Imperial College London

Dr Maarten van Reeuwijk

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Reader in Environmental Fluid Mechanics



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




Miss Rebecca Naessens +44 (0)20 7594 5990




331Skempton BuildingSouth Kensington Campus





Publication Type

96 results found

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 and three-dimensional spatial correlation functions are considered. It is found that the flow is characterized by large-scale spanwise vortices whose motion is felt in the entire flow field. Superimposed to this large-scale motion, a hierarchy of turbulent structures is present. The most coherent ones take the form of quasi-streamwise vortices and high and low streamwise velocity streaks. The topology of these interacting structures 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 of models based on coherent structure assumptions. Finally, the resulting scenario provides a phenomenological description of the elementary processes 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

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

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

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

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

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

Neamtu-Halic MM, Krug D, Mollicone J-P, van Reeuwijk M, Haller G, Holzner Met al., 2020, Connecting the time evolution of the turbulence interface to coherent structures, Journal of Fluid Mechanics, Vol: 898, Pages: 1-24, ISSN: 0022-1120

The surface area of turbulent/non-turbulent interfaces (TNTIs) is continuously produced and destroyed via stretching and curvature/propagation effects. Here, the mechanisms responsible for TNTI area growth and destruction are investigated in a turbulent flow with and without stable stratification through the time evolution equation of the TNTI area. We show that both terms have broad distributions and may locally contribute to either production or destruction. On average, however, the area growth is driven by stretching, which is approximately balanced by destruction by the curvature/propagation term. To investigate the contribution of different length scales to these processes, we apply spatial filtering to the data. In doing so, we find that the averages of the stretching and the curvature/propagation terms balance out across spatial scales of TNTI wrinkles and this scale-by-scale balance is consistent with an observed scale invariance of the nearby coherent vortices. Through a conditional analysis, we demonstrate that the TNTI area production (destruction) is localized at the front (lee) edge of the vortical structures in the interface proximity. Finally, we show that while basic mechanisms remain the same, increasing stratification reduces the rates at which TNTI surface area is produced as well as destroyed. We provide evidence that this reduction is largely connected to a change in the multiscale geometry of the interface, which tends to flatten in the wall-normal direction at all active length scales of the TNTI.

Journal article

Meyer D, Schoetter R, Riechert M, Verrelle A, Tewari M, Dudhia J, Masson V, van Reeuwijk M, Grimmond Set al., 2020, WRF-TEB: Implementation and evaluation of the coupled weather research and forecasting (WRF) and town energy balance (TEB) model, Journal of Advances in Modeling Earth Systems, Vol: 12, Pages: 1-18, ISSN: 1942-2466

Urban land surface processes need to be represented to inform future urban climate and building energy projections. Here, the single layer urban canopy model Town Energy Balance (TEB) is coupled to the Weather Research and Forecasting (WRF) model to create WRF‐TEB. The coupling method is described generically, implemented into software, and the code and data are released with a Singularity image to address issues of scientific reproducibility. The coupling is implemented modularly and verified by an integration test. Results show no detectable errors in the coupling. Separately, a meteorological evaluation is undertaken using observations from Toulouse, France. The latter evaluation, during an urban canopy layer heat island episode, shows reasonable ability to estimate turbulent heat flux densities and other meteorological quantities. We conclude that new model couplings should make use of integration tests as meteorological evaluations by themselves are insufficient, given that errors are difficult to attribute because of the interplay between observational errors and multiple parameterization schemes (e.g., radiation, microphysics, and boundary layer).

Journal article

Grylls T, Suter I, Van Reeuwijk M, 2020, Steady-state large-eddy simulations of convective and stable urban Boundary layers, Boundary-Layer Meteorology: an international journal of physical and biological processes in the atmospheric boundary layer, Vol: 175, Pages: 309-341, ISSN: 0006-8314

