3D Numerical Study Of Hill Mounted VAWT

The ground topography effect on the wind flow is significant. The knowledge of the flow behavior near ground is crucial in the development of wind power, especially in the choice of suitable sites and for estimation of energy production. In this paper, the numerical prediction of the flow over a three-dimensional hill model and the analysis of placement of Savonius turbines on top of the hill are presented. The numerical analysis is based on the finite volume method implemented in the ANSYS CFX 15 Software using the Shear-Stress Transport (SST) turbulence model. The numerical results for a conventional Savonius rotor and a vertical-axis spiral wind rotor are both satisfactory compared with experimental data. The performances of these turbines, installed on the hilltop, are studied for different height positions. Furthermore, the influence of the hill size on the extracted power is investigated. At TSR=1, the power coefficient of a conventional rotor is increased from 0.15 to 0.32 when the rotor is installed at a height of 0.25 m above the top of the hill, while it reaches 0.40 when the hill is two to three times higher. The helical Savonius rotor tested gives even higher power coefficient of 0.44

A Posteriori Bounds for Linear-Functional Outputs of Crouzeix-Raviart Finite Element Discretizations of the Incompressible Stokes Problem

We present a finite element technique for the efficient generation of lower and upper bounds to outputs which are linear functionals of the solutions to the incompressible Stokes equations in two space dimensions; the finite element discretization is effected by Crouzeix-Raviart elements, the discontinuous pressure approximation of which is central to our approach. The bounds are based upon the construction of an augmented Lagrangian: the objective is a quadratic "energy" reformulation of the desired output; the constraints are the finite element equilibrium equations (including the incompressibility constraint), and the intersubdomain continuity conditions on velocity. Appeal to the dual max-min problem for appropriately chosen candidate Lagrange multipliers then yields inexpensive bounds for the output associated with a fine-mesh discretization; the Lagrange multipliers are generated by exploiting an associated coarse-mesh approximation. In addition to the requisite coarse-mesh calculations, the bound technique requires solution only of local subdomain Stokes problems on the fine-mesh. The method is illustrated for the Stokes equations, in which the outputs of interest are the flowrate past, and the lift force on, a body immersed in a channel

Low Reynolds Number Vertical Axis Wind Turbine for Mars

A low Reynolds number wind turbine is designed to extract the power from wind energy on Mars. As compared to solar cells, wind turbine systems have an advantage on Mars, as they can continuously produce power during dust storms and at night. The present work specifically addresses the design of a 500 W Darrieus-type straight-bladed vertical-axis wind turbine (S-VAWT) considering the atmospheric conditions on Mars. The thin atmosphere and wind speed on Mars result in low Reynolds numbers (2000-80000) representing either laminar or transitional flow over airfoils, and influences the aerodynamic loads and performance of the airfoils. Therefore a transitional model is used to predict the lift and drag coefficients for transitional flows over airfoils. The transitional models used in the present work combine existing methods for predicting the onset and extent of transition, which are compatible with the Spalart-Allmaras turbulence model. The model is first validated with the experimental predictions reported in the literature for an NACA 0018 airfoil. The wind turbine is designed and optimized by iteratively stepping through the following tasks: rotor height, rotor diameter, chord length, and aerodynamic loads. The CARDAAV code, based on the “Double-Multiple Streamtube” model, is used to determine the performances and optimize the various parameters of the straight-bladed vertical-axis wind turbine

Wind turbine designs for urban applications: A case study of shrouded diffuser casing for turbines

The increased demand for renewable energy and the development of energy independent building designs have motivated significant research into the improvement of wind power technologies that target urban environments. However, the implementation of wind turbines in urban environments is still very limited. There have been some studies analyzing different designs of urban wind turbines either using computational fluid dynamics (CFD), wind tunnel tests or field data for existing or new turbine designs. This paper reviews the state-of-the-art of urban wind energy by examining the various types of urban wind turbine designs, with a view to understand their performance and the synergy between the turbines and the urban environments. It also considers a flanged diffuser shroud mechanism - a fluid machine, mounted on rooftop of buildings used as casing for small wind turbines to improve turbine performance by using mainly CFD. The diffuser shroud mechanism can draw the airflow over buildings utilizing its special features such as, cycloidal curve geometry at the inlet and a vortex generating flange at the outlet, to guide and accelerate the airflow inside. The performance of the fluid machine is optimized parametrically. The mechanism is modeled on a building rooftop in a real test site in Montreal, Canada with real statistical wind data. The CFD result confirms the functionality of the fluid machine to take advantage of the airflow over buildings in complex built-environments for wind power generation

