Complex Urban Geometry Research Articles

Simulation of Helicopter Community Noise in Complex Urban Geometry

Simulation of Helicopter Community Noise in Complex Urban Geometry

Identification of strength and location of stationary point source of atmospheric pollutant in urban conditions using computational fluid dynamics model

Abstract In this paper a method is presented which allows for estimation of the location and rate of an unknown point stationary source of passive atmospheric pollutant in a complex urban geometry. The variational formulation is used in which the cost function characterizing difference of the calculated and measured concentrations is minimized with respect to source coordinates and rate. The minimization problem is solved by a direct algorithm using a source-receptor function which is calculated by solving adjoint equation. The algorithm has been implemented in ADREA-HF computational fluid dynamics code and has been applied in complex urban geometry. Validation of the algorithm has been performed by simulation of a wind tunnel experiment on atmospheric dispersion among an array of rectangular obstacles. Good results of source parameters estimation have been achieved. The measuring sensors to be used in source estimation had been randomly selected out of 244 available sensors by a specially designed random sampling algorithm which allowed for estimation of the probability of ‘good’ source term estimation with the given number of sensors. In case of ‘perfect model’ (when synthetic measurements were used for source estimation) good results were achieved with 90% or higher probability for arbitrary measurement networks consisting of 15 or more sensors even though prior estimation of source location was not used in this case. Several tests had been performed with the use of real measurements which differed by prior estimation of source location. In all cases 90% or higher probability of obtaining good solution was reached only for measurement networks consisting of 150 or more sensors. Hence further improvement of the source estimation algorithm can be achieved first of all by improving the performance of forward model.

A CFD-based wind solver for an urban fast response transport and dispersion model

In many cities, ambient air quality is deteriorating leading to concerns about the health of city inhabitants. In urban areas with narrow streets surrounded by clusters of tall buildings, called street canyons, air pollution from traffic emissions and other sources may accumulate resulting in high pollutant concentrations. For various situations, including the evacuation of populated areas in the event of an accidental or deliberate release of chemical, biological and radiological agents, it is important that models should be developed that produce urban flow fields quickly. Various computational techniques have been used to calculate these flow fields, but these techniques are often computationally intensive. Most fast response models currently in use are at a disadvantage in these cases as they are unable to correlate highly heterogeneous urban structures with the diagnostic parameterizations on which they are based. In this paper, a novel variant of the popular projection method for solving the Navier–Stokes equations has been developed and applied to produce fast and reasonably accurate parallel computational fluid dynamics (CFD) solutions for flow in complex urban areas. This model, called QUIC-CFD represents an intermediate balance between fast (on the order of minutes for a several block problem) and reasonably accurate solutions. This paper details the solution procedure and validates this model for various simple and complex urban geometries.

A field study of factors influencing the concentrations of a traffic-related pollutant in the vicinity of a complex urban junction

The paper describes a field study focused on the dispersion of a traffic-related pollutant within an area close to a busy intersection between two street canyons in Central London. Simultaneous measurements of airflow, traffic flow and carbon monoxide concentrations ([CO]) are used to explore the causes of spatial variability in [CO] over a full range of background wind directions. Depending on the roof-top wind direction, evidence of both flow channelling and recirculation regimes were identified from data collected within the main canyon and the intersection. However, at the intersection, the merging of channelled flows from the canyons increased the flow complexity and turbulence intensity. These features, coupled with the close proximity of nearby queuing traffic in several directions, led to the highest overall time-average measured [CO] occurring at the intersection. Within the main street canyon, the data supported the presence of a helical flow regime for oblique roof-top flows, leading to increased [CO] on the canyon leeward side. Predominant wind directions led to some locations having significantly higher diurnal average [CO] due to being mostly on the canyon leeward side during the study period. For all locations, small changes in the background wind direction could cause large changes in the in-street mean wind angle and local turbulence intensity, implying that dispersion mechanisms would be highly sensitive to small changes in above roof flows. During peak traffic flow periods, concentrations within parallel side streets were approximately four times lower than within the main canyon and intersection which has implications for controlling personal exposure. Overall, the results illustrate that pollutant concentrations can be highly spatially variable over even short distances within complex urban geometries, and that synoptic wind patterns, traffic queue location and building topologies all play a role in determining where pollutant hot spots occur.

