Progress towards modelling unsteady forces using a drift-volume approach

Abstract

A kinetic-energy-based, semi-empirical model for unsteady force estimation has been de-veloped, inspired by the concept of the Darwinian drift volume. The limitations of potential flow added mass and vortex impulse approaches were explored, and compared to experimental results. An optical towing tank facility was used to measure the forces and flow around a circular flat plate normal to the free-stream for an acceleration moduli of 4. Lagrangian particle tracks were produced using Eulerian vector-fields and synthetic particle tracking velocimetry. Forces predicted by potential flow added mass theory and vortex impulse modelling exhibited poor agreement compared to measured forces. The kinetic-energy-based approach showed strong agreement with measured forces, and allowed for the direct calculation of an “energized mass”, analogous to added mass derived from potential flow. Contrary to classical theory, the energized mass was not constant in time, which contributed to additional work done by the body. Forces on the body predicted using the semi-empirical model captured the major trends present in the measured forces. The component of the force due to the rate of change of energized mass is shown to be a significant contribution to the total force.