Imperial College London

Professor Maarten van Reeuwijk

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Urban Fluid Mechanics



+44 (0)20 7594 6059m.vanreeuwijk Website CV




Miss Rebecca Naessens +44 (0)20 7594 5990




331Skempton BuildingSouth Kensington Campus





Publication Type

100 results found

Li T, Fellini S, van Reeuwijk M, 2023, Urban air quality: What is the optimal place to reduce transport emissions?, Atmospheric Environment, Vol: 292, ISSN: 1352-2310

We develop a linear model based on a complex network approach that predicts the effect of emission changes on air pollution exposure in urban street networks including NO–NO2–O3-chemisty. The operational air quality model SIRANE is used to create a weighted adjacency matrix A describing the relation between emissions of a passive scalar inside streets and the resulting concentrations in the street network. A case study in South Kensington (London) is used, and the adjacency matrix A0 is determined for one wind speed and eight different wind directions. The physics of the underlying problem is used to infer A for different wind speeds. Good agreement between SIRANE predictions and the model is observed for all but the lowest wind speed, despite non-linearities in SIRANE's model formulation. An indicator for exposure in the street is developed, and it is shown that the out-degree of the exposure matrix E represents the effect of a change in emissions on the exposure reduction in all streets in the network. The approach is then extended to NO–NO2–O3-chemisty, which introduces a non-linearity. It is shown that a linearised model agrees well with the fully nonlinear SIRANE predictions. The model shows that roads with large height-to-width ratios are the first in which emissions should be reduced in order to maximise exposure reduction.

Journal article

Grylls T, van Reeuwijk M, 2022, How trees affect urban air quality: It depends on the source, ATMOSPHERIC ENVIRONMENT, Vol: 290, ISSN: 1352-2310

Journal article

Suter I, Grylls T, Sützl B, Owens SO, Wilson CE, van Reeuwijk Met al., 2022, uDALES 1.0: a large-eddy-simulation model for urban environments, Geoscientific Model Development, Vol: 15, Pages: 5309-5335, ISSN: 1991-959X

Abstract. Urban environments increasingly move to the fore of climate and air quality research due to their central role in the population’s health and well-being. Tools to model the local environmental conditions, urban morphology and interaction with the atmospheric boundary layer play an important role for sustainable urban planning and policy-making. uDALES is a high-resolution, building-resolving large-eddy simulation code for urban microclimate and air quality. uDALES solves a surface energy balance for each urban facet and models multi-refection shortwave radiation, longwave radiation, heat storage and conductance, as well as turbulent latent and sensible heat fluxes. Vegetated surfaces and their effect on outdoor temperatures and energy demand can be studied. Furthermore a scheme to simulate emissions and transport of aerosols and some reactive gas species is present. The energy balance has been tested against idealized cases and the particle dispersion against field measurements, yielding satisfying results. uDALES can be used to study the effect of specific new constructions and building measures on the local micro-climate; or to gain new insight about the general effect of urban morphology on local climate, ventilation and dispersion. uDALES is available online under GNU General Public License and remains under maintenance and development.

Journal article

Puchol-Salort P, Boskovic S, Dobson B, van Reeuwijk M, Mijic Aet al., 2022, Water neutrality framework for systemic design of new urban developments, Water Research, Vol: 219, Pages: 1-13, ISSN: 0043-1354

The climate emergency and population growth threaten urban water security in cities worldwide. Growth, urbanisation, and changes to way of life have increased housing demand, requiring cities such as London to increase their housing stock by more than 15% over the next 10 years. These new urban developments will increase water demand, urban flood risk, and river water pollution levels; therefore, an integrated systems-based approach to development and water management is needed. Water Neutrality (WN) has emerged as a concept to frame the concerns about escalating water stresses in cities. We frame WN as a planning process for new urban developments that aims to minimise impacts on urban water security and offset any remaining stresses by retrofitting existing housing stock. In this work, we present a novel systemic design framework for future urban planning called CityPlan-Water, which guides how WN might be achieved to tackle current and future water pressures at a city scale. CityPlan-Water integrates spatial data with an integrated urban water management model, enabling urban design at a systems level and systematic assessment of future scenarios. We define a Water Neutrality Index that captures how successful a given urban planning scenario is in achieving WN and how multiple interventions could be combined at a city scale to improve WN. Results from CityPlan-Water suggest that it will be necessary to retrofit almost the same number of existing homes with WN design options to completely offset the impact imposed by proposed new developments. Combining options such as water efficient appliances, water reuse systems, and social awareness campaigns can offset the impact of new development on water demand by 70%, while to neutralise potential flood risk and water pollution at a city scale, interventions such as rainwater harvesting and Blue Green Infrastructure need to be added both in new urban developments and 432,000 existing London households. We see CityPlan-Wa

