Professor Alan Robins

This study investigates flow variability at different scales and its effects on the dispersion of a passive scalar in a regular street network by means of direct numerical simulations (DNS), and compared to wind tunnel (WT) measurements. Specific scientific questions addressed include: (i) sources of variability in the flow at street-network scale, (ii) the effects of such variability on both puff and continuous localised releases, (iii) additional sources of uncertainty related to experimental setups and their consequences. The street network modelled here consists of an array of rectangular buildings arranged uniformly and with periodic horizontal boundary conditions. The flow is driven by a body force at an angle of 45 degrees relative to the streets in the network. Sources of passive scalars were located near ground level at three different types of locations: a short street, an intersection between streets and a long street. Flow variability is documented at different scales: small-scale intra-street variations linked with local flow topology; inter-street flow structure differences; street-network scale variability; and larger-scale spatial variations associated with above-canopy structures. Flow statistics and the dispersion behaviour of both continuous and short-duration (puff) releases of a passive scalar in the street network are analysed and compared with the results of wind-tunnel measurements. Results agree well with the experimental data for a source location in an intersection, especially for flow statistics and mean concentration profiles for continuous releases. Larger differences arise in the comparisons of puff releases. These differences are quantified by computing several puff parameters including time of arrival, travel time, rise and decay times. Reasons for the differences are discussed in relation to the underlying flow variability identified, differences between the DNS and WT setup and uncertainties in the experimental setup. Implications for the propagation of short-duration releases in real urban areas are discussed in the light of our findings. In particular, it is highlighted that in modelling singular events such as accidental releases, characterising uncertainties is more meaningful and useful than computing ensemble averages.

We conducted experimental investigations on the effect of stable thermal conditions on rough-wall boundary layers, with a specific focus on their response to abrupt increases in surface roughness. For stably stratified boundary layers, a new analytical relation between the skin-friction coefficient, $C_f$, and the displacement thickness was proposed. Following the sharp roughness change, the overshoot in $C_f$ is slightly enhanced in stably stratified layers when compared with that of neutral boundary layers. Regarding the velocity defect law, we found that the displacement thickness multiplied by $\sqrt{2/C_f}$, performs better than the boundary layer thickness alone when describing the similarity within internal boundary layers for both neutral and stable cases. A non-adjusted region located just beneath the upper edge of the internal boundary layer was observed, with large magnitudes of skewness and kurtosis of streamwise and wall-normal velocity fluctuations for both neutral and stable cases. At a fixed wall-normal location, the greater the thermal stratification, the greater the magnitudes of skewness and kurtosis. Quadrant analysis revealed that the non-adjusted region is characterised by an enhancement/reduction of ejection/sweep events, particularly for stably stratified boundary layers. Spatially, these ejections correspond well with peaks of kurtosis, exhibit stronger intensity and occur more frequently following the abrupt change in surface conditions.

Cycling is a popular means of transport in cities, this experimental wind tunnel study models groups of cyclists' exposure to road vehicle emissions on a typical London street. Transport for London state that polluting vehicles are responsible for half of London's air pollution and research shows a direct link between poor air quality and increased rates of respiratory diseases. Cyclists are particularly at risk due to their increased inhalation rates and proximity to traffic, therefore expressing the importance and significance of this research. This is the first study to specifically look at the implications of cycling in groups, often the case in congested cycle lanes at peak hours. The results of this project, carried out in the Environmental Flow wind tunnel, confirm that pollutant concentration decreases rapidly with increased separation distance from an exhaust when a rider and vehicle are in line. However, cyclists at the front of a group of in-line riders are subjected to the least pollution when adjacent to polluting vehicles, regardless of their separation distance. Following other riders may therefore increase exposure to air pollution. The increased pollutant concentration observed in groups of riders is likely linked with the complex aerodynamic field generated by upstream cyclists, trapping the vehicle exhaust fumes among the riders. This is combined with the reduced wind speed within groups which is less effective at sweeping the pollutants away. These findings suggest policy makers should construct wider cycle paths, or even better, separate riders from the road. Meanwhile, cyclists should distance themselves from both vehicles and other riders to minimise exhaust emission exposure. Drivers should also be advised to maximise the space they leave cyclists on the road.

