Turbulence quantification and sediment resuspension in an oscillating grid chamber

Abstract

An oscillating grid chamber has been developed to study sediment suspension, desorption of compounds from the resuspended sediment, and air–water mass transfer. The chamber is designed to allow researchers to study desorption of contaminants from cohesive sediments and the flux of those contaminants to the vapor phase. The chamber uses a single vertically oscillating grid driven by a DC motor and closed-loop controller. Sediment to be studied is placed in the bottom of the chamber and entrained into the water column by the turbulence generated by the oscillating grid. A two-component laser Doppler velocimeter (LDV) was used to measure the turbulent velocity field inside the chamber. Detailed mapping of the turbulent kinetic energy (TKE) produced by this grid arrangement was compared with established grid-stirred systems. At distances closer to the grid than two grid bar spacings, large lateral gradients exist in the TKE. The suspension of cohesive sediments was also studied using this chamber. Steady-state suspended sediment concentrations were achieved within 30 min for a variety of turbulence levels. By adjusting the grid operating parameters, the TKE can be set to simulate the turbulence found either at the bed or free surface in open-channel flow systems. With some care, the oscillating grid chamber can be used as a simple laboratory analogue to study various environmental processes within the flow or at either the sediment–water or air–water interface.