Implicit Large Eddy Simulation of Environmental Urban Flows

Abstract

Many environmental flows are turbulent flows. Depending on the physical aspects of the wind and the urban topology, turbulence might result into unfavourable or even dangerous conditions for the pedestrians. Turbulence can also play a very important role in the transport of toxic pollutants from accidental or intentional releases. Thus, it is vital to understand its complex characteristics so that its features are accurately predicted when computational methods are used. Real urban environment involving separation and reattachment regions provides an excellent testcase for investigating such complex flows. This thesis is focused on analysing the physics involved in flows around building models pertinent to environmental flows in urban areas and to evaluate the applicability of Implicit Large-Eddy Simulation in simulating the specific type of flows. For this purpose, a number of high resolution schemes in the context of Implicit Large-Eddy Simulation (each representing di erent degrees of spatial discretisation accuracy) was assessed. The evaluation of the schemes involved direct validation against experimental data as well as comparisons with DNS and LES data regarding flows within roughness element arrays in staggered arrangements. Initially, the flow within an uniform height cubical matrix was simulated. Four numerical schemes were tested in three di erent grid resolutions. The results were found in very good agreement with the Laser Doppler Anemometry data and they even exhibit DNS-like characteristics in specific locations of comparisons. Thus, it was concluded that high order spatial discretisation schemes allow the accurate representation of reality even in relatively coarse computational meshes. The second case under investigation involved flows within a more realistic representation of urban topology. Results obtained within an array of sixteen elements with five di erent heights reveal that although the roughness of the area is increased, the wind’s velocity profile above the obstacles shares almost the same slope as in the case of the array of the four cubical element. It is believed that this thesis has expanded the range of applications in the context of Implicit Large Eddy Simulation using high resolution schemes and contributed in persuading the scientific community for its potentials.