Search

We consider large-eddy simulations (LES) of buoyant plumes from a circular source with initial buoyancy flux F0 released into a stratified environment with constant buoyancy frequency N and a uniform crossflow with velocity U. We make a systematic comparison of the LES results with the mathematical theory of plumes in a crossflow. We pay particular attention to the limits $${\tilde{U}\ll1}$$ and $${\tilde{U}\gg 1}$$, where $${\tilde{U}=U/(F_0 N)^{1/4}}$$, for which analytical results are possible. For $${\tilde{U}\gg 1}$$, the LES results show good agreement with the well-known two-thirds law for the rise in height of the plume. Sufficiently far above the source, the centreline vertical velocity of the LES plumes is consistent with the analytical z−1/3 and z−1/2 scalings for respectively $${\tilde{U}\ll 1}$$ and $${\tilde{U}\gg 1}$$. In the general case, where the entrainment is assumed to be the sum of the contributions from the horizontal and vertical velocity components, we find that the discrepancy between the LES data and numerical solutions of the plume equations is largest for $${\tilde{U}=O(1)}$$. We propose a modified additive entrainment assumption in which the contributions from the horizontal and vertical velocity components are not equally weighted. We test this against observations of the plume generated by the Buncefield fire in the U.K. in December 2005 and find that the results compare favourably. We also show that the oscillations of the plume as it settles down to its final rise height may be attenuated by the radiation of gravity waves. For $${\tilde{U}\ll 1}$$ the oscillations decay rapidly due to the transport of energy away from the plume by gravity waves. For $${\tilde{U}>rsim 1}$$ the gravity waves travel in the same direction and at the same speed as the flow. In this case, the oscillations of the plume do not decay greatly by radiation of gravity waves. [ABSTRACT FROM AUTHOR]

Profiles of wind and turbulence over an urban area evolve with fetch in response to surface characteristics. Sodar measurements, taken on 22 April 2002 during the Salford Experiment in the UK (Salfex), are here related to upstream terrain. A logarithmic layer up to z = 65m was observed in all half-hour averaged profiles. Above this height the profile showed a different vertical gradient, suggesting a change in surface cover upstream. The drag coefficient varied by a factor of two over only a 20° direction change. Turbulence intensity ( σ x ) for each wind component ( x) decreased with height, but the ratio suggested an underestimate of σ u compared to previous results. Mean urban and suburban cover fraction within the source area for each height decreased sharply between z = 20 and 50m, increasing slightly above. The near-convergence of cover fractions thus occurred for source areas of minimum length ≈ 2,200 m. In comparison, the mean length scale of heterogeneity L P was calculated from surface cover data to be 1,284 m, and the corresponding mean blending height h b was 175 m. Finally, the mean streamline angle, α, was negative and the magnitude decreased with height. An exponential fit to α for z ≤ 65m gave an e-folding height scale of 159 m. A simple relationship between this height scale and L P was assumed, giving L P ≈ 1,080 m, which is in reasonable agreement with the estimate from surface cover type. The results suggest that more emphasis is required on modelling and measuring surface-layer flow over heterogeneous urban canopies. [ABSTRACT FROM AUTHOR]

The possibility of obtaining urban roughness length from satellite-derived maps of land cover is explored, using turbulence measurements taken above the urban canopy. The upwind land cover is identified using a source-area model. Firstly, from four sets of measurements taken in Birmingham (U.K.), it is shown that in an area mainly split between the land-cover types ‘urban’ and ‘suburban’, suburban cover in the upwind source area is the more important contributor to increasing measured roughness length. A smaller dataset obtained in a second U.K. city (Salford, Greater Manchester) shows a similar result. A roughness-length aggregation model is also applied to the data from both sites. This assigns values of roughness length to each of the land-cover classifications and uses the source-area model to predict the resultant roughness length based on the wind properties. The results from this are shown to agree with the earlier findings. [ABSTRACT FROM AUTHOR]

The results of wind measurements taken in two studies at an urban site in Birmingham (UK) are presented. Displacement heights and roughness lengths are calculated that compare well with previous measurements in the urban canopy. Dimensionless windshear and vertical-velocity standard deviation are also shown to obey Monin–Obukhov similarity theory. The mean roughness lengths measured at the highest measuring position (45 m) increase with the fraction of upwind urban cover calculated using a source-area model. [ABSTRACT FROM AUTHOR]