Journal article

Christensen AK, Piggott MD, van Sebille E, van Reeuwijk M, Pawar Set al., 2022, Investigating microscale patchiness of motile microbes under turbulence in a simulated convective mixed layer., PLoS Comput Biol, Vol: 18

Microbes play a primary role in aquatic ecosystems and biogeochemical cycles. Spatial patchiness is a critical factor underlying these activities, influencing biological productivity, nutrient cycling and dynamics across trophic levels. Incorporating spatial dynamics into microbial models is a long-standing challenge, particularly where small-scale turbulence is involved. Here, we combine a fully 3D direct numerical simulation of convective mixed layer turbulence, with an individual-based microbial model to test the key hypothesis that the coupling of gyrotactic motility and turbulence drives intense microscale patchiness. The fluid model simulates turbulent convection caused by heat loss through the fluid surface, for example during the night, during autumnal or winter cooling or during a cold-air outbreak. We find that under such conditions, turbulence-driven patchiness is depth-structured and requires high motility: Near the fluid surface, intense convective turbulence overpowers motility, homogenising motile and non-motile microbes approximately equally. At greater depth, in conditions analogous to a thermocline, highly motile microbes can be over twice as patch-concentrated as non-motile microbes, and can substantially amplify their swimming velocity by efficiently exploiting fast-moving packets of fluid. Our results substantiate the predictions of earlier studies, and demonstrate that turbulence-driven patchiness is not a ubiquitous consequence of motility but rather a delicate balance of motility and turbulent intensity.

Journal article

Zhang Z, Paschalis A, Mijic A, Meili N, Manoli G, Van Reeuwijk M, Fatichi Set al., 2022, A mechanistic assessment of urban heat island intensities and drivers across climates, Urban Climate, Vol: 44, Pages: 1-18, ISSN: 2212-0955

The urban heat island effect (UHI) has been widely observed globally, causing climate,health, and energy impacts in cities. The UHI intensities have been found to largelydepend on background climate and the properties of the urban fabric. Yet, a completemechanistic understanding of how UHIs develop at a global scale is still missing. Usingan urban ecohydrological and land-surface model (urban Tethys-Chloris) incombination with multi-source remote sensing data, we performed simulations for 49large urban clusters across the Northern Hemisphere in 2009-2019 and analysed howsurface and canopy air UHIs (SUHI and CUHI, respectively) develop during day andnight. Biophysical drivers triggering the development of SUHIs and CUHIs have similardependencies on background climate, but with different magnitudes. In humid regionsdaytime UHIs can be largely explained by the urban-rural difference inevapotranspiration, whereas heat convection and conduction are important in aridareas. Plant irrigation can largely promote daytime urban evapotranspiration only inarid and semi-arid climates. During night, heat conduction from the urban fabric to theenvironment creates large UHIs mostly in warm arid regions. Overall, this studypresents a mechanistic quantification of how UHIs develop worldwide and proposesviable solutions for sustainable climate-sensitive mitigation strategies.

Journal article

Lim HD, Hertwig D, Grylls T, Gough H, van Reeuwijk M, Grimmond S, Vanderwel Cet al., 2022, Pollutant dispersion by tall buildings: laboratory experiments and Large-Eddy Simulation, Experiments in Fluids: experimental methods and their applications to fluid flow, Vol: 63, ISSN: 0723-4864

Pollutant dispersion by a tall-building cluster within a low-rise neighbourhood of Beijing is investigated using both full-scale Large-Eddy Simulation and water flume experiments at 1:2400 model-to-full scale with Particle Image Velocimetry and Planar Laser-Induced Fluorescence. The Large-Eddy Simulation and flume results of this realistic test case agree remarkably well despite differences in the inflow conditions and scale. Tall buildings have strong influence on the local flow and the development of the rooftop shear layer which dominates vertical momentum and scalar fluxes. Additional measurements using tall-buildings-only models at both 1:2400 and 1:4800 scales indicates the rooftop shear layer is insensitive to the scale. The relatively thicker incoming boundary layer affects the Reynolds stresses, the relative size of the pollutant source affects the concentration statistics and the relative laser-sheet thickness affects the spatially averaged results of the measured flow field. Low-rise buildings around the tall building cluster cause minor but non-negligible offsets in the peak magnitude and vertical location, and have a similar influence on the velocity and concentration statistics as the scale choice. These observations are generally applicable to pollutant dispersion of realistic tall building clusters in cities. The consistency between simulations and water tunnel experiments indicates the suitability of both methodologies.

