Near Threshold Sediment Transport by a Forced Jet Impinging on a Mobile Sediment Bed

Abstract

Although sediment transport has been extensively studied in the past, flows

such as rotorcraft brownout with large-scale coherent structures call many of the

simplifying assumptions into question. The objective of this study is to develop

a model for the prediction of sediment removal, referred to as erosion, based on

independent measurements of the single-phase flow and the evolution of bedforms

on the surface of a mobile sediment bed. A series of phase-resolved particle image

velocimetry (PIV) flow measurements have been conducted to quantify the stress

induced by an acoustically forced impinging jet, analagous to tip-vortices within the

rotor wake. The threshold conditions for incipient particle motion are quantified

through a series of PIV measurements of the single-phase flow at conditions found

to produce quantifiable erosion of the surface. A force balance approach is used

to develop a model, following the theory presented by Bagnold (1966), to predict

the transport of sediment due to the stress above the theshold. A series of surface

elevation measurements are analyzed to quantify the removal of sediment, for the

evaluation of the predicted model. An additional series of PIV measurements are

performed on a prototype bedform, modeled after the observed bedforms, to quantify

the changes in the flow field caused by their developement. The proposed model is

shown to provide a better prediction of the observed erosion than classical sediment

transport models, especially for cases close to the threshold conditions. For higher

speed cases however, the model dramatically over predicts the observed erosion.

Several physcially-based explanations are provided for this kink in the trend.