A numerical study on hydraulic resistance in rough bed flows

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Richard McSherry, Thorsten Stoesser, Roger Falconer

Thursday 2 july 2015

14:05 - 14:20h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 12B. Engineering - River

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The reliance on empirical or semi-empirical relationships in the estimation of hydraulic resistance or “roughness” continues to constitute a major source of uncertainty in the prediction of water levels in environmental flows. The present study aims to build on a more rigourous expression for the friction factor that was developed by Nikora (2009) and explicitly accounts for the contributions of roughness-induced dispersive stresses, turbulent stresses, secondary currents, flow non-uniformity and unsteadiness, by manipulating the double-averaged momentum equation. Nikora's methodology is applied to Large Eddy Simulations (LES) of laminar and turbulent flows over isolated three-dimensional roughness elements, with a view to confirming the proposed theoretical relationships and quantifying the various turbulent and form-induced contributions to the flow resistance. The LES code is first validated by replicating the experimental tests of Meinders and Hanjalic (1999), in which turbulent flow over a matrix of three-dimensional cubes was investigated. Profiles of mean and fluctuating quantities are shown to agree very well with the experimental data. Laminar and turbulent flows past wall-mounted cuboids and hemispheres are then simulated: the computed flow fields are subjected to a double averaging procedure to quantify exactly the contributions of dispersive and turbulent stresses to the overall bed friction. The variation of bed friction in the five roughness regimes that were hypothesized by Nikora (2009) is investigated in detail, in terms of the double-averaged statistics. References Nikora, V. 2009. Friction Factor for Rough-Bed Flows: Interplay of Fluid Stresses, Secondary Currents, Non-Uniformity, and Unsteadiness. Proc. of 33rd IAHR Congress, Vancouver (CD ROM). Meinders, E. R., Hanjalic, K., 1999. Vortex structure and heat transfer in turbulent flow over a wall-mounted matrix of cubes. Int. J. Heat and Fluid Flow, 20, 255-267.