Finite-Difference Time-Domain Modeling of Low-Amplitude Sonic Boom Diffraction Around Building Structures

Open Access
- Author:
- Cho, Sang Terry
- Graduate Program:
- Acoustics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 10, 2012
- Committee Members:
- Victor Ward Sparrow, Dissertation Advisor/Co-Advisor
Victor Ward Sparrow, Committee Chair/Co-Chair
Philip John Morris, Committee Member
John Brian Fahnline, Committee Member
Lyle Norman Long, Committee Member - Keywords:
- FDTD
finite-difference
numerical model
sonic boom
low-boom
diffraction - Abstract:
- Overland civilian supersonic fights have been banned since 1973 because of the unacceptably loud and intrusive sonic booms generated by supersonic aircraft. However, the continued advancement in aeronautics technology made it possible to design small supersonic business jets whose shaped, low-amplitude sonic booms are known to be less intrusive to the human hearing. As there is enough evidence that these low-amplitude sonic booms are likely to be acceptable to human hearing outdoors, the focus of the sonic boom research is shifting towards their indoor acceptability. This dissertation presents two numerical models that simulate the external pressure loading on building structures that are exposed to low-amplitude sonic booms. Obtaining accurate external pressure loading information through numerical simulations is an important step in the study of sonic boom transmission into the interior of a building. A three-dimensional finite-difference time-domain (FDTD) model has been developed, which simulates several NASA field measurements of low-amplitude sonic booms conducted around distinct building geometries. The FDTD model exhibits good agreement with the measurement data, accurately calculating the acoustic field near the building structures including the effects of diffraction. The validation of the FDTD model for single and multiple building cases indicates that it can be extended for different building shapes or configurations of multiple buildings. Considering the expected degradation of the accuracy of the FDTD model above its high-frequency limit, a hybrid model combining the low-frequency FDTD model result with a high-frequency ray-tracing model result is developed, using a complementary FIR filter pair and a time-alignment algorithm for the component waveforms. The ray-tracing model, written by Riegel [1], is a high-frequency approximation that does not take into account the effects of diffraction. The result of the hybrid model, tested for a limited number of simulation cases, points out some principal challenges in numerical modeling of the sonic boom induced pressure loading on buildings. However, both the FDTD and hybrid models exhibit potential as a tool for an accurate prediction of the exterior pressure loading on buildings. The dissertation also reports an interesting phenomenon named the "building spiking effect", in which the pressure waveforms recorded on the building wall directly exposed to the incident boom consistently show two amplied peaks with a positive overpressure. A physical explanation of the building spiking effect is given, attributed to the frequency-selective nature of diraction.