Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Measurements and Modeling of the Unsteady Flow around a Thin Wing(2018) Manar, Field; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unsteady separated flows are encountered in many applications (e.g. dynamic stall in helicopters and wind turbines). Recent efforts to better understand the problem of unsteady separated aerodynamics have been prompted by growing interest in creating small-scale flight vehicles, termed micro air vehicles (MAVs). Because of their small size, all MAVs operate at low Reynolds numbers. In that regime, flow separation is a common occurrence either due to Reynolds number effects or aggressive motion. The dominant and most studied feature of these flows is the leading edge vortex (LEV). The LEV receives its circulation from a shear layer emanating from the leading edge of the wing, where the production of circulation occurs. In spite of its importance to the flow and the resulting forces, the production of circulation has received relatively little experimental attention. To fill this gap, water tank experiments on a surging flat plate wing at a high angle of attack have been performed at varying Reynolds number, acceleration, angle of attack, and aspect ratio. These experiments measured time resolved forces, LEV location, LEV circulation, and leading edge circulation production. These data were then used to explore how the LEV and the circulation production reacts to changes in kinematic parameters. This resulted in the proposal of a new relationship between the wake state and the leading edge circulation production, termed the boundary layer analogy (BLA). Additionally, existing potential flow modeling techniques were implemented and evaluated against the present experimental data. This analysis focused on evaluating the suitability of applying the Kutta condition at the leading edge. The Kutta condition was found to be a valid leading edge condition capable of predicting the LEV circulation seen in experiments. Representing the shed wake with multiple vortices was found to be necessary to capture the dynamics of vortex roll up and shedding. Other models struggle to account for these events, though simpler models may offer a better route to intuitive understanding of the fluid dynamic origin of the forces. The experimental data collected here, coupled with the novel analysis of the modeling techniques in the light of the leading edge circulation measurements, constitutes a significant step forward in the modeling and understanding of unsteady separated flows.Item Assessment of Turbulence Length Scales in Hybrid RANS-LES Methods(2017) Jain, Nishan; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Separated flows are common in many scenarios of practical interest. Key examples of these scenarios include static stall over fixed wing aircraft and dynamic stall over rotorcraft blades. During rotor operation at high advance ratio, the stall events lead to loss in performance of the rotorcraft and may cause severe aerodynamic loads. In order to mitigate vibratory loads, it is important to evaluate the involved flow physics as accurately as possible. It is well known that a complex rotor flow field involving separation and reverse flow cannot be numerically predicted reliably by classical RANS model. At the other end, using high-fidelity approaches such as DNS and LES to resolve the rotor flow-field at practical Reynolds number is beyond the current computational capabilities. Therefore, the main objective of this work is to develop a high-fidelity modeling framework for capturing flow features that are important for predicting stall events while remaining computationally affordable. The framework employs and refines DES type hybrid RANS-LES methods along with specialized numerical techniques from literature to accurately resolve incipient separated flows under static and dynamic conditions. A baseline computational framework comprising of well established laminar-turbulent transition model, adverse pressure gradient (APG) correction and a low Mach number correction is selected as a starting point. By conducting simulations of flow over SC1095 airfoil at near-stall regime using the baseline framework, the importance of regulating eddy viscosity in the outer part of the shear layer is realized. Sub-grid length scales from the literature are implemented into the in-house computational solvers and their sensitivity in generating the eddy viscosity is investigated. A novel length scale called SSM length scale is proposed based on the properties of available length scales and the grid requirements in mildly separated flows. Proposed length scale demonstrated good predictive capabilities in mildly separated flows under static conditions by reducing eddy viscosity levels at the outer region boundary layer. Three-dimensional dynamic stall simulations are also conducted on flow over the modified VR12 airfoil. With SSM length scale, DDES method predictions agreed well with experimental data and captured the cycle-to-cycle variation of integrated aerodynamic quantities. The undesirable weakening of conventional shielding is observed due to proposed length scale in a highly resolved computational domain. A novel and stronger shielding formulation are proposed based on the properties of available length scales. The combination of new shielding and SSM length scale demonstrated good predictive capabilities in near stall regime without any undesirable effects. The combination also eliminated the need for adverse pressure gradient correction. The final computational framework proved to be robust towards grid resolution and varying flow separation and