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The aerodynamic behaviour and wake flow of a cluster of two-dimensional sharp-edged bluff bodies exhibits extremely complex unsteady phenomena in both near and far fields. Due to the high cost of wind-tunnel experiments and numerical simulations, a complete understanding of wake flows and a description of their characteristics are lacking. This paper presents large-eddy simulations (LES) in different flow/wind directions for a cluster of $$2 \times 2$$ 2 × 2 aligned square cylinders, at a separation distance in streamwise and cross-wind directions equal to cylinder side length, and at Reynolds number $${Re}=22,000$$ Re = 22 , 000 based on the single cylinder side length D. The case at $$0^\circ$$ 0 ∘ incidence shows an evident channel-type flow in the along-wind street/gap, and at its exit an irregularly pulsing jet with an intense shedding of large vortices. The wavelet analyses of the side force/lift coefficient and instantaneous velocities in the wake show that the characteristic length and time scales of the large vortical structures in the far-field wake are close to the cluster size 2D; this is the so called ‘cluster effect’. The cluster effect increases monotonically as the flow incidence angle increases. At a large incidence angle in the near-field wake, the cylinder-scale flow structures are much weaker compared to the cluster-scale structures. At the incidence angle of $$45^\circ$$ 45 ∘ , the overall wake flow and the aerodynamic characteristics are well scaled by the scale approximately equal to 2D. Nevertheless, the interaction between cylinders significantly affects the aerodynamics performance of the individual cylinders. The drag and lift coefficients of the individual cylinders differ substantially from each other in the cluster, and are significantly different from observations on a single isolated cylinder too.

Arrays of buildings with pitched roofs are common in urban and suburban areas of European cities. Large-eddy simulations are performed to predict the boundary-layer flows over flat and pitched-roof cuboids to gain a greater understanding of the impact of pitched roofs on urban boundary layers. The simulation methodology is validated for an array of flat roof cuboids. Further simulations show that changes in the type of grid conformity have a negligible effect on the mean flow field and turbulent stresses, while having a visible, but small, effect on the dispersive stresses for a given packing density. Comparisons are made for flat and $$45^\circ $$ 45 ∘ pitched roof cuboid arrays at packing densities of 16.7% and 33.3%. The interactions between pitched-roof buildings and their effect on the urban boundary layer are considerably different to those of flat-roof buildings. The pitched roofs at a packing density of 33.3% leads to significant changes in the mean flow field, the Reynolds stresses, and the aerodynamic drag. Further work investigates the effects of changes in turbulence level and atmospheric thermal stratification in the approaching flow. Importantly, in comparison to a flat-roof array, the pitched-roof one at a packing density of 33.3% evidently increase the friction velocity and greatly reduces the effects of stable stratification conditions and changes in inflow turbulence level.

A synthetic inflow turbulence generator was implemented in the idealised Weather Research and Forecasting large eddy simulation (WRF-LES v3.6.1) model under neutral atmospheric conditions. This method is based on an exponential correlation function and generates a series of two-dimensional slices of data which are correlated both in space and in time. These data satisfy a spectrum with a near “-5/3” inertial subrange, suggesting its excellent capability for high Reynolds number atmospheric flows. It is more computationally efficient than other synthetic turbulence generation approaches, such as three-dimensional digital filter methods. A WRF-LES simulation with periodic boundary conditions was conducted to provide prior mean profiles of first and second moments of turbulence for the synthetic turbulence generation method, and the results of the periodic case were also used to evaluate the inflow case. The inflow case generated similar turbulence structures to those of the periodic case after a short adjustment distance. The inflow case yielded a mean velocity profile and second-moment profiles that agreed well with those generated using periodic boundary conditions, after a short adjustment distance. For the range of the integral length scales of the inflow turbulence (±40 %), its effect on the mean velocity profiles is negligible, whereas its influence on the second-moment profiles is more visible, in particular for the smallest integral length scales, e.g. those with the friction velocity of less than 4 % error of the reference data at x/H=7. This implementation enables a WRF-LES simulation of a horizontally inhomogeneous case with non-repeated surface land-use patterns and can be extended so as to conduct a multi-scale seamless nesting simulation from a meso-scale domain with a kilometre-scale resolution down to LES domains with metre-scale resolutions.

