The effect of multi-scale roughness on the structure of atmospheric turbulence

It is estimated that by 2050 our cities will host 68% of the world’s population, as compared to the current 54%, and 33% in the 1960s [1]. This increase in urban population density is accomplished by the proliferation of tall buildings that maximise the provision of housing/office per footprint area at street level. High rises are also employed by architects to iconically shape our cities and as an expression of modern art [2]. These tall structures are often characterised by unusually shaped components, and terraces/balconies that are designed to make the most of the panoramic views. An assessment of the effects of these buildings on the wind and temperature fields within cities, which affect both pedestrian comfort and air quality at street level is, therefore, needed both as a social exercise, and to provide reference data to assist and influence policymakers and regulators. Ultimately, this will contribute to more accurate weather and air quality prediction tools. This type of data is currently scarce. Fundamentally, it is also important to further our understanding of how the turbulence building blocks are modified by the presence of tall infrastructures, which are characterised by several length scales: the building height, k, but also much smaller length scales (l_i<k) associated with the detailed building layouts and its local features. To complicate the problem, these buildings are subjected to an incoming Atmospheric Boundary Layer (ABL), which rarely has neutral stability.

The project is divided into two phases:

Phase I - A series of wind tunnel experiments will be designed and carried out to assess the turbulence structure over urban morphologies, characterized by tall buildings of uniform height and a packing density that is typical of megacity environments. These will be performed in the ‘A’ wind tunnel at the University of Surrey. The level of details characterising the model of the buildings will be progressively increased, to produce a multi-scale roughness. The turbulence structure developed over single- and multi-scale roughness will be compared to assess similarities/differences. Scaling laws based on the characteristic length scales of the morphologies will be explored. For all cases, the drag generated by the morphologies will be measured by instrumenting a building with static pressure ports on all faces. This drastically reduces the uncertainty inherent in the law of the wall in rough wall turbulent boundary layers. To incorporate the effect of spatially heterogeneous surfaces, several velocity profiles will be collected with Laser Doppler Anemometry (LDA) to allow for analysis of spatially averaged quantities.

Phase II - The effect of ABL stratification will be examined in the unique ‘EnFlo’ wind tunnel (NERC National facility) at the University of Surrey. Similar single- and multi-scale urban roughness will be investigated (via LDA). Concentration measurements via Fast-response Flame Ionisation Detector (FFID) are also planned to complement the velocity profiles, and inform the urban dispersion phenomenon as a function of both the atmospheric stability and buildings length scales.

[1] 2018 Revision of world Urbanization Prospect (2018). DESA, United Nations.

[2] Mignon Nixon (2005). Fantastic reality: Louise Bourgeois and a story of modern art. MIT Press, USA.

Candidates should hold a first or upper-second class Bachelor’s degree and a Master’s degree at distinction level (or equivalent experience/qualifications) in a subject appropriate to the PhD projects applied for (please see project descriptions).

Candidates with a lower class of Bachelors degree, but a good performance at the Masters level ("merit" or above) will also be considered.

To be eligible applicants must also have lived in the UK or EU for 3 years or more.