Thesis--Auburn University, 2009. / Abstract. Vita. Includes bibliographical references (p. 53-55).

Wake has a high impact on wind turbine performance and durability. Its impact, without a doubt, should be considered in wind resource assessment. However, wakes in the forest is a niche that is only examined by a couple of countries which area has a significant per cent land covered in forests. In this thesis, the wake over a homogenous forest is analysed, using the realisable k-e model. Simulations were performed using OpenFOAM software. Firstly, simulations were performed for one wind turbine to examine if RANS is a reliable tool for wake calculation. Secondly, the power production of two wind turbines in tandem are examined ,where one is in the wake of another one. Wind speed velocity analysis concluded that RANS credible predicts velocity deficit in the wake region and could be used in forested areas. For instance, the correlation coefficient for velocity mostly lies above 0.9. Also, the distance between wind turbine swhich would be erected in the forest could be reduced. The reduction could exceed up to 3D with only 35% power loss, which means that wind farms in forests could be more compact. Further studies could examine multiple wake interactions and how a combination of forest clearing, vegetation growth and snow cover affects wind farm performance over a lifespan.

The Cassini spacecraft orbited Saturn for over 13 years and collected stellar occultations using an Ultraviolet Imaging Spectrograph (UVIS). Chord occultations were analyzed using autocorrelations at minimum ring plane radius to visualize the structure and correlation in the azimuthal direction. These particle tracking occultations cut a chord across the rings in the path of the star. By taking the autocorrelation of these chord occultations, 8 out of the 66 showed clumping within the first 3.0 km in azimuth, representing signs of a structure. Six of those occultations could be moonlets or propellers as their minimum ring plane radii are in the Propeller Belt region. The Fast Fourier Transform Power Spectrum of the autocorrelation was also taken, and 6 of the 8 had high peak power outputs at certain wavelengths. It is also observed that five of the occultations may contain self-gravity wakes.

Superhydrophobic surfaces are a class of surfaces that have a microscale roughness imposed on an already hydrophobic surface, akin to a lotus leaf. These surfaces have been shown to produce significant drag reduction for both laminar and turbulent flows of water through large and small-scale channels. The goal of this thesis was to explore how these surfaces alter the vortex shedding dynamics of a cylindrical body when coated on its surface, thus leading to an alteration in drag and lift on these surfaces. A cylindrical body was chosen as it is a very nice representative bluff body and sets the stage for predicting the behavior of hydrofoils and other bluff bodies under flow with a slip boundary condition. In this work, a series of experiments were performed which investigated the effect of superhydrophobic-induced slip on the flow past a circular cylinder. In these experiments, circular cylinders were coated with a series of superhydrophobic surfaces fabricated from PDMS with well-defined micron-sized patterns of surface roughness or random slip surfaces fabricated by sanding Teflon cylinders or spray painting superhydrophobic paint on a smooth cylinder. The presence of the superhydrophobic surface was found to have a significant effect on the vortex shedding dynamics in the wake of the circular cylinder. When compared to a smooth, no-slip cylinder, cylinders coated with superhydrophobic surfaces were found to delay the onset of vortex shedding and increase the length of the recirculation region in the wake of the cylinder. For superhydrophobic surfaces with ridges aligned in the flow direction the separation point was found to move further upstream towards the front stagnation point of the cylinder and the vortex shedding frequency was found to increase. For superhydrophobic surfaces with ridges running normal to the flow direction, the separation point and shedding frequency trends were reversed. The vortices shed from these surfaces were found to be weaker and less interlaced leading to reduced circulation and lift forces on these cylinders. The effect of slip on bluff bodies and separating flow was dealt with in detail in this thesis and the results could be used to predict the impact of these surfaces on the flow past hydrofoils which combine skin friction dominated flow with separating flow.

