Drag force on a sediment particle from point velocity measurements: A spectral approach

Drag on a bed sediment particle was measured using a force sensor in a laboratory flume. The applied flow velocity was also measured using particle image velocimetry. A quasi-steady spectral model is proposed to predict the spectra of drag force on the particle using point velocity data. The quantity C(D)A (where C(D) and A are the coefficient of drag and exposed area of the particle, respectively), which was determined experimentally, is used in the model owing to practical limitations in estimating appropriate values of the individual quantities (C(D) and A). The predicted spectrum is found to closely match the measured force spectrum up to 1 Hz, indicating the important role of low-frequency streamwise velocity fluctuations in generating quasi-steady drag force on the particle. The model highlights the significance of increased nonlinearity due to higher-order streamwise velocity fluctuations beyond 10 Hz. However, the nonlinearity due to shedding of vortices at higher frequencies is accounted for by coupling the model with an admittance function based on the exposure of the particle. The coupled model is found to perform satisfactorily for all exposures, except when the particle is flushed with the bed. The limitation of the quasi-steady model at lower exposures is also supported by poor correlation between the measured velocity and force time histories.