Author(s): L. R. Andersson; I. A.S. Larsson; J. G.I. Hellstrom; P. Andreasson; A. G. Andersson

Linked Author(s): Robin Andersson

Keywords: Pressure measurements; Rough surface; Hydropower-tunnel; Laser scan; Friction factor

Abstract: Flow in hydropower tunnels is characterized by high Reynolds number and often very rough rock walls. Due to the roughness of the walls, the flow in the tunnel is highly disturbed, resulting in large fluctuations of velocity and pressure in both time and space. Erosion problems and even partial collapse of tunnel walls are in some cases believed to be caused by hydraulic jacking from large flow induced pressure fluctuations. The objective of this work is to investigate the effects of the rough walls on the pressure variations in time and space over the rock surfaces. Pressure measurement experiments were performed in a 10 m long Plexiglas tunnel where one of the smooth walls was replaced with a rough surface. The rough surface was created from a down-scaled (1:10) laser scanned wall of a hydraulic tunnel. The differential pressure was measured at the smooth surface between points placed at the start and end of the first four 2 m sections of the channel. 10 gauge pressure sensors where flush mounted on the rough surface, these sensors measure the magnitude and the fluctuations of the pressure on the rough surface. The measurements showed significant spatial variation of the pressure on the surface. To exemplify, sensors placed on protruding roughness elements showed low gauge pressure but high fluctuations. The differential pressure indicated a head loss through the tunnel which was almost four times higher than a theoretical smooth channel.

DOI: https://doi.org/10.15142/T360628160853

Year: 2016