Mechanical Engineering - Theses
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ItemTurbulent flow over spatially varying roughness( 2023-03)Large-scale heterogeneity in surface roughness, such as uneven biofouling on ships, can lead to phenomena such as internal boundary layers (IBLs) and mean secondary flows that increase drag. Contemporary studies in the field often lack near-wall measurements either due to limitations in experimental methods or prohibitive computational costs for direct numerical simulation (DNS). As a result, contemporary studies on heterogeneous roughness model the near-wall flow using the equilibrium log-law without fully accounting for its validity in the presence of spatial heterogeneity. This thesis considers heterogeneities idealised as alternating strips of roughness and uses DNS in conjunction with an immersed-boundary method to resolve the complex surface geometries. This thesis explores two main themes: the equilibrium log-law and flow phenomena associated with heterogeneous roughness. For the former, the equilibrium log-law is found to yield accurate predictions of the wall shear stress when applied to spanwise-heterogeneous roughness. Where velocity measurements are not available, predictions of the skin-friction coefficient can be made in the asymptotic limits of small and large wavelengths of spanwise heterogeneity. The latter theme is explored by perturbing the spanwise heterogeneity through introducing a moving roughness geometry and through introducing an oblique angle to the heterogeneity. The conventional intuition of mean secondary flows as counter-rotating vortex pairs is found to not generalise outside of spanwise-heterogeneous roughness. Finally, IBLs are identified in all obliquely heterogeneous cases where the roughness is not aligned with the flow.