Real gas simulation of hydrogen release from a high-pressure chamber

Hydrogen release from a high-pressure chamber is to be modeled in this paper. Two approaches are developed to investigate the real gas effects at high pressures. In the first method, an analytical model is developed to simulate time histories of stagnation properties of hydrogen inside the chamber, as well as sonic properties of hydrogen at the orifice. Corresponding thermodynamic relations, which describe specific heats, internal energy and speed of sound, are derived based on the Beattie–Bridgeman state equation. Regarding the second approach, a 3-D unstructured tetrahedral finite volume Euler solver is applied to numerically simulate the hydrogen release whereby the solver is modified to take into account the real gas effects. All the required modification for calculation of real gas Jacobian matrices, eigenvectors and Roe's average convective fluxes are described. Real gas effect is thus modeled by the same state equation. Numerical and analytical results are then compared for ideal and real gas conditions and, to conclude, an excellent agreement is reported

Parallel computations of finite element output bounds for conjugate heat transfer

This paper investigates the a posteriori finite element bound method applied to a heat transfer problem in a multi-material electronic components array. The temperature field is obtained by solving Poisson equations and convection–diffusion equations in different regions of the computational domain. The bound method calculates very sharp lower and upper bounds of the temperature of the hottest component which is assumed to be the engineering output of interest. This paper shows that for this two-dimensional problem the bound method can yield more than an 80-fold reduction in simulation time over a fine mesh calculation (330,050 d.o.f.) while still maintaining quantitative control over the accuracy of the engineering output of interest. Parallel implementation on a Beowulf cluster is also reported

Numerical simulation of high pressure hydrogen release through an expanding opening

Computational Fluid Dynamics is an effective tool to develop safety standards related to the sudden release of hydrogen from a high pressure reservoir. In this work, a three-dimensional in-house code is developed to numerically simulate the release of high pressure hydrogen (70 MPa) from a reservoir when the release area into air is expanding with time. Furthermore, high pressure hydrogen flows cannot be accurately simulated by the ideal gas equation; therefore the Abel-Noble real gas equation of state is applied. A transport equation is solved to find the concentration of hydrogen and air in the hydrogen-air mixture generated soon after release. The novelty of this work is to simulate and to study the flow when the release area enlarges rapidly. To obtain this capability, the solid boundaries of the release area are moved and the mesh follows based on a spring method. All the nodes in the mesh are moved at each time step accordingly to have a good quality mesh. Three initial diameters of 1.0 mm, 1.5 mm and 2.0 mm are tested for the release area, and opening wall speeds of 80 m/s and up to 300 m/s are discussed

CFD BASED SIMULATION OF HYDROGEN RELEASE THROUGH ELLIPTICAL ORIFICES

Computational Fluid Dynamics (CFD) is employed to investigate the hydrogen jet exiting through different shapes of orifices. The effect of orifice geometry on the structure, development and dispersion of a highly under-expanded hydrogen jet close to the exit is numerically investigated. Various shapes of orifices are evaluated, including holes with constant areas such as elliptical and circular openings, as well as, enlarging circular orifices. A three-dimensional in-house parallel code is exploited to simulate the flow using an unstructured tetrahedral finite volume Euler solver. The numerical simulations indicate that, for a high pressure reservoir hydrogen release, the area of the orifice is the main geometric parameter influencing the centerline pressure at the hydrogen-air interface and the transient peak temperature, while the elliptical or expanding orifices slightly mitigate the auto-ignition risks associated with the accidental release of hydrogen. Therefore, circular openings represent the most conservative geometry for the study of auto-ignition of hydrogen

A three-dimensional finite element approach for predicting the transmission loss in mufflers and silencers with no mean flow

A three-dimensional finite element method has been implemented to predict the transmission loss of a packed muffler and a parallel baffle silencer for a given frequency range. Iso-parametric quadratic tetrahedral elements have been chosen due to their flexibility and accuracy in modeling geometries with curved surfaces. For accurate physical representation, perforated plates are modeled with complex acoustic impedance while absorption linings are modeled as a bulk media with a complex speed of sound and mean density. Domain decomposition and parallel processing techniques are applied to address the high computational and memory requirements. The comparison of the computationally predicted and the experimentally measured transmission loss shows a good agreement