A numerical study of free-surface turbulence in channel flow

Direct numerical simulations of open‐channel flow indicate that turbulence at the free surface contains large‐scale persistent structures. They are ‘‘upwellings’’ caused by impingement of bursts emanating from the bottom boundary; ‘‘downdrafts’’ in regions where adjacent upwellings interact, and whirlpool‐like ‘‘attached vortices’’ which form at the edge of upwellings. The attached vortices are particularly long‐lived in the sense that once formed, unless destroyed by other upwellings, they tend to interact with each other and dissipate only slowly. If turbulence generation at the bottom wall is turned off by changing the boundary condition to free slip, then the upwellings (related to bursts) and downdrafts no longer form. The dominant structures at the free surface become the attached vortices which pair, merge, and slowly dissipate. In the central regions, as expected, the structure remains three dimensional throughout the decay process. Near the free surface, the structure appears to be quasi‐ two dimensional, as indicated by quantitative measures such as energy spectra, interwave number energy transfer, invariants of the anisotropy tensor, and length scales. In the decaying case, the quasi‐two‐dimensional region increases in thickness, with decay time, though the structure in the central regions of the flow remains three dimensional.