Wakes are present in many engineering flows. These flows include internal flows such as mixing chambers and turbomachinery as well as external flows like flow over high-lift or multi-element airfoils. Many times these wakes are exposed to flow conditions such as adverse pressure gradients and streamline curvature that alter the mean flow and turbulent structure of the wake. The ability to understand how pressure gradients and streamline curvature affects the structure of the wake is essential to predicting how the wake will affect the performance of the application in which it is found. The effects of pressure gradients and curvature of low-speed wakes has been extensively documented. As the transonic flow regime is becoming of more interest as gas speeds in turbomachinery increase this work fills a void in the body of wake knowledge pertaining to curved wakes in high speed flows. An under-resolved direct numerical simulation of transonic wake flow being shed by a cambered airfoil in the presence of adverse pressure gradients and streamline curvature is therefore presented here. It was observed that the turbulence characteristics arising from the cambered airfoil that generates the wake dominate the evolution of the wake for different distances downstream depending on the component of the Reynolds stresses that is being considered. These characteristics dissipated the most quickly in the shear stresses and endured the longest in the tangential normal stresses. Previous work in low-speed wakes has indicated that curvature creates new production terms that translate into asymmetry in the profiles of the wake. Curvature was observed to have limited influence on the evolution of the streamwise normal stresses and an extensive impact on the tangential normal stresses. The transport of the Reynolds shear stresses indicate that the asymmetry in this stress is caused indeed by curvature but through turbulent diffusion and not production. The k-ε turbulence model overpredicted the effect of curvature on the turbulence stresses in the wake. This led to accelerated wake decay and spread compared to the UDNS data.

An experimental investigation was carried out of the flow over a hemisphere-cylinder at angles of attack (α = 0° to 90°, Visualizations of skin-friction lines were conducted and were focused mainly on the development of the laminar separation bubble as a function of angle of attack, the conditions under which open and closed separation exist and the interaction between the separation bubble and the leeward vortices. A digital processing method was developed to convert flow visualizations to numerical data. Static pressure measurements over a large range of Reynolds numbers were obtained for two models with different sizes and the same length-to-diameter ratios. Detailed velocity fields, mapped out by a seven hole probe and a laser-Doppler velocimeter (LDV) probe, were carefully examined to provide information on the development of vortical structures on the surface of the model. Comparisons were made of the results obtained using these two instruments. The flowfield in the wake of the hemispherecylinder was also examined at an angle of attack α = 30°. A small bead was strategically placed near the nose to force vortex asymmetry. Difference sizes of bead were also tested to investigate the effectiveness on the asymmetric pattern. Hot-wire anemometers and a dynamic signal analyzer were employed to study the unsteady motion of leeward vortices. / Ph. D.

A method in which the transport and evolution of the bubble population in a surface ship wake is numerically simulated is presented. The simulation is accomplished by constructing an advective-diffusive transport model for the scalar bubble field and solving this model for late times after ship passage. The bubble population model requires convection velocities and turbulent diffusion information that is supplied by solving the Reynolds-averaged parabolized Navier-Stokes equations with a <i>k</i> - ∊ turbulence model. The mean flow equations are solved by approximating the differential equations with a second order accurate finite difference scheme. The resulting large, sparse, banded matrix is solved by applying a version of the conjugate gradient method. The method has proven to be efficient and robust for the free shear flow problems of interest here. The simulation is initiated with given information in a plane at some point downstream of the ship from which the solution is propagated. The model is executed for a single and a twin propeller ship at 15 knots. The simulation shows that the development of the hydrodynamic and bubble near wake is dominated by ship geometry via strong advective transport. The far wake is dominated by diffusion and bubble rise and dissolution. Thus relatively large changes in geometry have a limited influence on the far wake. / Ph. D.