We present a new method for the evaluation of field measurements of atmospheric dispersion at the smallest resolvable time scales. Our method has its origins in the distance-neighbour functions defined by Richardson as well as the auto correlations, spatial correlations and cross correlations commonly used in the turbulence community. We show that a robust quantification of the puff duration can be achieved, which is comparable to the puff duration derived by a dosage-based approach. The new method gives reasonable results without the need to apply thresholds when the concentration measurement is very noisy, which is very advantageous in the context of field measurements.

Simulations of the flow over a heaving NACA 0012 wing are conducted to study the separated flow phenomena for a pre-stall and post-stall wing condition. An extensively validated high fidelity large-eddy simulation (LES) approach is used to examine the unsteady aerodynamic loads and flow structures at Reynolds number Rec = 2 × 104 based on the chord. We consider reduced frequencies of k = 0.47 and 0.94 for a chord-normalized peak-to-peak amplitude of A/c=0.5 and angles of attack of 5∘ and 15∘, representing pre-stall and post-stall conditions respectively. Comparison to experiment shows good agreement for the phase-averaged lift, drag and moments of the heaving wing. Characteristic phenomena of dynamic stall are analysed with emphasis on the leading edge vortex (LEV) development. A series of instantaneous spanwise vorticity plots show significant spanwise perturbations in the reverse flow region that develops over the suction surface during the start of the downstroke, giving rise to instabilities in the detached shear layer. The instabilities give rise to the first occurrence of turbulence near the wing surface at the leading edge.

This work investigates the vertical and horizontal mass (scalar) flux of a contaminant emitted from an area source located in an array of blocks representing an urban environment. Arrays consisting of buildings with random and uniform heights and staggered and aligned arrangements were tested. Results shows that the vertical scalar flux close to the source can affect downwind clean zones. It is also shown that taller buildings increase the vertical scalar flux and the fluctuations of the vertical velocity above the smaller buildings. The vertical advective scalar flux was found to have an effect on dispersion in the vicinity of the building (a local effect), while the vertical turbulent fluxes are associated with pollutant transportation downwind above the smaller buildings (a non-local effect).

This study has looked at the development of the internal boundary layer (IBL) over a block array close to a sharp change in surface roughness and its effect on dispersion from a ground level source for ratios of the downstream distance to the roughness length of less than 300. This was done by comparing a Large--Eddy Simulation (LES) with inflow boundary conditions against a LES with inlet--outlet periodic boundary conditions and data from a wind tunnel experiment. The comparison showed that using inflow boundary conditions resulted in significantly better predictions of normal Reynolds stresses, mean concentration and scalar variance than when using periodic boundary conditions. In addition to established methods, an alternative approach based on the vertical Reynolds stress was used to evaluate the depth of the IBL as it developed over the array which enabled the location of the interface to be more clearly defined. It was confirmed that the IBL growth rate close to the change in surface roughness could be described by a power law profile, similar to the widely used power law formula with an exponent 0.8 for a ratio of the downstream distance to the roughness length greater than 1000. An analysis of mean concentration and turbulent scalar fluxes suggested that the presence of the IBL constrained the vertical development of the plume from a ground level source and so led to trapping of material in the canopy layer.

Computational fluid dynamics has become the method of choice for aerodynamic shape optimisation of complex engineering problems. However, the sensitivity of the final aerodynamic shape to numerical parameters has been largely underestimated to date. The purpose of this work is to investigate the influence that numerical parameters have on the optimisation results for two aerofoil problems (NACA 0012 and RAE 2822) in transonic flow, and to provide compact guidelines for best practice. Numerical parameters include: a) two parameterisation methods, Hicks–Henne bump functions and free-form deformation; b) numerical settings related to the tuning of each parameterisation method; and c) closure coefficients of Spalart–Allmaras (SA) turbulence model. All optimisations were performed using the open-source software tool SU2, and gradients were computed using the continuous adjoint method. It was found that: a) the optimisation result of NACA 0012 aerofoil exhibits strong dependence on all numerical parameters investigated, whereas the optimal design of RAE 2822 aerofoil is insensitive to those numerical settings; b) the degree of sensitivity reflects the difference in the design space, particularly of the local curvature on the optimised shape; c) the closure coefficients of SA model affect the final optimisation performance, raising the need for a good calibration of the turbulence model.

