Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item CONTRIBUTIONS TOWARDS THE UNDERSTANDING OF ROTOR-INDUCED DUST PARTICLE MOBILIZATION AND TRANSPORT(2014) Sydney, Anish Joshua; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To better understand the problem of rotor-induced particle motion and rotorcraft brownout, time-resolved, dual-phase particle image velocimetry and particle tracking velocimetry measurements were made in the flow produced by a small laboratory rotor that was hovering over a ground plane covered with a mobile sediment bed. To investigate the three-dimensionality of the wake and resultant particle field, flow measurements were made in vertical and horizontal planes around the rotor and near the ground. The primary goals of the work were to: 1. Characterize the fundamental flow physics of a rotor wake interacting with a sediment bed; 2. Investigate how rotor operating parameters, such as the disk loading, blade loading coefficient, and wake shedding frequency affected the mobilization, uplift and overall development of the particle field; 3. Examine the effects of placing a body between the rotor and the ground to understand how the interactions of the rotor wake with the body affected the transport of particles from the bed. The results showed that the rotor wake was very three-dimensional, with highly non-uniform velocities near the ground that resulted in the radially asymmetric mobilization, uplift and suspension of particles. The tip vortices were found to be the primary contributor to the uplift of particles, with the aperiodic variations in their trajectories near the ground causing intermittent particle mobilization events. These effects were caused, in part, by wave-like displacements that developed along the lengths of the tip vortices, which caused some parts of the filaments to convect closer to the ground than other parts and so uplift discrete bursts or plumes of particles. The quantity and distribution of uplifted particles were shown to be affected by the operating condition of the rotor, with the overall complexity of the rotor wake generally resulting in the formation of a highly three-dimensional and time-varying particle field. The rotor operating parameters were shown to interdependently alter the characteristics of the groundwash flow and the tip vortices produced by the rotor. Stronger wake vortices that impinged on the bed generally uplifted more particles, however, higher near-wall flow velocities over the bed also convected particles further downstream before they could be suspended. The near-wall flow developments were further complicated by the interaction of the rotor wake with a body, which significantly distorted the development of the rotor wake at the ground, the resulting near-wall flow velocities generally being lower in magnitude. The degree of wake distortion, however, was found to be sensitive to the cross-sectional shape of the body. In cases where there was direct impingement of the tip vortices on the body surfaces, the distortions to the wake caused lower near-wall flow velocities but still contained vortices that were able to suspend sediment particles radially closer to the rotor compared to the isolated rotor case.Item UNDERSTANDING BROWNOUT USING DUAL-PHASE PARTICLE IMAGE VELOCIMETRY MEASUREMENTS(2011) Sydney, Anish Joshua; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To better understand the development of brownout dust clouds generated by rotor- craft, the dual-phase flow environment produced by one- and two-bladed laboratory-scale rotors operating over a mobile sediment bed were studied. Three size ranges of character- ized glass microspheres were used to represent the sediment particles. Time-resolved flow visualization, particle image velocimetry, and particle tracking velocimetry were used to make the flow measurements. The high imaging rate of these systems allowed the time- history of the rotor wake interactions with the sediment bed to be documented, providing a better understanding of the transient processes and mechanisms that lead to the uplift of sediment and the formation of dust clouds near a rotor in ground effect operation. In particular, the fluid dynamics of the blade tip vortices near the bed were examined in detail, which were shown to have a primary influence on the mobilization of sediment. In general, the near-wall measurements documented at least five fundamental uplift and sediment transport mechanisms below the rotor: 1. Creep, 2. Modified saltation and saltation bombardment, 3. Vortex induced trapping, 4. Reingestion bombardment (local and global), and 5. Secondary suspension. In addition, a further mechanism related to the local unsteady pressure field induced by the convecting wake vortices was hypothesized to contribute to the uplift of sediment. The highest sediment entrainment levels occurred within the wake impingement zone, mainly from the erosion aspects of the tip vortices on the bed. Once entrained, significant quantities of sediment were intermittently trapped in the vortex-induced upwash field. Secondary sediment suspension was found to be more prevalent with the two-bladed rotor because of the propensity for merging of adjacent blade tip vortices and the resulting higher upwash velocities. The trapping of suspended sediment particles into the vortex flow was shown to cause recirculation of the particles back onto the sediment bed, thereby ejecting more sediment through bombardment mech- anisms.