Ground Effects on Mach Cutoff Phenomenon of Sonic Boom

The ground effects on sonic boom signatures have conventionally been accounted for by multiplication with a simple amplification factor. However, these effects are significant for a waveform shape within a Mach cutoff region, where shock waves are diffracted due to increased atmospheric temperature toward the ground. This study investigates the waveform transition due to ground effects using computational fluid dynamics analysis in a stratified atmosphere. The freestream Mach number at the cutoff altitude of 2 km is set to one. Computations are performed to analyze an N-wave over a horizontal rigid wall at four different altitudes of 2.4, 2.11, 1.8, and 1.5 km. The computational results show that a partly diffracted waveform with a spiked shape occurs when the wall is within the caustic region. When the wall is within the shadow zone, the reflected evanescent waves that propagate upward enhance the pressure fluctuation over a wide range, including the incident evanescent wave. Consequently, the pressure fluctuation within the shadow zone exceeds twice that without the wall. These results show that the ground effects are significant for evaluating focused sonic booms and evanescent waves.