Current blockage in sheared flow: Experiments and numerical modelling of regular waves and strongly sheared current through a space-frame structure

Abstract

Space-frame structures supporting marine renewable energy machines such as offshore wind turbines are exposed to complex hydrodynamic forces resulting from the coexistence of waves and currents. Previous investigations on the interaction of such a structure acting as an obstacle array with regular waves and in-line uniform current reported a reduced fluid loading due to current blockage. This paper documents laboratory-scale experimental evidence for reduced fluid loading on a truss structure exposed to regular waves with in-line sheared current in shallow water. Strongly sheared current of different speeds is generated and profiled using purposely-built wire resistance arrays in a wave-current flume, and a range of regular waves are created using a piston-type wavemaker. The global hydrodynamic force time history on a truss structure is measured for a range of sheared current speeds and regular wave heights. For all test cases, two loading configurations are considered, with the truss positioned end-on and diagonal to the incident flow direction. Comparisons are made with the analytical current blockage model for steady uniform current by Taylor (1991) and Taylor et al. (2013), and with the numerical simulations conducted in OpenFOAM using a porous tower model following the approach by Santo et al. (2015a). Under the same input condition, the diagonal loading configuration is observed to attract higher forces and therefore the orientation of the structure plays an important role when assessing the survivability of such structures. Overall, good agreement in terms of the peak forces and the shapes of force time histories is achieved for all cases with an inline current, all with a single and consistent value for each of the local Morison drag and inertia coefficients (here Cd∼2.1 and Cm∼2), with the coefficients defined in terms of the overall structure. In contrast, predictions using the present API recommendation with the same Cd and Cm result in force overpredictions for all cases of regular waves with in-line current. For steady sheared current flow through a porous tower, apart from the dominant lateral flow divergence, numerical flow visualisation reveals the existence of vertical flow interaction in the porous tower. This is attributed to the non-uniform loading with water depth and was not observed previously for uniform current flow. This study provides the first experimental validation and justification for the use of a simple porous block in representing a complex geometry of real space-frame structures when exposed to combined large regular waves and in-line current.