The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Eugene Yee - One of the best experts on this subject based on the ideXlab platform.
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Comparison of Wind-tunnel and Water-channel Simulations of Plume Dispersion through a Large Array of Obstacles with a Scaled Field Experiment
Boundary-Layer Meteorology, 2006Co-Authors: Eugene Yee, Ralph M. Gailis, Alexander Hill, Trevor Hilderman, Darwin KielAbstract:We report on measurements of the near-field dispersion of contaminant plumes in a large array of building-like obstacles at three Scales; namely, at full-Scale in a field experiment, at 1:50 Scale in a wind-tunnel simulation, and at 1:205 Scale in a water-channel simulation. Plume concentration statistics extracted from the physical modelling in the wind-tunnel and water-channel simulations are compared to those obtained from a field experiment. The modification of the detailed structure of the plume as it interacts with the obstacles is investigated. To this purpose, measurements of the evolution of the mean concentration, concentration fluctuation intensity, concentration probability density function, and Integral Time Scale of concentration fluctuations in the array plume obtained from the field experiment and the Scaled wind-tunnel and water-channel experiments are reported and compared, as well as measurements of upwind and within-array velocity spectra. Generally, the wind-tunnel and water-channel results on the modification of the detailed plume structure by the obstacles were qualitatively similar to those observed in the field experiments. However, with the appropriate scaling, the water-channel simulations were able to reproduce quantitatively the results of the full-Scale field experiments better than the wind-tunnel simulations.
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An impact–effect mathematical model incorporating the influence of exposures to fluctuating concentrations in a dispersing plume of pollutant in the atmosphere
Journal of exposure analysis and environmental epidemiology, 1999Co-Authors: Eugene YeeAbstract:The statistical properties of the impact or toxic load (pollutant concentration raised to an exponent and multiplied by exposure duration), obtained from fluctuating concentrations in a plume dispersing in the atmosphere, are investigated both analytically and experimentally. A general expression for the kth order moment of the impact is derived in terms of the k-Time point joint moment of the nth power of the fluctuating plume concentration field. Special cases of this general relationship are treated explicitly: (i) a simple model for the ensemble-mean impact (or equivalently, the ensemble-mean impact ratio) is derived on the basic hypothesis that the higher moments of concentration can be adequately modelled using an exponential probability density function (PDF), and this hypothesis is shown to give results that agree remarkably well with an extensive new set of concentration fluctuation measurements; and (ii) a model for the Integral Time Scale of the process obtained by raising the concentration to the nth power is formulated using Gifford's meandering plume model, and the latter is subsequently used to derive a simple expression for estimating the impact variance for all exposure Times, given the mean and mean-square concentrations and the plume concentration Integral Time Scale only. The results of this model for impact variance are favorably compared with some data from full-Scale field experiments. The impact PDF is found to be reasonably well-characterized by a clipped-normal PDF for exposure Times, t(e), of practical interest (e.g., t(e) approximately >5 s). The implications of these results, for determining the fraction of an exposed population that will experience a specified level of effect from a random impact arising from exposure to a fluctuating plume of pollutant, are discussed briefly.
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A Stochastic Time Series Model for Threshold Crossing Statistics of Concentration Fluctuations in non-Intermittent Plumes
Boundary-Layer Meteorology, 1999Co-Authors: D.j. Wilson, Eugene YeeAbstract:A numerical stochastic model is developed for the upcrossing rate across a specified threshold concentration. The model assumes that the concentration Time series at a given spatial point within a dispersing plume can be approximated as a first-order Markovian process designed to be consistent with a given Time-invariant concentration probability density function (pdf). The model requires only the specification of a concentration pdf with a given mean and variance and a concentration fluctuation Integral Time Scale. Predicted upcrossing rates are compared with atmospheric plume concentration data obtained from a point source near the ground. For this data set, a log-normal pdf is found to give better estimates of the threshold crossing rate than a gamma pdf.
