Mesh densities in the model for a CMMCT unit in free flow. 

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... to the world’s increasing electricity demand and envi- ronmental concerns, a vast amount of research and related technologies have been extensively developed to improve the thermal efficiency of power consuming energy systems [ 1, 2]. On the other hand, renewable and sustainable energy systems have become more and more attractive because they have less impact on the environment than conventional energy systems [3]. These alternatives include solar, wind, ocean, hydropower, and geothermal resources. Marine currents, which are generated from tidal move- ments and ocean circulation, offer a huge untapped potential to produce a large amount of sustainable power [4]. Marine current energy has been found to be the most promising option for massive ocean-energy generation in the near future [5]. In 2003, the world’s first commercial- scale marine current turbine with a power rating of 300 kW was successfully installed by Marine Current Turbines (MCT) Ltd and IT-Power [5]. In the long term, the total potential for ocean energy production is equal to that of onshore wind energy [6]. While marine energy systems are currently at the proof-of-concept stage and still under development, the axial three-bladed turbine is by far the most dominant concept in the wind industry [7, 8]. The marine environment is considerably more hostile than the low-level atmospheric conditions encountered by wind turbines; this is largely due to the greater density of water, which is approximately 1000 times that of air. Therefore, the system’s structure and its anchoring must be designed to withstand the large hydrodynamic forces it will en- counter in order to endure [9]. Since seawater is a saline solution where any metallic components will have to be protected, corrosion is another serious consideration [9]. Thus, there is an increased interest in the use of composites as alternative materials. Developed and automated in the Turbomachinery Laboratory at Michigan State University, a novel manufacturing approach similar to filament winding is able to produce the composite wheels in different configurations (Fig. 1). The advantage of using a filament winding method to manufacture high-performance and light-weight composite marine current turbines is that the production can be rapid, inexpensive and utilize com- mercially available winding machines [10, 11, 12]. Achieving satisfactory performance from the novelly designed marine current turbines requires a complete understanding of the influence of rotating speed, as well as orientation in an array when considering multiple turbines. Three di- mensional CAD and CFD tools integrated with traditional computational tools for design and manufacturing are used to evaluate these effects. The numerical simulation done using the commercial CFD code FLUENT employed the finite volume formulation with a non-structured mesh. Zoran Čarija compared data calculated using FLUENT with measured values cover- ing the entire operating range of a 20 MW Francis turbine in order to validate the CFD simulation [13]. It was observed that all computed hydraulic characteristics that were obtained from CFD showed very good conformity with values and trend-lines of measured characteristics over the whole operating range. Rao and Sivashanmugam presented an investigation comparing experimental and CFD FLUENT simulation results on power consumption in new energy saving turbine agitators, and showed that CFD-based predictions of power are very close to the experimental values [14]. Based on the agreement between CFD simulation and experimental results, FLUENT was utilized to study the power generation of CMMCTs. One of many possible wound composite wheel patterns was selected; this particular wheel has the pattern desig- nation 8B. Its shape is very different from a conventional turbine. The main parameters of the ducted turbine unit modeled are shown in Table 1, which is based on the numerical investigation results by Gaden and Bibeau. Fig. 2 displays the model of the wound wheel. In this study, pre- processing codes for NX and Gambit were used to create the single CMMCT unit, which includes a nozzle, a wound turbine wheel, and a diffuser, all in a free stream. When flow characteristics involving rotating machinery are an- alyzed, a relative coordinate system is generally adopted because it allows for convenient application of boundary conditions and the steady state solver. The velocity inlet and pressure outlet bondary conditions were employed, and the symmetry boundary condition was used for the side walls of the computational domain cube due to the minimally disruptive nature of the flow (Fig. 3). In FLUENT, no defining surfaces are considered walls [15]. Since Navier-Stokes equations are solved inside the domain, the no-slip boundary condition is applied to all walls in the domain. For the computational domain itself, unstructed 3D tetrahedral meshing has been employed due to its flex- ibility when solving in complicated geometries such as those in this study. In order to accurately simulate the flow between turbine blades, further mesh refinement is required within the flow passages. The total mode mesh size utilized 1,123,613 elements after careful mesh inde- pendence testing in order to obtain converged results. In this study, the mass flow rate and momentum change for a − 4 convergence tolerance of 10 were monitored. When the residuals stay at or below this number and do not change as the iterations continue, it can be concluded that the solution has converged. Fig. 4 shows the mesh density differences between the turbine, nozzle, diffuser, and free stream cube. CFD is fundamentally based on the governing equations of fluid dynamics. They are mathematical statements of the conservation laws of physics where the following physical laws are adopted [16, ...