The two purposes of this study were to determine at what turbulent scales in a high Reynold's number shear flow the transition to isotropy occurs and at what scales Taylor's 'frozen field' hypothesis is applicable. The flow studied was the wind at a height of z = 2 m. above a flat land surface. Four hot wire anemometers were mounted in a three dimensional array to collect data on the downwind turbulent velocity fluctuations. Cross spectra were computed from the observed data between three pairs of hot wires having the same spacing in different directions; these were varied between 1.8 m. and 2 cm. Knowing the observed spectrum of downwind velocity fluctuations and assuming the turbulence is isotropic, incompressible, and obeys Taylor's hypothesis, theoretical cross spectra were computed. The results of the comparison between the observed and theoretical cross spectra for different spacings revealed that in the flow studied the behaviour of the turbulence is consistent with the assumptions of both isotropy and Taylor's hypothesis for k₁z > 20, but for wave numbers less than this range either or both of the assumptions are not valid. However, between k₁z = 4 and k₁z = 20 the turbulence appears to be at least axisymmetric about the downstream direction and for k₁z > 3 that part of Taylor's hypothesis relating observed frequency at a stationary sensor to the downstream wave number component appears to be justified. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An experimental study of a mixing layer between two uniform parallel streams is undertaken for three velocity ratios: 0.81, 0.75 and 0.65. It is found that the mixing layer grows linearly with streamwise distance in each case, as anticipated. Except in the very intermittent region, the mean velocity profiles agree with Görtler's theoretical prediction based on the assumption that the eddy viscosity is constant across the flow. The measurement of turbulent intensities and shear stress provide evidence that the flow becomes self preserving. The non-dimensional longitudinal intensity is symmetrical about the average velocity point, and is independent of the velocity ratio; although other non-dimensional Reynolds stress distributions are roughly independent of velocity ratio, they are not symmetrical. The shear stress calculated from the two dimensional mean equation of motion agrees well with measured values. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The purpose of this work was to investigate the characteristics of the small scales of the turbulent velocity and temperature fields in the atmospheric boundary layer. Of particular interest were the distributions of the spatial derivatives, the behaviour of the structure functions and the nature of the interactions leading to the inertial transfer of energy between the larger and smaller scales of the turbulent flow. The observed statistical distributions of the differentiated downstream and X-wire signals did not accurately conform to the theoretically predicted logarithmic normal model although there was good general agreement. The distribution of the sum of the squares of the differentiated X-wire signals was described extremely well by the log normal model. The temperature derivatives followed the log normal distribution somewhat better than did the velocity derivatives, but a poor signal to noise ratio prevented conclusive results. The observed structure functions conformed to present theory for the velocity and temperature data. The inertial transfer of energy in the downstream component was found to behave as would be intuitively expected. Specifically the net rate of arrival of energy was zero for wavenumbers where viscosity was unimportant and positive where viscosity was important. Interactions of scale sizes differing by more than a factor of ten in size did not contribute significantly to the energy transfers. The flow of downstream energy was observed to be from larger to smaller scale sizes for horizontal scales as large as twenty meters. The mean inertial transfer of energy associated with interactions between downstream components only, was found to be approximately equal to the mean rate of energy loss due to the corresponding dissipation term. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