The broad implication of the present study is to elucidate the significance of the dynamic heaving motion in the aerodynamic performance of a double-element wing, which is currently considered a promising aspect of research for the improvement of the aerodynamic correlation between CFD, wind tunnel and track testing in race car applications. The relationship between the varying aerodynamic forces, the vortex shedding, and the unsteady pressure field around a heaving double-element wing is investigated numerically for a range of mean ride heights, frequencies and amplitudes. The analysis of the results shows that at high frequencies the interaction of the shear vorticity between the two elements of the wing results in the generation of cohering leading edge and trailing edge vortices on the flap, which are associated to the rapid variation of thrust and the enhancement of downforce. Importantly, both the occurrence and magnitude of these vortices are dependent upon the frequency, amplitude, and mean ride height. The addition of the flap significantly alters the frequency of the shed vortices in the wake, while the generation of downforce lasts longer in ground proximity. The comparison with the results of a static wing provides evidence that this dynamic motion of a race car wing can be beneficial in terms of performance, or detrimental in terms of aerodynamic correlation.

A synthetic-turbulence and temperature-fluctuation-generation method is developed and embedded in large-eddy simulations to investigate the effects of weak stable stratification (i.e. Richardson number $$Ri\le 1$$Ri≤1) on turbulence and dispersion following a simulated rural-to-urban transition. The modelling approach is validated by comparing predictions of mean velocity, turbulent stresses, and point-source dispersion against data from a wind-tunnel experiment that simulates a stable atmospheric boundary layer ($$Ri=0.21$$Ri=0.21) approaching a regular array of uniform rectangular blocks. The depth of the internal boundary layer (IBL) that develops from the leading edge of the block array is determined using the wall-normal turbulent stress method proposed by Sessa et al. (J Wind Eng Ind Aerodyn 182:189–291, 2018). This shows that the depth and growth rate of the IBL are sensitive to the thermal stability and the turbulence kinetic energy (TKE) prescribed at the inlet, such that the IBL depth reduces as the TKE of the inflow is reduced while maintaining the same Ri, or as the Ri is increased while maintaining the same inflow TKE. When a ground level line source is introduced it is found that increasing Ri evidently reduces the vertical scalar fluxes at the canopy height, while increasing the mean concentrations within the streets. Furthermore, as with IBL development it is found that for a given value of Ri the effect of stratification becomes more pronounced as the inflow level of TKE is reduced, affecting scalar fluxes within and above the canopy, and volume-averaged mean concentrations within the streets.

The performance of four synthetic turbulence generators which represent the majority of capabilities of i: digital-filter-based (DFM) and ii: forward-stepwise-based (FSM) generator categories is evaluated prior to transferring generator outputs into computational fluid dynamics simulations. In addition, a cheap-to-run and easy-to-code piecewise closed-form function that transforms onespatial- point skewness-kurtosis of a synthetic time-series to a target value is derived and presented. The main purpose of the study is to support model users in their decision process for choosing the most convenient type and their understanding of the models, and to extend the Gaussian nature of these models into non-Gaussianity for the first time. These have been carried out through a systematic exploration of model parameters-stages, which was observed absent in the literature. The evaluation test-bed contains three benchmarks, each of which focuses on an isolated aspect of turbulent flows: i: decaying homogeneous isotropic turbulence, ii: homogeneous shear turbulence and iii: plane channel flow with smooth walls. Results obtained reveal that: (i) the original DFM provides the highest level of reconstruction for input one-spatial-point second-order correlation tensors and two-spatial/temporal-point correlation functions; (ii) FSM yields the best trade-off between the computational cost and the level of reconstruction; (iii) the use of exponential-form correlation functions as a model approximation is more advisable than that of Gaussian-form, as the former removes the premature, sharp, flow-type-independent drop in power spectra observed for the latter; (iv) the proposed non-Gaussian functionality reconstructs the target skewness-kurtosis pairs of the test-bed flows virtually without altering their already-embedded statistics; (v) the Lund transformation changes existing statistics only in statistically inhomogeneous lateral directions of a flow when anisotropic Reynolds stresses are present; and (vi) a spatial variation of correlation functions on turbulence generation plane improves the overall reconstruction fidelity in terms of correlation functions and power spectra.

Indoor airflow can be complex due to large regions with no dominant flow direction and low velocities. An airborne material released from a surface indoors would be expected to result in a high degree of variability in concentration. It is currently not known how the spatially and temporally resolved concentration field from this type of source could be exploited for the detection of contaminants or vapour from concealed materials. Previous experimental and numerical work has provided information on flow and passive scalar transport in indoor environments but little on how different detection/search strategies could be employed in these environments. This work used large-eddy simulation to extensively study the turbulence fluctuations and the instantaneous vapour field in a widely studied, iso-thermal, benchmark test room, considering the effects of the source size and location. The work provides insight into vapour behaviour within indoor spaces and can have application to fields such as vapour detection, source localisation using autonomous systems, or exposure to toxic chemicals. As an example, we have interpreted the results in terms of current theories of chemical location by animals and the capability of detection dogs.

