A review of urban roughness sublayer turbulence

It is becoming increasingly important that we can understand and model flow processes in urban areas. Applications such as weather forecasting, air quality and sustainable urban development rely on accurate modelling of the interface between an urban surface and the atmosphere above. This review gives an overview of current understanding of turbulence generated by an urban surface up to a few building heights, the layer called the roughness sublayer (RSL). High quality datasets are also identified which can be used in the development of suitable parameterisations of the urban RSL. Datasets derived from physical and numerical modelling, and full-scale observations in urban areas now exist across a range of urban-type morphologies (e.g. street canyons, cubes, idealised and realistic building layouts). Results show that the urban RSL depth falls within 2 – 5 times mean building height and is not easily related to morphology. Systematic perturbations away from uniform layouts (e.g. varying building heights) have a significant impact on RSL structure and depth. Considerable fetch is required to develop an overlying inertial sublayer, where turbulence is more homogeneous, and some authors have suggested that the “patchiness” of urban areas may prevent inertial sublayers from developing at all. Turbulence statistics suggest similarities between vegetation and urban canopies but key differences are emerging. There is no consensus as to suitable scaling variables, e.g. friction velocity above canopy vs. square root of maximum Reynolds stress, mean vs. maximum building height. The review includes a summary of existing modelling practices and highlights research priorities

Numerical simulation of the influence of building‑tree arrangements on wind velocity and PM2.5 dispersion in urban communities

Airflow behavior and outdoor PM(2.5) dispersion depend significantly on the building-tree layouts and orientation towards the prevailing wind conditions. To investigate this issue, the present work evaluates the aerodynamic effect of different building-tree layouts on the outdoor PM(2.5) dispersions in the urban communities of Shijiazhuang City, China. The adopted numerical CFD technique was based on the standard k–ε model and the Disperse Phase Model (DPM). For this study, ten different building-tree arrangements were conceptualized and all these configurations were simulated by using Ansys Fluent software to quantify the implications on the outdoor PM(2.5) dispersion due to their presence. The results have shown that: (1) a wide building interval space could benefit the air ventilation and thus decrease PM(2.5) concentrations, however, this effectiveness is highly influenced by the presence of the trees; (2) the trees on the leeward side of a building tend to increase the local wind velocity and decrease the pedestrian-level PM(2.5) concentrations, while those on the windward side tend to decrease the wind velocity. The small distance with trees in the central space of the community forms a wind shelter, hindering the particle dispersion; and (3) the configuration of parallel type buildings with clustered tree layouts in the narrow central space is most unfavorable to the air ventilation, leading to larger areas affected by excessive PM(2.5) concentration

Modeling of wind load on tall buildings using computational fluid dynamics

Master'sMASTER OF ENGINEERIN

Proposed guidelines of using CFD and the validity of the CFD models in the numerical simulations of wind environments around buildings

Abstract unavailable please refer to PDFAbstract unavailable please refer to PD

Estimating the potential yield of small building-mounted wind turbines

The wind profile in the urban boundary layer is described as following a logarithmic curve above the mean building height and an exponential curve below it. By considering the urban landscape to be an array of cubes, a method is described for calculating the surface roughness length and displacement height of this profile. Firstly, a computational fluid dynamics (CFD) model employing a k-ϵ turbulence model is used to simulate the flow around a cube. The results of this simulation are compared with wind tunnel measurements in order to validate the code. Then, the CFD model is used to simulate the wind flow around a simple pitched-roof building, using a semi-logarithmic inflow profile. An array of similar pitched-roof houses is modelled using CFD to determine the flow characteristics within an urban area. Mean wind speeds at potential turbine mounting points are studied, and optimum mounting points are identified for different prevailing wind directions. A methodology is proposed for estimating the energy yield of a building-mounted turbine from simple information such as wind atlas wind speed and building density. The energy yield of a small turbine on a hypothetical house in west London is estimated. The energy yield is shown to be very low, particularly if the turbine is mounted below rooftop height. It should be stressed that the complexity of modelling such urban environments using such a computational model has limitations and results can only be considered approximate, but nonetheless, gives an indication of expected yields within the built environment

Effect of roof shape, wind direction, building height and urban configuration on the energy yield and positioning of roof mounted wind turbines

