Numerical Simulation of the Lux Vertical Axis Wind Turbine

Abstract

Wind energy can be characterized as a cheap, clean, and renewable energy source that is absolutely sustainable. With increasing demand for wind energy, it is productive to investigate the structural and operational factors that undermine the proficiency and the characteristic performance of the wind turbine. Of paramount importance to efficient wind energy generation is the aerodynamics of the wind turbine blades. The aerodynamic factors, such as drag, airfoil pro files, and wake interactions that often reduce the performance of the wind turbines, can be investigated through computational mathematics using computational fluid dynamics (CFD). CFD offers basic techniques and tools for simulating physical processes and proffers important insights into the ow data, which are demanding and costly to measure experimentally. In this thesis, we develop a simulation model in an open-source software package called OpenFOAM to investigate the performance characteristics of the Lux Vertical Axis Wind Turbine (VAWT). The Lux VAWT has a simpler design than its horizontal counterparts; however, its performance is affected by the unsteady aerodynamic due to a complex flow field. The turbulent flow field is governed by the incompressible Navier- Stokes equations. Simulations are carried out with an unsteady incompressible and dynamic flow solver, PimpleDyMFoam, on an unstructured mesh surface of the Lux VAWT geometry. The computational domain includes both the stationary and rotating mesh domains to accommodate the rotating motion of the turbine blades and the free-stream zone. The arbitrary mesh interface is applied as a boundary condition for the patches between the two domains to enable computation across disconnected but adjacent mesh domains. Meshing was done using two separate meshing tools, snappyHexMesh and ANSYS Mesher. The snappyHexMesh tool offered the most flexible and effective control over the mesh generation and quality. In order to derive the maximal power output from the Lux VAWT simulations, the Unsteady Reynolds Averaged Navier--Stokes (URANS) equations are solved with different time-stepping methods; the objective is to reduce the computational costs. While attempting to reduce the numerical diffusion from the non-transient terms of URANS, a stabilized trapezoidal rule with a second-order backward differentiation formula (TR--BDF2) time-stepping method was implemented in OpenFOAM. As a result, the transient aerodynamic forces of the blades, the torque, and power output are evaluated. The findings demonstrate that most of the transient aerodynamic force is generated along the axis of rotation of the rotor during one complete revolution. Similarly, the computations indicate that the BDF2 method results in the least computational cost and predicts a turbine power that is somewhat comparable to the experimental results. The difference between the simulation results and the experimental data is attributed partly to the pressure fluctuations on the turbine blades due to the mesh topology.