Browsing by Subject "Turbulence -- Mathematical models"
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Item Boundary layer characteristics just above sub-urban roughness(Texas Tech University, 2004-05) Dutta, Satya RanianWith regard to the reliable design of distribution lines (less than 60 ft) and other structures that stand just above the suburban roughness, it is very important to accurately study the complex turbulence structures in the region of suburban roughness sublayer. In this study, it has been found that the profiles of mean wind velocity, turbulence intensity and integral length scale parameters (longitudinal, lateral, and vertical) are not exactly the same as those used in ASCE 7, which considers the ideal case of turbulence above open terrain or well above the roughness. To accomplish this, wind tunnel experiments were carried out in Texas Tech University's wind tunnel facility and then the data was analyzed to determine the variation of turbulence and gust size just above the roughness. Very close simulations of open and suburban terrain were accomplished by respectively laying carpet and carpet plus 12 house models (in 4 rows) in the upstream section of the wind tunnel. To determine the integral length scales, cross correlation and auto correlation methods were employed with the help of Math CAD software. The profiles of different parameters were determined with respect to height and surface roughness. Also, to determine the effect of an abrupt change of terrain, a single row of 3 house models was placed in front of the line of measurement and the corresponding test data analyzed. It has been primarily concluded that the suburban roughness sublayer turbulence affects the gust effect factors which in turn affect the design wind load on distribution lines and the other structures that stand just above the suburban roughness.Item Comparison of wind tunnel pressure measurements on the Texas Tech building(Texas Tech University, 2004-05) Chowdary, Niaz AA recent trend has been observed concerning the mean extreme local pressure coefficients measured in wind tunnels and in full-scale experiments for flow around low buildings. It was observed that the magnitudes of the mean extreme pressure coefficients measured in wind tunnel studies have been increasing as the data acquisition methods and boundary simulation techniques have improved. Wind tunnel'test data on generic low building is being generated at the Boundary Layer Wind Tunnel Laboratory at the Texas Tech University to contribute to a large database that will be made available to designers. In the most recent of these experiments, two scaled models 1:100 and 1:50 of the Texas Tech University test building were investigated at two simulations in open terrain condition. Pressure data collected at several taps is analyzed for a large number of records roughly covering all angles of attack. The primary purpose of this thesis is to develop two simulations of the 10th and 90' percentile velocity and turbulence characteristics observed at WERFL and to compare point pressure coefficients between the three models, two simulations, and two other wind tunnel data sets (UWO & CSU). In addition, the study was extended to include comparisons of results obtained using the same model but tested in different wind tunnels (UWO & CSU). Tests were conducted for different wind simulations, and the data were used to develop a better understanding of the effect of flow simulation and model scale on wind load estimates for low-rise buildings. These were conducted in a boundary layer wind tunnel using simulations developed as part of a research program aimed at improving boundary layer simulations. The purpose of the improved simulations was to reduce the differences between wind tunnel data obtained at three different universities, two different scaled models and two different simulations, obtained from physical model studies in boundary layer wind tunnels. The comparison of the pressures measured on the 1:100 scale model at the Texas Tech University to those measured at two other institutions revealed that there are some differences. These differences appear to be due primarily to differences in the reference static and dynamic pressures used at the various institutions as indicated by the fact that the shape of the distribution of the pressures over the exterior of the building is maintained between the results from the various studies. Additional differences can be associated with differences in the local turbulence intensities present in the various simulations. Results indicate that without careful adjustments, the scale effects, and wind simulation variations can result in widely varying pressure coefficient predictions for the same structure. These discrepancies were attributed to the approach flow characteristics, Reynolds number effects, frequency response of the pressure measurement system, and sampling frequency of acquired data. The model and full-scale flow parameters and pressure coefficients are presented for low-turbulence nominal flow. The comparison of the model exhibits a better agreement for most of the pressure taps, including the critical corner region, than other reported wind tunnel results.Item Reynolds stress model for recirculating flows(Texas Tech University, 1993-05) Chok, Chee VuiThe progress of the Reynolds Stress Model (RSM) has been slow due to the numerical difficulty in ensuring coupling of Reynolds stresses with mean velocities and the lack of improvement in modeling. In the present study, a special interpolation technique is employed to compute the Reynolds stress gradients in a non-staggered grid arrangement to avoid any unrealistic zig-zag solution that occurs when linear interpolation is used. To improve the RSM, the dissipation rate equation is modified to give better prediction on recirculating flows. The Navier-Stokes equations, Reynolds stress equations and dissipation rate equation are solved in this study. The governing equations are discretized using a non-staggered grid finite-volume scheme. The discretized equations are then solved by an implicit, time-marching, pressure-correction based algorithm. Two test cases which are representative of most flows are selected to verify the present method. Both test cases only need orthogonal grid formulation, eliminating any anomaly such as numerical instability caused by a non-orthogonal grid formulation. As a first test case, the numerical predictions obtained from the present method are compared with the direct numerical simulation results for channel flow. A second test case, involving more complex physics, is selected to test both the turbulence model as well as the numerical algorithm. In the present study, the backward-facing step which exhibits an abrupt change in flow characteristics over the step expansion, is used to demonstrate the capability of the model to predict recirculating flows. The results of the present calculations including mean velocities, Reynolds stresses, dissipation rate, friction factor, pressure coefficient and reattachment length are compared with the standard k — £ model results and experimental data. The present RSM results agree with experimental data, and the recirculating bubble and reattachment length predicted by the present RSM are better than the standard k — e model predictions.Item Wind tunnel blockage corrections: a computational study(Texas Tech University, 2004-08) Sahini, DeepakWind tunnel blockage testing has been a wide spread traditional practice in the automobile industry for many years; but the tests conducted have been associated with the so called blockage effects, which arise due to the constrained flow nature inside the wind tunnel test section and over the blockages. These blockage effects need to be corrected in order to comprehend the test results similar to those of the actual road conditions. CFD has emerged as a tool to determine the blockage effects and provide corrections using computational techniques. In this present study, two such CFD packages, namely PHOENICS and AIRFL03D, are used for determining the wind tunnel blockage effects. The problems taken into consideration are both two dimensional and three dimensional flow cases. The test section domain height is varied so as to produce different blockage ratios, keeping the blockage dimensions constant. A two dimensional free stream case with blockage l/h ratio variation is tested and compared with experimental results. In the other cases, the two packages are compared with each other for pressure and velocity distributions and drag coefficients. A grid independent study was performed for one case. Finally, blockage correction equations are obtained for all the test cases.