Journal article

Hossain MR, Craske J, van Reeuwijk M, 2022, Reconstructing wall shear stress from thermal wall imprints, International Journal of Heat and Fluid Flow, Vol: 95, Pages: 108976-108976, ISSN: 0142-727X

We reconstruct the wall shear stress of plane Couette flow from thermal wall imprints generated by direct numerical simulation at using an imposed surface temperature flux and fixed temperature at the bottom and top boundary, respectively. We explore the strong correlation between wall shear stress and wall temperature by analysing their joint probability density function and cross variance spectrum, before developing a spectral model based on linear regression. We then use observed symmetries in the estimator parameters to reduce the degrees of freedom of the model. The reconstructed wall shear stress reproduces streamwise streaky structures well. The relative error in the -norm of is primarily associated with the absence of local maxima in the reconstructed wall shear stress.

Journal article

Cimarelli A, Fregni A, Mollicone JP, van Reeuwijk M, De Angelis Eet al., 2022, Structure of turbulence in temporal planar jets, Physics of Fluids, Vol: 34, ISSN: 1070-6631

A detailed analysis of the structure of turbulence in a temporal planar turbulent jet is reported. Instantaneous snapshots of the flow andthree-dimensional spatial correlation functions are considered. It is found that the flow is characterized by large-scale spanwise vorticeswhose motion is felt in the entire flow field. Superimposed to this large-scale motion, a hierarchy of turbulent structures is present. The mostcoherent ones take the form of quasi-streamwise vortices and high and low streamwise velocity streaks. The topology of these interactingstructures is analyzed by quantitatively addressing their shape and size in the different flow regions. Such information is recognized to be relevant for a structural description of the otherwise disorganized motion in turbulent free-shear flows and can be used for the assessment ofmodels based on coherent structure assumptions. Finally, the resulting scenario provides a phenomenological description of the elementaryprocesses at the basis of turbulence in free-shear flows.

Journal article

Meyer D, Grimmond S, Dueben P, Hogan R, van Reeuwijk Met al., 2022, Machine learning emulation of urban land surface processes, Journal of Advances in Modeling Earth Systems, Vol: 14, ISSN: 1942-2466

Can we improve the modeling of urban land surface processes with machine learning (ML)? A prior comparison of urban land surface models (ULSMs) found that no single model is “best” at predicting all common surface fluxes. Here, we develop an urban neural network (UNN) trained on the mean predicted fluxes from 22 ULSMs at one site. The UNN emulates the mean output of ULSMs accurately. When compared to a reference ULSM (Town Energy Balance; TEB), the UNN has greater accuracy relative to flux observations, less computational cost, and requires fewer input parameters. When coupled to the Weather Research Forecasting (WRF) model using TensorFlow bindings, WRF-UNN is stable and more accurate than the reference WRF-TEB. Although the application is currently constrained by the training data (1 site), we show a novel approach to improve the modeling of surface fluxes by combining the strengths of several ULSMs into one using ML.

Journal article

Brizzolara S, Mollicone J-P, van Reeuwijk M, Mazzino A, Holzner Met al., 2022, Transition from shear-dominated to Rayleigh-Taylor turbulence (vol 924, A10, 2021), JOURNAL OF FLUID MECHANICS, Vol: 934, ISSN: 0022-1120

Journal article

Jordan OH, Rooney GG, Devenish BJ, van Reeuwijk Met al., 2021, Under pressure: turbulent plumes in a uniform crossflow, Journal of Fluid Mechanics, Vol: 932, ISSN: 0022-1120

Direct numerical simulation is used to investigate the integral behaviour of buoyant plumessubjected to a uniform crossflow that are infinitely lazy at the source. Neither a plumetrajectory defined by the centre of mass of the plume zc nor a trajectory defined by thecentral streamline zU is aligned with the average streamlines inside the plume. Both zcand zU are shown to correlate with field lines of the total buoyancy flux, which impliesthat a model for the vertical turbulent buoyancy flux is required to faithfully predict theplume angle. A study of the volume conservation equation shows that entrainment due toincorporation of ambient fluid with non-zero velocity due to the increase in the surfacearea (the Leibniz term) is the dominant entrainment mechanism in strong crossflows. Thedata indicate that pressure differences between the top and bottom of the plume play aleading role in the evolution of the horizontal and vertical momentum balances and arecrucial for appropriately modelling plume rise. By direct parameterisation of the verticalbuoyancy flux, the entrainment and the pressure, an integral plume model is developedwhich is in good agreement with the simulations for sufficiently strong crossflow. Aperturbation expansion shows that the current model is an intermediate-range model validfor downstream distances up to 100b–1000b, where b is the buoyancy length scale basedon the flow speed and plume buoyancy flux.

