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Characteristics of turbulent unidirectional flow over rough beds: Double‐averaging perspective with particular focus on sand dunes and gravel beds
Author(s) -
McLean S. R.,
Nikora V. I.
Publication year - 2006
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2005wr004708
Subject(s) - drag , turbulence , geology , flow (mathematics) , mechanics , momentum (technical analysis) , surface finish , range (aeronautics) , shear stress , geotechnical engineering , physics , materials science , finance , economics , composite material
We address some unsolved methodological issues in modeling of natural rough‐bed flows by critically examining existing approaches that parameterize rough‐bed flows. These often utilize loosely defined variables. Here we suggest that double‐averaging (in time and in a volume occupying a thin slab in a plane parallel to the mean bed) provides a rigorous, straightforward alternative that can aid in the parameterization process. We further present two examples: two‐dimensional bed form and gravel bed flows. We argue that the double‐averaging approach based on momentum equations should serve as a better methodological basis for modeling, phenomenological developments, and parameterizations. These equations explicitly include drag terms and form‐induced momentum fluxes due to spatial heterogeneity of the time‐averaged flow in the near‐bed region. They also provide a solid basis for better definitions of basic flow variables including the shear stress partitioning into turbulent and form‐induced momentum fluxes, skin friction, and pressure (form) drag. We show that for a range of rough‐bed flows the vertical distribution of the double‐averaged velocity consists of two distinct regions: (1) a linear region below roughness tops, and (2) a logarithmic region above them.