The effects of geometry modifications on some properties of time-resolved flow measurements on the upstream symmetry plane of a junction vortex region were studied. A single linearized hot-wire was used to characterize the spatial distribution of the fluctuating flow turbulent kinetic energy. The distribution was integrated to give a measure of the turbulent kinetic energy content of the flow region where the largest fluctuations were seen in real-time smoke visualizations. The wire signal was also used for spectral estimations throughout the flow field. Six different cases were considered including a reference case with no geometry changes. The geometry modifications consisted of a large circular wraparound fillet, a small elliptic wraparound fillet, a small circular wraparound fillet, an upstream flow fence and, a large leading-edge triangular fillet. In the smoke visualizations, none of the modifications had any significant effect except the large triangular fillet which resulted in a reduced vortex structure. The large triangular fillet also appeared to be the most effective in significantly reducing the integrated turbulent kinetic energy. With regard to the turbulent kinetic energy, the elliptic wraparound fillet had an insignificant effect while the large and small wraparound fillets and the vortex generator fence had adverse effects. The spectra obtained over the flow field suggest that the upstream turbulent boundary layer was the driving force behind the junction vortex time-variant behavior. An examination of the low frequency end of the spectra suggests a weak local peak at about 11 Hz for the flow between the upstream separation point and the average vortex center. / M.S.

A Vortex-Lattice Method (VLM) has been developed for the analysis of three-dimensional lifting bodies undergoing arbitrary motions. So far, the present method has been applied to thin surfaces in translational motion. Several cases were tested, such as rectangular, delta, sweptback and trapezoidal planforms. Emphasis has been placed on the comparison of the results with available experimental data. Consequently, many of the strengths and limitations of the technique were made apparent. Good agreement has been obtained for the aerodynamic coefficients, with a relatively coarse discretization of the surface. Results are valid as long as there is no separation or vortex breakdown. However, accurate pressure distribution calculations need at least three or four times the number of panels used for aerodynamic coefficients calculations. The method gives a qualitative description of the wake configuration, but unrealistically high velocities are induced near the vortex elements. Work is underway to extend the technique to more general types of bodies and maneuvers. / Master of Science

The unsteady interaction of a vortex core with a NACA 0015 airfoil is studied in two dimensions. A two-component, three-beam Helium-Neon laser-Doppler Velocimetry system is used to take data in a water tunnel. Ensemble-averaged velocity fields are obtained in the region of the leading edge of the airfoil. Finite-difference algorithms were written to obtain vorticity and pressure in the data field. Computer animation of the unsteady vorticity was accomplished first with a Fortran code written for an Apple Macintosh computer and later with a commercial software package for a SUN Microsystems graphics terminal. / Master of Science

The vorticity shedding characteristics in attached and separated regions were investigated over three configurations, namely a backward facing circular arc, an ellipse at an angle of attack and a pitching airfoil. A fully automated data acquisition system was developed, including a two-component Laser-Velocimetry system in backscatter mode, an accurately controlled traversing mechanism and a MINK-11 minicomputer. Two-component velocity measurements were obtained over the above mentioned bodies, with steady and unsteady free streams. Emphasis was concentrated on the separation region, the free-shear layers and the wake downstream of these bodies. Two inviscid vortex models were developed to predict two different flow phenomena, namely the separated flow over a circular cylinder started impulsively from rest and propagating stall over a linear stationary cascade. / Ph. D.

Taylor vortex flow is one of the basic problems of nonlinear hydrodynamic stability. In contrast with the wide region of wavenumber predicted by the linear theory, experiments show that Taylor vortex flow only appears in a small region containing the critical wavenumber ß_er This phenomenon is called wave selection. In this work, several high-order perturbation methods and a numerical method are established. Both evolution and steady state of the How caused by single or several disturbances are studied. The existence of multiple steady states for disturbances with small wavenumber is discovered and proved. The stable and unstable steady state solutions and some associated phenomena such as jump phenomenon and hysteresis phenomenon are found. and explained. In the small region, the wavenumbers and initial amplitudes of disturbances determine the wavenumber of the flow. But outside this region, only the wavenumbers of the disturbances have effect on the wave selection. These results indicate that unstable solutions play a key role in wave selection. The side-band stability curve produced by the high-order perturbation methods is accurate at low Taylor numbers but incorrect at relatively high Taylor numbers. The relation of the unstable solutions and side-band stability is discussed. Besides, the overshoot and the oscillation phenomena during evolution are studied in detail. Connections between this work and experiments are discussed. / 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.

