In this experimental work both qualitative (flow visualisation) and quantitative (laser Doppler anemometry) methods were applied in a wind tunnel in order to describe the complex 3-dimensional flow field in a real environment (a street canyon intersection). The main aim was an examination of the mean flow, turbulence and flow pathlines characterising a complex 3-dimensional urban location. The experiments highlighted the complexity of the observed flows, particularly in the upwind region of the intersection. In this complex and realistic situation some details of the upwind flow, such as the presence of two tall towers, play an important role in defining the flow field within the intersection, particularly at roof level. This effect is likely to have a strong influence on the mass exchange mechanism between the canopy flow and the air aloft, and therefore the distribution of pollutants. This strong interaction between the flows inside and outside the urban canopy is currently neglected in most state-of-the-art local scale dispersion models.
The mean flow and turbulence fields and the turbulence energy dissipation rate ( ) in a vessel stirred by a hydrofoil impeller were studied using particle image velocimetry (PIV). The vessel diameter was 100 mm, and it was stirred by an impeller of diameter D ) T/3. The impeller Reynolds number was 40 000 to ensure fully turbulent flow in the vessel. Particular attention was paid to the effect of spatial resolution on the estimation of , which was determined directly from its definition, i.e., by measuring the instantaneous spatial gradients of the Reynolds stresses. The data provide new information on the quantification and distribution of in stirred vessels and indicate that estimates of dissipation based on a constant length scale across the vessel and the distribution of the turbulence levels may underestimate the maximum and overestimate the minimum levels of present in a stirred vessel.
-channel LDA techniques were employed to measure the turbulence energy dissipation rate in vessels stirred by Rushton impellers. The measurements were made in vessels of diameter T = 100 mm and 294 mm and stirred by impellers of diameter D = T/3. The impellers were rotated at speeds corresponding to Reynolds number of 40,000 to ensure fully-turbulent flow in the vessels. e was determined directly from measurements of the Reynolds stress gradients by analyzing the PIV images over interrogation areas down to 0.1 mm. Similar LDA data were obtained in the larger 294 mm vessel, with a resolution of around 50 lm. The fluctuating gradient results obtained with the two techniques compare well and show that direct measurement of the e distribution is feasible with both PIV and LDA methods.
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