ABSTRACT
ABSTRACT: Many environmental flows can be considered as shallow. Clear examples are found in low-land rivers, lakes and coastal areas. The large width to depth ratio of shallow flows can give rise to flow structures that have two-dimensional characteristics. Those eddies with vertical axes contribute to the transverse exchange of mass and momentum and are therefore important for mixing processes. In most cases large eddies are generated in the wake of an obstacle or in the unstable shear layer further downstream. With a strong vertical confinement and little dissipation, large structures could be formed by the merging of vortices of equal sign (upcascading). In this paper various experiments will be addressed that were aimed at revealing the mechanisms of the generation and evolution of large eddies in shallow flows. The flow configurations comprise the shallow grid turbulence and shallow mixing layers. The experiments reveal that all large eddy structures carry the signature of the vertical confinement. The growth in length scale can be due to large-scale instability of the shear layer, selective dissipation of small-scale turbulence, and merging of vortices. The latter is difficult to observe in a dissipative flow containing 3D-turbulence because of the disturbing effects of the fluctuations and the limited lifetime of an eddy. The observations made with the experiments provide us with information regarding modeling approaches. When a full 3D-LES modeling is not feasible, the 3D-turbulence can be parameterized whereas the flow structures can be resolved. However, care should be taken of the effect of the 3D-fluctuations on the evolution of the large eddies as well as the disturbing effects of secondary circulation.