This manuscript mainly explores the characteristics of turbulence quantities in the wake of tall building clusters of different array size (𝑁N) and building spacing (𝑊𝑆WS) arranged in an aligned and regular grid in the flow direction. Velocity fields are measured in a wind tunnel using three-dimensional laser Doppler anemometry. Results show a delayed recovery of 𝑢𝑟𝑚𝑠urms and 𝑣𝑟𝑚𝑠vrms (defined as the root-mean-square of the streamwise and lateral velocities, respectively) in the wake flow compared with the mean flow. Based on the turbulent fluctuations, the extents of the near-, transition- and far-wake regions in Mishra et al. (Boundary-Layer Meteorol., vol. 189, 2023, pp. 1–25) are revisited. In the near wake, we observe a significant reduction in 𝑢𝑟𝑚𝑠urms and 𝑣𝑟𝑚𝑠vrms in the wake of a 4×44×4 cluster compared with that of a single building. In the transition region, the turbulence intensity magnitudes within the cluster reduce to below their free-stream counterpart; this reduction is associated with the slowly varying nature of the normalised wake deficit in the streamwise direction. The recovery of the root mean square in the far-wake region is observed for 𝑥≥2.5𝑊𝐴x≥2.5WA (where 𝑊𝐴WA is the width of the cluster), with the mutual interaction of the wakes formed behind the individual buildings reducing with an increase in 𝑊𝑆WS, resulting in a faster recovery of the turbulent fluctuations. Finally, wavelet analysis suggests the existence of multi-scale vortex-shedding frequencies downwind of tall building clusters.

Wind tunnel experiments on regular arrays of buildings were conducted in the environmental wind tunnel in the EnFlo laboratory at the University of Surrey. The model canopy comprised a square array of 14×21 rectangular blocks (1h×2h) with height h = 70 mm. Preliminary measurements of velocity, turbulence and tracer concentrations were made for 3 wind directions: 0, 45 and 90°. The results from this first experimental campaign along with numerical simulations have shown that the canopy has obstacles sufficiently long compared with their heights to yield extensive flow channelling along streets. Across the whole of the downwind half of the long street the flow for the present canopy is closely aligned with the obstacle faces, despite the 45° flow orientation aloft. This supports the suggestion that the streets are long enough to be representative for street network modelling approaches; shorter streets would probably not be sufficient and it will be interesting to see how well network models can predict concentrations in the present canopy. The extensive array and the small scale of the model posed challenging problems for reaching the desired high accuracy needed to validate the numerical simulations. The improvements in the methodology will be presented and discussed at the conference. The wind tunnel data, along with LES and DNS simulations, are being used to understand the behaviour of flow and dispersion within regular array with a more realistic geometry than the usual cuboids. This integrated methodology will help developing parametrisations for improved street network dispersion models.

Air quality in cities is influenced not only by emissions and chemical transformations but also by the physical state of the atmosphere which varies both temporally and spatially. Increasingly, tall buildings (TB) are common features of the urban landscape, yet their impact on urban air flow and dispersion is not well understood, and their effects are not appropriately captured in parameterisation schemes. Here, hardware models of areas within two global mega-cities (London and Beijing) are used to analyse the impact of TB on flow and transport in isolated and cluster settings. Results show that TB generate strong updrafts and downdrafts that affect street-level flow fields. Velocity differences do not decay monotonically with distance from the TB, especially in the near-wake region where the flow is characterised by recirculating winds and jets. Lateral distance from an isolated TB centreline is crucial, and flow is still strongly impacted at longitudinal distances of several TB heights. Evaluation of a wake-flow scheme (ADMS–Build) in the isolated TB case indicates important characteristics are not captured. There is better agreement for a slender, shorter TB than a taller non-cuboidal TB. Better prediction of flow occurs horizontally further away and vertically further from the surface. TB clusters modify the shape of pollutant plumes. Strong updrafts generated by the overlapping wakes of TB clusters lift pollutants out of the canopy, causing a much deeper tracer plume in the lee of the cluster, and an elevated plume centreline with maximum concentrations around the TB mean height. Enhanced vertical spread of the pollutants in the near-wake of the cluster results in overall lower maximum concentrations, but higher concentrations above the mean TB height. These results have important implications for interpreting observations in areas with TB. Using real world ceilometer observations in two mega-cities (Beijing and Paris), we assess the diurnal seasonal variability of the urban boundary layer and evaluate a mixed layer height (MLH) empirical model with parameters derived from a third mega-city (London). The MLH model works well in central Beijing but less well in suburban Paris. The variability of the physical meteorology across different vertical scales discussed in this paper provides additional context for interpreting air quality observations.

Roadside concentrations of harmful pollutants such as NOx are highly variable in both space and time. This is rarely considered when assessing pedestrian and cyclist exposures. We aim to fully describe the spatio-temporal variability of exposures of pedestrians and cyclists travelling along a road at high resolution. We evaluate the value added of high spatio-temporal resolution compared to high spatial resolution only. We also compare high resolution vehicle emissions modelling to using a constant volume source. We highlight conditions of peak exposures, and discuss implications for health impact assessments. Using the large eddy simulation code Fluidity we simulate NOx concentrations at a resolution of 2 m and 1 s along a 350 m road segment in a complex real-world street geometry including an intersection and bus stops. We then simulate pedestrian and cyclist journeys for different routes and departure times. For the high spatio-temporal method, the standard deviation in 1 s concentration experienced by pedestrians (50.9 μg.m-3) is nearly three times greater than that predicted by the high-spatial only (17.5 μg.m-3) or constant volume source (17.6 μg.m-3) methods. This exposure is characterised by low concentrations punctuated by short duration, peak exposures which elevate the mean exposure and are not captured by the other two methods. We also find that the mean exposure of cyclists on the road (31.8 μg.m-3) is significantly greater than that of cyclists on a roadside path (25.6 μg.m-3) and that of pedestrians on a sidewalk (17.6 μg.m-3). We conclude that ignoring high resolution temporal air pollution variability experienced at the breathing time scale can lead to a mischaracterization of pedestrian and cyclist exposures, and therefore also potentially the harm caused. High resolution methods reveal that peaks, and hence mean exposures, can be meaningfully reduced by avoiding hyper-local hotspots such as bus stops and junctions.

