Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item DEVELOPMENT OF A LAGRANGIAN-LAGRANGIAN METHODOLOGY TO PREDICT BROWNOUT DUST CLOUDS(2012) Syal, Monica; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Lagrangian-Lagrangian dust cloud simulation methodology has been developed to help better understand the complicated two-phase nature of the rotorcraft brownout problem. Brownout conditions occur when rotorcraft land or take off from ground surfaces covered with loose sediment such as sand and dust, which decreases the pilot's visibility of the ground and poses a serious safety of flight risk. The present work involved the development of a comprehensive, computationally efficient three-dimensional sediment tracking method for dilute, low Reynolds number Stokes-type flows. The flow field generated by a helicopter rotor in ground effect operations over a mobile sediment bed was modeled by using an inviscid, incompressible, Lagrangian free-vortex method, coupled to a viscous semi-empirical approximation for the boundary layer flow near the ground. A new threshold model for the onset of sediment mobility was developed by including the effects of unsteady pressure forces that are induced in vortically dominated rotor flows, which can significantly alter the threshold conditions for particle motion. Other important aspects of particle mobility and uplift in such vortically driven dust flows were also modeled, including bombardment effects when previously suspended particles impact the bed and eject new particles. Bombardment effects were shown to be a particularly significant contributor to the mobilization and eventual suspension of large quantities of smaller-sized dust particles, which tend to remain suspended. A numerically efficient Lagrangian particle tracking methodology was developed where individual particle or clusters of particles were tracked in the flow. To this end, a multi-step, second-order accurate time-marching scheme was developed to solve the numerically stiff equations that govern the dynamics of particle motion. The stability and accuracy of this scheme was examined and matched to the characteristics of free-vortex method. One-way coupling of the flow and the particle motion was assumed. Particle collisions were not considered. To help reduce numerical costs, the methodology was implemented on graphic processing units, which gave over an order of magnitude reduction in simulation time without any loss in accuracy. Validation of the methodology was performed against available measurements, including flow field measurements that have been made with laboratory-scale and full-scale rotors in ground effect operations. The predicted dust clouds were also compared against measurements of developing dust clouds produced by a helicopter during taxi-pass and approach-to-touchdown flight maneuvers. The results showed that the problem of brownout is mostly driven by the local action of the rotor wake vortices and the grouping or bundling of vortex filaments near the sediment bed. The possibilities of mitigating the intensity of brownout conditions by diffusing the blade tip vortices was also explored. While other means of brownout mitigation may be possible, enhancing the diffusion of the tip vortices was shown to drastically reduce the quantity of mobilized particles and the overall severity of the brownout dust cloud.Item Contributions to the Aerodynamic Optimization of a Coaxial Rotor System(2008-10-23) Syal, Monica; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The present work analyses the aerodynamic complexities involved in the design of a coaxial rotor system in an attempt to maximize its performance in hover and forward flight. The aerodynamic methodologies of the simple momentum theory (SMT), the blade element momentum theory (BEMT), and a free vortex wake method (FVM) are used to help study this problem. It is shown that because of the inter-rotor aerodynamic interference effects, as well as the requirement of a torque balanced operating condition, the two rotors of the coaxial system generally operate at different thrust and different aerodynamic loadings. Therefore, for an optimally performing coaxial system, the geometric designs of the two rotors can expected to be different. To this end, parametric studies were performed to understand the effects of changes in inter-rotor spacings, blade twist, and blade planforms on both the upper and lower rotors. A more formal optimization was attempted by coupling FVM with an optimizer to find the best rotor geometry (if any) to maximize the figure of merit in hover or to minimize the total power required in forward flight. It was shown that the performance of the coaxial rotor system can, indeed, be improved significantly by having different blade geometries on the upper and lower rotors. However, it was also shown that the blade twist distribution has more significant effects on the rotor performance as compared to the blade planform shapes. The baseline geometry for all the optimization analyses had untwisted blades on both rotors. It was shown that a higher inter-rotor spacing is desired to reduce the interference effects between the two rotors in hovering flight. However, the spacing distance can be limited by the increased rotor weight and increased parasitic drag in forward flight. The results also show that a high blade twist is desired on the upper rotor to reduce the induced losses of the coaxial system, whereas a high blade twist on the bottom rotor increases the induced losses of the coaxial system. In forward flight, the results showed that at high advance ratios the aerodynamic interactions between the two rotors become smaller, and both rotors behave almost as isolated rotors. Parametric studies were also performed to study the effects of changing linear twist rates on both the rotors of the coaxial system in forward flight. The results showed that the total power required at an advance ratio of 0.25 is insensitive to the changes in the blade twist on upper and lower rotors. This outcome also showed that the optimum blade shapes obtained for hovering flight also offered better performance in forward flight.