Journal article

Puchol-Salort P, Lu J, Boskovic S, Mijic A, Dobson B, van Reeuwijk Met al., 2021, Urban water neutrality at different scales: CityPlan design and evaluation framework, egu proceedings

<jats:p>&amp;lt;p&amp;gt;London aims to build more than half a million households over the next 10 years to cope with the growing demand for housing in the UK. In this future scenario, urban water security levels will be threatened due to new development pressures combined with the climate emergency and exponential population growth in the city. In addition to this, there is a lack of agreement between the policy and decision-making sectors to decide what can be accepted as a sustainable urban development project and which are the physical and decision boundaries inside the city (i.e., while boroughs and wastewater zones present decision boundaries, new urban developments have physical boundaries only). In our previous work, we developed a new concept for urban Water Neutrality (WN) inside an operational framework called CityPlan to frame the concerns about rising water stresses in cities. This framework integrates spatial data with an integrated urban water management model, enabling urban design at systems level and delivering a new index that assesses possible future scenarios. Despite several studies related to WN, little evidence is yet available in the literature of how urban water neutrality can be achieved at different urban scales and if results might vary depending on the scale studied.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this work, we expand the CityPlan framework and present an innovative evaluation approach that sets several urban indicators to be tested at different urban scales. As part of the evaluation toolkit of CityPlan, we also develop the Water Efficiency Certificate (WEC) by boroughs using two novel criteria: the Housing Age Indicator (HAI) and the Device Efficiency Score (DES). The WEC evaluates the current situation of household water consumption and can be used to support predictions of water consumption under different scenarios, to study the potential for retrofitting existing residential buildings, and to de

Journal article

Boetti M, van Reeuwijk M, Liberzon A, 2021, Potential-enstrophy lengthscale for the turbulent/nonturbulent interface in stratified flow, Physical Review Fluids, Vol: 6, ISSN: 2469-990X

We study properties of the turbulent/nonturbulent interface (TNTI) between two layers of stratified fluids through direct numerical simulations (DNSs). Zero mean shear forcing creates moderate turbulence in one of the layers with the Taylor microscale Reynolds numbers in the mixed region of Reλ=35,44. We focus on the similarities and differences of the properties of stratified TNTIs due to two distinct types of forcing: (a) convection due to a boundary heat source and (b) agitation resembling a vertically oscillating grid experiment. Similarly to other stratified flows, the small scale dynamics of the TNTI in the present DNSs differ from what would be expected in comparable yet unstratified TNTIs. The interface cannot be indeed uniquely identified by the commonly used vorticity ω. Instead, the potential enstrophy Π2 is shown to be the most appropriate marker in these flow cases. It is emphasized that the Kolmogorov lengthscale ηK∼√ν/ω is not representative of the small scale dynamics of the interface. Hence, an alternative lengthscale, ηΠ, is defined, in analogy to the Kolmogorov scale, based on the potential enstrophy, ηΠ=(ν3/Π∗)1/6, being Π∗=|g/ρ0Π|. The conditionally averaged profiles of potential enstrophy Π2, enstrophy ω2, and turbulent kinetic energy dissipation ε of the two distinctly different turbulence forcing cases collapsed when scaled by ηΠ at different time instants in each simulation. This implies not only the self-similarity of the small scale statistics of the TNTI in either of the two cases, but also the similarity between the statistics of the two different turbulent flows in the proximity of TNTI.

Journal article

Puchol-Salort P, Mijic A, van Reeuwijk M, Boskovic S, Dobson Bet al., 2021, Urban Water Neutrality at City Scale: CityPlan Evaluation and Design Module, Earth and Space Science Open Archive

Journal article

Sützl BS, Rooney GG, Finnenkoetter A, Bohnenstengel SI, Grimmond S, Reeuwijk Met al., 2021, Distributed urban drag parameterization for sub‐kilometre scale numerical weather prediction, Quarterly Journal of the Royal Meteorological Society, Vol: 147, Pages: 3940-3956, ISSN: 0035-9009

