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

Abstract

The friction loss in a part of the rock-blasted unlined tunnel of the Litjfossen hydropower plant in Norway was determined from experimental and numerical studies. Remote sensing data from the prototype tunnel provided the input data for both the numerical model and the construction of a 1:15 scale model with an innovative milling approach. The numerical simulations were based on the solution of the Reynolds-averaged Navier–Stokes equations using the CFD program OpenFoam. Head loss measurements in the scale model were carried out by means of pressure measurements for a range of discharges and were compared against the results of the numerical model. The measured data were used to determine the Darcy–Weisbach and Manning friction factors of the investigated tunnel reach. The high-resolution remote sensing data were also used to test the applicability of existing approaches to determine the friction factor in unlined rock blasted tunnels. The results of the study show the usefulness of the chosen hybrid approach of experimental investigations and numerical simulations and that existing approaches for the determination of head losses in unlined tunnels need to be further refined.

Technical Note: On the Production and Accuracy of CNC-Manufactured Hydraulic Scale Models

“…The scale-model tests were designed to investigate the effect of the wall rock roughness on head losses and the friction factor in the tunnels [34,35]. It was therefore decided to construct models of rather straight tunnel sections to avoid the additional influence of bends, alcoves, and partial linings.…”

“…The reduced meshes were then used for creating the required input files for the milling at the Digital Building Fabrication Laboratory (DBFL) at TU Braunschweig [36]. The milling of the tunnel-models followed the strategy described in other studies [10,34]. The total length of the tunnel models exceeded both the spatial scale of the 5-axis CNC-controlled milling facility at the DBFL and the spatial dimensions of the base material used for milling, so that each tunnel-model was subdivided in five tunnel segments.…”

“…The blocks used for the milling can consist of different materials such as foams and metal alloys, and the accuracy of the milled surface or object depends on the accuracy of the DEM as well as the size of the milling head [10]. The milling technique is used in various hydraulic laboratories for the construction of scale models and has already been successfully applied to construct artificial self-affine rough beds, scaled versions of rough waterways, conduits, and hydraulic structures [10,22,23,[32][33][34]. The accuracy of milled models is directly visible by eye, but has seldomly been quantified.…”

“…We note that the focus of the TN is on the milling of the tunnel models. Information on the experimental setup and procedure as well as the results from the scale model tests are reported in further publications [34,35]. The technical note is structured as follows: Section 2 presents the available data and the workflow to construct the models, Section 3 contains the description of the results, and the discussion in Section 4 concludes the technical note.…”

“…Workflow for the production of 1:15 scale models of unlined rock blasted hydropower tunnels and the subsequent assessment of the accuracy of the milled tunnel parts. (a) point cloud section of the laserscan in prototype scale, (b) visualization of a part of the detailed scaled mesh, (c) visualization of the reduced mesh of the same location as in (b), (d) photograph taken during the milling of a segment, (e) setup of two combined segments for structure from motion, (f) point cloud created by structure from motion for coated/uncoated segments, (g) sketch visualizing the final assembly of a tunnel section from the milled segments.The milling of the tunnel-models followed the strategy described in other studies[10,34]. The total length of the tunnel models exceeded both the spatial scale of the 5-axis CNC-controlled milling facility at the DBFL and the spatial dimensions of the base material used for milling, so that each tunnel-model was subdivided in five tunnel segments.…”