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

Space-frame structures supporting marine renewable energy devices 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 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. (2015). Under the same input condition, the diagonal loading configuration is observed to attract higher forces and therefore it should not be ignored when assessing the survivability of such structures. Overall, good agreement in terms of the peak forces and the shapes of force time history is achieved, all with a single and consistent value for each of Cd and Cm. On the other hand, predictions using standard Morison with no blockage and 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 an 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 on 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

Electrical rotary joint apparatus for large space structures

A structural array and electrical rotary joint for transmitting an electrical power between large space structures having relative rotational movement is disclosed which includes large support framework structures which rotate relative to one another about a common axis of rotation. A rotary interface joint is defined between the structures. A cylindrical hub member is carried by one structure and a cylindrical hub member is carried by a support structure with a third hub member being concentrically within a fourth hub member for relative rotation. Tension connecting cables connect hub members with their associated outer structures whereby relative rotational movement between the structures is transmitted to the cylindrical hub members for unitary motion therewith. Electrical conductor brush members are carried by one hub and electrical contact rings are carried by another hub member in sliding electrical contact with the brushes for transmission of electrical power during relative rotational movement between the two support structures

Genetic Algorithm Optimization of Space Frame

Structural design of space frames requires appropriate form for a structure so that it can carry the imposed loads safely and economically. Traditional approaches towards the task of finding such forms for structures have been by the use of experimental models or by intuition and experience. The main objective of this paper is to develop and use reliable, creative and efficient computational tools for the linearly elastic analysis and optimum design of space frame structures under static loads. The use of SAP2000 can assist greatly in achieving a safe design. However, commercially available programs are not designed as optimization tools. In this study for optimization of multistory structures, home written MATLAB code interface program is designed to connect SAP2000 which is known as a commercial nonlinear finite element program and genetic algorithm optimization program. The design algorithm obtains minimum weight frames by selecting suitable sections from specified group list, with consideration actual design constraints like, strength, lateral displacement, inter story drift according to Load and Resistance Factor Design (LRFD). The improved method is tested on different two dimensional multi story moment resisting frames. It is concluded that this method can be used as a useful tool in engineering design and optimization

Semi-flexible Additive Manufacturing Materials for Modularization Purposes - A modular assembly proposal for a foam edge-based spatial framework

This paper introduces a series of design and fabrication tests directed towards the use of bendable 3D printing materials in order to simplify a foam bubble-based geometry as a frame structure for modular assembly. The aspiration to reference a spittlebug's bubble cocoon in nature for a light installation in the urban context was integrated into a computational workflow conditioning light-weight, material-, and cost savings along with assembly-simplicity. Firstly, before elaborating on the project motivation and background in foam structures and applications of 3D-printed thermoplastic polyurethane (TPU) material, this paper describes the physical nature of bubble foams in its relevant aspects. Subsequently this is implemented into the parametric design process for an optimized foam structure with Grasshopper clarifying the need for flexible materials to enhance modular feasibility. Following, the additive manufacturing iterations of the digitally designed node components with TPU are presented and evaluated. Finally, after the test assembly of both components is depicted, this paper assesses the divergence between natural foams and the case study structure with respect to self-organizing behavior

Performance of an ideal turbine in an inviscid shear flow

Although wind and tidal turbines operate in turbulent shear flow, most theoretical results concerning turbine performance, such as the well-known Betz limit, assume the upstream velocity profile is uniform. To improve on these existing results we extend the classical actuator disc model in this paper to investigate the performance of an ideal turbine in steady, inviscid shear flow. The model is developed on the assumption that there is negligible lateral interaction in the flow passing through the disc and that the actuator applies a uniform resistance across its area. With these assumptions, solution of the model leads to two key results. First, for laterally unbounded shear flow, it is shown that the normalised power extracted is the same as that for an ideal turbine in uniform flow, if the average of the cube of the upstream velocity of the fluid passing through the turbine is used in the normalisation. Second, for a laterally bounded shear flow, it is shown that the same normalisation can be applied, but allowance must also be made for the fact that non-uniform flow bypassing the turbine alters the background pressure gradient and, in turn, the turbines ‘effective blockage’ (so that it may be greater or less than the geometric blockage, defined as the ratio of turbine disc area to cross-sectional area of the flow). Predictions based on the extended model agree well with numerical simulations approximating the incompressible Euler equations. The model may be used to improve interpretation of model-scale results for wind and tidal turbines in tunnels/flumes, to investigate the variation in force across a turbine and to update existing theoretical models of arrays of tidal turbines