Michel Pavageau

WIND TUNNEL MEASUREMENTS OF CONCENTRATION FLUCTUATIONS IN AN URBAN STREET CANYON

Abstract A wind tunnel study was performed to examine some turbulent characteristics and statistical properties of the concentration field developing from the steady release of a tracer gas at street level in a canyon amidst urban roughness. The experiment was conducted with the approaching wind direction perpendicular to the street axis and, with a street width to building height aspect ratio equal to one. Concentration time series were recorded at 70 points within the test street cross-section and above. Mean concentrations, variances and related turbulent quantities, as well as other statistical quantities including quantiles were computed. Concentration spectra and autocorrelation functions were also examined. The emphasis is put here on the results concerning mean concentrations and the variance of concentration fluctuations. The main objective of this paper is to put forward potential benefits of the experimental approach taken in this study. Through a simple and already widely studied configuration it is aimed to show how, for modelling purposes, this approach can help improving our understanding of the mechanisms of dipersion of pollution from car exhausts in built-up areas and, with further measurements, how it could assist in drawing specifications for siting monitoring networks.

Observation of volume variation effects in turbulent free convection

Abstract This paper presents preliminary results of laboratory experiments designed to study the effects of specific volume variation as regards the basic physical mechanisms governing weakly compressible flows. Turbulent free convection developing in a large vertical open tunnel was investigated. Results were analyzed in the framework of an original formulation of the Navier–Stokes equations in which no simplifying assumptions regarding density variations were made, and where the terms representing specific volume fluctuation effects could be isolated and examined separately. Non-isovolume mechanisms were found to play a noticeable role in the transport equations of internal and turbulent energy, and had to be accounted for in budget analysis. In contrast, the analysis showed that the form of the transport equation of mean momentum obtained and used classically when the Boussinesq approximation applies could still be used in the present case, consistently with earlier observations in the atmosphere. Results put forward that the role played by non-isovolume mechanisms depends strongly on the mean velocity divergence and, with respect to the temperature field, on the dissipation rate of the temperature variance.