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The main motive for studying turbulent flow in an urban environment is to understand the processes governing momentum, heat and mass exchange between the atmosphere and a very inhomogeneous and aerodynamically rough surface. This exchange regulates the microclimate wherein about 40% of the current world population lives. An understanding of its mechanisms is essential for a variety of reasons and applications. The structure of the atmosphere close to this irregular surface is not homogeneous and there is reason for concern that traditional micrometeorological theories are inadequate to describe the turbulent transfer in this environment. The main objective of the present study is to investigate the turbulent transfer mechanism and the applicability of the Monin Obukhov similarity framework in an unstable suburban atmosphere. In addition the first full set of energy balance data including longer term directly-measured sensible and latent heat fluxes is presented. The results suggest that the (co)spectra in respect to shape and location of the peaks are relatively insensitive to surface features. They generally agree well with homogeneous surface layer data with the exceptions of u, T, uw and possibly q which all exhibit slight anomalies which may be attributed to particular surface features. The non-dimensional dissipation functions and most of the integral statistics results follow the trends predicted by similarity theory (i.e. they are a function of stability), however, the magnitudes are often smaller. Analysis of the correlation coefficients shows that under near neutral and slightly unstable conditions the transfers of momentum and heat are most efficient (and enhanced compared to the homogeneous surface layer) whereas the transfer efficiency of moisture is generally least efficient. This results in a dissimilar behaviour of heat and moisture. It is shown that the humidity statistics not only depend on surface boundary conditions but are also influenced by the entire PBL. Observational support in this respect is obtained from a time series analysis of humidity signals which shows the sporadic occurrence of strong, dry downdrafts (under mainly cloudy conditions) which result in positive contributions to the moisture flux. There is evidence that the present observation levels are sometimes within the roughness sub-layer. At around noon and in the early afternoon the Bowen ratio measured using the gradient approach was often larger than the Bowen ratio obtained from directly measured fluxes. This affects the turbulent fluxes derived from the Bowen ratio-energy balance approach. It is suggested that beside the inequality of the transfer efficiencies sampling problems affect the gradient measurements. The average diurnal energy balance is in general agreement with previous summertime observations from the same site. The results indicate that the storage heat flux, obtained as the energy balance residual using directly measured turbulent fluxes, peaks slightly earlier than predicted by the objective hysteresis model. / Arts, Faculty of / Geography, Department of / Graduate

Across the air-sea interface there is a transfer of momentum, heat and moisture. Knowledge of these is essential to the understanding of oceanic and atmospheric circulations. This study is an investigation of the vertical turbulent transfers of momentum, heat and moisture in the boundary layer of the atmosphere using an instrumented light aircraft. The data were collected at several altitudes between 18 m and 500 m in the Atlantic trade wind zone east of the island of Barbados. Since the tropical ocean is the primary source of heat input to the atmospheric heat engine, good estimates, in this region, of the transfers of heat and moisture and their vertical variations are essential to any global numerical atmospheric prediction scheme. The fluctuations of the velocity components, temperature and humidity and the transfers of momentum, heat and moisture were investigated, primarily by means of their spectra and cospectra. It was found that: ninety percent of the heat input to the atmosphere was in the form of latent heat; the sensible heat flux was positive (upward) at the small scales generated near the surface and negative at the large scales due to subsiding air; the latent heat flux was positive at all scales and similar in spectral distribution to the momentum flux; the flow appeared to be anisotropic even at scales one hundred times smaller than the distance from the boundary; the drag coefficient, from direct measurements of the momentum flux (or stress), was (1.45±0.08) x 10⁻³; shear generated turbulence was not entirely dissipated locally. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