One-dimensional urban models embedded in mesoscale numerical models may place several grid points within the urban canopy. This requires an accurate parametrization for shear stresses (i.e. vertical momentum fluxes) including the dispersive stress and momentum sinks at these points. We used a case study with a packing density of 33% and checked rigorously the vertical variation of spatially-averaged total shear stress, which can be used in a one-dimensional column urban model. We found that the intrinsic spatial average, in which the volume or area of the solid parts are not included in the average process, yield greater time–spatial average of total stress within the canopy and a more evident abrupt change at the top of the buildings than the comprehensive spatial average, in which the volume or area of the solid parts are included in the average.

The weight and structure of civil transport aircraft are mainly dictated by the loads they experience at the limit of the design envelope (e.g. turbulence, gusts and manoeuvres). During the design process aircraft loading is often predicted using simplified models. However, under extreme operating conditions aircraft experience erratic unsteady loads that are not well understood, and hence difficult to predict using state of the art reduced models. Achieving a more profound understanding of the flow structures dictating the aerodynamic loads under extreme conditions will be crucial for the design of efficient aircraft. An example of such conditions occurs when a wing starts heaving or encounters a gust near its stall angle (e.g. during landing); unsteady flows and transient effects, including flow instabilities and vortex shedding, take place, making the aerodynamic loads highly unpredictable (see for example Von Ellenrieder et al, J Fluid Mech, 490:129–138, 2003, [1]). These complex physical mechanisms can only be fully captured by experiments and accurate numerical simulations, both of which are currently expensive to be used during the design stages, but can provide useful insight. Direct numerical simulations (DNS), being free from simplified modelling assumptions, have the advantage of including all the relevant flow physics. On the other hand, the large eddy simulation (LES) technique can be used to simulate realistic flow conditions, but its accuracy in predicting complex flow phenomena including flow instability and vortex shedding is not clear. This work focuses on assessing the performance of OpenFoam’s LES solver for the prediction of the aerodynamic loads acting on a heaving wing at incidence through comparisons with DNS and experimental results.

We present results from laboratory and computational experiments on the turbulent flow over an array of rectangular blocks modelling a typical, asymmetric urban canopy at various orientations to the approach flow. The work forms part of a larger study on dispersion within such arrays (project DIPLOS) and concentrates on the nature of the mean flow and turbulence fields within the canopy region, recognis- ing that unless the flow field is adequately represented in computational models there is no reason to expect realistic simulations of the nature of the dispersion of pollutants emitted within the canopy. Comparisons between the experimental data and those ob- tained from both large-eddy simulation (LES) and direct numerical simulation (DNS) are shown and it is concluded that careful use of LES can produce generally excellent agreement with laboratory and DNS results, lending further confidence in the use of LES for such situations. Various crucial issues are discussed and advice offered to both experimentalists and those seeking to compute canopy flows with turbulence resolving models

Large-Eddy Simulation of the flow around an elastically mounted rectangular cylinder with an aspect ratio 4 was undertaken. 1DOF analysis of the heaving and torsional motions was performed under a free vibration. Various characteristics of the flow-field at lock-in are discussed. Subsequently, a divergence-free synthetic inflow generation approach was employed to analyse the effects of the freestream turbulence on the bridge response. The inflow turbulence intensity and integral length scales were systematically studied. The effect of turbulence intensity (up to 12%) was shown to deplete the structural response for both torsional and heaving motions. A variation of the tested integral length scales, which were order of the cylinder dimensions, had less effects (than a variation of the turbulence intensity) on the structure response.

Flow past stationary and oscillating square cylinders at Reynolds number 22,000 are studied using immersed boundary method (IBM) in large eddy simulation (LES), as the IBM is able to seamlessly simulate arbitrarily-moving bodies at a high efficiency. The square cylinders are forced to oscillate in a prescribed sinusoidal motion at reduced velocity 7.7, which corresponds to the resonance point. Two amplitude ratios A ∕ D = 0.05 and 0.1 based on a cylinder height D are studied. The accuracy of the current solver with the immersed boundary method is rigorously assessed against the reference data. To understand the effect of structural motion on the flow and turbulence, aerodynamics of the oscillating cylinder is compared with that of the stationary cylinder. It is found that the oscillation motion reduces the spanwise correlation of flow field and triggers earlier reattachment on the side faces. Consequently, more chaotic surface pressure is generated downstream the reattachment point, and a smaller correlation is observed between upper and lower surface pressures. This helps to fully understand the interaction of turbulence and the flow-induced motion of a structure.