A comprehensive investigation is carried out to establish best practice guidelines for the modelling of statistically steady-state non-neutral urban boundary layers (UBL) using large-eddy simulation (LES). These steady-state simulations enable targeted studies under realistic non-neutral conditions without the complications associated with the inherently transient nature of the UBL. An extensive set of simulations of convective and stable conditions is carried out to determine which simplifications, volumetric forcings, and boundary conditions can be applied to replicate the mean and turbulent (variance and covariance) statistics of this intrinsically transient problem most faithfully. In addition, a new method is introduced in which a transient simulation can be ‘frozen’ into a steady state. It is found that non-neutral simulations have different requirements to their neutral counterparts. In convective conditions, capping the boundary-layer height h with the top of the modelled domain to h/5 and h/10 (which is common practice in neutral simulations) reduces the turbulent kinetic energy by as much as 61% and 44%, respectively. Consistent with the literature, we find that domain heights lz≥5|L| are necessary to reproduce the convective-boundary-layer dynamics, where L is the Obukhov length. In stably stratified situations, the use of a uniform momentum forcing systematically underestimates the mechanical generation of turbulence over the urban canopy layer, and therefore leads to misrepresentations of both the inner- and outer-layer dynamics. The new ‘frozen-transient’ method that is able to maintain a prescribed flow state (including entrainment at the boundary-layer top) is shown to work well in both stable and convective conditions. Guidelines are provided for future studies of the capped and uncapped convective and stable UBL.

Journal article

Sutzl B, Van Reeuwijk M, 2020, bss116/citygenerator: Urban Landscape Generator v1.0

The Urban Landscape Generator is a tool that creates randomised urban landscapes with fractal-like street networks, based on urban morpholgy parameters. This version v1.0 provides the functionality of creating urban layouts based on building density, proportion of green space, and wind-facing frontal area for a fixed wind direction. Three different fractal types ('hierarchical', 'cascade', 'random') can be chosen, as well as the level of randomness in building-layout and -height generation.


Nair V, Heus T, Van Reeuwijk M, 2020, Dynamics of subsiding shells in actively growing clouds with vertical updrafts, Journal of the Atmospheric Sciences, Vol: 77, Pages: 1353-1369, ISSN: 0022-4928

The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulation. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell which grows with time. A self-similar regime is observed for first and second order moments when normalized with respective maximum values. Internal scales derived from integral properties of the flow problem are identified. Self-similarity analysis conducted by normalizing using these scales reveal that contrary to classical self similar flows, the turbulent kinetic energy budget terms and velocity moments scale according to the buoyancy and not with the mean velocity. The shell thickness is observed to increase linearly with time. The buoyancy scale remains time-invariant and is set by the initial cloud-environment thermodynamics. The shell accelerates ballistically with a magnitude set by the saturation value of the buoyancy of the cloud-environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. Relations are obtained for predicting the shell thickness and minimum velocities by linking the internal scales with external flow parameters. The values thus calculated are consistent with the thickness and velocities observed in typical shallow cumulus clouds. The entrainment coefficient is a function of the initial state of the cloud and the environment, and is shown to be of the same order of magnitude as fractional entrainment rates calculated for large scale models.

Journal article

van Reeuwijk M, Holzner M, 2020, Direct simulation of turbulent entrainment in a temporal plane jet

We report on properties of the viscous superlayer using direct simulation of a temporal plane jet at Re = 2500. The local entrainment velocity vn scales with the Kolmogorov velocity with a prefactor that depends on the enstrophy threshold. The entrainment flux vnS is practically independent of the threshold choice, as is observed from the mean entrainment velocity ue = vnS/A, where S is the enstrophy isosurface area and A is the surface area after projecting onto the homogeneous directions. A decomposition of vn into a mean and turbulent contribution indicates that turbulent fluctuations are the dominant mechanism by which the interface propagates even in this viscosity-dominated region of the flow.

Conference paper

Verso L, van Reeuwijk M, Liberzon A, 2019, Transient stratification force on particles crossing a density interface, International Journal of Multiphase Flow, Vol: 121, Pages: 1-13, ISSN: 0301-9322

We perform a series of experiments to measure Lagrangian trajectories of settling and rising particles as they traverse a density interface of thickness h using an index-matched water-salt-ethanol solution. The experiments confirm the substantial deceleration that particles experience as a result of the additional force exerted on the particle due to the sudden change in density. This stratification force is calculated from the measurement data for all particle trajectories. In absence of suitable parameterisations in the literature, a simple phenomenological model is developed which relies on parameterisations of the effective wake volume and recovery time scale. The model accurately predicts the particle trajectories obtained in our experiments and those of Srdić-Mitrović et al. (1999). Furthermore, the model demonstrates that the problem depends on four key parameters, namely the entrance Reynolds number Re1, entrance Froude number Fr, particle to fluid density ratio ρp/ρf, and relative interface thickness h/a.

