Progress towards modelling unsteady forces using a drift-volume approach

Classical potential flow added mass theory does not appear to account for all the force experienced by an accelerating body, especially in separated flow. The drift-volume method has shown good agreement with experiment for a highly accelerated case [1], and has been extended in this study to a kinematics-based force model. An optical towing tank facility is used to measure the forces and flow around a circular flat plate normal to the free stream fora range of acceleration moduli. Lagrangian particle tracks were produced using the Eulerian vector-fields and synthetic particle tracking velocimetry. Compared to classical theory, when the “added mass" is determined using the drift volume approach, an additional rate-of-change of added mass term appears, and is shown to be a significant component of the total force. In addition, the particle tracks revealed that the vortex ring development behind the flat plate made up a remarkable portion of the drift volume. From this observation a model for the unsteady forces via drift volume development is proposed, using a conservation of mass scheme[2]. An enstrophy-history method as described by Rosiet al.[3] is used to determine the role of entrainment in the vortex ring growth, incorporated into the unsteady force model