International audience; The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume 16 model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over 18 the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind Denise Hertwig et al. direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications.

A Met Office/Natural Environment Research Council Joint Weather and Climate Research Programme workshop brought together 50 key international scientists from the UK and international community to formulate the key requirements for an Urban Meteorological Research strategy. The workshop was jointly organised by University of Reading and the Met Office.

Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes are dominant, whereas the tall building produces an increase of the magnitude of advective scalar fluxes that yields the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street-canyon dispersion models and enhance their capabilities in areas with tall buildings.

Synthetic turbulence for inflow conditions formulated on a 2-D plane generally produces unphysically large pressure fluctuations in direct numerical and large-eddy simulations. To reduce such artificial fluctuations a divergencefree method is developed with incompressible flow solvers. The procedure of the velocity-pressure solvers is slightly modified on a vertical plane near (rather than at) the inlet by inserting the synthetic turbulence on that plane during the procedure. Simple analytic and numerical error estimations are used to show that the impact of the modified solvers on solution accuracy is small. The final synthetic turbulence satisfies the divergence-free condition. No additional CPU time is required to achieve this condition. The method was tested via simulations of a plane channel flow with Re_tau = 395. Reynolds stresses, wall skin friction and power spectra of velocity fluctuations are compared with those obtained from using periodic inlet-outlet boundary conditions. In particular, the variances and power spectra of pressure fluctuations are shown to be accurately predicted only when the divergence-free inlet condition is used.

The importance of vortex induced vibrations (VIV) in the design of bridge decks is becoming more apparent (Wu and Kareem, 2012). Generally, experimental literature presents a bridge deck situated in a smooth flow, though bridges are usually subjected to the inherent turbulence of atmospheric wind. According to the results of Matsumoto et al. (1993), the effects of turbulence on VIV are rather complicated, this is mainly due to an interaction between the vortices in the wake, and vortices induced by the structure motion. The impacts of turbulence on the motion-induced forces are uncertain due to a limited understanding of this issue. Wu and Kareem (2012) also present a conjecture, suggesting that turbulence can enhance or diminish the structural response, depending on whether the dominant vortices are generated from the motion of the structure, or vortex shedding in the wake. This numerical investigation aims to verify this hypothesis, while studying the effects of turbulence on the structural response, aerodynamic forces, correlations, and wake characteristics for a dynamic bridge deck. Some effects of freestream turbulence on a rectangular bridge deck undergoing VIV are presented. Heaving motion of the model under a smooth flow is presenting with a peak r.m.s. response y / D∼ 3% in a close agreement with the equivalent wind tunnel experiment. Introducing turbulence into the flow reduces the peak rms response by 50%, which is in accordance with observations in the literature. This abstract also reports the effects of turbulence on the spanwise correlation of pressure on the bridge deck surfaces. Due to the dominance of the structural motion, the correlations for the dynamic cases at lock-in were markedly increased in comparison to the equivalent static case. The presence of turbulence has also shown to reduce the correlation across the span for both static and dynamic cases. It can therefore be deduced that turbulence reduces the strength of the leading edge (motion-induced) vortices of the structure, and accordingly its VIV response. Sensitivity studies of the effects of turbulence intensity and integral length scales were conducted. Effects of turbulence on the pitching motion were studied as well. Detailed results will be reported in the conference.

Large-eddy simulations of the dispersion from scalar line sources at various locations within a fully developed turbulent channel flow at Re = uh/ν = 10,400 are presented. Both mean and fluctuating scalar quantities are compared with those from the single available set of experimental data (Lavertu and Mydlarski, J Fluid Mech 528:135–172, 2005) and differences are highlighted and discussed. The results are also discussed in the context of scalar dispersion in other kinds of turbulent flows, e.g. homogeneous shear-flow. Initial computations at a much lower Reynolds number are also reported and compared with the two available direct numerical simulation data sets.

