Unsteady Force Estimation Using a Lagrangian Drift-Volume Approach
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A novel Lagrangian force estimation technique for unsteady fluid flows has been developed, using the concept of a Darwinian drift volume to measure the unsteady force on an accelerating body. A methodology using multiple drift volumes is described and evaluated on an experimental test case containing highly-separated, vortical flow. The inherent advantage of the force estimation technique presented is that, unlike many modern Eulerian techniques, gradient operations and near-body measurements are not required to be calculated. These noise amplifying processes are avoided since the drift volume is calculated only from particle displacements in the flow field, reducing the importance of having high quality acceleration and spatial gradient data near walls and in regions of high shear. The resultant unsteady force estimates from the proposed technique are shown to align with the measured drag force during high accelerations, a region in which comparable methods suffer. The critical aspects of understanding unsteady flows, relating to peak and time-resolved forces, often lie within the acceleration phase of the motions, which are well-captured by the drift-volume approach. Therefore, this Lagrangian force estimation technique opens the door to fluid-dynamic analyses in areas that, until now, were inaccessible by conventional means.