Marine cables often comprise an integral part of a larger structural system, such as an offshore drilling platform. They are also used to support marker buoys, as well as anchored or towed instrument arrays. Consequently, the resonant flow-induced oscillation of these cables, caused by vortex shedding, is extremely undesirable because of the associated damaging phenomena. The present study, which employs hot wire anemometry as the principal investigative tool, was undertaken to examine the behavior in the near wake of a flexible, helically wound, high aspect ratio (L/d = 107) marine cable in a linear shear flow (steepness parameter β = 0.0053) at centerline Reynolds numbers between 2.0 x 10³ ≤ Re_c ≤ 4.2 x 10⁴. Particular attention was paid to lock-on or “synchronization” related changes associated with uniform and sheared flow past the cable when it was forced to vibrate in the first mode. The study was extended to include an analysis of the effects on vortex shedding synchronization phenomena generated by placing spherical bluff bodies along the cable span. The frequency and reduced velocity boundaries of the lock-on regions, for both the cable and sphere-cable combination, as a function of vibration amplitude up to a/d = 0.322, were measured, plotted and analyzed. The critical reduced velocity of the vibrating cable was U*ᵣ = 5.50, and for the sphere-cable combination U*ᵣ = 3. 39. A sudden increase in the vortex formation length in the near wake of the vibrating cable occurred during perfect synchronization (U*ᵣ = 5.50). Shortly thereafter, the formation length returned to stationary cable values. At Re_c = 2.96 x 10³, the spanwise vortex shedding frequencies behind the stationary and vibrating cable in a linear shear flow (with and without the presence of spherical bluff bodies) were measured, plotted and the results coup a red. A stable cellular vortex shedding frequency structure, strengthened by the synchronous vibration, existed along the span of the cable. The presence of and spacing between the spherical bodies along the cable span significantly affected the sparwise character of the near wake cellular structure. The length of the locked-on region was substantially increased (almost doubled) when the spherical bodies were present along the span of the cable. / Ph. D.

The archetypal high temperature superconductor Bi₂Sr₂CaCu₂O[subscript[8+δ]] has been extensively investigated. However, until now, little has been known about the behaviour of the magnetic vortices inside the Vortex Glass and liquid state. µSR measurements have shown a negative skewness for the field probability distributions in these regimes. Such a negative skewness has only recently been explained as being a direct consequence of three-body correlations between vortices in a similar layered superconductor. With a new understanding and knowledge of the physics of these systems, it is instructive to re-visit the superconductor Bi₂Sr₂CaCu₂O[subscript[8+δ]] to explain the evolution of these three-body correlations occurring here. Comparing this with the ion-irradiated superconductors (of the same Bi₂Sr₂CaCu₂O[subscript[8+δ]] material), allows one to observe how three-body correlations between vortices evolve differently to that in the pristine material. Moreover, in the region of the macroscopic irreversibility line, entropically driven disorder exists below the matching field B[subscript[ϕ]] , whilst there is the appearance of relatively straight vortex lines at fields above B[subscript[ϕ]] . Such phenomena suggest a significant di fference in the evolution of three-body correlations compared with the unirradiated material. There has been much work conducted on the interplay between superconductivity and magnetism in materials of reduced dimensions. Work presented here on the ferromagnet/superconductor trilayer system (Permalloy/Nb/Permalloy) has shown a novel magnetic profile at the interface between the ferromagnetic and superconducting boundary, where, contrary to what is expected, the magnetism appears to be significantly suppressed at the interface before increasing towards the centre of the Nb layer.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy