. The ensemble average C R (x 2 ,x 3 ) = C(x 2 − x 2C ,x 3 − x 3C ) of these maps, to be referred to as the "relative" concentration map [15], is shown in Fig. 2(d).…”

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confidence: 99%

Relative dispersion of a passive scalar plume in turbulent shear flow

Vanderwel

Tavoularis

2014

Phys. Rev. E

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“…By monitoring how the trail expands over time, we obtain an estimate of D, the di¡usivity coe¤cient of the chemical in the trail. As the movement of the trail-makers and trackers result in Re510, the consequences of eddy di¡usivity (Okubo 1971;Csanady 1973) is considered absent in our still-water observation vessels; laminar £ow is assumed (i.e. the ambient £uid is at rest).…”

Section: Trail Locationmentioning

confidence: 99%

The fluid physics of signal perception by mate-tracking copepods

Yen

Weissburg

Doall

1998

Phil. Trans. R. Soc. Lond. B

111

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Within laboratory-induced swarms of the marine copepod Temora longicornis, the male exhibits chemically mediated trail-following behaviour, concluding with £uid mechanical provocation of the mate-capture response.The location and structure of the invisible trail were determined by examining the speci¢c behaviour of the female copepods creating the signal, the response of the male to her signal, and the £uid physics of signal persistence. Using the distance of the mate-tracking male from the ageing trail of the female, we estimated that the molecular di¡usion coe¤cient of the putative pheromonal stimulant was 2.7 Â10 À5 cm 2 s À1 , or 1000 times slower than the di¡usion of momentum. Estimates of signal strength levels, using calculations of di¡usive properties of odour trails and attenuation rates of £uid mechanical signals, were compared to the physiological and behavioural threshold detection levels. Males ¢nd trails because of strong across-plume chemical gradients; males sometimes go the wrong way because of weak along-plume gradients; males lose the trail when the female hops because of signal dilution; and mate-capture behaviour is elicited by suprathreshold £ow signals. The male is stimulated by the female odour to accelerate along the trail to catch up with her, and the boundary layer separating the signal from the chemosensitive receptors along the copepod antennule thins. Di¡usion times, and hence reaction times, shorten and behavioural orientation responses can proceed more quickly. While`perceptive' distance to the odour signal in the trail or the £uid mechanical signal from the female remains within 1^2 body lengths (55 mm), the`reactive' distance between males and females was an order of magnitude larger. Therefore, when nearest-neighbour distances are 5 cm or less, as in swarms of 10 4 copepods m À3 , mating events are facilitated. The strong similarity in the structure of mating trails and vortex tubes (isotropic, millimetre^centimetre scale, 10:1 aspect ratio, 10 s persistence), indicates that these trails are constrained by the same physical forces that in£uence water motion in a low Reynolds number £uid regime, where viscosity limits forces to the molecular scale. The exploratory reaches of mating trails appear inscribed within Kolmogorov eddies and may represent a measure of eddy size. Biologically formed mating trails, however, are distinct in their £ow velocity and chemical composition from common small-scale turbulent features; and mechanoreceptive and chemoreceptive copepods use their senses to discriminate these di¡erences. Zooplankton are not aimless wanderers in a featureless environment. Their ambit is replete with clues that guide them in their e¡orts for survival in the ocean.

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“…Furthermore, and in line with this argument, I is greater for the lower emission rate. It is possible that dispersion in the wind shear [35,36] contributes to the vertical asymmetry in the plume. However, the asymmetry is also visible very near the stack (at x/H = 0.1) where the size of the plume is limited and the wind velocity changes by more than about 6 % through the plume and the turbulence intensity by <2 %.…”

Section: Intermittency Of the Concentration Time Seriesmentioning

confidence: 99%

Statistical properties of concentration fluctuations in two merging plumes

2013

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The statistics of the fluctuating concentration field within a plume is important in the analysis of atmospheric dispersion of toxic, inflammable and odorous gases. Previous work has tended to focus on concentration fluctuations in single plumes released in the surface layer or at ground level and there is a general lack of information about the mixing of two adjacent plumes and how the statistical properties of the concentration fluctuations are modified in these circumstances. In this work, data from wind tunnel experiments are used to analyse the variance, skewness, kurtosis, intermittency, probability density function and power spectrum of the concentration field during the mixing of two identical plumes and results are compared with those obtained for an equivalent single plume. The normalised variance, skewness and kurtosis on the centre-lines of the combined plume increase with distance downwind of the stack and, in the two-source configuration, takes lower values than those found in the single plumes. The results reflect the merging process at short range, which is least protracted for cases in which the sources are in-line or up to 30 • off-line. At angles of 45 • and more, the plumes are effectively side-by-side during the merging process and the interaction between the vortex pairs in each plume is strong. Vertical asymmetry is observed between the upper and the lower parts of the plumes, with the upper part having greater intermittency (i.e. the probability that no plume material is present) and a more pronounced tail to the concentration probability distribution. This asymmetry tends to diminish at greater distances from the source but occurs in both buoyant and neutral plumes and is believed to be associated with the 'bending-over' of the emission in the cross-flow and the vortex pair that this generates. The results allowed us to identify three phases in plume development. The first, very near the stack, is dominated by turbulence generated within the plume and characterised by concentration spectra with distinct peaks corresponding to scales comparable with those of the counter-rotating vortex pair. A second phase follows at somewhat greater distances 123Environ Fluid Mech downwind, in which there are significant contributions to the concentration fluctuations from both the turbulence internal to the plume and the external turbulence. The third phase is one in which the concentration fluctuations appear to be controlled by the external turbulence present in the ambient flow.

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Turbulent Diffusion in the Environment

Cited by 763 publications

References 0 publications

Relative dispersion of a passive scalar plume in turbulent shear flow

Relative dispersion of a passive scalar plume in turbulent shear flow

The fluid physics of signal perception by mate-tracking copepods

Statistical properties of concentration fluctuations in two merging plumes

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