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  • Author or Editor: C. R. Murthy x
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C. R. Murthy

Abstract

Based on the turbulent entrainment hypothesis of Morton et al. and the eddy diffusivity hypothesis, approximate theories have been developed for the rise of a buoyant chimney plume in the atmospheric boundary layer, incorporating wind shear effects by assuming a power law type of wind velocity profile, UZp , for the boundary layer. The two theoretical approaches suggest that the mean path of a hot plume in a neutral atmospheric boundary layer can be represented by a power law Z*∝X* n ,where the exponent n is related to the wind velocity profile exponent p by n=⅔(1+p), and is less than the value of ⅔ reported in the case of uniform wind. Wind tunnel test results show fair agreement with the theoretical results.

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C. R. Murthy

Abstract

Experimental data on the diffusion of fluorescent dye patches were obtained in Lake Ontario, to study large-scale horizontal diffusion characteristics. In each experiment, a slug of water-soluble rhodamine dye solution was introduced at appropriate depths. The growth of the diffusing dye patch was followed up to 80 h after dye release, using fluorometric sampling. The data covered a length scale (i.e., patch size) of 100 m to 15 km and the corresponding eddy diffusivities varied from 102 to 106 cm2 s−1. Several horizontal diffusion characteristics are constructed based on a simple theoretical framework. Although the diffusion characteristics cannot be justified entirely from theoretical arguments, they could be viewed as purely statistical since they have been constructed from experimental data obtained in widely varying environmental conditions and provide useful guidelines for modeling practical diffusion problems.

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C. R. Murthy

Abstract

A group of ten sub-surface drogues were released in a cluster in the epilimnion 15 km offshore near Oshawa, Lake Ontario, and the subsequent drift and horizontal spread were followed for 72 h using the radar/decca navigation system of the Canadian Survey Ship Limnos. Experimental data suggest periodic shrinking and expansion of the drogue group at approximately 16–18 h corresponding to the local inertial period. This is attributed to the generation of convergence and divergence flow fields as the inertial currents are modified by the sloping shore line or a sloping thermocline. Thus, mesoscale motions such as convergence and divergence of the flow field as well as turbulent dispersion due to random eddies are important considerations in the kinematics of floatables.

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Y. R. Rao
and
C. R. Murthy

Abstract

The recently developed wavelet transform, conventional spectral, and empirical orthogonal function decompositions have been applied to the vertical temperature profiles characterized with an isolated upwelling event and several other small-scale structures. Although empirical orthogonal function provides optimal decomposition of variance, it does not reveal much information about the transition of energy to small-scale oscillations. The localized characteristics that are not detected properly in the power spectral analysis are well represented by wavelets in the frequency–time domain.

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Y. R. Rao
and
C. R. Murthy

Abstract

Simultaneous measurements of Eulerian and Lagrangian currents along the north shore of Lake Ontario are analyzed to provide the mean flow properties and horizontal turbulent exchange characteristics in the coastal boundary layer (CBL). The summer coastal boundary layer is characterized by a frictional boundary layer (FBL) of a width of ∼3 km, in which shore and bottom friction affects the flow. In this regime the currents are predominantly shore parallel and persistent. The outer boundary layer also called an inertial boundary layer (IBL), typically of the order of 5–6 km wide, is a consequence of the adjustment of inertial oscillations to the lateral boundary.

During the summer season within the CBL, the current motions are associated with thermocline displacements. The eastward (westward) wind stress causes thermocline elevation (depression) causing upwelling (downwelling). The mean subsurface westward currents associated with downwelling events are typically stronger in comparison to weak eastward flow during upwelling. Further, upwelling events are characterized by reduced low frequency motion (>1 day) and significant near-inertial (∼17 h) currents. The width of the CBL decreases during upwelling and increases during downwelling. Internal waves generated by baroclinic seiches during these events have periods from 11 to 17 hours. The near-surface horizontal exchange coefficients calculated from Lagrangian measurements are higher than those from subsurface Eulerian values. Upwelling events show that the turbulent kinetic energy is higher than mean flow kinetic energy (MKE) in the CBL, and cross-shore turbulent exchange increases in the IBL. During downwelling the alongshore exchange coefficients are higher in the FBL, whereas cross-shore exchanges are higher in the IBL. Downwelling events are also characterized by increased contribution from the MKE rather than the turbulent kinetic energy.

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C. R. Murthy
and
D. S. Dunbar

Abstract

Properties of the nearshore flow have been observed in some detail in Douglas Point, Lake Huron. Time series flow data obtained from a network of current meters deployed in a coastal chain perpendicular to the local shoreline have been analyzed to resolve the mean flow properties, horizontal turbulence characteristics and the kinetic energy (in the mean flow and fluctuations) within the coastal boundary layer. The variability of these parameters as a function of the distance from shore for an episode during which persistent short-parallel currents prevailed for several days has revealed two distinct boundary layers, an inner boundary layer dominated by bottom and shore friction (FBL—frictional boundary layer) and an outer boundary layer as a consequence of the adjustments of inertial oscillations to the lateral boundary (IBL—inertial boundary layer). If one takes the width of the frictional boundary layer as the distance to the point where the kinetic energy of currents peak, it is ∼2 km at Douglas Point, Lake Huron. As the width of the inertial boundary layer, one may take the distance to the point where inertial oscillations begin to dominate shore-parallel flow. This width is ∼8–10 km at Douglas Point, Lake Huron.

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C. R. Murthy
and
G. T. Csanady

Abstract

Experimental data were obtained in Lake Huron on the distribution of mean concentration and meansquare fluctuation about the center of gravity of a diffusing plume of fluorescent dye. Some of the mean concentration profiles showed a skewness attributable to the vertical current shear, while others were approximately Gaussian. The ratio of rms concentration fluctuation to local mean appears to be a quasi-universal distribution, in the sense that the typical amplitude of this ratio depends on turbulence intensity, but otherwise individual distributions are similar, their length-scale being equal to the length-scale of the mean concentration distribution.

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A. El-Shaarawi
and
C. R. Murthy

Abstract

A generalized statistical model is proposed to determine the probability density function of the concentration history measured at a fixed point in the wake of a continuous point source in coastal currents. An application of the model using the experimental data indicates that the log-normal distribution is adequate for many practical problems. Some general statistical characteristics of the concentration field are computed from the model.

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J. O. Blanton
and
C. R. Murthy

Abstract

Observations of currents across a nearshore zone from 2 to 6 km offshore indicate that unsteady longshore flow and complete reversals in flow are usually accompanied by large values of lateral shear. These values often approach and may exceed 10−4 sec−1, near the value of the Coriolis parameter at mid-latitude. At times when lateral shear is high, other turbulent properties such as variance at a point are also high. The variations of lateral shear are highly temporal and can be qualitatively related to the cycles of cyclone-anti-cyclone activity in the area. High shear values usually do not coincide with high winds, but are usually related to the inability of the nearshore currents to adjust to a slowly varying wind regime. Simple momentum arguments suggest that the time for adjustment decreases as water depth nearshore decreases.

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