Large-eddy simulation for flow and dispersion in urban streets

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 fluctuation

Offshore Turbine Arrays: Numerical Modeling and Experimental Validation

The interaction between wind turbines in a large wind farm needs to be better understood to reduce array losses and improve energy production. A numerical test bed for an array of offshore wind turbines was developed in the open-source computational fluid dynamics (CFD) framework OpenFOAM. It provides a computational tool which can be used in combination with physical model turbine array studies in the Flow Physics Facility (FPF) at UNH as well as other test facilities. Turbines were modeled as actuator disks with turbulence sources to reduce computational cost. Both k-ϵ and k-ω SST turbulence models were utilized to capture the flow in the near-wall, wake, and free stream regions. Experimental studies were performed in the FPF to validate the numerical results and to provide realistic initial and boundary conditions, for example turbulent boundary layer inlet velocity profiles. Mesh refinement and boundary condition studies were performed. Numerical simulations were executed on a custom-built server, designed to be the head node of a future CFD cluster. The entire project was built on open-source software to facilitate replication and expansion. The numerical model provides building blocks for simulations of large wind turbine arrays, computational resources permitting. The numerical model currently replicates a three by one array of wind turbines in the FPF, and provides detailed insight into the array fluid dynamics

LES validation of urban flow, part II: eddy statistics and flow structures

Time-dependent three-dimensional numerical simulations such as large-eddy simulation (LES) play an important role in fundamental research and practical applications in meteorology and wind engineering. Whether these simulations provide a sufficiently accurate picture of the time-dependent structure of the flow, however, is often not determined in enough detail. We propose an application-specific validation procedure for LES that focuses on the time dependent nature of mechanically induced shear-layer turbulence to derive information about strengths and limitations of the model. The validation procedure is tested for LES of turbulent flow in a complex city, for which reference data from wind-tunnel experiments are available. An initial comparison of mean flow statistics and frequency distributions was presented in part I. Part II focuses on comparing eddy statistics and flow structures. Analyses of integral time scales and auto-spectral energy densities show that the tested LES reproduces the temporal characteristics of energy-dominant and flux-carrying eddies accurately. Quadrant analysis of the vertical turbulent momentum flux reveals strong similarities between instantaneous ejection-sweep patterns in the LES and the laboratory flow, also showing comparable occurrence statistics of rare but strong flux events. A further comparison of wavelet-coefficient frequency distributions and associated high-order statistics reveals a strong agreement of location-dependent intermittency patterns induced by resolved eddies in the energy-production range. The validation concept enables wide-ranging conclusions to be drawn about the skill of turbulence-resolving simulations than the traditional approach of comparing only mean flow and turbulence statistics. Based on the accuracy levels determined, it can be stated that the tested LES is sufficiently accurate for its purpose of generating realistic urban wind fields that can be used to drive simpler dispersion models

Application and assessment of a GPU-based LES method for predicting dynamic wind loads on buildings

This study presents the assessment of a fast Large Eddy Simulation method for estimating dynamic wind loads on buildings using a GPU-based CFD software, which produces statistically converged results on a nine-million-cell mesh in approximately 6 hours. The surface pressure distribution of a cuboid building model was validated with experimental data obtained in an atmospheric boundary layer wind tunnel and compared with field measurements. Although due to the applied equidistant Cartesian grid the large gradients near the edges are not fully resolved, good overall agreement was found for the mean and fluctuating pressure distributions (correlation coefficient: 0.90/0.73, FAC2: 0.92/0.98, FB: −/0.06, MG: −/0.95, NMSE: −/0.10, VG: −/1.08). It was shown that the numerical model is able to produce matching turbulent spectra in an intermediate frequency range within the inertial subrange, limited by the domain size and the spatial resolution. Mesh refinement for capturing large gradients as well as for expanding the frequency limits can be achieved by using a GPU with higher VRAM capacity for the simulation. The continuing advancement of the presented model is a promising development for estimating dynamic wind loads on buildings and identifying design problems fast enough for the engineering practice, without high-performance computing

LES validation of urban flow, part I: flow statistics and frequency distributions

Essential prerequisites for a thorough model evaluation are the availability of problem-specific, quality-controlled reference data and the use of model-specific comparison methods. The work presented here is motivated by the striking lack of proportion between the increasing use of large-eddy simulation (LES) as a standard technique in micro-meteorology and wind engineering and the level of scrutiny that is commonly applied to assess the quality of results obtained. We propose and apply an in-depth, multi-level validation concept that is specifically targeted at the time-dependency of mechanically induced shear-layer turbulence. Near-surface isothermal turbulent flow in a densely built-up city serves as the test scenario for the approach. High-resolution LES data are evaluated based on a comprehensive database of boundary-layer wind-tunnel measurements. From an exploratory data analysis of mean flow and turbulence statistics, a high level of agreement between simulation and experiment is apparent. Inspecting frequency distributions of the underlying instantaneous data proves to be necessary for a more rigorous assessment of the overall prediction quality. From velocity histograms local accuracy limitations due to a comparatively coarse building representation as well as particular strengths of the model to capture complex urban flow features with sufficient accuracy are readily determined. However, the analysis shows that further crucial information about the physical validity of the LES needs to be obtained through the comparison of eddy statistics, which is focused on in part II. Compared with methods that rely on single figures of merit, the multi-level validation strategy presented here supports conclusions about the simulation quality and the model's fitness for its intended range of application through a deeper understanding of the unsteady structure of the flow

Numerical Simulation of Snow Deposition Around living Snow Fences

In this study, computational fluid dynamics (CFD) was used to investigate the air flow around porous snow fences to gain insight into snow transport and deposition in the vicinity of fences. Numerical simulations were performed to validate the CFD approach using experimental data from a wind tunnel study. Subsequent simulations were used to test the use of a porosity model to represent fence geometry and determine the effect of fence spacing for fences comprised of multiple rows. The results demonstrate that CFD simulations can reproduce the aerodynamics around porous fences. Additionally, the flow field generated with a porosity model is in close agreement with that from a model with explicit representation of fence porosity. Simulations of fences comprised of two rows spaced at various distances demonstrate that when the row spacing is small the fence behaves as a single row

Numerical modelling of the aerodynamic interference between helicopter and ground obstacles

Helicopters are frequently operating in confined areas where the complex flow fields that develop in windy conditions may result in dangerous situations. Tools to analyse the interaction between rotorcraft wakes and ground obstacles are therefore essential. This work, carried out within the activity of the GARTEUR Action Group 22 on “Forces on Obstacles in Rotor Wake”, attempts to assess numerical models for this problem. In particular, a helicopter operating in hover above a building as well as in its wake, one main rotor diameter above the ground, has been analysed. Recent tests conducted at Politecnico di Milano provide a basis for comparison with unsteady simulations performed, with and without wind. The helicopter rotor has been modelled using steady and unsteady actuator disk methods, as well as with fully resolved blade simulations. The results identify the most efficient aerodynamic model that captures the wakes interaction, so that real-time coupled simulations can be made possible. Previous studies have already proved that the wake superposition technique cannot guarantee accurate results if the helicopter is close to the obstacle. The validity of that conclusion has been further investigated in this work to determine the minimum distance between helicopter and building at which minimal wake interference occurs