Source Inversion for Contaminant Plume Dispersion in Urban Environments Using Building-Resolving Simulations

AbstractThe ability to determine the source of a contaminant plume in urban environments is crucial for emergency-response applications. Locating the source and determining its strength based on downwind concentration measurements, however, are complicated by the presence of buildings that can divert flow in unexpected directions. High-resolution flow simulations are now possible for predicting plume evolution in complex urban geometries, where contaminant dispersion is affected by the flow around individual buildings. Using Bayesian inference via stochastic sampling algorithms with a high-resolution computational fluid dynamics model, an atmospheric release event can be reconstructed to determine the plume source and release rate based on point measurements of concentration. Event-reconstruction algorithms are applied first for flow around a prototype isolated building (a cube) and then using observations and flow conditions from Oklahoma City, Oklahoma, during the Joint Urban 2003 field campaign. Stochastic sampling methods (Markov chain Monte Carlo) are used to extract likely source parameters, taking into consideration measurement and forward model errors. In all cases the steady-state flow field generated by a 3D Navier–Stokes finite-element code (FEM3MP) is used to drive thousands of forward-dispersion simulations. To enhance computational performance in the inversion procedure, a reusable database of dispersion simulation results is created. It is possible to successfully invert the dispersion problems to determine the source location and release rate to within narrow confidence intervals even with such complex geometries. The stochastic methodology here is general and can be used for time-varying release rates and reactive flow conditions. The results of inversion indicate the probability of a source being found at a particular location with a particular release rate, thus inherently reflecting uncertainty in observed data or the lack of enough data in the shape and size of the probability distribution. A composite plume showing concentrations at the desired confidence level can also be constructed using the realizations from the reconstructed probability distribution. This can be used by emergency responders as a tool to determine the likelihood of concentration at a particular location being above a threshold value.

Interaction of Nocturnal Low-Level Jets with Urban Geometries as Seen in Joint Urban 2003 Data

Abstract Because accurate modeling of atmospheric flows in urban environments requires sophisticated representation of complex urban geometries, much work has been devoted to treatment of the urban surface. However, the importance of the larger-scale flow impinging upon the urban complex to the flow, transport, and dispersion within it and downwind has received less attention. Building-resolving computational fluid dynamics (CFD) models are commonly employed to investigate interactions between the flow and three-dimensional structures that make up the urban environment; however, such models are typically forced with simplified boundary conditions that fail to include important regional-scale phenomena that can strongly influence the flow within the urban complex and downwind. This paper investigates the interaction of an important and frequently occurring regional-scale phenomenon, the nocturnal low-level jet (LLJ), with urban-scale turbulence and dispersion in Oklahoma City, Oklahoma, using data from the Joint Urban 2003 (JU2003) field experiment. Two simulations of nocturnal tracer release experiments from JU2003 using Lawrence Livermore National Laboratory’s Finite-Element Model in 3 Dimensions and Massively Parallelized (FEM3MP) CFD model yield differing levels of agreement with the observations in wind speed, turbulence kinetic energy (TKE), and concentration profiles in the urban wake, approximately 750 m downwind of the central business district. Profiles of several observed turbulence parameters at this location indicate characteristics of both bottom-up and top-down boundary layers during each of the experiments. These data are consistent with turbulence production due to at least two sources, the complex flow structures of the urban area and the region of strong vertical wind shear occurring beneath the LLJs present each night. Strong LLJs occurred each night, but their structures varied considerably, resulting in significant differences in the magnitudes of the turbulence parameters observed during the two experiments. Because FEM3MP was forced only with an upwind velocity profile that did not adequately represent the LLJ, the downward propagation of TKE observed during the experiments was absent from the simulations. As such, the differing levels of agreement between the simulations and observations during the two experiments can, in part, be explained by their exclusion of this important larger-scale influence. The ability of the Weather Research and Forecast Model (WRF) to simulate accurate velocity fields during each night was demonstrated, and the use of regional-scale simulation data was identified as a promising approach for representing the effects of important regional-scale phenomena such as the LLJ on urban-scale simulations.

Pollution dispersion at complex street configurations: covered roadways

Dispersion of vehicular pollutants emitted from covered roadways is particularly important in describing and controlling local air quality conditions at many complex urban roadway geometries. We have conducted a joint measurement and numerical modelling study to describe, understand and simulate the kinematics of the atmospheric transport and dispersion processes, and to assess the representativeness of various numerical model simulations. The measurement program was conducted in an environmental boundary-layer wind tunnel and involved the controlled line-source release of a tracer gas (ethane, C2 H6) and subsequent in situ sampling of concentration fields. The covered section was open to one side; two aspect ratios (width to height) for the covered section were tested 3.6 and 7.2. With the open side of the covered section to leeward of the ambient flow, pedestrian-level concentrations were observed to be fairly uniform within the covered section and were an order of magnitude greater than those downwind. An abrupt transition zone exists at the trailing edge of the covered section. A Navier-Stokes model (Fluent) realistically simulated the major transport and dispersion characteristics. A simple Gaussian line source equation was also evaluated because its numerical analogues (e.g. CAL3QHC) are often used for regulatory applications involving similarly complex geometries. As expected, this approach does not yield representative results. However, using the Gaussian model with the measured concentrations, we are able to characterise the downwind profile of the vertical dispersion (sigma) function and quantify the enhanced dispersion in the transition zone.