This paper focuses on investigating thermal buoyancy effects on dispersion of approach flows and local heat transfer from/to buildings by taking the DAPPLE site as the test case but for varying wind direction and magnitude. Firstly, based on the BT tower data, only weakly unstable conditions (i.e. Richardson number |Ri

Towards meeting the objective of simulating heat transfer processes in urban areas, the study of dispersion from a scalar (ground) surface area source has been addressed as a first step, as dispersion from such a source is in some ways analogous to heat transfer from the surface. Two different urban-like geometries are considered in this study: an array with uniform height cubes and an array with random height cuboids. Some point measurement dispersion experiments in a wind tunnel have previously been carried out in identical arrays using a naphthalene sublimation technique. Large-eddy simulations (LES) of these experiments have been performed as a validation study and the details, presented here, demonstrate the influence of the roughness morphology on the dispersion processes and the power of LES for obtaining physically important scalar turbulent flux information.

Further to our previous large-eddy simulation (LES) of flow over a staggered array of uniform cubes, a simulation of flow over random urban-like obstacles is presented. To gain a deeper insight into the effects of randomness in the obstacle topology, the current results, e.g. spatially-averaged mean velocity, Reynolds stresses, turbulence kinetic energy and dispersive stresses, are compared with our previous LES data and direct numerical simulation data of flow over uniform cubes. Significantly different features in the turbulence statistics are observed within and immediately above the canopy, although there are some similarities in the spatially-averaged statistics. It is also found that the relatively high pressures on the tallest buildings generate contributions to the total surface drag that are far in excess of their proportionate frontal area within the array. Details of the turbulence characteristics (like the stress anisotropy) are compared with those in regular roughness arrays and attempts to find some generality in the turbulence statistics within the canopy region are discussed.

Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor. The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities, the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations.

Wind turbines operate in the atmospheric boundary layer and their rotating mechanisms provide complicated aerodynamic phenomena within their operating environments. When the upstream wind is uniform and normal to the plane of a rotating blade, rotational effects (i.e. rotational augmentation) emerge. Rotational augmentation means that stall occurs at a higher angle of attack on the rotating blade section than it does on an analogous stationary airfoil.

Validation of large-eddy simulation (LES) computations of flows over an array of cubes with imposed periodic inflow-outflow condition is described. Then, a method of generation of appropriate inflow data for such flows is proposed and the results are compared with those using periodic boundary conditions in the streamwise direction and wind tunnel measurements [3]. Finally, some results obtained using the inflow method for an oscillatory through- flow combined with a steady current are discussed.

Large-eddy simulation (LES) has been applied to calculate the turbulent flow over staggered wall-mounted cubes and staggered random arrays of obstacles with area density 25%, at Reynolds numbers between 5 × 10^3 and 5 10^6, based on the free stream velocity and the obstacle height. Re = 5 × 10^3 data were intensively validated against direct numerical simulation (DNS) results at the same Re and experimental data obtained in a boundary layer developing over an identical roughness and at a rather higher Re. The results collectively confirm that Reynolds number dependency is very weak, principally because the surface drag is predominantly form drag and the turbulence production process is at scales comparable to the roughness element sizes. LES is thus able to simulate turbulent flow over the urban-like obstacles at high Re with grids that would be far too coarse for adequate computation of corresponding smooth-wall flows. Comparison between LES and steady Reynolds-averaged Navier-Stokes (RANS) results are included, emphasising that the latter are inadequate, especially within the canopy region.

The influence of local surface heating and cooling on flow over urban-like roughness is investigated using large-eddy simulations. By adjusting the incoming or outgoing heat flux from the ground surface, various degrees of local thermal stratification, represented by a Richardson number $$(Ri_\tau )$$ , were attained. Drag and heat transfer coefficients, turbulence structure, integral length scales, and the strength of quadrant events that contribute to momentum and heat fluxes were obtained and are compared with locally stable, neutral and unstable flows. With increasing $$Ri_\tau $$ , or equivalently as the flow characteristics change from local thermal instability to stability, a gradual decline in the drag and heat transfer coefficients is observed. These values are found to be fairly independent of the type of thermal boundary condition (constant heat flux or constant temperature) and domain size. The maps of anisotropy invariants showed that for the values of $$Ri_\tau $$ considered, turbulence structures are almost the same in shape for neutral and unstable cases but differ slightly from those in the stable case. The degree of anisotropy is found to decrease as $$Ri_\tau $$ increases from $$-2$$ to 2.5. Compared to the neutral case, the integral length scales are shortened in the streamwise and vertical direction by ground cooling, but enhanced in the vertical direction with ground heating. Quadrant analysis showed that an increase in floor heating increases the strength of ejections above the canopy. However, the contributions of updrafts or downdrafts to the heat flux are found not to be significantly influenced by the type of local thermal stratification for the values of $$Ri_\tau $$ considered. From the octant analysis, the transport mechanisms of momentum and heat above the canopy are found to be very similar in both locally unstable and stable flows.

