Normal Force and Pitch Moment for a 6:1 Prolate Spheroid at Moderate Reynolds Numbers

Ideally, air- and waterborne vehicles would be designed primarily with computers using computational fluid dynamics (CFD) tools to predict the forces acting on the vehicle. Despite significant advancements in computational power in recent decades, three-dimensional flow over doubly-curved bodies cannot currently be modeled with the desired level of accuracy in an acceptable amount of time. Additional experimental observations are one pathway to improve computational design tools.

The prolate spheroid has long been a favored reference for the development of CFD tools. It is simple to describe mathematically, yet the flow around it is highly three-dimensional, featuring laminar to turbulent transition, laminar and turbulent separation, reattachment, and multiple sets of trailing vortices, depending on the flow conditions. These phenomena provide a range of specific features against which to compare numerical results. However, gross measures, including forces and moments, provide a first-order reference in the validation process.

The normal force and pitch moment were measured for a 6:1 prolate spheroid in the large towing tank facility at the U.S. Naval Academy for Reynolds numbers from 0.4-9×10⁶ and angles of attack from 2.5° to 20°. The results of this work agree well with previous studies and expand the range of Reynolds numbers significantly. A detailed description of materials and methods is provided to support CFD model validation.