Imperial College London

Professor Maarten van Reeuwijk

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Urban Fluid Mechanics
 
 
 
//

Contact

 

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

 
 
//

Assistant

 

Miss Rebecca Naessens +44 (0)20 7594 5990

 
//

Location

 

331Skempton BuildingSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

116 results found

van Reeuwijk M, Cui L, Hughes G, 2024, Mixing and entrainment in inclined gravity currents

<jats:p>We explore the dynamics of inclined temporal gravity currents using direct numerical simulation, and find that the current creates an environment in which the flux Richardson number, gradient Richardson number and turbulent flux coefficient are constant across a large portion of the depth of the outer layer. Changing the slope angle modifies these mixing parameters, and the flow approaches a maximum Richardson number of approx. 0.15 as the angle tends to zero, for which the entrainment coefficient E-&gt;0.The turbulent Prandtl number remains O(1) for all slope angles, demonstrating that E-&gt;0 is not caused by a switch-off of the turbulent buoyancy flux. Instead, E-&gt;0 occurs as the result of the turbulence intensity going to zero as the angle tends to zero, due to the flow requiring larger and larger shear to maintain the same level of turbulence. We develop a conceptual model which is in excellent agreement with the DNS data.</jats:p>

Other

Alsulaiman N, Van Reeuwijk M, Piggott M, 2024, Applications of Data Assimilation and Parameter Calibration with Multi-Resolution Measurements of Seawater Temperature for Hydrodynamic Modeling of Shallow, Tidal Environments

<jats:p>This study investigates the integration of seawater temperature data acquired through in-situ and remote sensing methods, aiming to enhance the accuracy of hydrodynamic models. Our focus is on a shallow bay influenced by semidiurnal tides, where continuous thermal discharges from a coastal power plant significantly impact the local temperature field. The investigation addresses the model's uncertainty in capturing the variability of thermal effluents, particularly regarding the input descriptions for the discharge rate (Q) and excess temperature (&amp;#916;T) added to the ambient waters. These parameters display seasonal variations that reflect the energy consumption trends of the local population, introducing complexity into seawater temperature modeling. We aim to assess the effectiveness of using different data types in two key application areas: (A) generating better initial conditions through data assimilation with the Ensemble Kalman Filter (EnKF) technique, and (B) automating the calibration of the model parameters for the description of the thermal discharge. To explore these applications, we conduct a twin experiment that replicates the bay's real-world conditions, allowing for a comprehensive evaluation of the impact of integrating temperature data of varying resolutions on the assimilation and calibration processes. Our goal is to determine the most effective spatiotemporal scales for these applications, and to provide recommendations for modeling approaches in similar tidal environments.</jats:p>

Other

Del Ponte AV, Fellini S, Marro M, van Reeuwijk M, Ridolfi L, Salizzoni Pet al., 2024, Influence of Street Trees on Turbulent Fluctuations and Transport Processes in an Urban Canyon: A Wind Tunnel Study, Boundary-Layer Meteorology, Vol: 190, ISSN: 0006-8314

The presence of vegetation within urban canyons leads to non-trivial patterns of the concentration of airborne pollutants, as a result of the complex structure of the velocity field. To investigate the relationship between concentration, velocity fields and vegetation density, we have performed wind-tunnel experiments in a reduced-scale street canyon, oriented perpendicular to the external wind flow, within which we placed a steady ground-level line source of a passive tracer. The aerodynamic behavior of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon, according to three different densities. The canyon ventilation was investigated by acquiring one-point simultaneous statistics of concentration and velocity over a dense grid of points within the canyon. The results show that the presence of trees hinders the upward mean vertical velocity at the rooftop, causes a reduction of the turbulent kinetic energy inside the canyon, and reduces the energy content of the large scales. The scalar concentration is conversely characterized by an enhanced level of turbulent fluctuations, whose magnitude is not dampened increasing the tree density. Within the canyon, high tree density inhibits turbulent mass fluxes, which are instead enhanced at roof level, where the mean component of the scalar flux is however hindered. A statistical analysis of concentration time series reveals that the lognormal distribution is suitable to model concentration fluctuations and extreme events, in dispersing plumes emitted by a linear source.

