The dependency of the friction factor on the flow direction was investigated experimentally in a milled scale model of an unlined rock blasted tunnel under pressurized flow conditions by reversing the flow direction. The experimental data were used to highlight the significance of anisotropic roughness structures and variations in the cross-sectional area on the flow resistance. It is hypothesized that local sudden expansions of the cross-sectional area, which are followed by gradual contractions, contribute significantly to the directional dependency of the friction factor due to potential flow separation. For the reversed case, i.e. when sudden large-scale contractions were followed by gradual expansions, 15% lower friction factors were observed. The results were also used to highlight the scale dependency of these topographical features, the dependency of the friction factor from the tunnel driving direction, and to show the need for the development of methods that can be used to parameterize the directional dependency of hydraulic roughness and friction factors.

在加压流动条件下，通过反转流动方向，在无衬里爆破隧道的铣削比例模型中实验研究了摩擦系数对流动方向的依赖性。实验数据用于强调各向异性粗糙结构和横截面积变化对流动阻力的重要性。据推测，横截面积的局部突然膨胀，随后是逐渐收缩，由于潜在的流动分离，对摩擦系数的方向依赖性有显着贡献。对于相反的情况，即当突然的大规模收缩之后逐渐膨胀时，观察到摩擦系数降低了 15%。结果还被用来突出这些地形特征的尺度依赖性，