Localized roughness effects in non-uniform hydraulic waterways

Abstract

Hydropower tunnels are generally subject to a degree of rock falls. Studies explaining this are scarce and the current industrial standards offer little insight. To simulate tunnel conditions, high Reynolds number flow inside a channel with a rectangular cross-section is investigated using particle image velocimetry and pressure measurements. For validation, the flow is modelled using large-eddy simulation (LES) and a Reynolds-averaged Navier-Stokes (RANS) approach with the k − ε turbulence model. One wall of the channel has been replaced with a rough surface captured using laser scanning. The results indicate flow-roughness effects deviating from the standard non-asymmetric channel flow, and hence cannot be properly predicted using spatially averaged relations. These effects manifest as localized bursts of velocity connected to individual roughness elements. The bursts are large enough to affect both temporally and spatially averaged quantities. Both turbulence models show satisfactory agreement for the overall flow behaviour, where LES also provided information for in-depth analysis.

Experimental and Numerical Determination of the Head Loss of a Pressure Driven Flow through an Unlined Rock-Blasted Tunnel

“…Simulations carried out by [30] using a computational grid derived from terrestrial laser scanning (TLS) data of a hydropower-tunnel and a 3D-modelling approach based on the Reynolds-averaged Navier-Stokes (RANS) equations with the k-ε turbulence model showed the possibility to study local pressure variations due to a complex tunnel geometry. A similar conclusion was reported based on the numerical simulations of physical scale model experiments by [29]. However, it should be kept in mind that CFD models require a rigorous verification and validation process.…”

“…Thus, such scale models are prone to both laboratory and scale effects e.g., [24,26] which may be minimized by further developing existing technologies for scale model production to avoid simplifying assumptions regarding wall roughness characteristics. This becomes possible through the use of computer numerically controlled (CNC) production techniques e.g., [27,28] and a recent study of [29] showed the possibilities to gain further insights into roughness effects in non-uniform hydraulic waterways of hydropower systems using this technique.…”