Analysis of three-dimensional flow in a cylindrical sediment oxygen demand chamber

Abstract

The rate of consumption of dissolved oxygen at the bottom of a river or coastal water is often measured in situ by a sediment oxygen demand (SOD) chamber. A continuous flow is generated above the sediment, and the SOD is determined from the difference in oxygen concentrations in the inflow and outflow. The steady three-dimensional swirling laminar flow field inside a cylindrical SOD chamber previously used in field investigations is computed using the finite volume method on an unstructured tetrahedral mesh. The numerical predictions reveal a highly complicated flow characterized by (i) a mainly tangential and near-uniform flow along the circumference in the horizontal layers; (ii) significant reverse bottom currents; and (iii) strong swirl induced by the jet momentum in the vertical section. The computed scalar field suggests that good mixing is achieved within the chamber. The computed velocity and scalar field are well supported by laboratory velocity measurements using laser-Doppler anemometry (LDA), and measured concentrations in a tracer experiment. The present study elucidates the fluid mechanics of an important type of SOD chamber design often used in environmental water quality studies.