provided highly accurate aerodynamic characteristics for rotorcraft airfoils exhibiting stall events in the complete angle of attack range.Item An Experimental Study of Static and Oscillating Rotor Blade Sections in Reverse Flow(2015) Lind, Andrew Hume; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The rotorcraft community has a growing interest in the development of high-speed helicopters to replace outdated fleets. One barrier to the design of such helicopters is the lack of understanding of the aerodynamic behavior of retreating rotor blades in the reverse flow region. This work considers two fundamental models of this complex unsteady flow regime: static and oscillating (i.e., pitching) airfoils in reverse flow. Wind tunnel tests have been performed at the University of Maryland (UMD) and the United States Naval Academy (USNA). Four rotor blade sections are considered: two featuring a sharp geometric trailing edge (NACA 0012 and NACA 0024) and two featuring a blunt geometric trailing edge (ellipse and cambered ellipse). Static airfoil experiments were performed at angles of attack through 180 deg and Reynolds numbers up to one million, representative of the conditions found in the reverse flow region of a full-scale high-speed helicopter. Time-resolved velocity field measurements were used to identify three unsteady flow regimes: slender body vortex shedding, turbulent wake, and deep stall vortex shedding. Unsteady airloads were measured in these three regimes using unsteady pressure transducers. The magnitude of the unsteady airloads is high in the turbulent wake regime when the separated shear layer is close to the airfoil surface and in deep stall due to periodic vortex-induced flow. Oscillating airfoil experiments were performed on a NACA 0012 and cambered ellipse to investigate reverse flow dynamic stall characteristics by modeling cyclic pitching kinematics. The parameter space spanned three Reynolds numbers (165,000; 330,000; and 500,000), five reduced frequencies between 0.100 and 0.511, three mean pitch angles (5,10, and 15 deg), and two pitch amplitudes (5 deg and 10 deg). The sharp aerodynamic leading edge of the NACA 0012 airfoil forces flow separation resulting in deep dynamic stall. The number of associated vortex structures depends strongly on pitching kinematics. The cambered ellipse exhibits light reverse flow dynamic stall for a wide range of pitching kinematics. Deep dynamic stall over the cambered ellipse airfoil is observed for high mean pitch angles and pitch amplitudes. The detailed results and analysis in this work contributes to the development of a new generation of high-speed helicopters.Item A Coupled CFD/CSD Investigation of the Effects of Leading Edge Slat on Rotor Performance(2012) Mishra, Asitav; Baeder, James D.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A coupled Computational Fluid Dynamic (CFD) and Computational Structural Dynamics (CSD) methodology is extended to analyze the effectiveness of a leading edge slat (LE-Slat) for mitigating the adverse effects of dynamic stall on rotor blade aerodynamic and dynamic response. This involved the following improvements over the existing CFD methodology to handle a multi-element airfoil rotor: incorporating the so-called Implicit Hole Cutting method for inter-mesh connectivity, implementing a generalized force transfer routine for transferring LE-Slat loads onto the main blade, and achieving increased parallelization of the code. Initially, the structured overset mesh CFD solver is extensively validated against available 2-D experimental wind tunnel test cases in steady and unsteady flight conditions. The solver predicts the measurements with sufficient accuracy for test cases with both the baseline airfoil and that with two slat configurations, S-1 and S-6. As expected, the addition of the slat is found to be highly effective in delaying stall until larger angles for the case of a static airfoil and ameliorating the effects of dynamic stall for a 2-D pitching airfoil. The 3-D coupled CFD/CSD model is extensively validated against flight test data of a UH-60A rotor in a high-altitude, high-thrust flight condition, namely C9017, characterized by distinct dynamic stall events in the retreating side of the rotor disk. The validated rotor analysis tool is then used to successfully demonstrate the effectiveness of a LE-Slat in mitigating (or eliminating) dynamic stall on the rotor retreating side. The calculations are performed with a modified UH-60A blade with a 40%-span slatted airfoil section. The addition of the slat is effective in the mitigation (and/or elimination) of lift and moment stall at outboard stations, which in turn is accompanied by a reduction of torsional structural loads (upto 73%) and pitch link loads (upto 62%) as compared to the baseline C9017 values. The effect of a dynamically moving slat, actuating between slat positions S-1 and S-6, is thoroughly investigated, firstly on 2-D airfoil dynamic stall, and then on the UH-60A rotor. Three slat actuation strategies with upto [1, 3, 5]/rev harmonics, respectively, are considered. However, it is noted that the dynamic slat does not necessarily result in better rotor performance as compared to a static slat configuration. The coupled CFD/CSD platform is further used to successfully demonstrate the capability of the slat (S-6) to achieve upto 10% higher thrust than C9017, which is beyond the conventional thrust limit imposed by McHugh's stall boundary. Stall mitigation due to the slat results in a reduction of torsional load upto 54% and reduction of pitch link load upto 32% as compared to the baseline C9017 flight test values, even for an increase in thrust of 10%.