The transport of small particles across the aerodynamic boundary layer that developed over a smooth, flat, acrylic plate and their subsequent deposition was investigated. The velocity boundary layer over the flat plate was characterized for a wind tunnel mainstream velocity of 2 m/s. Particle deposition was quantified with respect to location on the experimental plate with a microscope. The deposition of 0.8, 0.9, 1.1, and 2.0 micron diameter unit density, polystyrene latex microspheres on to oil-coated, uncoated, upper, and lower surfaces was investigated. Although experimental deposition velocities exhibited run-to-run variation, they were significantly greater than those reported in the literature. A turbulent flow deposition model which included eddy diffusion, Brownian diffusion, inertial, and gravitational deposition mechanisms underestimated the experimental deposition velocities. The experimental plate was nonconductive and could not be electrically grounded. It appears the electrostatic attraction mechanism was responsible for the increased experimental deposition velocities; this mechanism was not included in the deposition model. There was no significant resuspension of 42 micron diameter microspheres deposited to an initially moist experimental plate after 6 hours in the wind tunnel at a mean air velocity of 2 m/s. / Ph. D.

The strongly time variant flow in an incompressible, turbulent junction vortex formed at the base of a streamlined cylinder with a circular leading edge placed normal to a flat surface was investigated. The investigation centered around spectral analysis and time resolved measurements of the velocity fluctuations to characterize the time variant flow on the plane of symmetry. All the measurements were performed with a two-color, two-component, frequency shifted laser Doppler velocimeter. Spectral analysis methods for randomly sampled data occurring from the LDV were evaluated under various simulated and real flow situations. The real flow situations studied were the vortex shedding flow behind a cylinder and the two-dimensional turbulent boundary layer. The spectral estimates obtained from the discretized lag product method were found to be better than those obtained from the direct transform method. It was found that the exact lag product method does not offer significant improvements in the spectral estimates to offset its computational slowness. The mean velocity vectors in the junction vortex showed a single vortex on the plane of symmetry and a singular separation point upstream of the cylinder. The time resolved measurements showed the instantaneous separation point on the plane of symmetry to be randomly oscillating between two limits. Maximum possible excursions of the junction vortex position and size were also obtained form the time resolved measurements. The turbulence intensities in the junction vortex were found to be at least two to three times higher than typical two-dimensional boundary layer values. The histograms of instantaneous velocity fluctuations deviated from the expected Gaussian distributions and were found to have multiple peaks. The spectral content of the junction vortex flow was investigated. The overall character of the junction vortex flow was found to be similar to a two-dimensional turbulent boundary layer, with greater amplification perceived in the lower frequencies relative to the higher frequencies. The spectra at locations above the time mean center of the junction vortex showed distinct peaks around 20-30 Hz, unlike boundary layer flows. / Ph. D.

An experimental study of the incompressible, three-dimensional, turbulent flow separation around the base of a bluff obstacle on a flat surface is described. The bluff obstacle is a streamlined, right circular cylinder mounted with its axis normal to the flat surface. The flow environment is characterized by a body Reynolds number of 183,000, based on the diameter of the circular cylinder. The study includes surface flow visualizations, surface pressure measurements, and mean flow measurements. The mean flow measurements consist of total pressure, static pressure, and velocity distributions in three planes around the base of the streamlined cylinder. The results show the presence of a large, dominant vortex in the junction between the cylinder and the flat surface. This vortex was found to consist of low total pressure fluid from the boundary layer flow upstream of the junction. In addition to the three-dimensional flow measurements, extensive measurements in the two-dimensional turbulent boundary layer on the flat surface are reported. These results show the existence of small, but statistically significant, spanwise variations in the nominally two-dimensional turbulent boundary layer. A systematic approach for estimating the wall shear stress from velocity profile data in a two-dimensional turbulent boundary layer based on the method of least squares is presented. / Ph. D.

