UC San Diego

The FastCTD is a shipboard system consisting of a high-speed winch and profiling body developed at Scripps Institution of Oceanography. The instrument package onboard the profiling body (“fish”) can be used to measure temperature, salinity, and pressure rapidly and accurately. The winch and telemetry system allows the fish to be profiled vertically through the ocean at very high speeds (4-6 m/s). The combination enables characterization of ocean phenomena with large vertical scales that change rapidly in time, such as oceanic internal waves. Central to its performance is the ability to accurately acquire data while traveling at high velocities, without any interruptions. The fin configuration induces a rotation in a predetermined direction, curtailing vertical unpredictable and unstable flight of the FastCTD, but in doing so, results in excessive rotations. This interferes with the data telemetry through the tow cable, since the conductors can break due to extreme twisting. Evaluating the effect of the fins on the flight dynamics of the fish may lead to the development of a more robust design, and better performance.

The optimization is carried out in two steps. The first is a thorough analysis of data recorded by the onboard inertial measurement unit, in order to quantitatively and qualitatively understand the instability. The second is a first order computational fluid dynamic study of the fins and control surfaces, and their influence with varying external conditions. The simulations reveal that a steeper angle of descent is useful in mitigating the degree of rotation. Two straightforward design changes involving the positioning of the fins are proposed, and their effectiveness of influencing the angle of attack are studied, as a first step toward possible future designs of the FastCTD.