The measurement of ventilated supercavity interface vibrations

This paper discusses the issues encountered in attempting to perform the first measurements of the interface velocity fluctuations of a ventilated supercavity. A ventilated supercavity is formed and maintained when an upstream cavitator on an undersea vehicle generates a separated flow which is "ventilated," or continuously supplied with gas. This causes the separated flow region or "cavity" to grow and envelop the vehicle. Three laser Doppler vibrometers (LDVs) were used in an attempt to measure the air-water interface vibrations of a ventilated supercavity enveloping a vehicle model in a water tunnel facility. The presence of multiple vibrating surfaces including the tunnel window, model, and interface made determining the velocity spectrum due to the interface alone difficult. The measurements were further complicated by the LDV's electronic noise whose spectrum bears a striking resemblance to that of turbulence velocity fluctuations and, thus, to the spectrum anticipated for the interface. Comparison of the spectra measured with the three LDVs suggests that at least two of the spectra obtained may contain substantial contributions from the ventilated supercavity interface.