A recently developed, height-distributed urban drag parametrization is tested with the London Model, a sub-kilometre resolution version of the Met Office Unified Model over Greater London. The distributed-drag parametrization requires vertical morphology profiles in the form of height-distributed frontal-area functions, which capture the full extent and variability of building heights. London's morphology profiles are calculated and parametrized by an exponential distribution with the ratio of maximum to mean building height as the parameter. A case study evaluates the differences between the new distributed-drag scheme and the current London Model setup using the MORUSES urban land-surface model. The new drag parametrization shows increased horizontal spatial variability in total surface stress, identifying densely built-up areas, high-rise building clusters, parks, and the river. Effects on the wind speed in the lower levels include a lesser gradient and more heterogeneous wind profiles, extended wakes downwind of the city centre, and vertically growing perturbations that suggest the formation of internal boundary layers. The surface sensible heat fluxes are underpredicted, which is attributed to difficulties coupling the distributed momentum exchange with the surface-based heat exchange.

Journal article

Brizzolara S, Mollicone J-P, van Reeuwijk M, Mazzino A, Holzner Met al., 2021, Transition from shear-dominated to Rayleigh-Taylor turbulence, Journal of Fluid Mechanics, Vol: 924, Pages: 1-13, ISSN: 0022-1120

Turbulent mixing layers in nature are often characterised by the presence of a mean shear and an unstable buoyancy gradient between two streams of different velocities. Depending on the relative strength of shear versus buoyancy, either the former or the latter may dominate the turbulence and mixing between the two streams. In this paper, we present a phenomenological theory that leads to the identification of two distinct turbulent regimes: an early regime, dominated by mean shear, and a later regime dominated by buoyancy. The main theoretical result consists of the identification of a cross-over timescale that distinguishes between the shear- and the buoyancy-dominated turbulence. This cross-over time depends on three large-scale constants of the flow, namely, the buoyancy difference, the velocity difference between the two streams and the gravitational acceleration. We validate our theory against direct numerical simulations of a temporal turbulent mixing layer compounded with an unstable stratification. We observe that the cross-over time correctly predicts the transition from shear- to buoyancy-driven turbulence, in terms of turbulent kinetic energy production, energy spectra scaling and mixing layer thickness.

Journal article

Grylls T, Suter I, Sützl B, Owens S, Meyer D, van Reeuwijk Met al., 2021, uDALES: large-eddy-simulation software for urban flow, dispersion, and microclimate modelling, Journal of Open Source Software, Vol: 6, Pages: 1-4, ISSN: 2475-9066

With continuing urbanization, challenges associated with the urban environment such as airquality, heat islands, pedestrian thermal comfort, and wind loads on tall buildings, are increasingly relevant. Our ability to realistically capture processes such as the transport of heat,moisture, momentum and pollutants, and those of radiative transfer in urban environmentsis key to understanding and facing these challenges (Oke et al., 2017). The turbulent natureof the urban flow field and the inherent heterogeneity and wide range of scales associatedwith the urban environment result in a complex modelling problem. Large-eddy simulation(LES) is an approach to turbulence modelling used in computational fluid dynamics to simulate turbulent flows over a wide range of spatial and temporal scales. LES is one of the mostpromising tools to model the interactions typical of urban areas due to its ability to resolve theurban flow field at resolutions of O(1 m, 0.1 s), over spatial domains of O(100 m), and timeperiods of O(10 h). Although there are many scalable LES models for atmospheric flows, toour knowledge, only few are capable of explicitly representing buildings and of modelling thefull range of urban processes (e.g. PALM-4U Resler et al. (2017); Maronga et al. (2020); orOpenFoam Weller et al. (1998)).

Journal article

van Reeuwijk M, Grylls T, 2021, Tree model with drag, transpiration, shading and deposition: Identification of cooling regimes and large-eddy simulation, EGU General Assembly 2021, Publisher: Copernicus GmbH, Pages: 1-1

Conference paper

Nair V, Heus T, van Reeuwijk M, 2021, A Lagrangian study of interfaces at the edges of cumulus clouds, Journal of the Atmospheric Sciences, Vol: 78, Pages: 2397-2412, ISSN: 0022-4928