Journal article

Fenton D, Cimarelli A, Mollicone JP, van Reeuwijk M, De Angelis Eet al., 2024, A Turbulent Plume in Crossflow, ERCOFTAC Series, Pages: 356-361

The behaviour of a turbulent forced buoyant plume subjected to uniform crossflow is investigated utilising Direct Numerical Simulation (DNS) employing a fourth-order finite difference scheme and third-order Adams-Bashforth temporal integration. The flow features are assessed phenomenologically in the statistically steady state obtained by averaging 1,200 instantaneous 3D fields. Preliminary results on the structure of turbulent production and dissipation are shown in view of future discussions on LES modeling assumptions.

Book chapter

Quilelli Correa Rocha Ribeiro R, Gryspeerdt E, van Reeuwijk M, 2023, Retrieving cloud sensitivity to aerosol using ship emissions in overcast conditions, Geophysical Research Letters, Vol: 50, ISSN: 0094-8276

The interaction between aerosols and clouds is one of the major uncertainties in past climate change, affecting the accuracy of future climate projections. Ship tracks, trails left in clouds through the addition of aerosol in the ship exhaust plume, have become a key observational tool for constraining aerosol-cloud interactions. However, manyexpected tracks remain undetected, presenting a significant gap in current knowledge of aerosol forcing. Here we leverage a plume-parcel model to simulate the impact of aerosol dispersion for 2957 cases off California’s coast on cloud droplet number concentration (CDNC) enhancements. Plume-parcel models show a large sensitivity to updraft uncertainties, which are found to be a primary control on track formation. Using these plume-parcel models, updraft values consistent with observed CDNC enhancements are recovered, suggesting that relying solely on cloud-top radiative cooling may overestimate in-cloud updrafts by around 50%, hence overstating the cloud sensitivity to aerosols.

Journal article

Huang J, van Reeuwijk M, Burridge H, 2023, Local entrainment across a TNTI and TTI in a turbulent forced fountain, Journal of Fluid Mechanics, Vol: 977, ISSN: 0022-1120

Local instantaneous exchanges of volume, momentum and buoyancy across turbulent/non-turbulent interfaces (TNTIs) and turbulent/turbulent interfaces (TTIs) are studied using data from direct numerical simulations of a turbulent forced fountain. We apply a novel algorithm that enables independent calculation of the instantaneous local entrainment and detrainment fluxes, and therefore, for the first time, the entrainment and detrainment coefficients according to the fountain model (Bloomfield & Kerr, J. Fluid Mech., vol. 424, 2000, pp. 197–216) are determined explicitly. Across the interface between the fountain and the ambient fluid, which is a TNTI, only volume entrainment occurs, and it is well predicted by the fountain model. Across the interface between the rising upflow and falling downflow within the fountain, which is a TTI, both entrainment and detrainment of volume, momentum and buoyancy occur – with the magnitude of both entrainment and detrainment typically being large compared with the net for all exchanges. However, the model seems to be unable to capture the momentum exchanges due to its ignorance of the pressure. We find that each conditional entrainment and detrainment rate, of volume, momentum and buoyancy, can be described accurately by Gaussian profiles, while the net exchange that is the superposition of the entrainment and detrainment cannot. Moreover, the entrainment exchange rate has its maximum closer to the fountain centreline than that of detrainment, explaining the tendency for net entrainment closer to the fountain centreline and net detrainment further away.

Journal article

Brizzolara S, Mollicone J-P, van Reeuwijk M, Holzner Met al., 2023, Entrainment at multi-scales in shear-dominated and Rayleigh–Taylor turbulence, European Journal of Mechanics - B/Fluids, Vol: 101, Pages: 294-302, ISSN: 0997-7546

A partially turbulent flow continuously incorporates irrotational fluid into the turbulent region, a phenomenon known as entrainment. Although entrainment locally acts at viscous scales, the thin interface separating the turbulent from the irrotational region is extremely convoluted, and twisted in such a way that renders the global entrainment flux scale-independent. Despite turbulent entrainment being widely recognized as a multi-scale process, the theoretical basis for quantifying the entrainment flux at multi-scales is lacking. In this paper we derive an equation that allows us to quantify the local entrainment velocity at multi-scales. This is done by defining the local entrainment velocity as the propagation speed of an iso-surface of filtered enstrophy relative to the coarse-grained velocity field, and using the filtered enstrophy budget to split the total velocity into its individual components, i.e. viscous, inviscid, baroclinic and sub-filter. The equation is used to investigate the entrainment at multi-scales in simulated turbulent mixing layers, where turbulence is sustained by either a mean shear or an unstable buoyancy gradient (Rayleigh–Taylor turbulence).