<div>Initial experiments on separated shock/boundary-layer interactions were carried out within the Boeing/AFOSR Mach-6 Quiet Tunnel. Measurements were made of hypersonic laminar-turbulent transition within the separation above a compression corner. This wind tunnel features freestream fluctuations that are similar to those in</div><div>flight. The present work focuses on the role of traveling instabilities within the shear layer above the separation bubble.</div><div>A 7 degree half-angle cone with a slice and a finite-span compression ramp was designed and tested. Due to a lack of space for post-reattachment sensors, early designs of this</div><div>generic geometry did not allow for measurement of a post-reattachment boundary layer. Oil flow and heat transfer measurements showed that by lengthening the ramp, the post-reattachment boundary layer could be measured. A parametric study was completed to determine that a 20 degree ramp angle caused reattachment at 45% of the</div><div>total ramp length and provided the best flow field for boundary-layer transition measurements.</div><div>Surface pressure fluctuation measurements showed post-reattachment wave packets and turbulent spots. The presence of wave packets suggests that a shear-layer</div><div>instability might be present. Pressure fluctuation magnitudes showed a consistent transition Reynolds numbers of 900000, based on freestream conditions and distance</div><div>from the nosetip. Pressure fluctuations grew exponentially from less than 1% to roughly 10% of tangent-wedge surface pressure during transition.</div><div>A high-voltage pulsed plasma perturber was used to introduce controlled disturbances into the boundary layer. The concept was demonstrated on a straight 7 degree half-angle circular cone. The perturbations successfully excited the second-mode instability at naturally unstable frequencies. The maximum second-mode amplitudes prior to transition were measured to be about 10% of the mean surface static pressure. </div><div>The plasma perturber was then used to disturb the boundary layer just upstream of the separation bubble on the cone with the slice and ramp. A traveling instability was measured post-reattachment but the transition location did not change for any tested condition. It appears that the excited shear-layer instability was not the dominant mechanism of transition.</div>

Turbulent collapse and recovery are both observed to occur abruptly in the atmospheric stable boundary layer (SBL). The understanding and predictability of turbulent recovery remains limited, reducing numerical weather prediction accuracy. Previous studies have shown that regime occupation is the result of the net effect of highly variable processes, from turbulent to synoptic scales, making stochastic methods a compelling approach. Idealized stable boundary layer models have shown that under some circumstances, regimes can be related to the stable branches of model equilibria, and an additional unstable equilibrium is predicted. This work seeks to determine the extent to which the SBL regime occupation can be explained using a one-dimensional stochastic differential equation (SDE). The drift and diffusion coefficients of the SDE of an input time series are approximated from the statistics of its averaged time tendencies. These approximated coefficients are fit using Gaussian Process Regression. Probabilistic estimates of the system's equilibrium points are then found and used to create an empirical bifurcation diagram without making any prior assumptions on the dynamical form of the system. This data driven bifurcation diagram is compared to modelled predictions. The analysis is repeated on several meteorological towers around the world to assess the influence of local meteorological settings. This work provides empirical insights into the nature of regime dynamics and the extent to which the SBL displays hysteresis. / Graduate

Shear sheltering is defined as the effect of the mean flow velocity profile in a boundary layer on the turbulence caused by an imposed gust. In aeroacoustic applications turbulent boundary layers interacting with blade trailing edges or roughness elements are an important source of sound, and the effect of shear sheltering on these noise sources has not been studied in detail. Since the surface pressure spectrum below the boundary layer is the primary driver of trailing edge and roughness noise, this thesis considers the effect that shear sheltering has on the surface pressure spectrum below a boundary layer. This study presents a model of the incoming turbulence as a vortex sheet at a specified height above the surface and shows, using canonical boundary layers and approximations to numerical results, how the mean flow velocity profile can be manipulated to alter the surface pressure spectrum and hence the associated trailing edge noise. The results from this model demonstrate that different mean velocity profiles drive significant changes in the unsteady characteristics of the flow. The surface pressure fluctuations results also suggest that boundary layers where the shear in the mean velocity profile is significant can be beneficial for the reduction of trailing edge noise at particular frequencies. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

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Measurements of surface shear stress magnitude and direction are reported for a three-dimensional, pressure driven, turbulent boundary layer around a wing body junction. Measurements were made using a dual-beam oil film laser interferometer at 56 locations. An iterative procedure was developed which increased the precision of the data extracted from the data records. Skin friction directions computed using a least square error fit were compared to angles obtained from surface oil flows, hot wire anemometry, and LDV measurements. Also, the magnitude of the skin friction coefficients were compared to independently obtained skin friction coefficients. The data agreed to within experimental error outside the effects from the vortex legs present along the side of the wing-body. No accurate data was available for quantitative comparison under the effects of the vortex, but the magnitudes followed the qualitative trends expected. This method failed badly in the region of large three-dimensional effects and requires further study in this area of application. / Master of Science

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