Journal article

Van Reeuwijk M, Holzner M, Caulfield CP, 2019, Mixing and entrainment are suppressed in inclined gravity currents, Journal of Fluid Mechanics, Vol: 873, Pages: 786-815, ISSN: 0022-1120

We explore the dynamics of inclined temporal gravity currents using directnumerical simulation, and find that the current creates an environment in which the flux Richardson number $Ri_f$, gradient Richardson number $Ri_g$, and turbulent flux coefficient $\Gamma$ are constant across a large portion of the depth. Changing the slope angle $\alpha$ modifies these mixing parameters, and the flow approaches a maximum Richardson number $Ri_\textrm{max}\approx 0.15$ as $\alpha \rightarrow 0$ at which the entrainment coefficient $E \rightarrow 0$. The turbulent Prandtl number remains $O(1)$ for all slope angles, demonstrating that $E\rightarrow 0$ is not caused by a switch-off of the turbulent buoyancy flux as conjectured by Ellison (1957). Instead, $E\rightarrow 0$ occurs as the result of the turbulence intensity going to zero as $\alpha\rightarrow 0$, due to the flow requiring larger and larger shear to maintain the same level of turbulence. We develop an approximate model valid for small $\alpha$ which is able to predict accurately $Ri_f$, $Ri_g$ and $\Gamma$ as a function of $\alpha$ and their maximum attainable values. The model predicts an entrainment law of the form $E=0.31(Ri_\textrm{max}-Ri)$, which is in good agreement with the simulation data. The simulations and model presented here contribute to a growing body of evidence that an approach to a marginally or critically stable, relatively weakly stratified equilibrium for stratified shear flows may well be a generic property of turbulent stratified flows.

Journal article

Grylls T, Le Cornec CMA, Salizzoni P, Soulhac L, Stettler MEJ, Van Reeuwijk Met al., 2019, Evaluation of an operational air quality model using large-eddy simulation, Atmospheric Environment: X, Vol: 3, ISSN: 2590-1621

The large-eddy simulation (LES) model uDALES is used to evaluate the predictive skill of the operational air quality model SIRANE. The use of LES in this study presents a novel approach to air quality model evaluation, avoiding sources of uncertainty and providing numerical control that permits systematic analysis of targeted parametrisations and assumptions.A case study is conducted over South Kensington, London with the morphology, emissions, meteorological conditions and boundary conditions carefully matched in both models. The dispersion of both inert (NOx) and reactive (NO, NO2 and O3) pollutants under neutral, steady-state conditions is simulated for a south-westerly and westerly wind direction. A quantitative comparison between the two models is performed using statistical indices (the fractional bias, FB, the normalised mean squared error, NMSE, and the fraction in a factor of 2, FAC2).SIRANE is shown to successfully capture the dominant trends with respect to canyon-averaged concentrations of inert NOx (FB = -0.08, NMSE = 0.08 and FAC2 = 1.0). The prediction of along-canyon velocities is shown to exhibit sources of systematic error dependant on the angle of incidence of the mean wind (FB = -0.18). The assumption of photostationarity within SIRANE (deviations from equilibrium of up to 170% exist close to busy roads) is also identified as a significant source of systematic bias resulting in over- and underpredictions of NO2 (FB = -0.18) and O3 (FB = 0.14) respectively. The validity of the assumed uniform in-canyon concentration is assessed by analysing the pedestrian, leeward and windward concentrations resolved in uDALES. The use of canyon-averaged concentrations to predict pedestrian level exposure is shown to result in significant underestimations. Linear regression is used to effectively capture the relationship between pedestrian- and canyon-averaged concentrations in uDALES. Correction factors are derived (m ≈ 1.62 and R 2 = 0.92 for inert NOx) th

Journal article

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