Interfaces at the edge of an idealised, non-precipitating, warm cloud are studied using Direct Numerical Simulation (DNS) complemented with a Lagrangian particle tracking routine. Once a shell has formed, four zones can be distinguished: the cloud core, visible shell, invisible shell and the environment. The union of the visible and invisible regions is the shell commonly referred to in literature. The boundary between the invisible shell and the environment is the Turbulent-NonTurbulent Interface (TNTI) which is typically not considered in cloud studies. Three million particles were seeded homogeneously across the domain and properties were recorded along individual trajectories. The results demonstrate that the traditional cloud boundary (separating cloudy and non-cloudy regions using thresholds applied on liquid condensate or updraft velocity) are some distance away from the TNTI. Furthermore, there is no dynamic difference between the traditional liquid-condensate boundary and the region extending to the TNTI. However, particles crossing the TNTI exhibit a sharp jump in enstrophy and a smooth increase in buoyancy. The traditional cloud boundary coincides with the location of minimum buoyancy in the shell. The shell pre-mixes the entraining and detraining air and analysis reveals a highly skewed picture of entrainment and detrainment at the traditional cloud boundary. A preferential entrainment of particles with velocity and specific humidity higher than the mean values in the shell is observed. Large-eddy simulation of a more realistic setup detects an interface with similar properties using the same thresholds as in the DNS, indicating that the DNS results extrapolate beyond their idealised conditions.

Journal article

Auwerter LC-C, Cheeseman C, Templeton M, Van Reeuwijk Met al., 2021, Quantifying the durability of a friction-reducing surface with recoverable super-hydrophobicity, Journal of Hydraulic Engineering, Vol: 147, Pages: 1-10, ISSN: 0733-9429

The durability of superhydrophobic surfaces in fully immersed conditions is a major obstacle to their application in engineering applications. We perform an experimental study to measure the friction factor fd as a function of time for a new superhydrophobic surface that is capable of recovering the Cassie-Baxter wetting state. Values of fd were obtained by measuring the pressure drop and volume flux of a turbulent water flow in a 1.5 m long duct containing one superhydrophobic wall. The Reynolds number of the flow was approximately 4.5×104 for all experiments. Reductions in fd were 29%–36% relative to a hydraulically smooth surface. The Cassie-Baxter state could be recovered by blowing air through the porous surface for 10 min. The durability of the drag-reduction, as quantified by the relaxation time T in which the surface loses its superhydrophobic characteristics, were measured to be between 10 and 60 min depending on the initial head. The relaxation time T was highly dependent on the pressure difference across the surface. In contrast to models based on Darcy flow through a porous medium, the study indicates that there seems to be a critical pressure difference beyond which the Cassie-Baxter state cannot be sustained for the material under consideration.

Journal article

Cimarelli A, Mollicone J-P, van Reeuwijk M, De Angelis Eet al., 2021, Spatially evolving cascades in temporal planar jets, Journal of Fluid Mechanics, Vol: 910, ISSN: 0022-1120

Starting from an alternative decomposition of the turbulent field, a multi-dimensional statistical formalism for the description and understanding of turbulence in free-shear flows is proposed and applied to the symmetries of planar temporal jets. The theoretical framework is based on the exact equation for the second-order moment of the two-point velocity increment and allows us to trace, for the first time, the spatially evolving cascade processes at the basis of turbulence mixing and entrainment. Fascinating reverse energy cascade mechanisms are found to be responsible for the generation of long and wide structures in the interface region. Analogously to two-dimensional turbulence, the energy provided by these spatially ascending reverse cascades is found to be eventually dissipated by viscosity at large scales through friction shearing processes involving a thin cross-flow layer of these large-scale structures. Finally, the external non-turbulent region of the jet is also found to be active from an energetic point of view. It is found that pressure-mediated non-local phenomena of displacement of almost quiescent fluid give rise to non-turbulent fluctuations that in time, through transitional mechanisms, would contribute to the growth of the turbulent jet. Overall, the unexpected paths taken by the scale-energy flux in the combined physical/scale space, which are a substantial novelty with respect to known descriptions of turbulent mixing and entrainment, may have major repercussions on our theoretical understanding and modelling, as anticipated here by reduced equations capable of giving a simple scale-dependent description of the rich dynamics of the flow.

Journal article

Puchol-Salort P, OKeeffe J, van Reeuwijk M, Mijic Aet al., 2021, An urban planning sustainability framework: systems approach to blue green urban design, Sustainable Cities and Society, Vol: 66, Pages: 1-14, ISSN: 2210-6707

The climate emergency and population growth are challenging water security and sustainable urban design in cities worldwide. Sustainable urban development is crucial to minimise pressures on the natural environment and on existing urban infrastructure systems, including water, energy, and land. These pressures are particularly evident in London, which is considered highly vulnerable to water shortages and floods and where there has been a historical shortage of housing. However, the impacts of urban growth on environmental management and protection are complex and difficult to evaluate. In addition, there is a disconnection between the policy and decision-making processes as to what comprises a sustainable urban development project.We present a systems-based Urban Planning Sustainability Framework (UPSUF) that integrates sustainability evaluation, design solutions and planning system process. One of the features of this master planning framework is the spatial representation of the urban development in a Geographical Information System to create an operational link between design solutions and evaluation metrics. UPSUF moves from an initial baseline scenario to a sustainable urban development design, incorporating the requirements of governance and regulatory bodies, as well as those of the end-users. Ultimately, UPSUF has the potential to facilitate partnership between the public and the private sectors.

