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Mitigation of structural-acoustic mode coupling in a modal test of a hollow structure

Conference Proceedings of the Society for Experimental Mechanics Series

Schultz, Ryan S.; Pacini, Ben

A phenomenon in which structural and internal acoustic modes couple is occasionally observed during modal testing. If the structural and acoustic modes are compatible (similar frequencies and shapes), the structural mode can split into two separate modes with the same shape but different frequencies; where one mode is expected, two are observed in the structural response. For a modal test that will inform updates to an analytical model (e.g. finite element), the test and model conditions should closely match. This implies that a system exhibiting strongly coupled structural-acoustic modes in test should have a corresponding analytical model that captures that coupling. However, developing and running a coupled structural-acoustic finite element model can be challenging and may not be necessary for the end use of the model. In this scenario, it may be advantageous to alter the test conditions to match the in-vacuo structural model by de-coupling the structural and acoustic modes. Here, acoustic absorption material was used to decouple the modes and attempt to measure the in-vacuo structural response. It was found that the split peak could be eliminated by applying sufficient acoustic absorbing material to the air cavity. However, it was also observed that the amount of acoustic absorbing material had an effect on the apparent structural damping of a second, separate mode. Analytical and numerical methods were used to demonstrate how coupled systems interact with changes to damping and mode frequency proximity while drawing parallels to the phenomena observed during modal tests.

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TYPE Conference Poster YEAR 2017
ScopusOSTIDOI

Evaluation of microphone density for finite element source inversion simulation of a laboratory acoustic test

Conference Proceedings of the Society for Experimental Mechanics Series

Schultz, Ryan S.; Walsh, Timothy W.

Simulation of the response of a system to an acoustic environment is desirable in the assessment of aerospace structures in flight-like environments. In simulating a laboratory acoustic test a large challenge is modeling the as-tested acoustic field. Acoustic source inversion capabilities in Sandia’s Sierra/SD structural dynamics code have allowed for the determination of an acoustic field based on measured microphone responses—given measured pressures, source inversion optimization algorithms determine the input parameters of a set of acoustic sources defined in an acoustic finite element model. Inherently, the resulting acoustic field is dependent on the target microphone data. If there are insufficient target points, then the as-tested field may not be recreated properly. Here, the question of number of microphones is studied using synthetic data, that is, target data taken from a previous simulation which allows for comparison of the full pressure field—an important benefit not available with test data. By exploring a range of target points distributed throughout the domain, a rate of convergence to the true field can be observed. Results will be compared with the goal of developing guidelines for the number of sensors required to aid in the design of future laboratory acoustic tests to be used for model assessment.

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TYPE Conference Poster YEAR 2016
ScopusOSTIDOI

Performing direct-field acoustic test environments on a sandia flight system to provide data for finite element simulation

Conference Proceedings of the Society for Experimental Mechanics Series

Stasiunas, Eric C.; Schultz, Ryan S.; Ross, Michael R.

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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TYPE Conference Poster YEAR 2016
ScopusOSTIDOI
Results 76–95 of 95
Results 76–95 of 95