PhD ThesisThe increasing interest among architects and planners in designing environmentally friendly buildings has led to a desire to explore and integrate renewable sources of energy within the built environment. Roof mounted wind turbines is a technology that presents a high potential for integration within the built environment. However, there is a state of uncertainty regarding the viability of these wind turbines. This thesis argues that part of this uncertainty is attributed to uninformed decisions about positioning and locating urban wind turbines. This is underpinned by lack of consideration to the wind accelerating effect of different roof shapes, buildings’ heights and surrounding urban configurations. This thesis aims to investigate the effect of different roof shapes on wind acceleration and positioning of roof mounted wind turbines covering different buildings’ heights within different urban configurations under different wind directions. To achieve the aim of the thesis, the commercial Computational Fluid Dynamics (CFD) code Fluent 12.1, implementing the Realizable k-ε turbulence model, is used to simulate wind flow around different roof shapes, different buildings’ heights and different urban settings. Predictions are comparatively analysed to identify the optimum roof shape for mounting wind turbines. Simulation results indicate that the barrel vaulted roof has the highest wind accelerating effect. The barrel vaulted roof shape case was carried further to investigate the effect of building height and surrounding urban configurations on the energy yield and positioning of roof mounted wind turbines. The optimum mounting location for each of the investigated roof shapes namely: flat, domed, gabled, pyramidal, barrel vaulted and wedged roofs is identified. Results from the investigation predict a possible increase up to 56.1% in energy yield in the case of a barrel vaulted roof if an informed wind assessment above buildings’ roofs is carried out. However, changing the building height and surrounding urban configuration had an effect on choosing the optimum mounting location and the energy yield at that location.A studentship from the School of Architecture, Planning and Landscape,Newcastle University. Arab British Chamber of Commerce. Newcastle University International Postgraduate Scholarship

The influence of advertisement boards, street and source layouts on CO dispersion and building intake fraction in three-dimensional urban-like models

Heavy traffic flows commonly result in large vehicular pollutant exposure in near-road buildings. Street layouts and pollutant source settings are key factors. Advertisement boards are sometimes adopted for business purpose, but their impacts on pollutant dispersion and exposure are still unclear. Thus, this paper numerically investigates the influence of aspect ratios (building height/street width, H/Ws=1 or 2; *Revised Manuscript with No Changes Marked Click here to view linked References H=30m), source locations and advertisement-board settings on the flow, carbon monoxide (CO) dispersion and exposure within three-dimensional urban-like models under the parallel approaching wind to the main streets. Personal intake fraction (P_iF) represents the fraction of total vehicular emissions inhaled averagely by each person of a population. Spatial mean P_iF is named as and that for the entire building as building intake fraction (B). With span-wise CO source fixed in target secondary streets of No 2 or 13 (S2 or S13), is particularly large in target streets. B decreases exponentially toward downstream from the target street and S13 cases attain greater B and larger exponential decreasing rates. Cases with H/Ws=2 experiences more limited upward dispersion and subsequently smaller (0.155-0.339ppm) of entire target street than cases with H/Ws=1 (0.375-0.731ppm). For cases with stream-wise CO source along the main street (Smain), B first rises quickly toward downstream, then adjusts to equilibrium values (0.051-0.063ppm). Finally, with span-wise source, vertical and double-layer advertisement boards produce stronger upward CO transportation and greater B than lateral and single-layer types, while with Smain source, the double-layer and lateral types produce larger B and shorter exposure adjustment distance

Simulação numérica do escoamento atmosférico sobre edifícios altos para determinação do efeito de vizinhança

Numerical simulations of atmospheric flow were carried out in this study in order to evaluate the neighborhood effects on the wind loading over standard model-scale tall buildings. The computational models were developed by solving the steady-state Reynolds Averaged Navier-Stokes equations (RANS equations) with turbulence treated by a k-ε model. Two building positioning scenarios were simulated: scenario-1 consisted of the isolated configuration of a standard model-scale building and scenario-2 was composed of the standard building with a selected neighborhood. Both scenarios were analyzed for wind incidence angles of zero, 45, and 90 degrees. The numerical results were obtained in terms of pressure and force coefficients which allowed the determination of neighborhood factors. The simulations showed that the neighborhood influences the mean wind loading on the faces of the standard building, sometimes amplifying the load (in the case of incident winds at zero and 90 degrees), sometimes attenuating the acting forces (in the case of incident winds at zero and 45 degrees). The numerical results were compared with experimental data and showed similar orders of magnitude suggesting that the simulations correctly describe the physical behavior of the wind action.Nesse trabalho foram realizadas simulações numéricas do escoamento atmosférico com o principal objetivo de avaliar os efeitos de vizinhança em edifícios altos. Os modelos computacionais foram desenvolvidos mediante o uso do método RANS, sendo o fechamento da turbulência tratado por um modelo do tipo k-ε. Foram simulados dois cenários de posicionamento de edifícios: o cenário-1 consistiu na configuração isolada de um edifício padrão e o cenário-2 foi composto pelo edifício padrão com uma vizinhança selecionada. Ambos os cenários foram analisados para ângulos de incidência de ventos de zero, 45 e 90 graus. Os resultados numéricos foram obtidos em termos de coeficientes de pressão e força, os quais permitiram a determinação dos fatores de vizinhança. As simulações mostraram que a vizinhança influencia de forma significativa a atuação do vento sobre as fachadas do edifício padrão, ora amplificando o carregamento, ora atenuando as forças atuantes. Os resultados numéricos foram comparados com experimentos e mostraram ordens de grandeza semelhantes, sugerindo que as simulações descrevem de forma adequada o comportamento físico da ação dos ventos.