Exploring the role of bed fluidization on ripple formation in highly turbulent flows

Abstract:

Although ripples and dunes are typically found in flows dominated by mean shear or oscillations, ripples have also been observed in turbulent flows in absence of either (Johnson & Cowen 2020). Bedforms can also develop without visible suspension or bedload transport. Despite observation of ripples found absent mean shear or visible transport, the physical mechanisms by which these ripples are generated remain unknown. This research aims to uncover methods of ripple production and bed fluidization in dynamic coastal environments where turbulence is the dominant mechanism for sediment transport.

We experimentally explore the formation of ripples from bed fluidization. Fluidization may be caused by instantaneous pressure gradients of the turbulent flow penetrating into the porous bed. This allows buried sediments to rise and mix with upper layers of sediments. The present work will quantify the extent to which the bed fluidizes in order to determine how ripples form in the absence of mean bed shear and will highlight the depth to which turbulence penetrates a dense porous sediment bed.

Experiments are conducted in a water tank, 0.84m by 0.84m by 1m. A 16 by 16 array of randomly actuated synthetic jets (Variano et al. 2004, Johnson & Cowen 2018) is positioned at the top of the tank to generate turbulence without mean flow with variable control to allow scaling to environmental flow conditions. Particle image velocimetry (PIV) measurements are conducted to provide spatio-temporal velocity data of the turbulent flow. The bottom of the tank is comprised of several layers of different colored sands. During experiments in which turbulence is forced above the sediment bed, liftoff events occurring in the measurement region can be detected in the PIV images, and time-lapsed photography monitors large-scale ripple evolution. Over time, turbulence causes ripples to form. As ripples form, sediment cores are collected via “freeze coring,” in which a hollow aluminum wedge filled with liquid nitrogen is inserted into the bed and sediment freezes onto the wedge face. The cores are used to determine the extent of bed fluidization, as indicated by mixing between the colored layers. Preliminary experiments will be presented to substantiate this hypothesis.