Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    NUMERICAL SIMULATION AND VALIDATION OF HELICOPTER BLADE-VORTEX INTERACTION USING COUPLED CFD/CSD AND THREE LEVELS OF AERODYNAMIC MODELING
    (2014) Amiraux, Mathieu; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Rotorcraft Blade-Vortex Interaction (BVI) remains one of the most challenging flow phenomenon to simulate numerically. Over the past decade, the HART-II rotor test and its extensive experimental dataset has been a major database for validation of CFD codes. Its strong BVI signature, with high levels of intrusive noise and vibrations, makes it a difficult test for computational methods. The main challenge is to accurately capture and preserve the vortices which interact with the rotor, while predicting correct blade deformations and loading. This doctoral dissertation presents the application of a coupled CFD/CSD methodology to the problem of helicopter BVI and compares three levels of fidelity for aerodynamic modeling: a hybrid lifting-line/free-wake (wake coupling) method, with modified compressible unsteady model; a hybrid URANS/free-wake method; and a URANS-based wake capturing method, using multiple overset meshes to capture the entire flow field. To further increase numerical correlation, three helicopter fuselage models are implemented in the framework. The first is a high resolution 3D GPU panel code; the second is an immersed boundary based method, with 3D elliptic grid adaption; the last one uses a body-fitted, curvilinear fuselage mesh. The main contribution of this work is the implementation and systematic comparison of multiple numerical methods to perform BVI modeling. The trade-offs between solution accuracy and computational cost are highlighted for the different approaches. Various improvements have been made to each code to enhance physical fidelity, while advanced technologies, such as GPU computing, have been employed to increase efficiency. The resulting numerical setup covers all aspects of the simulation creating a truly multi-fidelity and multi-physics framework. Overall, the wake capturing approach showed the best BVI phasing correlation and good blade deflection predictions, with slightly under-predicted aerodynamic loading magnitudes. However, it proved to be much more expensive than the other two methods. Wake coupling with RANS solver had very good loading magnitude predictions, and therefore good acoustic intensities, with acceptable computational cost. The lifting-line based technique often had over-predicted aerodynamic levels, due to the degree of empiricism of the model, but its very short run-times, thanks to GPU technology, makes it a very attractive approach.
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    A Physics-Based Approach to Characterizing Helicopter External Noise Radiation from Ground-Based Noise Measurements
    (2008) Greenwood, Eric; Schmitz, Fredric H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis describes a new method of characterizing the external noise radiation of a helicopter suitable for use in the generation of ground noise contours for community land use planning and assessing the acoustic observability of helicopter flight plans. This work is an extension of the semi-empirical Rotorcraft Noise Model / Quasi-Static Acoustic Mapping (RNM/Q-SAM) methodology of characterizing helicopter externally radiated noise using acoustic radiation hemispheres. Current methods of interpolation of data on RNM acoustic radiation spheres are found to lead to high levels of inaccuracy when using sparse microphone arrays. A new method of interpolation based on the theory of radial basis functions is developed in this thesis and shown to lead to significantly improved accuracy. This thesis also extends the RNM/Q-SAM methodology to turning flight conditions. New test procedures are developed for steady turning flight conditions and then used in the acoustic flight testing of the Bell 206B helicopter. The extended RNM/Q-SAM method is applied to the resulting data set in order to generate the first acoustic radiation hemispheres for a helicopter in steady turning flight across a range of flight path angles. The results indicate that the extended Quasi-Static Acoustic Mapping technique is valid for steady turning flight Blade-Vortex Interaction noise. Furthermore, steady turning flight alone is shown not to lead to large increases in externally radiated noise compared to similar straight-line flight conditions. This indicates that high BVI noise levels reported during turns in prior research were most likely caused by transient maneuvers and not turning flight alone.