Modeling the performance of a winged sub-surface float for acoustic profiling and wave measurements

Abstract

Near-surface submerged floats have been used to support upward-looking acoustic Doppler profilers to collect current and/or directional wave measurements. However, mooring drawdown and tilt in strong currents pose a design challenge to keep the instruments sufficiently level and close to the surface to collect good quality data. We describe a concept for a relatively small winged float utilizing current-generated lift to minimize drawdown in very high currents, while also maintaining a stable attitude for the sensor. The winged float design consists of a streamlined body with double (box) wings, having modest static buoyancy and tail fins to control the body's attitude while maintaining a near-constant angle of attack of its wings. We model the performance of the winged float by incorporating iteratively the dynamic lift and drag of the wings as well as the static buoyancy and drag of the body itself. Standard aerodynamic formulas are used to estimate wing lift and drag under varying current conditions. Model results indicate that a float equipped with 2 wings, each having a 0.75m chord length and 1.5m span (2.25m2 total wing area) outperforms a 65 inch syntactic float when the current profile is characterized by a strong near-surface current and has a comparable performance to a 65 inch float when the current profile has a sub-surface maximum. We conclude that a winged float, combined with a properly designed mooring, can provide a suitable platform for long-term wave and near-surface current profile measurements in strong current regimes without the need for a surface buoy or massive buoyancy modules.