Thesis (Ph. D.)--Technische Hogeschool Delft, 1986. / Errata slip inserted. Summary in English and Dutch; acknowledgements in Dutch. Vita. Includes bibliographical references (p. 105-111).

Thesis (Ph.D.)--Memorial University of Newfoundland, 2003. / Bibliography: leaves 215-232.

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

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

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1980. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 196-201. / byLee-Lueng Fu. / Ph.D.

No description available.

Boundary layer separation is usually an unwanted phenomenon in most technical applications as for instance on airplane wings, on ground vehicles and in internal flows such as diffusers. If separation occurs it leads to loss of lift, higher drag and results in energy losses. It is therefore important to be able to find methods to control and if possible avoid separation altogether without introducing a too heavy penalty such as increased drag, energy consuming suction etc. In the present work we study one such control method, namely the use of vortex generators (VGs), which are known to be able to hinder turbulent boundary layer separation. We first study the downstream development of streamwise vortices behind pairs and arrays of vortex generators and how the strength of the vortices is coupled to the relative size of the vortex generators in comparison to the boundary layer size. Both the amplitude and the trajectory of the vortices are tracked in the downstream direction. Also the influences of yaw and free stream turbulence on the vortices are investigated. This part of the study is made with hot-wire anemometry where all three velocity components of the vortex structure are measured. The generation of circulation by the VGs scales excellently with the VG blade height and the velocity at the blade edge. The magnitude of circulation was found to be independent of yaw angle. The second part of the study deals with the control effect of vortex generators on three different cases where the strength of the adverse pressure gradient (APG) in a turbulent boundary layer has been varied. In this case the measurements have been made with particle image velocimetry. It was found that the streamwise position where the VGs are placed is not critical for the control effect. For the three different APG cases approximately the same level of circulation was needed to inhibit separation. In contrast to some previous studies we find no evidence of a universal detachment shape factor H12, that is independent of pressure gradient. / QC 20101119

A new turbulence model designed for two-dimensional, steady and unsteady boundary layers in strong adverse pressure gradients is described. The model is developed in a rational way based on an understanding of the flow physics obtained from recent experimental observations. The turbulent shear stress is given by a mixing length model, but the variation of the mixing length in the outer region is not constant; it varies according to an integral form of the turbulence kinetic-energy equation. This approach allows for the history effects of the turbulence to be taken into account in an approximate but rational way. The form of the near-wall mixing length model is derived based on the rigorous distribution of the shear stress near the wall, and it takes into account the pressure and convection terms which become important in strong adverse pressure gradients. Since the significance of the normal stresses in turbulent kinetic-energy production is increasing as separation is approached, a model accounting for this contribution is incorporated. The model is calibrated using available experimental data. These data also indicate a change in turbulence structure near and through separation. Such a change can be significant and is accounted for here using an empirical function. The complete model was tested against steady and unsteady, two-dimensional experimental cases with adverse pressure gradient up to separation. Improved predictions compared to those obtained with other turbulence models were demonstrated. The general and rational approach that led to the derivation of the model allows the straightforward extension of the model in the region of separation. The further extension to steady and unsteady, three-dimensional cases is indicated. / Ph. D.

This thesis is a study of the effects of distributed roughness elements on a two-dimensional turbulent boundary layer. Measurements were taken on a total of ten rough wall configurations: four involving Gaussian spikes, and six with circular cylindrical posts. Rough wall flows are particularly suited to study with Laser Doppler Velocimetry (LDV) due to the fact that measurements are required near a solid surface, as well has in highly turbulent fluid. The LDV system used in this study is a fine resolution (~50 micron), three-component, fiber optic system. All mean velocities, Reynolds stresses, and triple products are measured. This study is unique in the range and variety of roughness cases for which data was taken. The data show that the flow over a rough wall is characterized by high levels of turbulence near the roughness element peaks at the interface between low-speed, near-wall fluid and the higher speed fluid above. Behind an element, high-momentum fluid sweeps toward the wall, and there is a small region of ejection of low-momentum fluid. Cylindrical elements typically have larger magnitudes of turbulent stresses at their peaks compared to Gaussian elements. Trends in mean velocity profile parameters such as displacement height, roughness effect, and wake parameter are examined with respect to roughness element geometry and spacing. / Master of Science

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