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Coherent vortices are generated in flow fields due to flow interaction with sharp solid surfaces. Such vortices generate significant disturbances in the flow and affect its further development. In this dissertation attention is focused on the interaction of vortices with solid or free liquid/air surfaces. We examine vortices with their axis parallel or normal to the surface. Three main cases were examined: the interaction of a vortex pair propagating towards a solid boundary, the interaction of spanwise vortices in a turbulent boundary layer, and finally the interaction of spanwise vortices with a flat-plate wake and a free liquid surface. These problems hold significance in several engineering applications, including investigations into trailing wing tip vortices and their interaction with the ground, vortical effects on the development of turbulent boundary layers and free surface signatures and their detection in ship/submarine wakes. Data are acquired with a laser Doppler velocimetry system (LDV) and with Particle-Image Velocimetry (PIV), using a high-speed digital video camera. The LDV system measures two components of velocity along appropriately chosen planes. Grids of data were acquired for different pitch rates of a disturbing flap that generates vortices. Phase-averaged vorticity and turbulence level contours are estimated and presented. It is found that vortices with diameter the order of the boundary layer quickly diffuse and disappear while their turbulent kinetic energy spreads uniformly across the entire boundary layer. Larger vortices have a considerably longer life span and in turn feed more vorticity into the boundary layer. Trailing edge vortices are generated in a water tunnel by sharp hinged motions of a flap. These vortices are allowed to reconnect with the free surface and mix with a turbulent free shear layer. The flow is conditionally sampled via frame grabbing of free surface shadowgraphs. It is found that the vortex core bends away from the plane of the shear layer. Moreover, contrary to earlier findings, organized velocity fluctuations decrease as the free surface is approached. / Ph. D.

We present an analysis of the effects of heating, suction, and compressibility on the stability characteristics of boundary-layer flows within the framework of a complete nonparallel, linear, spatial stability theory. Included in the theory are disturbances due to velocity, Pressure, temperature, density, and transport properties as well as 'variations of the fluid properties with temperature. The method of multiple scales is used to account for the nonparallelism of the mean flow and equations are derived for the evolution of the disturbance amplitude and wave number vector. / Ph. D.

The subharmonic instability in two-dimensional boundary layer on a flat plate is analyzed using the parametric instability model and the resonant triad model. The problems arising from both models are solved numerically using the shooting technique and results are presented. It is found that in the presence of a strong interaction (e.g., large amplitude of the two-dimensional wave), results from the resonant triad model are inaccurate as compared with the experimental data and the t results from the parametric instability model. This is mainly because the resonant triad model is a weakly nonlinear model, and it does not account for the modification of the eigenfunctions of the interacting waves which really takes place as we find out from the experiments. The parametric instability model is a powerful model, despite all the assumptions included. The model, however, does not introduce a clear understanding of how the subharmonic mode originates from the three-dimensional Tollmien-Schlichting modes. For a weak interaction results from the resonant triad model and the parametric instability model get close to each other. / Master of Science

The Reynolds stresses in a three-dimensional turbulent boundary layer were examined using octant analysis. The representative flow was a pressure driven, three-dimensional turbulent boundary layer on the leeside (x/L=0.76-0.78, φ=105°-130°) of a 6:1 prolate spheroid at 10° angle of attack. The Reynolds number for the flow was Re_L=4.2x10⁺⁶. The LDV data of Chesnakas, Simpson, and Madden (1994) were the basis of examination. This data set employed a post-processing technique for refining the radial location of the measurements. A least-squares fit of the Spalding wall law was used to both correct the measurement locations and estimate the wall shear stress. This paper presents a previously unpublished assessment of the technique. Octant analysis was performed on the corrected data under free-stream and wall-collateral coordinates. (The wall-collateral coordinate system is aligned with the mean tangential velocity in the buffer-layer.) The octant analysis led to the development of a structural model that extends the sweep/ejection process to three dimensions. Ejections and sweeps produce w' through the same mechanism that produces u'; they transport fluid across a spanwise velocity gradient. The model's results remain consistent with coordinate rotation. The model also describes the asymmetries that evolve between ejections and sweeps with spanwise fluctuations (w') of opposite sign. These asymmetries cause non-zero u'w' and v'w' in the buffer layer. Comparison of the two coordinate systems reveals that wall-collateral coordinates provides a simpler foundation for octant analysis. The sweep and ejection octants maintain a nearly equal distribution of velocity events throughout the buffer and lower log layers. Also, the spanwise velocity profile monotonically decreases to a constant value at the boundary layer edge, simplifying application of the sweep/ejection model to spanwise fluctuations. Comparison with other 3DTBL experiments suggests that the wall-collateral coordinates are more closely aligned with the quasi-streamwise vortex structures than free-stream coordinates. The octant analysis also reveals structural behavior consistent with the four mechanisms revealed by the direct numerical simulation of Sendstad and Moin (1992). / Master of Science