Three-dimensional (3D) turbulent boundary layers (TBLs) are ubiquitous in most engineering applications, but most turbulence models used to simulate these flows are built on two-dimensional turbulence theory, limiting the accuracy of simulation results. To improve the accuracy of turbulence modeling capabilities, a better understanding of 3DTBL physics is required. This dissertation outlines the experimental investigation of the attached 3D TBL flow over the Benchmark Validation Experiments for RANS/LES Investigations (BeVERLI) Hill using laser Doppler velocimetry in the Virginia Tech Stability Wind Tunnel. The mean flow and turbulence behavior of the boundary layer are studied and compared with turbulence theories to identify the validity of these assumptions in the BeVERLI Hill flow. It is shown that the pressure gradients and curvature of the hill have a significant effect on the turbulence behavior, including significant history effects at all stations due to the changing pressure gradient impact through the height of the boundary layer. Supplementing the experimental results with analysis from rapid distortion theory and simulations, it is shown that the stations lower on the hill are significantly affected by the non-linear history effects due to the varying upstream origins of the flow passing through those stations. Stations closer to the hill apex pass through a region of extremely strong favorable pressure gradient and hill constriction, resulting in behavior that matches qualitatively with the results from rapid distortion theory and provides insights into the physical mechanisms taking place in these regions of the flow. Despite the misalignment of the mean flow angle (γ_FGA) and turbulent shear stress angle (γ_SSA) throughout all of the profiles, the proposed 3D law of the wall of van den Berg (1975), which incorporates pressure gradient and inertial effects and relies on the assumption that γ_FGA=γ_SSA, is able to predict the flow behavior at more mildly non-equilibrium stations. This suggests that models that currently rely on assumptions founded on the two-dimensional law of the wall could be improved by incorporating van den Berg's model instead. The total shear stress distribution at selected stations on the BeVERLI Hill are all significantly reduced below equilibrium two-dimensional (2D) levels, indicating that turbulence models built on this assumptions will not be able to accurately simulate the 3D turbulence behavior. / Doctor of Philosophy / As an object moves through a fluid or a fluid moves past an obstacle, fluid sticks to the solid boundary of the object because of the fluid's viscosity, resulting in zero velocity on the surface (known as the "no-slip" condition). There then exists a region where the flow velocity increases from zero to the freestream velocity - this region is known as the boundary layer. The nature of the boundary layer developing around a body significantly influences how the body and fluid interact and is critical to practical items of engineering interest, such as estimating how much drag a vehicle will experience. Most bodies of engineering interest are three-dimensional (3D), like an aircraft or a car, and thus induce a three-dimensional boundary layer, but many turbulence theories used in computational fluid dynamics simulations are based on simplified two-dimensional (2D) flow behavior studied in laboratories. To further improve the accuracy of simulations, a better understanding of three-dimensional turbulent boundary layer flows is required. This dissertation outlines a study of three-dimensional turbulent flows by analyzing the three-dimensional turbulent boundary layer over the Benchmark Validation Experiments for RANS/LES Investigations (BeVERLI) Hill using laser Doppler velocimetry (LDV) in the Virginia Tech Stability Wind Tunnel. LDV uses the Doppler shift principle to measure the fluid velocity and turbulence at different points in the flow. Through analysis of the fluid velocity and turbulence in the flow, it is shown that the turbulence and flow behavior at certain stations are heavily influenced on the upstream flow history. Stations closer to the bottom of the hill are more influenced by the upstream flow history, while stations closer to the top of the hill experience such strong acceleration due to the local favorable pressure gradient and hill curvature that the upstream history has a more linear influence. In general, the turbulence on the hill is reduced due to the acceleration over the surface below 2D levels and does not match with the 2D fundamental relationships often used in turbulence theories for simulations. Thus, simulations that rely on these assumptions will not be able to accurately predict the details of the 3D flow. A proposed 3D model for the mean velocity behavior by van den Berg (1975) will perform better in simulations than the typical 2D law used in some turbulence model assumptions.

The Generation IV Very High Temperature Reactor (VHTR) design is being actively studied in various countries for application due to its inherent passive safe design, higher thermal efficiencies, and proposed capability of providing high temperature process heat. The pebble bed core is one of two core designs used in gas reactors. In the pebble bed core there are mechanisms present which can cause the flow to laminarize, thus reducing its heat transfer effectiveness. Wind tunnel experiments were conducted using Particle Image Velocimetry (PIV) to investigate boundary layer laminarization due to flow acceleration and convex curvature effects. The flow was subject to acceleration and curvature both separately and together and the flow behavior characterized with velocity flow profiles, mean boundary layer parameters, and turbulence quantities. Laminarization was identified and the influence of acceleration and curvature was characterized. / Graduation date: 2013

No description available.

No description available.

