Improvements in numerical airflow modeling around multiple buildings

The main objective of the thesis was to provide faster, yet reliable and simple modeling tool for simulation of outdoor airflow around multiple buildings. The intent was to improve the existing zero-equation turbulence models mainly developed and used for modeling of airflows in indoor environments. The zero-equation turbulence model adopts the local mean velocity and distance to the nearest wall surface as the characteristic velocity and length scale to estimate the turbulent viscosity. However, the coefficients in the model equation require assessment for each particular application. For that reason, the wind tunnel modeling of complex airflow around four scaled student dorm buildings at the Penn State University campus has been carried out in the closed loop, low speed wind tunnel at the Department of Aerospace Engineering at the Pennsylvania State University using triple hot wire anemometry to collect the velocity time series. The obtained results on 1:250 scale modeled buildings provided the base for calibration of the coefficients in the zero-equation turbulent model. The derived expression for calibration coefficient implements both the bulk and turbulent Reynolds number as relevant characteristics of approaching wind flow. The calibrated zero-equation turbulence model has been incorporated in commercial CFD software PHOENICS and tested against the wind tunnel results. The simulation results show satisfactory agreement with measured data for longitudinal velocity components. The prediction of vertical velocity component was less accurate and inferior performance of the model was observed for lateral velocity component prediction. Limitations in measurement equipment and negligence of all mean strain rate tensor components including lateral and vertical velocity gradients are two most likely sources of discrepancy between the numerical and wind tunnel experimental results. Further estimate of exponents in the derived expression for calibration coefficient is recommended for different buildings layouts.;Experimental study with regular array of cubes placed in the boundary layer wind tunnel provided the data for additional validation of the developed model. The experimental results comprised of streamwise and vertical velocity profiles along the centerline of the modeled building cube array. The developed model for calibration coefficients in the expression for turbulent eddy viscosity in the zero-equation turbulence model demonstrated good agreement in streamwise velocity prediction with experimentally acquired results. Implemented turbulence model showed comparative competitiveness with more advanced one-point turbulence closure models, such as two equation "k-epsilon" and modified "Kato-Launder" version of the "k-epsilon " model, although, the full adjustment of the calibration coefficients in the model was not feasible due to lack of input information that was not measured in the wind tunnel experiment.;Part of the thesis effort focused on possible improvements in the description of inlet boundary conditions of incoming wind. For that purpose, an extensive literature review on the atmospheric surface layer flows, their characterization and parameterization has been performed. In addition, the systematic overview of various strategies for synthetic turbulent wind speeds generation has been discussed in detail. As an outcome, the most suitable techniques for implementation in CFD models are described with sufficient level of detail for easy coding. The described techniques should provide substantial foundation for the future implementation of unsteady Reynolds averaged Navier-Stokes equations (URANS) turbulence modeling approach for simulation of outdoor airflow in complex urban settings.;The developed methodology for calibration coefficients in the zero-equation turbulence models represents the major contribution of this study. Further improvements of the model are possible with provision of experimental data with high spatial resolution using advanced non-invasive measurement techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA). More comprehensive set of experimental data would enable further refinement of the proposed model in terms of adjustment of the exponents in the proposed equation for calibration coefficients. Instead of using single constant value, the exponents should be defined as a function of plan area density, frontal area density and other morphological parameters already established to describe various buildings layouts and boundary layer flows in urban environments. (Abstract shortened by UMI.)...