Comparison of Laser Doppler Vibrometry and Digital Image Correlation measurement techniques for applications vibroacoustics

Laser-Doppler Vibrometry (LDV) is a state-of-the-art contactless measurement technique able of capturing the velocity response at a single measurement point, up into the high-frequency range with high accuracy. With scanning LDV, even the velocity field of the surface of a test object can be observed from a sequence of repeated measurements. A 3D LDV system is able to measure all 3 spatial components of velocity, in-plane as well as out-of-plane. However, the most common 1D LDV systems are only able to measure the velocity component in the direction of the laser beam. With proper setup of the system, this corresponds to the out-of-plane component. An alternative contactless measurement technique that had arisen in the last decade is the Digital Image Correlation (DIC). This technique allows to measure the displacement fields of structures based on a full-field method using a series of images taken over time at discrete time intervals. While the measurement time of both methods highly depend on the set parameters, the DIC has the advantage that in-plane velocities are measured without additional effort. In this paper a comparison between measured data acquired using both techniques will be performed. The targeted application is conducting measurements on lightweight structures in the vibroacoustic frequency range. To this end, the upper frequency limit shall be determined, at which the DIC data quality is comparable to the LDV data, regarding out-of-plane components. Additionally, both data sets will be used to estimate structural intensity vector fields for quality and performance estimation

The natural Helmholtz-Hodge decomposition for open-boundary flow analysis

pre-printThe Helmholtz-Hodge decomposition (HHD), which describes a flow as the sum of an incompressible, an irrotational, and a harmonic flow, is a fundamental tool for simulation and analysis. Unfortunately, for bounded domains, the HHD is not uniquely defined, traditionally, boundary conditions are imposed to obtain a unique solution. However, in general, the boundary conditions used during the simulation may not be known known, or the simulation may use open boundary conditions. In these cases, the flow imposed by traditional boundary conditions may not be compatible with the given data, which leads to sometimes drastic artifacts and distortions in all three components, hence producing unphysical results. This paper proposes the natural HHD, which is defined by separating the flow into internal and external components. Using a completely data-driven approach, the proposed technique obtains uniqueness without assuming boundary conditions a priori. As a result, it enables a reliable and artifact-free analysis for flows with open boundaries or unknown boundary conditions. Furthermore, our approach computes the HHD on a point-wise basis in contrast to the existing global techniques, and thus supports computing inexpensive local approximations for any subset of the domain. Finally, the technique is easy to implement for a variety of spatial discretizations and interpolated fields in both two and three dimensions

Force sensing to reconstruct potential energy landscapes for cluttered large obstacle traversal

Visual sensing of environmental geometry allows robots to use artificial potential fields to avoid sparse obstacles. Yet robots must further traverse cluttered large obstacles for applications like search and rescue through rubble and planetary exploration across Martain rocks. Recent studies discovered that to traverse cluttered large obstacles, multi-legged insects and insect-inspired robots make strenuous transitions across locomotor modes with major changes in body orientation. When viewed on a potential energy landscape resulting from locomotor-obstacle physical interaction, these are barrier-crossing transitions across landscape basins. This potential energy landscape approach may provide a modeling framework for cluttered large obstacle traversal. Here, we take the next step toward this vision by testing whether force sensing allows the reconstruction of the potential energy landscape. We developed a cockroach-inspired, minimalistic robot capable of sensing obstacle contact forces and torques around its body as it propelled forward against a pair of cluttered grass-like beam obstacles. We performed measurements over many traverses with systematically varied body orientations. Despite the forces and torques not being fully conservative, they well-matched the potential energy landscape gradients and the landscape reconstructed from them well-matched ground truth. In addition, inspired by cockroach observations, we found that robot head oscillation during traversal further improved the accuracies of force sensing and landscape reconstruction. We still need to study how to reconstruct landscape during a single traverse, as in applications, robots have little chance to use multiple traverses to sample the environment systematically and how to find landscape saddles for least-effort transitions to traverse