Journal article

Yu T, Suetzl BS, van Reeuwijk M, 2023, Urban neighbourhood classification and multi-scale heterogeneity analysis of Greater London, Environment and Planning B: Urban Analytics and City Science, Vol: 50, Pages: 1534-1558, ISSN: 2399-8083

We study the compositional and configurational heterogeneity of Greater London at the city- and neighbourhood-scale using Geographic Information System (GIS) data. Urban morphometric indicators are calculated including plan-area indices and fractal dimensions of land cover, frontal area index of buildings, evenness, and contagion. To distinguish between city-scale heterogeneity and neighbourhood-scale heterogeneity, the study area of 720 km2 is divided into 1 × 1 km2 neighbourhoods. City-scale heterogeneity is represented by categorisation of the neighbourhoods using a k-means clustering algorithm based on the morphometric indicators. This results in six neighbourhood types ranging from “greenspace” to “central business district”. Neighbourhood-scale heterogeneity is quantified using a hierarchical multi-scale analysis for each neighbourhood type. The analysis reveals the dominant length scales for land-cover and neighbourhood types and the resolutions with the most information gain. We analyse multi-scale anisotropy and show that small-scale features are homogeneous, and that anisotropy is present at larger length scales.

Journal article

Vouriot CVM, Higton TD, Linden PF, Hughes GO, van Reeuwijk M, Burridge Het al., 2023, Uniformly distributed floor sources of buoyancy can give rise to significant spatial inhomogeneities within rooms, Flow: Applications of Fluid Mechanics, Vol: 3, Pages: 1-18, ISSN: 2633-4259

Displacement ventilation, where cool external air enters a room through low-level vents and warmer airleaves through high-level vents, is characterised by vertical gradients in pressure arising from the warmerindoor temperatures. Models usually assume that horizontal variations of temperature difference are smallin comparison and are, therefore, unimportant. Small-scale laboratory experiments and computational fluiddynamics were used to examine these flows, driven by a uniformly heated floor. These experiments andsimulations show that the horizontal variations of temperature difference can be neglected for predictions of the bulk ventilation rate; however, they also evidence that these horizontal variations can be significant andplay a critical role in establishing the pattern of flow within the room — this renders the horizontal position ofthe low- and high-level vents (relative to one another) important. We consider two cases: single-ended (whereinlet and outlet are at the same end of the room) and opposite-ended. In both cases the ventilation flow rateis the same. However, in the opposite-ended case a dead zone is established in the upper part of the roomwhich results in significant horizontal variations. We consider the formation of this dead zone by examiningthe streamline patterns and the age of air within the room. We discuss the implications for occupant exposureto pollutants and airborne disease.Impact Statement:Exposure to indoor air pollution and airborne diseases are major factors in human health and well being.Guidance on appropriate ventilation rates is typically based on bulk ventilation rates, either in terms of theamount supplied per individual or as air exchange rates for a space. Such bulk measures assume homogeneousconditions within a space while, in practice, there are often significant spatial variations in properties. Thispaper shows that in displacement ventilation, where it is commonly assumed that horizontal variations arenegligible

Journal article

Puchol-Salort P, Boskovic S, Dobson B, Krivtsov V, Rico-Carranza E, van Reeuwijk M, Whyte J, Mijic Aet al., 2023, Integrated Urban Planning Decision-Making Process Towards Water Neutral Solutions

