Numerical modeling of flow characteristics in a rotating annular flume

A rotating annular flume (RALF) has been constructed at the Center of Coast al and Land-Margin Research (CCALMR) to study the biogeochemistry of sedime nt-water interfaces. The flume was designed to allow for evolving, integrat ed measurements of physical, chemical, and biological parameters, as often as possible in a real-time, computer-controlled mode. Several numerical mod els have or are being developed/applied to provide a virtual representation of the flume, with the dual objective of assisting the design of experimen ts and of assessing our level of understanding of processes and process int eractions. We will concentrate here on the characterization of the flow in the flume, a basic but interestingly complex problem. The operational chall enge is to minimize secondary circulation and lateral variability of shear stress, factors that prevent the flume flow to match the idealized concept of an endless channel flow. Satisfactory minimization of these factors can be achieved by allowing both the top and the bottom rings of the flume to r otate in contrary directions, a concept introduced by earlier research effo rts and verified in RALF via Acoustic Doppler Velocimeter (ADV) measurement s and 3D numerical modeling. Once logistics (e.g., in the form of the size of the ADV's sampling volume and of the vertical discretization of the nume rical grids) are appropriately handled, observations and model results show good agreement. This agreement legitimates the use of the model as a desig n and investigative tool, in particular to define optimal rotation ratios o f the top and bottom rings. The ratios that minimize secondary flow and lat eral variability of shear stress are distinct. This is a logical (generatin g mechanisms are different) but often not recognized aspect of the operatio n of annular flumes. (C) 2000 Elsevier Science B.V. All rights reserved.