Doctor of Philosophy

dissertationWith modern computational resources rapidly advancing towards exascale, large-scale simulations useful for understanding natural and man-made phenomena are becoming in- creasingly accessible. As a result, the size and complexity of data representing such phenom- ena are also increasing, making the role of data analysis to propel science even more integral. This dissertation presents research on addressing some of the contemporary challenges in the analysis of vector fields--an important type of scientific data useful for representing a multitude of physical phenomena, such as wind flow and ocean currents. In particular, new theories and computational frameworks to enable consistent feature extraction from vector fields are presented. One of the most fundamental challenges in the analysis of vector fields is that their features are defined with respect to reference frames. Unfortunately, there is no single ""correct"" reference frame for analysis, and an unsuitable frame may cause features of interest to remain undetected, thus creating serious physical consequences. This work develops new reference frames that enable extraction of localized features that other techniques and frames fail to detect. As a result, these reference frames objectify the notion of ""correctness"" of features for certain goals by revealing the phenomena of importance from the underlying data. An important consequence of using these local frames is that the analysis of unsteady (time-varying) vector fields can be reduced to the analysis of sequences of steady (time- independent) vector fields, which can be performed using simpler and scalable techniques that allow better data management by accessing the data on a per-time-step basis. Nevertheless, the state-of-the-art analysis of steady vector fields is not robust, as most techniques are numerical in nature. The residing numerical errors can violate consistency with the underlying theory by breaching important fundamental laws, which may lead to serious physical consequences. This dissertation considers consistency as the most fundamental characteristic of computational analysis that must always be preserved, and presents a new discrete theory that uses combinatorial representations and algorithms to provide consistency guarantees during vector field analysis along with the uncertainty visualization of unavoidable discretization errors. Together, the two main contributions of this dissertation address two important concerns regarding feature extraction from scientific data: correctness and precision. The work presented here also opens new avenues for further research by exploring more-general reference frames and more-sophisticated domain discretizations

Inlet swirl distortion effects on the generation and propagation of fan rotor shock noise

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 195-200).A body-force-based fan model for the prediction of multiple-pure-tone noise generation is developed in this thesis. The model eliminates the need for a full-wheel, three-dimensional unsteady RANS simulation of the fan blade row, allowing Euler calculations to be used to capture the phenomena of interest. The Euler calculations reduce numerical wave dissipation and enable the simultaneous computation of source noise generation and propagation through the engine inlet to the far-field in non-uniform flow. The generated shock Mach numbers are in good agreement with experimental results, with the peak values predicted within 6%. An assessment of the far-field acoustics against experimental data showed agreement of 8 dB on average for the blade-passing tone. In a first-of-its-kind comparison, noise generation and propagation are computed for a fan installed in a conventional inlet and in a boundary-layer-ingesting serpentine inlet for a free-stream Mach number of 0.1. The key effect of boundary layer ingestion is the creation of streamwise vorticity which is ingested into the inlet, resulting in co- and counter-rotating streamwise vortices in the inlet. The fan sound power level increases by 38 dB due to this distortion, while the vortex whose circulation is in the same direction as the fan rotation enhances the sound power attenuation within the inlet duct such that the far-field overall sound pressure levels are increased by only 7 dB on average. The far-field spectra are altered in the following manner due to inlet distortion: (1) tones at up to 3 times the blade-passing frequency are amplified; and (2) tones above one-half of the blade-passing frequency are attenuated and appear to be cut-off. To quantify the effects of serpentine inlet duct geometry on the generation and propagation of multiple-pure-tone noise, a parametric study of inlets is conducted. The conclusions are that (1) the ingestion of streamwise vorticity alters multiple-pure-tone noise more than changes in inlet area ratio or offset ratio do; and (2) changes in the far-field spectra relative to the conventional inlet results are only weakly affected by the duct geometry changes investigated and are instead predominantly caused by flow non-uniformities. A response-surface correlation for the effects of inlet geometry on far-field noise is also developed.by Jeff Defoe.Ph.D