<jats:p>Urban water security levels will be threatened during the next few years due to new development pressures combined with the climate emergency and increasing population growth in cities. In the UK, London&amp;#8217;s planning authorities have a target of more than half a million households for the next 10 years. This new housing will increase the current impacts on urban consumer demand, flood risk, and river water quality indicators. In our previous work, we developed a new concept for urban Water Neutrality (WN) inside an integrated urban planning sustainability framework called CityPlan to deal with water stress and urban complexity issues. This framework integrates the UK&amp;#8217;s planning application process with systemic design solutions and evaluation, all being spatially represented in a GIS platform. With the new digital era, there is a constantly increasing number of spatial datasets that are openly available from different sources, but most of them are disaggregated and difficult to understand by key urban stakeholders such as Local Planning Authorities, housing developers, and water companies. Moreover, there are several Multi-Criteria Decision Support Tools (MCDST) that address water management challenges in the literature; but there is still little evidence of one that evaluates the impacts and opportunities to allocate water neutral urban developments.In this work, we expand the CityPlan framework and present an innovative fully data-driven approach to test WN indicators at different urban scales. WaNetDST integrates GIS spatial data with a series of rules for development impact and offset opportunity based on the current properties of the urban land. This integration is linked to a new scoring system from expert advice that maps strategic areas for water neutral interventions and links the most impactful zones with others more prone to be intervened. The tool connects different urban scales with a series of case study areas: f

Other

Quilelli Correa Rocha Ribeiro R, Gryspeerdt E, Van Reeuwijk M, 2023, Retrieving cloud sensitivity to aerosol using ship emissions

<jats:p>Aerosol-cloud interactions are one of the key uncertainties in understanding future climate change. A commonly used method for constraining these interactions is using ship tracks. Aerosol-containing plumes from ships can develop into linearly shaped clouds identifiable in satellite images, isolating the aerosol impact on clouds. Previous studies have shown that ship tracks form more commonly in clean conditions, but even accounting for this, many ships that might be expected to form ship tracks do not. This leads to uncertainties in aerosol-cloud interactions and their climate impact. &amp;#160;Ship track formation depends on the aerosol-containing plumes from the ship being sufficiently concentrated upon reaching the cloud. The cloud must also be sensitive to aerosol. In focus are updraft-limited clouds: smaller updrafts promote slower cooling as a cloud parcel rises, higher critical supersaturation values and lower aerosol activation fractions. It is not clear which of these are more important, but it is vital to understand them if we are using ship tracks to retrieve cloud sensitivity to aerosol.&amp;#160;&amp;#160; &amp;#160;We develop a plume-parcel model to address these issues to estimate cloud droplet enhancements in ship tracks. Ship aerosol concentrations at the cloud height were modelled as plumes, simulating the shorter timescales of injection. Droplet number concentration enhancements were estimated using K&amp;#246;hler theory for over one hundred thousand ships off the coast of California. &amp;#160;Using a constant updraft, the model was able to achieve reasonable enhancements (r2 ranging between (0.32, 0.4)). These enhancements were shown to be significantly sensitive to the choice of the updraft. In order to examine the hypothetical updraft values required for activation, an optimisation algorithm was developed to fit updrafts to cloud enhancement observations; a 1-1 correlation was achieved between observed a

Other

Arshad M, Cheng S, van Reeuwijk M, Sherwin SJ, Weinberg PDet al., 2023, Modification of the swirling well cell culture model to alter shear stress metrics, BIOTECHNOLOGY AND BIOENGINEERING, Vol: 120, Pages: 1254-1268, ISSN: 0006-3592

Journal article

Huang J, Burridge HC, van Reeuwijk M, 2023, The internal structure of forced fountains, Journal of Fluid Mechanics, Vol: 961, Pages: 1-29, ISSN: 0022-1120

We study the mixing processes inside a forced fountain using data from directnumerical simulation. The outer boundary of the fountain with the ambient is aturbulent/non-turbulent interface. Inside the fountain, two internalboundaries, both turbulent/turbulent interfaces, are identified: 1) theclassical boundary between upflow and downflow which is composed of the loci ofpoints of zero mean vertical velocity; and 2) the streamline that separates themean flow emitted by the source from the entrained fluid from the ambient (theseparatrix). We show that entrainment due to turbulent fluxes across theinternal boundary is at least as important as that by the mean flow. However,entrainment by the turbulence behaves substantively differently from that bythe mean flow and cannot be modelled using the same assumptions. This presentsa challenge for existing models of turbulent fountains and other environmentalflows that evolve inside turbulent environments.

