1,366 research outputs found
Connectivity of concentration clouds in turbulent flows
No full textWe study two cases of dispersion of neutral scalar species from point sources into a simulated boundary layer. Using
large-eddy simulation, we calculate the concentration fluctuations of passive plumes from an elevated (ES) and a
ground-level (GLS) source in a turbulent boundary layer over a rough wall. The mean concentration, relative
fluctuations and spectra are found in good agreement with the wind-tunnel measurements. Concentration clouds have
been analyzed by applying a multi-cloud tracking and conditional sampling algorithm to LES data. The space-time
development of the concentration clouds are examined. In particular, we focus our main interest on the quantification
of the connectivity of the concentration clouds, which is a very tough issue because of the immense dataset.
Keywords: concentration cloud, quantification of connectivity, multi-cloud tracking, conditional samplin
Modelling extreme concentration from a source in a turbulent flow over rough wall
No full textThe concentration fluctuations in passive plumes from an elevated and a groundlevel
source in a turbulent boundary layer over a rough wall were studied using
large eddy simulation and wind tunnel experiment. The predictions of statistics
up to second order moments were thereby validated. In addition, the trend of relative
fluctuations far downstream for a ground level source was estimated using
dimensional analysis. The techniques of extreme value theory were then applied
to predict extreme concentrations by modelling the upper tail of the probability
density function of the concentration time series by the Generalised Pareto Distribution.
Data obtained from both the simulations and experiments were analysed in
this manner. The predicted maximum concentration (?0) normalized by the local
mean concentration (Cm) or by the local r.m.s of concentration fluctuation (crms),
was extensively investigated. Values for ?0/Cm and ?0/crms as large as 50 and 20
respectively were found for the elevated source and 10 and 15 respectively for the
ground-level source
Large-eddy simulation of turbulent flow over a rough surface
No full textA family of wall models is proposed that exhibits more satisfactory performance than previous models for the large-eddy simulation (LES) of the turbulent boundary layer over a rough surface.The time and horizontally averaged statistics such as mean vertical profiles of wind velocity, Reynolds stress, turbulent intensities, turbulent kinetic energy and also spectra are compared with wind-tunnel experimental data. The purpose of the present study is to obtain simulated turbulent flows that are comparable with wind-tunnel measurements for use as the wind environment for the numerical prediction by LES of source dispersion in theneutral atmospheric boundary layer
LES and extreme value theory - modelling the upper tail of the PDF of concentration in turbulent dispersion
No full textA Neutrally Stratified Boundary-Layer: A Comparison of Four Large-Eddy-Simulation Computer Codes
No full textA neutrally stratified Ekman layer limited in depth by a stress-free rigid lid has been simulated using four different large-eddy simulation computer codes. This is the second large-eddy simulation intercomparison study
in a series of simulations of different flows aimed at a systematic analysis of large-eddy simulation sensitivity to subgrid scale model assumptions and numerical methods. The resolution was chosen to be coarse so that
the results might be sensitive to the details of the various subgrid scale model formulations. Stochastic backscatter of subgrid kinetic energy and scalar variance was included in one subgrid scale model.
Profiles of first and second moments, budgets of resolved-scale second moments, and spectra have been analyzed. It is found that significant differences in the lowest third of the boundary layer result from moderate
changes in subgrid scale eddy diffusivities or from the inclusion of backscatter.
To isolate effects of the various numerical methods, a set of tests with a fixed subgrid scale K-profile were made. When run with a common subgrid scale model the results from the four large-eddy simulation codes converge. This test indicates that the large-eddy simulation results are more sensitive to the subgrid scale model formulations than to the various numerics
Direct and large-eddy simulation of transient buoyant plumes : a comparison with an experiment
No full textDirect numerical simulation of turbulent Couette-Poiseuille flow with zero skin friction
No full textThe near-wall scaling of mean velocity U(y) is addressed for the case of zero skin friction on one wall of a fully turbulent channel flow. The present DNS results can be added to the evidence in support of the conjecture that U is proportional to √yw in the region just above the wall at which the mean shear dU/dy = 0
Noncommuting filters and dynamic modelling for LES of turbulent compressible flow in 3D shear layers
No full textAbstract in publication
Scaling and intermittency in ocean turbulence: analysis of coastal water optical properties and sea surface temperature (SST)
No full textWe consider here some scaling and intermittency properties of oceanic turbulence, with a general aim of considering the impact of turbulence on the bio-optical dynamics. For that purpose, we tried two different approaches, using in situ and satellite data. For the in situ study we adopted one dimensional and for the satellite two dimensional approaches. Different techniques such as Fourier power spectrum, Empirical mode of decomposition (EMD), Hilbert spectral analysis (HSA) have been used for analyzing the intermittency characteristics of the in situ data. For analyzing the multi-scale properties of the satellite images, we have considered Structure functions (SF) and Fourier power spectrum (1D and 2D). The general objective is to understand the multi-scale oceanic variability using scaling tools developed in the field of intermittent turbulence studies
The organization of industry in the P.R. China: A new start or resort to old concepts?
No full textThe Chinese have let it be known since the death of Mao Zedong and the purge of the “Gang of Four” that they would in future draw on credits from foreign governments as well as other sources to finance imports of technology. According to western conjectures China’s capital requirements may be between US $ 25 and 50 bn or even a good deal higher. Many observers have taken the view that the Chinese have thereby put all their former development principles behind them. The author of the following article was in the P.R. China about the middle of this year at the invitation of the National Planning Commission. His assessment is that the P.R. China is far from embarking on an entirely new course in its development policy
- …
