Perpustakaan Digital ITB

Marine biofouling on ship hulls is a well-documented driver of increased frictional drag and fuel consumption, but the fouling itself typically accumulates unevenly across a hull's wetted surface, producing a heterogeneous, patchy roughness condition; standard drag-prediction methods, however, are derived almost entirely from uniformly rough test surfaces, leaving a gap between how hull roughness actually occurs and how it is conventionally assessed. This study investigates how heterogeneous surface roughness, varying in both coverage fraction and spatial arrangement, affects wall shear stress in a turbulent channel flow, a controlled setting that isolates frictional drag from the wave-making and form-drag effects present in full-scale ship testing. Ten wall configurations, spanning fully smooth and fully rough baselines together with partial-coverage configurations arranged on the Top wall only, the Bottom wall only, and both walls simultaneously, were tested at three flow speeds using a streamwise pressure-drop method, with coverage fraction and position varied systematically across the channel's two independent walls. The results show that the skin friction coefficient depends on both coverage fraction and spatial arrangement: a naive model assuming drag scales linearly with rough area poorly describes the measured response, while a position-weighted model accounting for roughness proximity to the measurement location provides a substantially closer match, and at matched 50% coverage, roughness placed on the Top wall produced 1.35 to 1.55 times the skin friction coefficient of the same coverage placed on the Bottom wall. Expressed in physical units of drag force, this effect occupies the same order of magnitude as published ship fouling powering penalties, supporting its plausible significance for real hull fouling assessment: current practice, which typically records only total fouled area, may be insufficient to characterize the true drag penalty of a given hull condition, since the same coverage fraction can correspond to meaningfully different drag depending on where that fouling is actually located.