Wingtip vortices are an important problem in aerodynamic and hydrodynamic engineering because of their contribution to induced drag, tip cavitation, and wake turbulence. These effects decrease equipment efficiency and lifespan, which increases application costs. Biology provides an inspiring solution to this problem in avian flight through the spreading of primary feathers. Previous studies have shown increased lift to drag ratio and efficiency of wings and propeller blades through modified wingtip geometry. The goal of this project is to optimize the tip geometry (primary feather angle) of a test wing for minimal tip vortex strength using genetic algorithms to mimic natural design evolution. Ultrasonic transducers are used to measure the wing tip vortex circulation in wind tunnel tests for each candidate design. Although neither angle of attack series converged completely, there was partial convergence in each. Due to the fluctuations in the low angle of attack tests, the parent selection algorithm was altered for the high angle of attack series, which resulted in improved convergence trends. A genetic algorithm that used uniform crossover breeding, a 20% mutation rate, and roulette wheel parent selection methods was used to generate an improved tip geometry at a low angle of attack of 6° and a freestream velocity of 15.25 m/s over the course of 17 generations. This improved design consisted of three key features, a staggered leading edge, a drastic mid-section vertical separation, and an upswept trailing edge. A second algorithm, which employed uniform crossover, a 20% mutation rate, and an elitist selection roulette parent selection, provided an improved tip geometry for a 12° angle of attack at a freestream velocity of 11.5 m/s. This improved design consisted of three key features, a downswept leading edge, a drastic mid-section vertical separation, and an upturned trailing edge. Both results showed that the wing tip vortex strength can be reduced by approximately 20% by manipulating tip geometry and that the trailing edge traits produce the most prominent effects on vortex strength.

Previous research has indicated aperiodicity in the positions of tip vortices emitted from a helicopter rotor blade in hover. The objective of the current study is to develop an analysis of the tip vortex aperiodicity in hover and to validate it with measurements on a reduced-scale, 1m diameter, four-bladed rotor. A “vortex ring emitter model” (VREM) was developed to study the statistics of the tip vortices emitted from a rotor blade during hover. In order to better model the rotor wake, a number of independent vortex blobs were used to describe a vortex ring. An empirical model for viscosity was also considered which helped model the core radius growth of the vortex ring with vortex age. A parametric analysis was then performed to obtain a comprehensive qualitative and quantitative convergence study of the time step, viscosity parameter, initial core size, number of rings shed, number of blobs and overlap factor. It was observed that the solution converged rapidly for all the parameters used. The locations of tip vortex cores for vortex ages ranging from 0◦ to 260◦ were measured on the reduced-scale rotor using a stereo PIV system. The blade loading for the reduced scaled rotor was Ct /σ = 0.044 and the blade rotational speed was 1520 RPM, which corresponds to a tip Reynolds number of 248,000. The 95 % confidence region for the position of tip vortex cores exhibited an anisotropic, aperiodic pattern, approximating an ellipse. It was seen that the principal axis of this ellipse appeared to be aligned perpendicular to the slipstream boundary. The analytical model showed good correlation with experimental data in terms of the orientation and extent of the anisotropy. Moreover, an estimate of the total thrust produced and spanwise loading along the rotor blade was also obtained and compared with Blade Element Momentum Theory (BEMT). It was seen that by using more blobs to represent a vortex ring, the solution converged to the BEMT estimate. / text

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Aeroacoustic investigations regarding different types of helicopter noise have indicated that the most annoying noise is caused by impulsive blade surface pressure changes in descent or forward flight conditions. Blade Vortex Interaction (BVI) is one of the main phenomena producing significant impulsive noise by the unsteady fluctuation in blade loading due to the rapid change of induced velocity field during interaction with vortices shed from previous blades. The tip vortex core structure and the blade vortex miss distance were identified as having a primary influence on BVI. In this thesis, piezoelectrically modulated and/or vectored blowing at the rotor blade tip is theoretically investigated as an active technique for modifying the structure of the tip vortex core as well as for increasing blade vortex miss distance. The mechanisms of formation and convection of rotor blade tip vortices up to and beyond 360 degrees wake age are described based on the CFD results for the baseline cases of a hovering rotor with rounded and square tips. A methodology combining electromechanical and CFD modeling is developed and applied to the study of a piezoelectrically modulated and vectored blowing two-dimensional wing section. The thesis is focused on the CFD analysis of rotor flow with modulated tangential blowing over a rounded blade tip, and with steady mid-plane blade tip blowing, respectively. Computational results characterizing the far-wake flow indicate that for steady tangential blowing the miss distance can be doubled compared to the baseline case, which may lead to a significant reduction in BVI noise level if this trend shown in hover can be replicated in low speed forward flight. Moreover, near-wake flow analysis show that through modulated blowing a higher dissipation of vorticity can be obtained.