All Angle-of-Attack Hydrodynamic Model for Underwater Vehicles Validated by Tank Test Data

Abstract

An underwater vehicle’s design and operation requires prediction of its performance at various velocities and angles-of-attack or of sideslip. Traditional models based upon headway motion at substantial speeds use coefficient-based equations of motion. Simulations based upon these coefficients are not valid for hover, low speeds, or high angles-of-attack or of sideslip. To remedy this severe limitation, nonlinear hydrodynamic models valid for all attitudes of underwater vehicles have been developed and are presented here. These models are derived from the physics of hydrodynamic phenomena. Forces and moments for the total vehicle are obtained by relying on body-buildup techniques. For the vehicle’s hull, the models are profile drag, lift, crossflow force, and added mass. For appendages such as fins, the models are lift and drag when unstalled, normal force when stalled, transition between unstalled and stalled conditions, and hull interference effects. Whenever the equations contain parametric coefficients such as added-mass, drag, and lift, values are specified for all angles-of-attack and sideslip with a minimal use of empirical look-up tables. These models represent the state-of-the-art in low speed hydrodynamics at all angles-of-attack. The hydrodynamic models presented here have been improved and validated by analysis and comparison with test data. Sub-scale versions of two different vehicles have been tested in tow-tanks at all angles-of-attack. The models have been implemented in a C language computer code which runs at high speed with no iteration required. This code is utilized regularly in faster-than-real-time vehicle performance simulations.