Journal article

Grylls T, van Reeuwijk M, 2021, Tree model with drag, transpiration, shading and deposition: Identification of cooling regimes and large-eddy simulation, Agricultural and Forest Meteorology, Vol: 298-299, Pages: 1-19, ISSN: 0168-1923

Trees play an important role in the urban heat island effect and urban air quality due to their impact on the transfer of radiation, momentum, heat, moisture and pollution. However, the effects of trees are hard to quantify due to their complex interactions with urban surfaces and the turbulent atmosphere overhead. We present a complete tree model for large-eddy simulations (LES) that represents the effects of trees on drag, transpiration, shading and deposition at resolutions of O(1 m, 0.1 s) whilst minimising the number of model parameters. The tree model avoids the necessity to resolve the leaf temperature via a derivation of the Penman-Monteith equation and distinguishes between cooling via transpiration and shading. The latent heat flux is further broken down into radiative and advective components in order to better understand the mechanism behind transpirational cooling (e.g. the ‘oasis’ effect).The new tree model is investigated analytically to provide insight into tree cooling regimes, and is applied to field studies to contextualise the analysis. The combined cooling effect of trees due to transpiration and shading processes can be reduced to a four-dimensional parameter space. The net tree cooling () and tree cooling ratio () parameters are defined to enable a systematic categorisation of the thermal effect of a tree into five regimes: net heating, net reduction (shading dominated), net reduction (transpiration dominated), net cooling (shading dominated) and net cooling (transpiration dominated). Existing parameterisations for tree cooling are reviewed, illustrating their limitations and highlighting the need for complete models to determine tree cooling.The tree model is implemented into the LES model uDALES. The drag and canopy energy balance models are validated, and results are presented for domains that are 1) fully covered by trees; 2) partially covered by trees; and 3) have a single line of trees. These simulations provide physical insi

Journal article

Mader J, Van Reeuwijk M, Craske J, 2021, Confined turbulent convection driven by a combination of line and distributed sources of buoyancy, Physical Review Fluids, Vol: 6, Pages: 1-25, ISSN: 2469-990X

We study the flow and thermal stratification of a closed domain subjected to different combinations of line and distributed surface heating and cooling. Our observations are drawn from a set of direct numerical simulations in which the ratio of the strength of the distributed sources to the localised sources \HfRb is varied and shown to play a decisive role in determining the system’s statistically steady state. Domains of sufficient horizontal extent that are (\HfRb=0) produce a stable two-layer stratification. The planar plumes generated by each line source are connected by a large scale circulation over the full depth of the domain and induce secondary circulations within each layer. As the distributed component of the heating, and therefore \HfRb, increases, the buoyancy difference between the layers decreases, before being destroyed when \HfRb>1. For increasing \HfRb∈[0,1], we observe an increasing tilt of the interface between the layers and the eventual disappearance of the secondary circulation cells. The mean buoyancy transport between the two layers of the stable stratification is dominated by the plumes for all $

Journal article

van Reeuwijk M, Vassilicos JC, Craske J, 2021, Unified description of turbulent entrainment, JOURNAL OF FLUID MECHANICS, Vol: 908, ISSN: 0022-1120

Journal article

Sützl BS, Rooney GG, van Reeuwijk M, 2021, Drag distribution in idealized heterogeneous urban environments, Boundary-Layer Meteorology, Vol: 178, Pages: 225-248, ISSN: 0006-8314

Large-eddy simulations of nine idealized heterogeneous urban morphologies with identical building density and frontal area index are used to explore the impact of heterogeneity on urban airflow. The fractal-like urban morphologies were generated with a new open-source Urban Landscape Generator tool (doi:10.5281/zenodo.3747475). The vertical structure of mean flow and the dispersive vertical momentum transport within the roughness sublayer are shown to be strongly influenced by the building morphologies. The friction velocity and displacement height show high correlations with the maximum building height rather than the average height. Well-known roughness parametrizations of the logarithmic layer cannot adequately capture the large spread observed in the large-eddy simulation data. A generalized frontal area index Λf is introduced that characterizes the vertical distribution of the frontal area in the urban canopy. The vertically distributed stress profiles, which differ significantly per simulation, are shown to roughly collapse upon plotting them against Λf. The stress distribution representing urban drag can be fitted with a third degree polynomial. The results can be used for more detailed and robust representations of building effects in the development of urban canopy models.