A critical evaluation of the Baldwin-lomax (Bl) turbulence model for shock/shear layer interactions, reversed flow, and curved, asymmetric wakes is made. No general definition for reference line is available for wakes, and difficulties exist for length scale prediction in complex flows. An entropy envelope for shear layers, and the locus of maximum entropy to define the wake centerline is proposed. The range of the Bl model is limited to the entropy envelope. This provides all relevant modeling data, and allows general application of existing reversed flow corrections. The total enhancements are flow adaptive and form the Dynamic Bl model. This robust model is more accurate in complex boundary layers and wakes. The Dynamic Bl model is applied to a supersonic fan cascade at the design incidence. Sharp differences in turbulent viscosities were seen between the the original, Baseline Bl, and Dynamic Bl models. Only slight differences exist in the overall cascade solutions. This includes loss factors which were produced by different mechanisms. The Baseline BL model predicted separation on the SS surface and larger standing vorticies off the TE. The Dynamic Bl model predicted attached boundary layers, smaller standing vorticies off the TE, but uniformly higher skin friction. The shock structure in the cascade may reduce the flow field dependence on specific viscosity profile characteristics, so these may be less important than overall turbulence levels. / Master of Science

Turbulent flow measurements were performed in the wake of a slender axisymmetric body in the presence of a flat plate strut and an image plane representing the "rigid lid" approximation to a free surface. The tests were performed in a wind tunnel at a nominal Reynolds number of 6.0 x 10⁵. All turbulent flow parameters were measured at three streamwise stations. A Yawhead probe was used for the mean flow measurements, and a Constant Temperature Anemometer System with a 'x'-wire probe was used to obtain the turbulent flow characteristics. The presence of the image plane was found to increase the velocity defect and the static pressure as the image plane was approached. A redistribution among the various components of velocity fluctuations was noted near the "rigid lid" plane. The transverse component was enhanced at the expense of the normal component. The image plane also was found to influence the magnitudes and radial spread of turbulence intensities and Reynolds stresses. Some interactions between the wake of the axisymmetric body and that of the plate strut were observed. Overall, the mean velocities and the turbulence quantities indicated symmetry about the image plane throughout the wake. / M.S.

The transition to turbulence of plane wakes is characterized by the development of the velocity-fluctuation field from a spectrum of weak random background noise in the initial laminar wake to a nearly featureless broad spectrum of intense fluctuations within the turbulent wake. This transition has also been described as a sequence of instabilities and wave-wave interactions. In the initial small-amplitude stage,. a narrow, but continuous, band of dominant instability modes centered near the most unstable mode, known also as the fundamental mode, grow exponentially at rates that can be calculated from the linearized Navier-Stokes equations. As these modes grow, the nonlinear terms become more important and cannot be neglected anymore. The effect of these terms is to introduce wave-wave interactions that lead to quadratic energy transfer between the different spectral components of the velocity-fluctuation field. While the consequences of these interactions, such as broadening of the power spectra, have been observed in many experiments, the characteristics of these interactions have only been examined in limited cases. Previous measurements of the auto-bispectrum showed that three-wave interaction processes are important in the transitioning wake. However, quantification of these processes can only be obtained from measurement of the nonlinear energy transfer rates resulting from the nonlinear wave-wave interactions. Such quantification is very important for understanding the effects of the different mechanisms involved in the transition and final breakdown to turbulence. An understanding of these mechanisms and their effects can then be used to control the transition by enhancing certain mechanisms and reducing the role of others through external excitation. In this work, quantitative estimates of the auto-bispectrum, linear and quadratic coupling coefficients and the resulting energy transfer rates between the interacting waves at different locations are presented in controlled and natural transitions of the plane wake. The results show that, in both natural and controlled transitions, the underlying nonlinear dynamics are similar. Basically, nonlinear interactions between the instability modes result in energy transfer to harmonic bands as well as low-frequency difference components. These components play an important role in the transfer of energy to the sidebands and the valleys between the peaks. The results also show that, while energy-transfer rates in natural transition are lower than in controlled transition, the random nature of wave excitation in natural transition causes energy transfer to a band of low-frequency components which leads to energy transfer to many sidebands and results in a spectrum that differs dramatically from the one obtained in the controlled case where two instabilities are excited. / Master of Science