Page 27 - Summer 2020
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and, perhaps more importantly, significantly more data can be gathered with ground-based acquisition systems.
The advantages of ground-based acoustic testing come with a challenge, however, in that one needs to engi- neer specific acoustic fields that emulate what would be seen in-flight. Given the large-domain sizes and high- frequency ranges of interest, these problems typically have many wavelengths in the domain, thus making HPC with finite elements an attractive solution strategy.
Ground-based testing goes hand in hand with compu- tational acoustics modeling. Figure 3A shows a typical experimental setup of a ground-based acoustics test at Sandia National Labs, and Figure 3B shows a corre- sponding finite-element model of a representative test (Schultz et al., 2015). The ground-based test can tell us with high confidence the mechanical response from a certain loading environment at specific sensor locations. However, numerical simulations are used to test large numbers of different loading environments and provide the response at all points of a model, something impossi- ble to do with experiments alone. These models typically reach sizes of hundreds of millions of finite elements.
Example: Orion Capsule in Ground-Based Acoustic Test
As an example of ground-based acoustic testing, we pres-
ent a numerical model of a reverberation chamber test on the Orion capsule. Reverberation (reverb) chambers are rooms designed to produce a diffuse sound field around an object of interest, which is a common condi- tion in flight environments. A diffuse sound field is an acoustic environment where the acoustic energy density is the same at all locations. By understanding structural response to a diffuse field, including absorption coef- ficients and transmission loss, the structural behavior during launch and reentry can be characterized.
The sound absorbability is determined by the change in reverb time of the test object. Acoustic excitation can be accomplished with a variety of different source arrange- ments. Reverb room tests are very important for ground testing flight objects that will be excited to uniformly high random pressure loads while in use.
To demonstrate a simulation of a reverb chamber test, we present a numerical simulation of a three-quarter scale version of the Orion capsule (crew module; National
 Figure 3. A: an engineer setting up a ground-based acoustic test of a weapon system. Instrumentation is being put into place in preparation for acoustic excitation to evaluate the structural response to high-amplitude acoustic fields. B: results of the computational acoustics simulation corresponding to the physical test in A. The simulation results provide pressure, acceleration, and stress values in the weapons system at every point in time in the simulation. These numerical results are used to evaluate the weapon response in the simulated acoustics environment. The finite-element mesh is represented by the grid, and the colors represent the magnitude of the acoustic pressure field at an instant in time.
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