Current measurement by lagrangian drifting buoys - Problems and potential

This paper discusses the current use of drogued Lagrangian free drifting buoys in terms of the two main problems Which limit their utility - drogue Lagrangian effectiveness (i.e., slippage) and overall drogue survivability. Scale model test data of drogue drag coefficient and behavior are summarized for numerous drogue designs and shapes. The benefits of certain designs are discussed and estimates of drogue slippage as a function of error-inducing drag force are present ed for representative drogues. The measurement of the steady flow horizontal drag coefficient of a full seal window shade drogue is described and compared with the scale model value. The in-situ measurement of the vertical drag coefficient of two full-scale window shade drogues are summarized and estimates made of buoy-drogue vertical interaction forces and conditions for slack line. The results of a full scale, instrumented window shade drogue slippage test are described for the case of a window shade drogue suspended beneath a Nova University minibuoy. The resulting measurement of drogue dynamic behavior and slippage estimates are presented and discussed in relation to the mathematical correctability of current velocity estimates from drogued buoy trajectories.