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A Study of Concentration Fluctuations in Instantaneous Clouds Dispersing in the Atmospheric Surface Layer for Relative Turbulent Diffusion: Basic Descriptive Statistics
Boundary-Layer Meteorology, 1998Co-Authors: Eugene Yee, P. R. Kosteniuk, C. A. Biltoft, J. F. BowersAbstract:A series of tracer experiments studying the statistical properties of concentration fluctuations in clouds dispersing in the atmospheric surface layer is described and analyzed. Experiments were conducted at downwind fetches between about 200 and 1200 m, under a wide range of atmospheric conditions ranging from very unstable to moderately stable stratification. The present experiments have addressed basic requirements not met by past field experiments involving instantaneously released clouds; namely, the experiments provided repeat realizations of instantaneously released clouds measured with high-resolution concentration detectors, accompanied by the contemporaneous acquisition of high-quality meteorological and turbulence measurements. Extensive analyses are performed on the cloud concentration data in the framework of relative diffusion. Ensembles of cloud concentration realizations have been constructed. From these ensembles, crosswind and Time profiles of the ensemble-mean concentration, concentration variance, ensemble-mean dosage, and dosage variance are obtained. The behaviour of the Time profiles of the Integral Time Scale of cloud concentration fluctuations is studied. The use of surface-layer similarity theory for the analysis of the downwind variation of a number of cloud quantities (e.g., cloud size and duration, cloud centre ensemble-mean concentration and dosage, cloud centre concentration and dosage variance, cloud centre Integral Time Scale) is shown to be an effective basis for ordering these quantities. Furthermore, a number of approximate universal relationships describing the behavior of these cloud quantities has been derived. Finally, it is shown that the Scaled crosswind and Time profiles of ensemble-mean concentration and concentration variance as well as the Scaled Time profiles of the concentration fluctuation Integral Time Scale exhibit self-similar forms that are independent of atmospheric stratification and downwind fetch.
Nicolas Mordant - One of the best experts on this subject based on the ideXlab platform.
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Three-Dimensional Structure of the Lagrangian Acceleration in Turbulent Flows
Physical review letters, 2004Co-Authors: Nicolas Mordant, Alice M. Crawford, Eberhard BodenschatzAbstract:We report experimental results on the three-dimensional Lagrangian acceleration in highly turbulent flows. Tracer particles are tracked optically using four silicon strip detectors from high energy physics that provide high temporal and spatial resolution. The components of the acceleration are shown to be statistically dependent. The probability density function of the acceleration magnitude is comparable to a log-normal distribution. Assuming isotropy, a log-normal distribution of the magnitude can account for the observed dependency of the components. The Time dynamics of the acceleration components is found to be typical of the dissipation Scales, whereas the magnitude evolves over longer Times, possibly close to the Integral Time Scale.
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The 3D structure of the Lagrangian acceleration in turbulent flows
Physical Review Letters, 2004Co-Authors: Nicolas Mordant, Alice M. Crawford, Eberhard BodenschatzAbstract:We report experimental results on the three dimensional Lagrangian acceleration in highly turbulent flows. Tracer particles are tracked optically using four silicon strip detectors from high energy physics that provide high temporal and spatial resolution. The components of the acceleration are shown to be statistically dependent. The probability density function (PDF) of the acceleration magnitude is comparable to a log-normal distribution. Assuming isotropy, a log-normal distribution of the magnitude can account for the observed dependency of the components. The Time dynamics of the acceleration components is found to be typical of the dissipation Scales whereas the magnitude evolves over longer Times, possibly close to the Integral Time Scale.
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measurement of lagrangian velocity in fully developed turbulence
Physical Review Letters, 2001Co-Authors: Nicolas Mordant, P Metz, Olivier Michel, Jeanfrancois PintonAbstract:We have developed a new experimental technique to measure the Lagrangian velocity of tracer particles in a turbulent flow, based on ultrasonic Doppler tracking. This method yields a direct access to the velocity of a single particule at a turbulent Reynolds number $R_{\lambda} = 740$. Its dynamics is analyzed with two decades of Time resolution, below the Lagrangian correlation Time. We observe that the Lagrangian velocity spectrum has a Lorentzian form $E^{L}(\omega) = u_{rms}^{2} T_{L} / (1 + (T_{L}\omega)^{2})$, in agreement with a Kolmogorov-like scaling in the inertial range. The probability density function (PDF) of the velocity Time increments displays a change of shape from quasi-Gaussian a Integral Time Scale to stretched exponential tails at the smallest Time increments. This intermittency, when measured from relative scaling exponents of structure functions, is more pronounced than in the Eulerian framework.
Eberhard Bodenschatz - One of the best experts on this subject based on the ideXlab platform.
