Fluid Mechanics of Triangular Sediment Oxygen Demand Chamber

Benthal respiration rates are often measured in situ by a sediment oxygen demand (SOD) chamber in which a continuous flow is generated above the sediment. The steady threedimensional turbulent flow field inside a triangular SOD chamber (previously used in field investigations) is computed using the renormalization group (RNG) k-(epsilon) model on an unstructured tetrahedral mesh. The numerical predictions reveal a highly complicated flow characterized by (1) a jet flow near the level of the inlet, with strong downflow near the outlet end; (2) significant reverse bottom currents; and (3) strong secondary circulations in the triangular cross section. Good mixing is achieved, with mean near-bottom velocities about 10 times greater than that determined from the inflow discharge and cross-sectional area. The computed velocity field is well supported by laboratory velocity measurements using laser-Doppler anemometry (LDA). The implications on SOD chamber design are also discussed with reference to computed flow fields in representative dome-shaped and rectangular chambers used in field application. The present study explains the previous large discrepancies in SOD field measurement using chambers of different designs, and points to the importance of hydrodynamics of SOD chambers.