Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item CFD/Quasi-Steady Coupled Trim Analysis of Diptera-type Flapping Wing MAV in Steady Flight(2016) Badrya, Camli; Baeder, James D.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The nuances in flapping wing aerodynamics are not yet fully understood to the extent where concepts can be translated to practical designs. Trimmed flight is a fundamental concept for aircraft in general. It describes the flight condition when there are no accelerations on the vehicle. From an engineering perspective, trim estimation is essential for performance analysis and flapping wing vehicle design. Without an efficient trim algorithm, trial-and-error based identification of the trimmed wing kinematics is computationally expensive for any flight condition, because the large number of simulations required make the process impractical. In a global sense the nature of forces produced by flapping wings closely resemble those on a helicopter blade, such that an analogy can be drawn between the two. Therefore, techniques developed for helicopter performance calculations are adapted and applied to the flapping wing platform particularly for analyzing steady flight. Using a flight dynamic model of the insect, which comes embedded with simplified quasi- steady wing aerodynamics and is coupled to high-fidelity CFD analysis, trim solutions are obtained in realistic time frames. This procedure is analogous to rotorcraft periodic coupling for trim. This multi-fidelity approach, where many quasi-steady calculations are combined with a judicious number of CFD simulations, may be used in parametric sweeps and design studies to improve hover and cruise performance. It was shown that the coupled trim methodology based on the QS model is capable of driving the CFD towards a stable trim solution. In forward flight the trim procedure tilts the stroke plane resulting in lift generation during downstroke and propulsive force during upstroke. The airloads, thrust and power are affected by the trim parameters, and the CFD/QS methodology accurately accounted for these inter-dependencies. Also it is observed that power initially decreases as an insect goes from hover to forward flight. Furthermore, the lift-to-power ratio versus average lift was identified as a principal efficiency metric to assess the performance of flapping-wing vehicles for a given geometry and kinematic parameters.Item Investigation of Aerodynamics of Flapping Wings for Miro Air Vehicle Applications(2013) Malhan, Ria Pavnish; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A coupled CFD-CSD solver was used to simulate the aerodynamics of a flexible flapping wing. The CFD solver is a compressible RANS (Reynolds Averaged Navier Stokes) solver. Multibody dynamics solver `MBDyn', was used as the structural solver to take into account non linear shell straining, making it possible to analyze low aspect ratio wings with large deformations. Validation of the two codes was carried out independently. The solvers were then coupled using python and validated against prior experiments and analysis on spanwise and chordwise flexible wings. As realistic MAV wings are extremely flexible and lightweight, under the effect of high inertial and aerodynamic forces, they undergo large non linear deformations over a flap cycle. However, there is a dearth of experimental data on well characterized flapping wings (with known structural and mass properties) at MAV-scale Reynolds numbers. Systematic experiments were carried out on rigid and flexible flapping wings in an open jet wind tunnel and forces were measured using a test bed. Pure flapping of rigid wings did not generate sufficient propulsive force and may not be a viable configuration. Passive pitching of rigid wing generated both, target vertical and propulsive forces. Dynamic wing twist was then incorporated using flexible wings. A flexible wing was fabricated using a combination of unidirectional carbon fiber strips (chordwise ribs), carbon rod (leading edge spar) and mylar film (membrane). Structural model of the wing (combination of beam and shell elements) was developed and then coupled to the CFD model. CFD-CSD analysis of flexible wing was carried out and good correlation was obtained for all the configurations. This comprehensive experimental data set can also be used to validate other aeroelastic analyses of the future. Further, the analysis was used to gain more insights into flow physics. It was observed that as a result of flexibility, by taking advantage of unsteady flow features, a lighter, simpler mechanism could be used to produce larger forces than a rigid wing. The validated, comprehensive analysis developed in this work may serve as a design tool for deciding configurations and wing kinematics of next generation MAVs.Item A Comparative Framework for Maneuverability and Gust Tolerance of Aerial Microsystems(2012) Campbell, Renee; Humbert, James S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerial microsystems have the potential of navigating low-altitude, cluttered environments such as urban corridors and building interiors. Reliable systems require both agility and tolerance to gusts. While many platform designs are under development, no framework currently exists to quantitatively assess these inherent bare airframe characteristics which are independent of closed loop controllers. This research develops a method to quantify the maneuverability and gust tolerance of vehicles using reachability and disturbance sensitivity sets. The method is applied to a stable flybar helicopter and an unstable flybarless helicopter, whose state space models were formed through system identification. Model-based static H-infinity controllers were also implemented on the vehicles and tested in the lab using fan-generated gusts. It is shown that the flybar restricts the bare airframe's ability to maneuver in translational velocity directions. As such, the flybarless helicopter proved more maneuverable and gust tolerant than the flybar helicopter. This approach was specifically applied here to compare stable and unstable helicopter platforms; however, the framework may be used to assess a broad range of aerial microsystems.Item Unsteady Low Reynolds Number Aerodynamics of a Rotating Wing(2012) Kolluru Venkata, Siddarth; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Micro air vehicles (MAVs) are small, unmanned aircraft useful for reconnaissance. The small size of MAVs presents unique challenges as they operate at low Reynolds numbers O(10^4), and they share a flight regime with insects rather than conventional aircraft. The low Reynolds number regime is dominated by poor aerodynamic characteristics such as low lift-to-drag ratios. To overcome this, birds and insects utilize unsteady high lift mechanisms to generate sufficient lift. A leading edge vortex (LEV), one of these unsteady lift mechanism, is thought to be responsible for the high lift generated by these natural fliers, but the factors which contribute to the formation, stability, and persistence of LEVs are still unclear. The objectives of this study are to: 1) qualitatively understand the formation of the LEV by evaluating the effect of wing acceleration profiles, wing root geometry, Reynolds number, and unsteady variations of pitch, 2) quantify whether high lift can be sustained at low Reynolds numbers on a rotary wing in continuous revolution, and 3) determine the effect of wing flexibility on the unsteady lift coefficient. Experiments were performed on a rotating wing setup designed to model the translational phase of the insect wing stroke during hover. Experiments were performed in a water tank at Reynolds numbers between 5,000 and 25,000, and the flow was investigated using dye flow visualization, as well as lift and drag force measurements. A rigid wing and a simple one degree-of-freedom flexible wing were tested. Dye flow visualization on a rotating wing showed the formation of a coherent LEV near the wing root. The LEV became less coherent further outboard, and eventually burst. As the wing continued to rotate, the location where the LEV burst moved inboard. Dye injection within the burst vortex showed the formation of multiple small scale shedding vortices that traveled downstream and formed a region of recirculating flow (i.e., a burst vortex). Parameter variations in this experiment included velocity profiles, acceleration profiles, and Reynolds numbers. High lift and drag coefficient peaks were measured during the acceleration phase of the wing stroke at Reynolds numbers of 15,000 and 25,000. After the initial peak, the coefficients dropped, increased, and eventually attained a ``steady-state" intermediate value after 5 chord-lengths of travel, which they maintained for the remainder of the first revolution. When the wing began the second revolution, both the lift and drag coefficients decreased, and leveled out at a second intermediate value. Force measurements on a chordwise flexible wing revealed lower lift coefficients. For all of the cases tested, high lift was achieved during the acceleration phase and first revolution of the wing stroke, though values dropped during the second revolution.