A large-eddy simulation with transitional structure function(TSF) subgrid model we previously proposed was performed to investigate the turbulent flow with thermal influence over an inhomogeneous canopy, which was represented as alternative large and small roughness elements. The aerodynamic and thermodynamic effects of the presence of a layer of large roughness elements were modelled by adding a drag term to the three-dimensional Navier–Stokes equations and a heat source/sink term to the scalar equation, respectively. The layer of small roughness elements was simply treated using the method as described in paper (Moeng 1984, J. Atmos Sci. 41, 2052–2062) for homogeneous rough surface. The horizontally averaged statistics such as mean vertical profiles of wind velocity, air temperature, et al., are in reasonable agreement with Gao et al.(1989, Boundary layer meteorol. 47, 349–377) field observation (homogeneous canopy). Not surprisingly, the calculated instantaneous velocity and temperature fields show that the roughness elements considerably changed the turbulent structure within the canopy. The adjustment of the mean vertical profiles of velocity and temperature was studied, which was found qualitatively comparable with Belcher et al. (2003, J Fluid Mech. 488, 369–398)’s theoretical results. The urban heat island(UHI) was investigated imposing heat source in the region of large roughness elements. An elevated inversion layer, a phenomenon often observed in the urban area (Sang et al., J Wind Eng. Ind. Aesodyn. 87, 243–258)’s was successfully simulated above the canopy. The cool island(CI) was also investigated imposing heat sink to simply model the evaporation of plant canopy. An inversion layer was found very stable and robust within the canopy.

A family of wall models is proposed that exhibits more satisfactory performance than previous models for the large-eddy simulation (LES) of the turbulent boundary layer over a rough surface. The time and horizontally averaged statistics such as mean vertical profiles of wind velocity, Reynolds stress, turbulent intensities, turbulent kinetic energy and also spectra are compared with wind-tunnel experimental data. The purpose of the present study is to obtain simulated turbulent flows that are comparable with wind-tunnel measurements for use as the wind environment for the numerical prediction by LES of source dispersion in the neutral atmospheric boundary layer.

The water-heat transfer process between land and atmosphere in Haibei alpine meadow area has been systematically observed. A multi-layer coupling model for land-atmosphere interaction was presented with special attention paid to the moisture transfer in leaf stomata under unsaturated condition. A profound investigation on the physical process of turbulent transfer inside the vegetation has been performed with a revised formula of water absorption for root system. The present model facilitates the study of vertically distributed physical variables in detail. Numerical simulation was conducted according to the transfer process of Kinesia humility meadow in the area of Haibei Alpine Meadow Ecosystem Station, CAS. The calculated results agree well with observation.

Based on the turbulent energy and structure function model, a subgrid stress model named transitional structure function (TSF) is proposed, which exhibits more satisfactory performance than the previous one when applied in turbulent flow within and above forest canopy. The time and horizontal averaged statistics such as mean vertical profiles of wind velocity, Reynolds stress, turbulent kinetic energy etc., and instantaneous fields as well are in general agreement with earlier observations, at least in a qualitative sense.

Numerical investigations on submerged cavitating slender axisymmetric bodies with gravity effect are conducted in this paper. The developed cavity shape and the hydrodynamic characteristics on cavitating axisymmetric bodies are calculated. While these cavities originate from both the shoulder and the tail of bodies, we focus our main attention on the cavities originated from the shoulder. The boundary element method is used to solve the potential flow problem, and a new iteration method named Adaptive Modified Newton Iteration is developed to determine the cavity shape.

The study investigates thermal stratification effects of approach flows on dispersion in urban environments. This is in some ways analogous to a well developed non-neutral flow (e.g. through a large urban area) approaching a neighborhoodscale urban region, where the effect of the local heat transfer was assumed less important. A generic urban-type geometry, i.e. a group of staggered cubes, wastaken as the first test case. The DAPPLE site, which was about a one-km2 region near the intersection of Marylebone Road and Gloucester Place in central London, was taken as the second test case. Only weakly unstable conditions (i.e.bulk Richardson number Rb >= ?0.2) of approach flows were considered, with adiabatic boundary conditions at the ground and building surfaces. A number of numerical experiments including with various Rb were performed. The modelled mean concentration for Rb = ?0.1 gave the best agreement with the field data at all DAPPLE stations. This suggests that stratification effects on dispersion inweakly unstable conditions (e.g. in London) are not negligible.