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Three-Dimensional Structure of the Lagrangian Acceleration in Turbulent Flows
Physical review letters, 2004Co-Authors: Nicolas Mordant, Alice M. Crawford, Eberhard BodenschatzAbstract:We report experimental results on the three-dimensional Lagrangian acceleration in highly turbulent flows. Tracer particles are tracked optically using four silicon strip detectors from high energy physics that provide high temporal and spatial resolution. The components of the acceleration are shown to be statistically dependent. The probability density function of the acceleration magnitude is comparable to a log-normal distribution. Assuming isotropy, a log-normal distribution of the magnitude can account for the observed dependency of the components. The Time dynamics of the acceleration components is found to be typical of the dissipation Scales, whereas the magnitude evolves over longer Times, possibly close to the Integral Time Scale.
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The 3D structure of the Lagrangian acceleration in turbulent flows
Physical Review Letters, 2004Co-Authors: Nicolas Mordant, Alice M. Crawford, Eberhard BodenschatzAbstract:We report experimental results on the three dimensional Lagrangian acceleration in highly turbulent flows. Tracer particles are tracked optically using four silicon strip detectors from high energy physics that provide high temporal and spatial resolution. The components of the acceleration are shown to be statistically dependent. The probability density function (PDF) of the acceleration magnitude is comparable to a log-normal distribution. Assuming isotropy, a log-normal distribution of the magnitude can account for the observed dependency of the components. The Time dynamics of the acceleration components is found to be typical of the dissipation Scales whereas the magnitude evolves over longer Times, possibly close to the Integral Time Scale.
Leonardo P. Chamorro - One of the best experts on this subject based on the ideXlab platform.
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On the evolution of the Integral Time Scale within wind farms
Energies, 2018Co-Authors: Liu Huiwen, Imran Hayat, Yaqing Jin, Leonardo P. ChamorroAbstract:A wind-tunnel investigation was carried out to characterize the spatial distribution of the Integral Time Scale ( T u ) within, and in the vicinity of, two model wind farms. The turbine arrays were placed over a rough wall and operated under high turbulence. The two layouts consisted of aligned units distinguished only by the streamwise spacing ( Δ x T ) between the devices, set at five and ten rotor diameters d T (or S x = Δ x T / d T = 5 and 10). They shared the same spanwise spacing between turbines of 2.5 d T ; this resulted in arrays of 8 × 3 and 5 × 3 horizontal-axis turbines. Hotwire anemometry was used to characterize the instantaneous velocity at various vertical and transverse locations along the central column of the wind farms. Results show that T u was modulated by the wind farm layout. It was significantly reduced within the wind farms and right above them, where the internal boundary layer develops. The undisturbed levels above the wind farms were recovered only at ≈ d T / 2 above the top tip. This quantity appeared to reach adjusted values starting the fifth row of turbines in the S x = 5 wind farm, and earlier in the S x = 10 counterpart. Within the adjusted zone, the distribution of T u at hub height exhibited a negligible growth in the S x = 5 case; whereas it underwent a mild growth in the S x = 10 wind farm. In addition, the flow impinging the inner turbines exhibited T u / T i n c u < 1 , where T i n c u is the Integral Time Scale of the overall incoming flow. Specifically, T u → β T i n c u at z = z h u b , where β < 1 within standard layouts of wind farms, in particular β ≈ 0.5 and 0.7 for S x = 5 and 10.
P. Henrik Alfredsson - One of the best experts on this subject based on the ideXlab platform.
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Scaling Laws in Canopy Flows: A Wind-Tunnel Analysis
Boundary-Layer Meteorology, 2013Co-Authors: Antonio Segalini, Jens H. M. Fransson, P. Henrik AlfredssonAbstract:An analysis of velocity statistics and spectra measured above a wind-tunnel forest model is reported. Several measurement stations downstream of the forest edge have been investigated and it is observed that, while the mean velocity profile adjusts quickly to the new canopy boundary condition, the turbulence lags behind and shows a continuous penetration towards the free stream along the canopy model. The statistical profiles illustrate this growth and do not collapse when plotted as a function of the vertical coordinate. However, when the statistics are plotted as function of the local mean velocity (normalized with a characteristic velocity Scale), they do collapse, independently of the streamwise position and freestream velocity. A new scaling for the spectra of all three velocity components is proposed based on the velocity variance and Integral Time Scale. This normalization improves the collapse of the spectra compared to existing scalings adopted in atmospheric measurements, and allows the determination of a universal function that provides the velocity spectrum. Furthermore, a comparison of the proposed scaling laws for two different canopy densities is shown, demonstrating that the vertical velocity variance is the most sensible statistical quantity to the characteristics of the canopy roughness.