CFD Simulation Results

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Chapter VII

Simulation Results

7.0 Introduction

In this chapter the simulation results of the marine current energy conversion system has been discussed fully. Flow 3D software package has been used to develop the computational fluid dynamic model of the marine current turbine. Also Matlab/Simulink has been selected to carry out the overall modeling task of MCECS. The software package allows user to use Simulink in conjunction with Matlab to develop algorithm, numerical computation and data visualization. A complete simulation model of the MCECS is developed using SimPower Systems toolbox of Matlab/Simulink Software.

7.1 CFD Simulation Results

CFD simulations predict the motion of a flow within a domain by discretizing the governing Navier-Stokes equations and solving the resulting set of algebraic equations.

The commercial CFD package FLOW 3D was used to solve the incompressible Navier-Stokes equations using finite difference approximations. To ensure numerical stability, a combination of central and upwind differences is used for the convective terms. One important feature known as the General Moving Object (GMO) can simulate the complex motions of floating bodies in steep waves. The motions of the bodies can be prescribed or they can be coupled to the motion of the fluid. It allows for extremely complicated motions and flows. For the simulation, the rotor was first drawn in

144 | P a g e SolidWorks. The geometry file was imported into FLOW 3D as an STL file. The thickness of the blades of the rotor was made two time actual so that the grid employed could discretize them. The density of the rotor was adjusted to keep the total inertia of the rotor equal to the original prototype. The accuracy of any CFD simulation depends on the type of grid utilized (e.g. structured or unstructured) and obviously the grid resolution. Structured grids are the most desirable because they inherently provide highly accurate numerical solutions. As such, they have generally been used in CFD simulations of turbines. The disadvantage of using structured grids is that the grid generation process is a manually intensive and time consuming task. In contrast, the creation of unstructured grids is a more automated process that can be performed easily. However, when using unstructured grids, we should insure that the grid quality (e.g. grid element skewness, aspect ratio, and density) does not adversely impact the accuracy of the numerical solution.

Fig. 7.1a shows the computational domain grid set up in FLOW 3D to analyze the performance characteristics of marine current turbine. However, here each had a cell size of 5mm. Both the finer mesh and coarse size grid were used and they produced basically the same results. The rotor was modeled as a general moving object with freedom to rotate around the z- axis. The boundary conditions used for the simulation are shown in Fig. 6.1b. The k-ε Reynolds-averaged Navier-Stokes (RANS) model was used to simulate turbulence in the flow.A uniform steady velocity profile was assumed at the inlet of the computational domain. Thus we made the simplifying assumption that the turbulence intensity was 5% and uniform at the inlet of the computational domain. A pressure outlet boundary condition was applied at the outlet. To reduce flow

145 | P a g e recirculations on the far-field boundaries and to improve the stability of the numerical solution, symmetry boundary condition was applied. A rotating reference frame model was used to simulate the rotation of the blade and hub. This method simulates rotation without the need to physically rotate the computational grid by forming the governing equations for the rotor domain in a reference frame that rotates with the turbine, while the outer domain remains in a stationary coordinate system. Fig 7.2 plots the power coefficient of the turbine with respect to tip speed ratio.

Fig. 7.1(a)

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Fig. 7.1(b)

Fig. 7.1 CFD Software FLOW 3D (a) Computational Domain Grid (b) Boundary condition of Marine Current Turbine

Fig. 7.2 Power Coefficient Curve of Marine Current Turbine

147 | P a g e In order to run the turbine at the optimum speed proportional integral controller is incorporated in FLOW -3D subrotine so that turbine can operate at optimum speed for different flow speed. Fig 7.3a shows that the flow speed is varied from 0.8m/s to 1.25m/s.

The rotor speed at different flow speed is captured in Fig. 7.3b. The output shows that

Fig. 7.3(a)

Fig. 7.3(b)

Fig. 7.3 Response of FLOW 3D Marine Current Speed (a) Step change in flow speed (b) Turbine Optimum Rotor Speed with Proportional Integral controller

148 | P a g e PI controller can successfully operate the system at optimum speed. The optimization of the rotor design (pitch angle, geometry, number of blades) done using CFD. Simulation shows that rotor with curved blades improves the efficiency of the turbine. It should be noted that the CFD model only includes the turbine blade and hub geometries. Other turbine components, such as the clamp support, were not considered in this simulation.

Nevertheless, the reasonable agreement between the CFD results and the blade element momentum method calculations provides confidence in the results obtained with both numerical methods.The CFD simulation represents is a step in the development of a CFD methodology to characterize the turbine performance. In future research should be done to develop a numerical model by considering realistic inlet velocity profiles and turbulence levels. Also include structural elements of the turbine, such as the support, in future simulations.