Journal article

Neamtu-Halic MM, Mollicone J-P, van Reeuwijk M, Holzner Met al., 2020, Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification, Journal of Turbulence, Vol: 22, Pages: 393-412, ISSN: 1468-5248

We investigate the enstrophy dynamics in relation to objective Eulerian coherent structures (OECSs) and their impact on the enstrophy and scalar transport near the turbulent/non-turbulent interface (TNTI) in flows with and without stable stratification. We confirm that vortex-stretching produces enstrophy inside the boundaries of the OECSs, while viscous diffusion transfers the enstrophy across the boundaries of the structures. Although often overlooked in the literature, viscous dissipation of enstrophy within the boundaries of vortical structures is significant. Conversely, for the weakly stratified flows also investigated here, the effect of the baroclinic torque is negligible. We provide evidence that the OECSs advect the passive/active scalar and redistribute it via molecular diffusion. Finally, we use conditional analysis to show that the typical profiles of the enstrophy and scalar transport equation terms across the TNTI are compatible with the presence of OECSs positioned at the edge between the turbulent sublayer and the turbulent core region. We show that when these profiles are further conditioned to the presence of OECSs, their magnitude is considerably higher.

Journal article

Puchol-Salort P, Van Reeuwijk M, Mijic A, Okeeffe Jet al., 2020, An urban planning sustainability framework: systems approach to blue green urban design, Publisher: Earth ArXiv

The climate emergency and population growth are challenging water security and sustainable urban design in cities worldwide. Sustainable urban development is crucial to minimise pressures on the natural environment and on existing urban infrastructure systems, including water, energy, and land. These pressures are particularly evident in London, which isconsidered highly vulnerable to water shortages and floods and where there has been a historical shortage of housing. However, the impacts of urban growth on environmental management and protection are complex and difficult to evaluate. In addition, there is adisconnection between the policy and decision-making processes as to what comprises asustainable urban development project.Here we present a systems-based Urban Planning Sustainability Framework (UPSUF) that integrates sustainability evaluation, design solutions and planning system process. One of the features of this master planning framework is the spatial representation of the urban development in a Geographical Information System to create an operational link between design solutions and evaluation metrics. UPSUF moves from an initial baseline scenario to a sustainable urban development design, incorporating the requirements of governance and regulatory bodies, as well as those of the end-users. It evaluates the impact on the built andthe natural environments through the concept of urban ecosystem services, and makes the process for sustainable design more accurate and reliable. Ultimately, UPSUF has the potential to facilitate partnership and constructive dialogue between the public and the private sectors.

Working paper

Le Cornec CMA, Molden N, van Reeuwijk M, Stettler MEJet al., 2020, Modelling of instantaneous emissions from diesel vehicles with a special focus on NOx: Insights from machine learning techniques, Science of The Total Environment, Vol: 737, Pages: 1-13, ISSN: 0048-9697

Accurate instantaneous vehicle emissions models are vital for evaluating the impacts of road transport on air pollution at high temporal and spatial resolution. In this study, we apply machine learning techniques to a dataset of 70 diesel vehicles tested in real-world driving conditions to: (i) cluster vehicles with similar emissions performance, and (ii) model instantaneous emissions. The application of dynamic time warping and clustering analysis by NOx emissions resulted in 17 clusters capturing 88% of trips in the dataset. We show that clustering effectively groups vehicles with similar emissions profiles, however no significant correlation between emissions and vehicle characteristics (i.e. engine size, vehicle weight) were found. For each cluster, we evaluate three instantaneous emissions models: a look-up table (LT) approach, a non-linear regression (NLR) model and a neural network multi-layer perceptron (MLP) model. The NLR model provides accurate instantaneous NOx predictions, on par with the MLP: relative errors in prediction of emission factors are below 20% for both models, average fractional biases are −0.01 (s.d. 0.02) and −0.0003 (s.d. 0.04), and average normalised mean squared errors are 0.25 (s.d. 0.14) and 0.29 (s.d. 0.16), for the NLR and MLP models respectively. However, neural networks are better able to deal with vehicles not belonging to a specific cluster. The new models that we present rely on simple inputs of vehicle speed and acceleration, which could be extracted from existing sources including traffic cameras and vehicle tracking devices, and can therefore be deployed immediately to enable fast and accurate prediction of vehicle NOx emissions. The speed and the ease of use of these new models make them an ideal operational tool for policy makers aiming to build emission inventories or evaluate emissions mitigation strategies.

Journal article

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