This dissertation presents experimental measurements and analysis of the surface pressure fluctuations beneath several turbulent boundary layers of practical interest. Pressure fluctuations in turbulent boundary layers are a source of noise and vibration that can accelerate structural fatigue. Pressure fluctuations and their correlation with velocity fluctuations is an important diffusive mechanism of turbulence transport. The approach was to study the statistics of both the surface pressure and the velocity field through new measurements of the fluctuating surface pressure and existing measurements of the velocity field and the covariance of the surface pressure and fluctuating velocity components. Measurements were made in three types of flows. The first type of flow was a zero pressure gradient, two-dimensional, turbulent boundary layer (Re(theta) = 7300 and Re(theta) = 23400). The two-dimensional flows serve as a baseline for comparison to the other three-dimensional flows and validate the experimental techniques used in the present study through comparison with existing measurements. The second type of flow was a three-dimensional, pressure-driven, turbulent boundary layer that forms away from a wing-body junction. Two of this type of boundary layer were studied - Re(theta) = 5940 and Re(theta) = 23200. The third type of flow was the separating flow about the leeside of a 6:1 prolate spheroid at angle of attack. Measurements were made at two angles of attack, 10° and 20°, and two axial locations, x/L = 0.600 and x/L = 0.772, in this type of flow. Spectral scaling is discussed and various scaling combinations of the spectral power density of surface pressure fluctuations beneath two-dimensional boundary layers that cover a wide range of Reynolds number (1400 < Re(theta) < 23400) are presented. The spectral power density of surface pressure fluctuations beneath the separating flow on the leeside of a 6:1 prolate spheroid at 10° angle of attack collapse when normalized using viscous scales. However, the spectral power density of surface pressure fluctuations beneath highly three-dimensional flow contain nearly constant spectral levels within a middle to high frequency range. The nearly constant spectral levels are due to a lack of overlapping frequency structure between the large-scale motions and the viscous-dominated motions since each of these types of motion may have different flow histories due to the three-dimensional flow structure. This effect amplifies the importance of the middle frequency range to p' as compared to two-dimensional flows. In terms of instrumentation, accurate p' measurements in a three-dimensional flow require accurate high frequency (f > 20 kHz) p measurements. The lack of similarity in the shape of the spectral power density preclude a direct extension of "universal" generalizations that are true for surface pressure fluctuations beneath two-dimensional boundary layers. The resulting RMS surface pressure fluctuation distributions reflect the importance of the high frequency wall region contributions. Scaling parameters for the p spectra beneath three-dimensional flows must incorporate local flow structure in order to be successful. Analysis based on the Poisson equation shows that variation of the high frequency spectral levels are related to the variation in near-wall mean velocity gradients and v² structure. In the 6:1 prolate spheroid flow, near regions of crossflow separation there is a local minimum in RMS surface pressure fluctuations, whereas around reattachments and under the large shed vortices there is a local maximum in RMS surface pressure fluctuations. Measurements of the correlation coefficient between surface pressure and velocity fluctuations show that there can be sources of p away from the wall in three-dimensional flows. Sources of p away from the wall are significant in terms of fluid-structure interaction since they contribute low frequency fluctuations. Structures typically have low resonant frequencies. Sources of p away from the wall are also significant in terms of radiated sound since they are likely to interact with the free-stream and be radiated away as sound. / Ph. D.

Thesis (Ph.D.)--George Mason University, 2008. / Vita: p. 178. Thesis director: Zafer Boybeyi. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computational Sciences and Informatics. Title from PDF t.p. (viewed June 30, 2008). Includes bibliographical references (p. 168-177). Also issued in print.

No description available.

Component failures due to excessive flow-induced vibration in reactor cores and steam generators affect the performance and reliability of nuclear power stations around the world. Numerous investigations on the axial flow within tube bundles have identified the presence of potentially damaging low frequency, large-scale co- herent patterns. This Thesis reports a first detailed investigation into the effect of a streamwise separation between each pair of consecutive rod bundles on the flow structure. Turbulent flow in an annular channel with a sudden expansion between the inner cylinders is studied numerically. The calculations are performed using wall-resolved Large Eddy Simulation (LES) for which the generation of turbulent inlet conditions is achieved through the development and evaluation of the precur- sor and recycling inlet methods in the framework of Code Saturne. Results are presented for a fully concentric configuration in which three streamwise separation lengths based on the inner rod diameter, Drod, are considered, namely 2 × Drod (‘2Dc' case), 4×Drod (‘4Dc' case) and 6×Drod (‘6Dc' case). It is shown that the flow structure in the inter-rod spacing is highly dependent on the length of the streamwise gap. The characteristic flow features discerned in the narrow ‘2Dc' case suggest a striking similarity with flow over d-type rib-roughness, whereas the larger gap lengths in the ‘4Dc' and ‘6Dc' configurations lead to flow patterns comparable to those in k-type roughness. The effect of downstream eccentricity is examined for configurations with a streamwise gap length of 2 × Drod (‘2De' case) and 6 × Drod (‘6De' case). The results reveal that the misalignment of the downstream rod with respect to the upstream cylinder has a significant impact on the flow behaviour in the narrow ‘2De' case, whilst the ‘6De' case presents reasonable agreement with its concentric counterpart.

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Approved for public release; distribution is unlimited / The horizontal separations between convective thermal plumes, and features within a thermal plume, were determined through the use of an acoustic sounder, an anemometer and extensive data analysis. The mean, standard deviation, median and mode were calculated for the computed correlation lengths of the acoustic sounder data sampled in time intervals of 2, 5 and 10 minutes. The data sampled at 2 and 5-minute intervals emphasized features within an individual thermal plume. The mean correlation distances found for 2 and 5-minute intervals were 81 meters Å 70 meters and 89 meters Å 72 meters, respectively. Their medians were 61 meters and 69 meters; and their modes were 41 meters and 50 meters, respectively. The 10-minute time interval statistics used a low pass filter to emphasize larger scale features. The mean correlation length was 494 meters Å 373 meters, the median was 391 meters and the mode was 316 meters. These distances represent the distance between the center of a plume and the center of a quiet region adjacent to that plume. / Captain, United States Army