Journal article

Nair V, Devenish B, van Reeuwijk M, 2023, Effect of Gravity on Particle Clustering and Collisions in Decaying Turbulence, FLOW TURBULENCE AND COMBUSTION, ISSN: 1386-6184

Journal article

Li T, Fellini S, van Reeuwijk M, 2023, Urban air quality: What is the optimal place to reduce transport emissions?, Atmospheric Environment, Vol: 292, Pages: 1-12, 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 ad jacency 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, Pages: 1-14, ISSN: 1352-2310

Large-eddy simulation (LES) is used to systematically analyse the impacts of trees on air quality in idealised street canyons. The LES tree model includes radiation, transpiration, drag and deposition effects. The superposition of background concentrations and local emissions is used to construct realistic urban scenarios for fine particulate matter (PM2.5) and nitrogen oxides (NOx). Both neutral and convective atmospheric conditions are considered to assess the importance of buoyancy effects and the role of tree shading and transpiration. Tree impact on local air quality is shown to be driven by the balance between the rate at which they actively remove pollutants from the air (deposition) and the way in which they alter the transport of pollutants within and out of the street canyon (dispersion). For pollutant species or street types where the concentration field is dominated by background levels (such as PM2.5), deposition will generally dominate and thus local air quality will improve. For pollutants and street types where local emission sources dominate (e.g. NOx on a busy road), the dispersion effects of trees become more prominent and can lead to elevated concentrations where mixing or exchange is significantly inhibited. Mixing in the convective simulation is more vigorous than in the neutral simulation which results in substantial differences in in-canyon flow fields and exchange velocities, highlighting the importance of incorporating thermal effects when studying urban trees. Increased residency times, and thus deposition, under neutral conditions suggest that trees can have amplified effects under conditions conducive of poor air quality. For the cases considered, trees largely act to improve air quality with the exception of localised hotspots. The competing effects of trees — specifically deposition versus altered exchange with the atmosphere — are also incorporated in a simple integral model that predicts whether or not the air quality wil

Journal article

Suter I, Grylls T, S├╝tzl 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

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

Christensen AK, Piggott M, 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 COMPUTATIONAL BIOLOGY, Vol: 18, ISSN: 1553-734X

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

Gryspeerdt E, Louro Coelho M, Smith T, Suarez De La Fuente S, Quilelli Correa Rocha Ribeiro R, van Reeuwijk Met al., 2022, Measuring cloud sensitivity to aerosols at a global scale using isolated aerosol sources

<jats:p>&amp;lt;p&amp;gt;The sensitivity of clouds to anthropogenic aerosol perturbations remains one of the largest uncertainties in the human forcing of the climate system. A key difficulty is in isolating the impact of aerosols from large-scale covariability of aerosol and cloud properties. Natural experiments, where aerosol is produced independently of the cloud and meteorological properties, provide a pathway to address this issue. These aerosol sources often modify cloud properties, leaving linear cloud features known as shiptracks (when formed by a ship) or pollution tracks (more generally).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this work, we use a database of point sources of aerosol over both land and ocean to identify clouds that are sensitive to aerosol and to measure their response. Using a neural network to identify when a point source is modifying the cloud, we are able to measure the sensitivity of individual clouds to aerosol at a global scale, looking at over 400 million cases.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We find the probability of track formation is strongly dependent on the background cloud and meteorological state, similar to previous regional studies. With our global database, we identify regions that are strongly susceptible to aerosol perturbations, even where aerosol sources are rare. We find that there are several regions that are highly susceptible to aerosol, but that have been previously overlooked due to a low frequency of pollution tracks. &amp;amp;#160; &amp;amp;#160;&amp;lt;/p&amp;gt;</jats:p>

Conference paper

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 – CORRIGENDUM, Journal of Fluid Mechanics, Vol: 934, Pages: 1-1, ISSN: 0022-1120

Journal article

Vouriot CVM, Burridge HC, van Reeuwijk M, 2022, Assessing exposure to infected breath in naturally ventilated classrooms

A typical UK naturally ventilated classroom in wintertime is considered using CFD simulations. The ratio between actual exposure to infected breath and proxy exposure is calculated from point measurements of CO2. At the breathing plane, the proxy exposure is within a factor 2 of the actual far field exposure for measurements taken within approximately 90 % of the room cross-sectional area. While this is significant it remains small compared to the typical uncertainties associated with the parametrisation of many airborne diseases. As such CO2 measurements remain a useful and cost-effective way to infer exposure and thus the risk of far-field airborne infection.

Conference paper

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

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00475063&limit=30&person=true