The understanding of the behaviour of pollutants released in urban sites is of paramount importance for a number of reasons, mainly related to human health. Furthermore, the particular present international political situation adds further concerns, as the deliberate discharge of toxic material in populated areas is a serious threat. Wind tunnel experiments were performed in order to study flow and pollutant dispersion in a real urban environment. The work is part of a larger EPSRC funded project (DAPPLE, Dispersion of Air Pollution & Penetration into the Local Environment) involving six British Universities.

In this study, we evaluated several simple natural ventilation models of cross ventilation and single-sided ventilation with data measured in a full-scale field study in London. In the field study, the ventilation rate in a naturally ventilated office was measured using a tracer gas technique with CO2. Internal temperatures were measured using a vertical temperature array. The external temperature, wind speed and direction were measured at a nearby weather station. In addition, a 1:200 scale model of the urban area within 300 m of the test room was built in a wind tunnel to measure the pressure coefficients. The ventilation models were evaluated with input data from two sources. Wind data from a nearby airport and pressure coefficients from the literature were used, as is common practice. Alternatively, wind data measured at the local weather station and the pressure coefficients measured from wind tunnel experiments were used. The results showed that, regardless of the input data sources, the cross-ventilation model in general gives reasonable predictions. For single-sided ventilation, several empirical models were evaluated and poor predictions were obtained using the models. We discuss ways in which models of natural ventilation might be improved in the future. •A full-scale field study in a naturally ventilated office room in London.•Ventilation rates of both cross-ventilation and single-sided ventilation were measured.•Wind-tunnel experiments on a reduced-scale model of the building and surroundings, measuring pressure coefficients.•Measured local wind data and pressure coefficients were compared with commonly used models.•Evaluation of simple analytical ventilation models were conducted.

The large eddy simulation (LES) code Fluidity was used to simulate the dispersion of NOx traffic emissions along a road in London. The traffic emissions were represented by moving volume sources, one for each vehicle, with time-varying emission rates. Traffic modelling software was used to generate the vehicle movement, while an instantaneous emissions model was used to calculate the NOx emissions at 1 s intervals. The traffic emissions were also modelled as a constant volume source along the length of the road for comparison. A validation of Fluidity against wind tunnel measurements is presented before a qualitative comparison of the LES concentrations with measured roadside concentrations. Fluidity showed an acceptable comparison with the wind tunnel data for velocities and turbulence intensities. The in-canyon tracer concentrations were found to be significantly different between the wind tunnel and Fluidity. This difference was explained by the very high sensitivity of the in-canyon tracer concentrations to the precise release location. Despite this, the comparison showed that Fluidity was able to provide a realistic representation of roadside concentration variations at high temporal resolution, which is not achieved when traffic emissions are modelled as a constant volume source or by Gaussian plume models.

Wind tunnel experiments on regular arrays of buildings were conducted in the environmental wind tunnel in the EnFlo laboratory at the University of Surrey. The model canopy comprised a square array of 14×21 rectangular blocks (1h × 2h) with height h = 70 mm. Preliminary measurements of velocity, turbulence and tracer concentrations were made for 3 wind directions: 0, 45 and 90◦. The results from this first experimental campaign along with numerical simulations have shown that the canopy has obstacles sufficiently long compared with their heights to yield extensive flow channelling along streets. Across the whole of the downwind half of the long street the flow for the present canopy is closely aligned with the obstacle faces, despite the 45◦ flow orientation aloft. This supports the suggestion that the streets are long enough to be representative for street network modelling approaches; shorter streets would probably not be sufficient and it will be interesting to see how well network models can predict concentrations in the present canopy. The extensive array and the small scale of the model posed challenging problems for reaching the desired high accuracy needed to validate the numerical simulations. The improvements in the methodology will be presented and discussed at the conference. The wind tunnel data, along with LES and DNS simulations, are being used to understand the behaviour of flow and dispersion within regular array with a more realistic geometry than the usual cuboids. This integrated methodology will help developing parametrisations for improved street network dispersion models

Preliminary wind tunnel experiments for the FUTURE project. Using the 'A tunnel' facility at the EnFlo lab, University of Surrey. Each file consists of the mean velocity values (U, V) measured at different locations (x,y,z) in the wake of a group of tall buildings arranged in a regular array. Additionally, the reference velocity measured at the tunnel inlet, reference temperature and atmospheric pressure are also included in each file.