Publications

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Aero-acoustic loading has been established as the primary source of excitation for a Flight System at Sandia National Laboratories. However, flight data of this system does not exist, limiting estimations of system or component response in this environment. Therefore, an experimental acoustic simulation was performed on a heavily-instrumented Flight System, using the direct-field acoustic test (DFAT) method with a multi-input multi-output (MIMO) control system. The combination of DFAT and MIMO resulted in attaining uniform and gradient acoustic fields as high as 127 dB OASPL. This paper will discuss the design of the test, the speaker and controller configurations, and the test results of this unique test method. Additionally, an overview of the method used to apply the measured test data to the pressure-loading finite element simulations of the Flight System will be discussed as well.

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The acoustic field generated during a Direct Field Acoustic Test (DFAT) has been analytically modeled in two space dimensions using a properly phased distribution of propagating plane waves. Both the pure-tone and broadband acoustic field were qualitatively and quantitatively compared to a diffuse acoustic field. The modeling indicates significant non-uniformity of sound pressure level for an empty (no test article) DFAT, specifically a center peak and concentric maxima/minima rings. This spatial variation is due to the equivalent phase among all propagating plane waves at each frequency. The excitation of a simply supported slender beam immersed within the acoustic fields was also analytically modeled. Results indicate that mid-span response is dependent upon location and orientation of the beam relative to the center of the DFAT acoustic field. For a diffuse acoustic field, due to its spatial uniformity, mid-span response sensitivity to location and orientation is nonexistent.

Before a spacecraft can be considered for launch, it must first survive environmental testing that simulates the launch environment. Typically, these simulations include vibration testing performed using an electro-dynamic shaker. For some spacecraft however, acoustic excitation may provide a more severe loading environment than base shaker excitation. Because this was the case for a Sandia Flight System, it was necessary to perform an acoustic test prior to launch in order to verify survival due to an acoustic environment. Typically, acoustic tests are performed in acoustic chambers, but because of scheduling, transportation, and cleanliness concerns, this was not possible. Instead, the test was performed as a direct field acoustic test (DFAT). This type of test consists of surrounding a test article with a wall of speakers and controlling the acoustic input using control microphones placed around the test item, with a closed-loop control system. Obtaining the desired acoustic input environment - proto-flight random noise input with an overall sound pressure level (OASPL) of 146.7 dB-with this technique presented a challenge due to several factors. An acoustic profile with this high OASPL had not knowingly been obtained using the DFAT technique prior to this test. In addition, the test was performed in a high-bay, where floor space and existing equipment constrained the speaker circle diameter. And finally, the Flight System had to be tested without contamination of the unit, which required a contamination bag enclosure of the test unit. This paper describes in detail the logistics, challenges, and results encountered while performing a high-OASPL, direct-field acoustic test on a contamination-sensitive Flight System in a high-bay environment.

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In order to create an analytical model of a material or structure, two sets of experiments must be performed-calibration and validation. Calibration experiments provide the analyst with the parameters from which to build a model that encompasses the behavior of the material. Once the model is calibrated, the new analytical results must be compared with a different, independent set of experiments, referred to as the validation experiments. This modeling procedure was performed for a crushable honeycomb material, with the validation experiments presented here. This paper covers the design of the validation experiments, the analysis of the resulting data, and the metric used for model validation.

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