Using a recently developed inflow generation approach (Kim et al., 2013), Large-Eddy Simulation (LES) of the flow over the Commonwealth Advisory Aeronautical Council (CAARC) standard tall building has been performed. The surface pressure fluctuations were calculated and are shown to be in reasonable agreement with the measurements in the literature. Numerical experiments were conducted to investigate the effects of the inflow turbulence intensity and integral length scales. Previous research, e.g. Surry and Djakovich (1995) and Huang et al. (2007), suggests a presence of periodicity in the lateral forces on tall buildings, indicating a correlation of the surface pressures on the building's lateral faces. This paper also focuses on the correlations of surface pressure, local wall-normal forces and cross-wind velocity.

Further to our previous work – simulations of flow and dispersion in an oblique wind over the DAPPLE site (Xie ZT & Castro IP, Atmospheric Environment, 2009, Vol.43, 2174-2185)– large-eddy simulations of flows and dispersion over the same site in a wind perpendicular to Marylebone Road and the windward surfaces of most of the buildings were performed. The DAPPLE site is located at the intersection of Marylebone Road and Gloucester Place in central London. In order to investigate the effects of wind direction on flows and dispersion, the velocity and scalar fields in the perpendicular wind were compared with those in the oblique wind. Furthermore, realistic wind conditions measured on the BT Tower at 190 m above street level were processed and used to drive the numerical simulations of flows and dispersion at the DAPPLE site. This leads to significant predictive improvements of the dispersion compared with field measurements, which provides validation and confidence for coupling mesoscale meteorological models, e.g. the UK Met Office’s Unified Model and the NCAR’s Weather Research & Forecasting Model, with the street-scale large-eddy simulation of urban environments

Large-eddy simulations (LES) with our recently developed inflow approach (Xie &Castro, 2008a) have been used for flow and dispersion within a genuine city area -the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Plin Central London. Numerical results up to second-order statistics are reported fora computational domain of 1.2km (streamwise) x 0.8km (lateral) x 0.2km (in fullscale), with a resolution down to approximately one meter in space and one secondin time. They are in reasonable agreement with the experimental data. Such a comprehensiveurban geometry is often, as here, composed of staggered, aligned, squarearrays of blocks with non-uniform height and non-uniform base, street canyons andintersections. Both the integrative and local effect of flow and dispersion to thesegeometrical patterns were investigated. For example, it was found that the peaksof spatially averaged urms, vrms, wrms and < u0w0 > occurred neither at the meanheight nor at the maximum height, but at the height of large and tall buildings. Itwas also found that the mean and fluctuating concentrations in the near-source fieldis highly dependent on the source location and the local geometry pattern, whereasin the far field (e.g. >0.1km) they are not. In summary, it is demonstrated thatfull-scale resolution of around one meter is sufficient to yield accurate prediction ofthe flow and mean dispersion characteristics and to provide reasonable estimationof concentration fluctuations

In this paper we raise several issues, e.g. resolution, Reynolds number dependency, mesh quality and inflow boundary conditions for Large-Eddy Simulation (LES) of street scale flows, scalar dispersion and heat transfer within urban areas. Some of the issues are addressed extensively and some LES results of test cases are presented. The other issues are discussed and commented for further study. Finally we attempt to foresee prospects for the use of LES for urban environments with a computational domain size up to a few kilometers and a resolution down to one meter.

The concentration fluctuations in passive plumes from an elevated and a groundlevel source in a turbulent boundary layer over a rough wall were studied using large eddy simulation and wind tunnel experiment. The predictions of statistics up to second order moments were thereby validated. In addition, the trend of relative fluctuations far downstream for a ground level source was estimated using dimensional analysis. The techniques of extreme value theory were then applied to predict extreme concentrations by modelling the upper tail of the probability density function of the concentration time series by the Generalised Pareto Distribution. Data obtained from both the simulations and experiments were analysed in this manner. The predicted maximum concentration (i0) normalized by the local mean concentration (Cm) or by the local r.m.s of concentration fluctuation (crms), was extensively investigated. Values for i0=Cm and i0=crms as large as 50 and 20 respectively were found for the elevated source and 10 and 15 respectively for the ground-level source.