Structural design considerations of a profiling free vehicle used to measure ocean turbulence

A second-generation High-Resolution Profiler is being developed to make vertical profiles of water velocity over scales from centimeter to full-ocean depth, to measure internal waves and turbulence. The profiler will measure microstructure with airfoil shear probes, fine structure with an acoustic-travel-time current sensor, large-scale water velocity with an electromagnetic field sensor, profiler motion with accurate accelerometers and compass, and a GPS to obtain positions at the start and end of each profile. This paper discusses some of the design considerations of the vehicle structure and the acoustic current sensor. Structural flow-induced vibration of the sensor and profiler contaminate the velocity measurements and can reduce the profiler's sensitivity to low levels of turbulence. The profiler structure, the shear probes and acoustic velocity sensor, and their attachment to the profiler, were made stiff, and the profiler shortened to raise the lowest structural vibration mode to nearly 30 Hz, above the frequency range most sensitive for measuring turbulent dissipation. We are using modified MAVS electronics to make the acoustic measurement of water velocity with a sensor head that is customized for this application. The acoustic path-length has been increased to 25 cm to improve accuracy. Estimates are made of the flow distortion, upstream of the profiler bow, in the measurement volume.