Effect of Froude number and spacing ratio on drag force and drag coefficient over engineered roughness elements

This study experimentally investigates the influence of discrete transverse roughness elements on flow resistance in a rectangular open-channel flume. Experiments were conducted with three longitudinal spacing ratios (S/K = 6, 12, and 24) under steady discharges of 15, 20, and 25 L s -1 . The relationships between the Froude number (Fr) and both drag force (F D ) and drag coefficient (C D ) were analyzed. Results show that as Fr increases, the drag force generally increases in most configurations (particularly at larger spacing), while the drag coefficient exhibits an overall decreasing trend with Fr in the majority of cases, consistent with the normalization by dynamic pressure. Closer spacing (S/K = 6) tends to produce lower drag forces due to wake sheltering effects, whereas larger spacing (S/K = 24) generally results in higher drag forces owing to more fully developed flow structures and stronger form drag. These findings highlight the significant role of spacing ratio in modulating roughness-induced resistance.