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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

This report documents the measurements of turbulence quantities resulting from vortices embedded in a zero pressure gradient turbulent boundary layer. Turbulent boundary layers are found in most flow regimes over large scale vehicles and have been studied for many years. Various systems to control separation of boundary layers have been proposed, but vortex generators have proven to be an economical choice as they are often used to fix deficiencies in a flow field after large scale production of a vehicle has commenced. In order to better understand the interaction between vortex generators and the boundary layer in which they are embedded, an experiment has been performed using through non-invasive Comprehensive Laser Doppler Velocimeter. The results show that normalization on edge velocity is appropriate for comparison with previous work. The 1/S parameter and vq^2 parameter were found to be most appropriate to correlate the Reynolds stresses and triple products, respectively. The higher inflow edge velocity and greater momentum thickness, creating a lower vortex generator to boundary layer height ratio, result in a more diffuse vortex as compared to previous work conducted in the same wind tunnel, with the same geometry, but with different inflow conditions. / Master of Science

Turbulent boundary layers flowing over a smooth surface were studied to understand the influence of varying pressure gradients and flow Reynolds number on the boundary layer growth and mean turbulent properties. The test was conducted in the Virginia Tech Stability Wind Tunnel with a 0.914 m chord length, NACA 0012 Airfoil in the test section. This airfoil was rotated to different angles of attack to induce varying pressure gradients on the boundary layer developing on the test section walls. Mean pressure measurements, boundary layer pressure measurements, and time-resolved, wall-normal, stereoscopic particle image velocimetry (TR-PIV) measurements were made. The TR-PIV data was acquired at a chord-based Reynolds number of 1.2 million, 2 million, and 3.5 million, at a sampling rate of 1 kHz, in two different camera configurations. The boundary layer pressure measurements were acquired at different flow Reynolds numbers ranging between 0.76 million and 3.5 million. Both adverse and favorable pressure gradients of varying intensities were imposed on the boundary layer by rotating a 0.914 m chord NACA 0012 airfoil to angles of attacks between -{10}^o and {12}^o. Measurements at varying streamwise locations enabled the study of boundary layer flow development under changing pressure gradients. The pressure gradient influences were observed in the boundary layer characteristic properties, on the mean velocities, and on the Reynolds stresses present in the flow. The pressure gradient influences were found to be consistent at varying Reynolds numbers, but the intensity of their effects was influenced by the flow Reynolds number. Moreover, the influence of pressure gradients and flow Reynolds numbers was evident in both outer and inner scales. The test data acquired was also validated with previous works. / M.S. / The interaction of turbulent boundary layers and smooth surfaces is prevalent in our world. It plays a vital role in various phenomena, such as, aircraft stall, cabin noise, and structural vibrations. Varying flow conditions influence the behavior of boundary layers and the extent of their implications. The effects of pressure gradients and the level of turbulence, described by the Reynolds numbers, on turbulent boundary layer flow was studied. This was done through an experiment conducted at the Virginia Tech Stability Wind Tunnel facility. The test data was acquired through boundary layer pressure measurements and Time-Resolved, Stereoscopic Particle Image Velocimetry (TR-PIV) at varying streamwise locations in the test section. A 0.914 m chord, NACA 0012 airfoil was placed in the test section and its angle of attack was varied to -{10}^o,0^o,\ \ and\ {12}^o to induce a favorable, minimum, and an adverse pressure gradient, respectively. The TR-PIV measurements were acquired at a sampling rate of 1 kHz and in two different camera configurations. The flow Reynolds number was based on the airfoil chord length (Re_c) and was varied to 1.2 million, 2 million, and 3.5 million for the TR-PIV tests. The boundary layer pressure measurements were acquired using an array of 30 Pitot probes placed in the boundary layer of the flow. The flow Reynolds number for these test runs ranged between 0.76 million and 3.5 million. The acquired data was used to analyze the mean statistical properties of turbulent boundary layers primarily focusing on the mean velocities, boundary characteristic parameters, Reynolds normal stresses, and Reynolds shear stresses. The results showed that the nature of pressure gradient influences on the mean properties of turbulent boundary layers remained consistent regardless of the flow Reynolds number. However, the intensity of the pressure gradient effects was influenced by the flow Reynolds number. These observations were made at various streamwise data acquisition locations through which the evolution of the flow was also studied. Lastly, the results obtained in this experiment were validated with previous works.

L'approche de simulation numérique directe est utilisée pour la simulation d'un écoulement en canal pleinement turbulent afin d'étudier l'influence des grandes échelles de l'écoulement ainsi que la dynamique du transport des contraintes de Reynolds et de la vorticité. Les simulations sont réalisées sur un domaine de calcul de grande taille afin de pouvoir capturer l'intégralité des grandes structures de l'écoulement, et portent sur une gamme relativement étendue de nombres de Reynolds (Reτ =180, 395, 590 et 1100) allant des écoulements faiblement turbulents à des écoulements modérément turbulents. L'invariance remarquable des fluctuations de vorticité normale est expliquée à travers une analyse spectrale de la vorticité. L'étude des différents termes du transport de l'intensité turbulente de la vorticité révèle par ailleurs que le pic de production de la vorticité transverse est situé à proximité immédiate de la paroi et pourrait ouvrir la voie à des stratégies de réduction de la traînée basées sur la réduction de la vorticité transverse. Le transport des contraintes de Reynolds dans la couche interne et dans la couche de recouvrement est également étudié. A proximité des parois, la dépendance des termes de transport avec le nombre de Reynolds dans les échelles internes montre que ces dernières ne suffisent pas à caractériser la dynamique des contraintes de Reynolds dans cette zone. Cette insuffisance des échelles internes nous a amenés à nous intéresser plus particulièrement au processus de production à travers les statistiques de la production conditionnées par le passage par niveau des fluctuations de la vitesse normale ou longitudinale. Cette étude nous a permis d'identifier les fluctuations qui contribuent le plus à la production et celles qui sont à l'origine de la dépendance avec le nombre de Reynolds. / The direct numerical simulations of a fully turbulent channel flow are investigated to study the large scales effects on the flow quantities such as the Reynolds stresses and vorticity transport processes. Large computational domains are used so as to cover the largest scales of the flow. The simulations are performed in a wide range of Reynolds numbers (Reτ=180, 395, 590 and 1100) going from weakly to moderately high Reynolds number turbulent flows. The invariance of the wall-normal vorticity fluctuations scaled in wall variables in the inner layer versus the Reynolds number is analyzed using a spectral analysis. The vorticity transport equations are investigated in detail, presumably for the first time. The transport mechanism of the Reynolds shear stresses are subsequently analyzed in the inner layer and the overlapping zone. In the wall layer, different terms of the Reynolds stresses transport expressed in inner scales depend on the Reynolds number. This scaling failure lead us to focus on the statistics of the production when the streamwise or normal velocity fluctuations cross a given level, through the conditional Palm statistics. The main aim is to identify those amplitudes of the fluctuations that contribute more to the production and those which are responsible for the production Reynolds dependence.