Correlation and validation of a CFD based hydrodynamic and dynamic model for a towed undersea vehicle

Abstract

Describes the modeling of the statics and dynamics of a towed undersea vehicle, taking the reader through the process from CFD modeling, correlation using wind tunnel test data, and in-water steady state and manoeuvering validation. A Vehicle Control Technologies (VCT) model that had previously been developed without the benefit of modern CFD methods is compared to model scale wind tunnel force and moment data as well as full-scale in-water data. The hydrodynamic model is then updated with recent technology and again compared to the same model scale and full-scale data. Advancements in hydrodynamic technology over the last few years have taken hydrodynamics to a new plateau of modeling accuracy, which rivals those of experimentally based approaches, and are accomplished at a much lower cost. In the new methodologies, contributions to the body and fin surfaces are computed using a combination of techniques, which are both CFD based and semi-analytical. The CFD code developed by VCT tracks the hull vorticity and computes the point where the hull wake is shed based on a method similar to that used to compute the Kutta condition for a wing. Interference effects, such as fin-to-fin and fin-to-body, are now computed using the vorticity based CFD models. In addition to improving the static terms to the equations of motion, the CFD code improves the computation of the added mass terms and the computation of damping effects due to the vehicle hull. The CFD based model has been correlated to wind tunnel force and moment data. Wind tunnel data included body build-up configurations as well as the fully appended vehicle. The body build up process is important to understanding the effect of wing upwash on the control surfaces. The in-water comparisons for pitch angle and sternplane angle (both the steady state trim conditions and the transient response during dynamic manoeuvers) were made without further adjustments to the wind tunnel correlated model.