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The structure of a stable smooth wall zero pressure gradient turbulent boundary layer is investigated experimentally in order to determine the dominant outer region structure and to develop a hypothetical generalized boundary layer flow model. Three hot wire configurations, two vertically separated X-wires and a leading straight wire, a horizontal rake of 5 straight wires, and a vertical rake of 5 straight wires were used in the experiments, conducted at Reɵ = 8200. The basis for data reduction procedures came from crosscorrelations and the Variable Interval Time Average (VITA) technique. Three structure types are reported in the literature to be important: streaks and counter rotating streamwise vorticity, wall scaled hairpins or ring vortices, and large scale (0(ઠ)) bulges. A simple pictorial model consisting of three Reɵ dependent interdeveloping stages, which integrate all three structure types, is presented and discussed in relation to the literature and experiments performed. The rake data indicate that the positive ([formula omitted]u/[formula omitted]t) VITA detected velocity front has a scale much larger than that of the wall scaled eddies which typically have a scale of 100-300 y[formula omitted], and that this velocity front exhibits characteristics that are consistent with the trailing velocity front described in the model. The general convection velocity from basic crosscorrelations and the convection velocity of the positive VITA detected velocity front both had values 90-100% of the local mean velocity over most of the boundary layer. Evidence of small scale structure concentration on the downstream edge of the trailing velocity front is presented. A new method used to determine the average structure inclination angle associated with the trailing velocity front is presented and demonstrates that the generalized structure inclination angle, calculated from basic crosscorrelations between vertically separated sensors, does not indicate structure shape, but is associated with the bulk flow associated with the structure. The new method appears to give results that are consistent with flow visualization and more accurately estimates the inclination angle associated with the most dominant feature of the outer flow, the positive VITA velocity front. Although the model presented is somewhat crude and further development and refinement are required, the model appears to agree with most data in the literature, as well as the present experimental results. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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The relationship between dilatation, displacement speed, flame tangential strain rate and flame normal velocity gradient for a premixed turbulent flame in a corrugated/wrinkled flame regime is analyzed. The decomposition of dilatation into the flame tangential and normal strains and their relationship with curvature is studied. Three-dimensional, fully compressible direct numerical simulations (DNS) of premixed flames in a cube have been performed using a uniform 256^3 grid. For the turbulent case, decaying isotropic homogeneous turbulent velocity field is considered with an initial turbulence spectrum imposed. Simple single-step chemistry with an Arrhenius reaction rate is used. This simplification is valid as the flame considered is in the corrugated/wrinkled regime where the flame thickness is smaller than the smallest scales of turbulence. A single laminar flame is initially inserted into the turbulent field. A strongly linear relationship between dilatation and curvature has been seen which is due to the high correlation of displacement speed with curvature. The correlation between tangential strain rate and curvature is shown to be negative with a breakdown due to the curvature reaching the scale of the flame thickness at the cusps. To isolate the effect of heat release and turbulence, cases of a laminar sinusoidal wrinkled flame and a turbulent 𝛕=0 flame have been carried out. For a laminar sinusoidal wrinkled flame, a negative correlation between a^𝛕 and curvature was seen. This contradicts previous hypotheses (Haworth and Poinsot, 1992) (Chakraborty and Cant, 2004) where the negative correlation between a^𝛕 and curvature was explained to be due to different turbulence levels in front and behind the flame. Turbulence and alignment of flame surface with expansive tangential strains is shown to be responsible for the scatter seen in a^n and a^𝛕 relationships with curvature. Changing the peak reaction location towards the front of the flame did not change the trend in the plots of dilatation, tangential and normal strain rates versus curvature, confirming that dilatation relationship with curvature in particular is not due to any curvature distortion of the flame interior. However, it did thicken the flame and reduce the dilatation (and consequently its components, an and at) plot versus curvature and the magnitude of their curvature dependence. / Thesis / Master of Applied Science (MASc)

Surface pressure fluctuations developed by turbulent flow within a boundary layer is a major cause of flow noise from a body and an issue which reveals itself over a wide range of engineering applications. Modified boundary layers (MBLs) inspired by the down coat of an owl’s wing has shown to reduce the acoustic effects caused by flow noise. This thesis investigates the mechanisms that modified boundary layers can provide for reducing the surface pressure fluctuations in a boundary layer. This study analyzes various types of MBLs in a wall jet wind tunnel through computational fluid dynamics and numerical surface pressure spectrum predictions. A novel surface pressure fluctuation spectrum model is developed for use in a wall jet boundary layer and demonstrates high accuracy over a range of Reynolds numbers. Non-dimensional parameters which define the MBL’s geometry and flow environment were found to have a key role in optimizing the acoustic performance. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

Numerical simulations of incompressible, two-dimensional, monochromatically and bichromatically forced laminar free shear layers are performed on the basis of a vorticity-velocity formulation of the complete Navier-Stokes equations employing central finite differences. Spatially periodic shear layers developing in time (temporal model) are compared with shear layers developing in the stream-wise direction (spatial model). The regimes of linear growth and saturation of the fundamental are quantitatively scrutinized, the saturation of the subharmonic and vortex merging are investigated, and the effects of a forcing phase-shift between fundamental and subharmonic. For the spatial model the appearance of an unforced subharmonic was also examined. It was found that contrary to temporal shear layers a significant control of vortex merging by means of a forcing phase-shift and vortex shredding are not possible in spatial shear layers due to strong dispersion.

Detailed current profiles between the sediment-water interface and 20 cm above it reveal a viscous sublayer in the bottom boundary layer on the Oregon continental shelf. Data from three field experiments are used to test fundamental assumptions about boundary layer flow in the ocean. The first study, discussed in Chapter 1, evaluates the hypothesis that, in the absence of the obvious influence of topographic irregularities, the flow behaves like a universally similar, neutrally-buoyant flow over a smooth wall. The second study, discussed in Chapter 2, evaluates the influence which irregular small-scale topography may have on the near-bed flow, while the third, discussed in Chapter 3, examines streamwise velocity fluctuations in the viscous Sublayer and buffer layer and evaluates the hypothesis that spectra from the viscous sublayer and buffer layer of laboratory and geophysical boundary layer flows can be reduced to universal forms. although the thickness of the viscous sublayer scales with v/u, as required by universal similarity, the non-dimensional sublayer thickness is not as constant as in neutrally-buoyant laboratory flows. Even in the absence of the obvious effects of bottom irregularities, the near-bed flow is not as simple as smooth-walled boundarylayer flows in the laboratory. In the second study, it is shown that when the near-bed flow experiences resistance due to form drag as well as skin friction, the constant stress boundary layer assumption is not valid close to the sediment-water interface. Th the third study, it is shown that non-dimensionalized spectra of streamwise velocity fluctuations in the viscous sublayer and buffer layer at the ocean floor are very similar to those found in the laboratory. / Graduation date: 1982

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