Combined Experimental/Numerical Development of Propulsor Evaluation Capability

Propulsor design methods utilize Computational Fluid Dynamics (CFD) to develop initial propulsor configurations and predict the full-scale in-water performance of these optimal designs. However, like all numerical models, these CFD models need experimental validation to provide a sufficient level of confidence in the design. The actual data needed to validate CFD models include propulsor inflow velocities and thrust and are impractical to collect for full-scale vehicles. As a result, the in-water propulsor performance can be significantly different than CFD predictions. Another approach in the propulsor design process is to experimentally test a subscale version of the vehicle and appropriately scale results. This scaling is often unreliable due to differences between open water conditions and the flow in the laboratory facility. This paper presents a method to combine CFD modeling with subscale experiments to improve full-scale propulsor performance prediction. Laboratory experiments were conducted on subscale generic torpedo models in the 12^″ × 12^″ water tunnel located at the Naval Undersea Warfare Center in Newport, Rhode Island. This model included an operational ducted post-swirl propulsor. Laser Doppler Velocimetry was used to measure several velocity profiles along the torpedo hull. The experimental data were used in this project to validate the CFD models constructed using the commercial CFD software, Fluent®. Initially, axisymmetric two-dimensional simulations investigated the bare body, hull only case, and a shrouded body without the propulsor. These models were selected to understand the axisymmetric flow development and investigate methods to best match the propulsor inflow. A variety of turbulence models including the realizable k-epsilon model and the Spallart-Almaras model were investigated and ultimately the numerical and experimental velocity profiles were found to match within 3%. Based on these water tunnel simulations, differences between the flow in the facility and open water could then be characterized. These differences quantified both the effect of Reynolds number as well as local flow acceleration due to tunnel blockage effects. Full 3-D flow simulations were then conducted with an operating propulsor and compared with the corresponding subscale experimental data. Finally, simulations were conducted for full-scale tests and compared with actual in-water data. While the in-water data was limited to propulsor rpm and vehicle velocity, the operating advance ratio could be determined as well as the estimated vehicle thrust. This provided a method to utilize CFD/experiments to bridge the gap between subscale and full-scale tests. The predicted in-water advance ratio of 1.87 was very close to the measured value of 1.75.