Stably stratified flows are common in the environment such as in the atmospheric· boundary layer, the oceans, lakes and estuaries. Understanding mixing and dispersion in these flows is of fundamental importance in applications such as the prediction of pollution dispersion and for weather and climate prediction/models. Mixing efficiency in stratified flows is a measure of the proportion of the turbulent kinetic energy that goes into increasing the potential energy of the fluid by irreversible mixing. This can be important for parameterizing the effects of mixing in stratified flows. In this research, fully resolved direct numerical simulations (DNS) of the Navier-Stokes equations are used to study transient turbulent mixing events. The breaking of internal waves in the atmosphere could be a source of such episodic events in the environment. The simulations have been used to investigate the mixing efficiency (integrated over the duration of the event) as a function of the initial turbulence Richardson number Ri = N2L2/U2, where N is the buoyancy frequency, L is the turbulence length scale, and u is the turbulence velocity scale. Molecular effects on the mixing efficiency have been investigated by varying the Prandtl number Pr = V/K, where v is the viscosity and K is the scalar diffusivity. Comparison of the DNS results with grid turbulence experiments has been carried out. There is broad qualitative agreement between the experimental and DNS results.· However the experiments suggest a maximum mixing efficiency of 6% while our DNS gives values about five times higher. Reasons for this discrepancy are investigated. The mixing efficiency has also been determined using linear theory. It is found that the results obtained for the very stable cases converge on those obtained from DNS suggesting that strongly stratified flows exhibit linear behaviour. Lagrangian analysis of mixing is fundamental in understanding turbulent diffusion and mixing. Dispersion models such as that of Pearson, Puttock & Hunt (1983) are based on a Lagrangian approach. A particle-tracking algorithm (using a cubic spline interpolation scheme following Yeung &Pope, 1988) was developed and incorporated into the DNS code to enable an investigation into the fundamental aspects of mixing and diffusion from a Lagrangian perspective following fluid elements. From the simulations, the ensemble averaged rate of mixing as a function of time indicates clearly that nearly all the mixing in these flows occurs within times of order 3 Vu. The mean square vertical displacement statistics show how the stable stratification severely inhibits the vertical displacement of fluid elements but has no effect on displacements in the transverse direction. This is consistent with the Pearson, Puttock & Hunt model. The important link that asymptotic value of the mean square vertical displacement is a measure of the total irreversible mixing that has occurred in the flow is made. However the results show that the change in density of the fluid elements is only weakly correlated to the density fluctuations during the time when most of the mixing occurs, which contradicts a key modeling assumption of the PPH theory. Improvements to the parameterization of this mixing are investigated. Flow structures in stably stratified turbulence were examined using flow visualization software. The turbulence structure for strong stratification resembles randomly scattered pancakes that are flattened in the horizontal plane. It appears that overturning motions are the main mechanism by which mixing occurs in these flows. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2002.

L'objectif de cette thèse est d'étudier numériquement l'aéroacoustique à faibles nombres de Mach(M inf 0.3) pour un écoulement de couche limite turbulente épaisse affleurant une cavité, sur la base de simulations numériques à grandes échelles (LES). Un profil de vitesse en loi puissance et pour une couche limite d'équilibre ont servi comme conditions en entrée du domaine de calcul. La couche limite d'équilibre, sans et avec gradient de pression adverse, a été résolue par une approche asymptotique basée sur une formulation déficitaire avec un nouveau modèle de longueur de mélange. Ce dernier a été validé pour améliorer les comparaisons avec les expériences et les simulations numériques directes. Des simulations LES ont permis de regarder l'influence de l'épaisseur de la couche limite turbulente amont sur le mode d'oscillation d'une cavité L/D=4. Un accord satisfaisant avec les expériences d'Haigermoser et l'émergence du mode de cisaillement a été obtenu pour la vitesse amont de 5.8m/s. Le mode était de type sillage pour les deux autres cas tests (20et40m/s). Finalement, une simulation 3D a montré que le mode de sillage est un artefact du calcul 2D. En utilisant l'analogie de Lighthill-Curle et les champs de pression in stationnaire issus de la simulation, nous avons déterminé les niveaux de pression sonore dans le champ proche et lointain. Conformément aux expériences d'Haigermoser, une faible directivité vers l'amont est trouvée. Le mode de sillage influence très fortement les niveaux de pression acoustique. / The objective of this thesis is to study numerically the aeroacoustics of low Mach number (M inf 0.3) fow with thick turbulent boundary layer past a cavity based on Large Eddy Simulation (LES). Velocity profiles from power law and equilibrium turbulent boundary layer were imposed as inlet conditions on the computational domain. The equilibrium turbulent boundary layer profles (zero and adverse pressure gradient) have been generated using a symptotic approach with an improved mixing length model. A good agreement is observed between the computed boundary layer profiles and the profiles obtained from experiments and direct numerical simulations. LES results present the infuence of the thickness of the incoming turbulent boundary layers on the mode of oscillation in the shallow cavity of L/D=4. An agreement with the experiments of Haigermoser and the shear mode have been found for the upstream velocity 5.8m/s. Wake mode was observed for the other two test cases at 20 and 40m/s. A 3D cavity simulation is performed to show that the wake mode observed in the 2D calculations is an artifact. The hydrodynamic pressure feld obtained from the 2D simulation is used as an input to the acoustic analogy (Lighthill-Curle's analogy), to compute the acoustic pressure feld at the near and far feld of the cavities. Conforming the experiments of Haigermoser, a weak directivity of sound propagation was observed. Shear mode infuences the sound pressure levels strongly.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Microfiche copy available in Archives and Engineering. / Vita. / Includes bibliographical references. / by Nathan Clay Martin II. / Sc.D.