Separation of scales on a broad, shallow turbulent flow

CARRASCO MILLAN, A; VIONNET CA

JOURNAL OF HYDRAULIC RESEARCH

IAHR Secretariat, Madrid, Spain

Lugar: Madrid, Spain; Año: 2004 vol. 42 p. 630 - 638

0022-1686

Two different mechanisms acting on a broad, shallow turbulent flow whose energy spectrum is separated by a gap is investigated experimentally. The study concentrates on the spectral response evolution of the shallow flow approaching an opening induced by a lateral contraction, located halfway in streamwise direction of a large rectangular basin. The geometrical forcing, induced by the lateral contraction that breaks the plan-form symmetry of an otherwise broad and uniform shallow flow, produces an energy concentration in the range of low wavenumbers (large-scale) by an inverse cascade process. The geometrical forcing continues to pile up energy until scales the size of the entire flow geometry are strongly excited. The topographical forcing, induced by the presence of a fixed scour hole located at the contraction end that breaks an otherwise flat bottom, produces an energy concentration in the range of high wavenumbers (small-scale) by trapping some flow modes. The topographical forcing is only local, and has the effect of acting as an eddy viscosity on the large-scale turbulent motion. The problem presented is relevant to determine the fate of suspended sediment downstream of the opening of relief bridges located on flat floodplains in periods of high waters.