Marco Raciti Castelli

Modeling Strategy and Numerical Validation for a Darrieus Vertical Axis Micro-Wind Turbine

This paper presents a model for the evaluation of optimal spatial grid node distribution in the CFD analysis of a Darrieus vertical axis micro wind turbine, by analyzing the trends over a 360° rotation of some indicators of near-blade mesh quality. To this purpose, a complete validation campaign has been conducted through a systematic comparison of numerical simulations with wind tunnel experimental data. Both two-dimensional and three-dimensional grids, characterized by average y+ values of 30 and 1, have been tested by applying some statistical techniques as a guidance in selecting the appropriate grid configuration and corresponding turbulence model. Finally, the tip downstream recirculation zone due to the finite blade extension and the influence of spokes have been analyzed, achieving a numerical quantification of the influence of induced drag and spokes drag on overall rotor performance.Copyright

Effect of Blade Inclination Angle on a Darrieus Wind Turbine

This paper presents a model for the evaluation of energy performance and aerodynamic forces acting on a small helical Darrieus vertical axis wind turbine depending on blade inclination angle. It consists of an analytical code coupled to a solid modeling software capable of generating the desired blade geometry depending on the desired design geometric parameters, which is linked to a finite volume CFD code for the calculation of rotor performance. After describing and validating the model with experimental data, the results of numerical simulations are proposed on the bases of five machine architectures, which are characterized by an inclination of the blades with respect to the horizontal plane in order to generate a phase shift angle between lower and upper blade sections of 0 deg, 30 deg, 60 deg, 90 deg, and 120 deg for a rotor having an aspect ratio of 1.5. The effects of blade inclination on tangential and axial forces are first discussed and then the overall rotor torque is considered as a function of azimuthal position of the blades. Finally, the downstream tip recirculation zone due to the finite blade extension is analyzed for each blade inclination angle, achieving a numerical quantification of the influence of induced drag on rotor performance, as a function of both blade element longitudinal and azimuthal positions of the blade itself.

Innovative Discrete-Vortex Model for Dynamic Stall Simulations

An innovative model based on the vortex theory is presented with the aim of simulating the two-dimensional airfoil dynamic behavior at pitching reduced frequencies related to vertical axis wind-turbine operative conditions. The model relies on the introduction of a second separated wake from the suction side to correctly account for the aerodynamic effects of stall conditions and is provided with correction models whose aim is to consider the dynamic evolution of the shed vortices and of the separation point. The model receives as input experimental data to estimate the nonoscillating steady-state separation point for different angles of attack. A validation procedure confirmed the model capabilities to provide reliable numerical estimations of the lift coefficient for a pitching airfoil compared to experimental tests and computational-fluid-dynamics approaches based on the unsteady Reynolds-averaged Navier–Stokes equations complemented with the k-ω shear-stress transport turbulence model. In particular, ...

Numerical Investigation of Laminar to Turbulent Boundary Layer Transition on a Naca 0012 Airfoil for Vertical-Axis Wind Turbine Applications

Laminar to turbulent transition in vertical-axis wind turbines has a dramatic effect on overall rotor performance, especially in fast rotating machines, where reliable prediction of the total drag coefficient for high values of tip speed ratio is one of the most critical topic in CFD simulations. This paper presents a 2D numerical investigation of the capability of the γ–θ transition model to predict the laminar to turbulent transition and consequent friction drag over a NACA 0012 airfoil for a Reynolds number of 360,000, which is typical of vertical-axis wind turbine blades during operation. The analized range of angles of attack varies from 0 deg to 10 deg. The commercial CFD solver ANSYS FLUENT® is used. In particular, the sensitivity to grid resolution is investigated for four different architectures: a completely unstructured mesh, a hybrid structured-triangular one and two distinct hybrid structured-triangular meshes where the wake region behind the airfoil is discretized using a fully structured grid. The effect of freestream turbulence intensity on the transition onset is also analyzed. Finally, CFD results are compared to experimental data, although affected by some uncertainty, and to the predictions of an interactive program for the design and analysis of subsonic isolated airfoils (XFOIL), showing a very good agreement provided that the value of freestream turbulence intensity is known.

Evaluation of the wind potential in the province of Belluno (Italy)

Anemometric data from a measurement campaign performed in the whole province of Belluno (Italy) have been analyzed in order to evaluate its wind potential. Data collected from 15 anemometric stations, placed at a height of 10 m over the ground, have been considered. The temporal evolution of the average wind velocity in each measurement site has been computed, as well as the Weibull distribution, allowing a comparison between measured data and the prediction of the Italian Wind Atlas.

Numerical Simulation of a Straight-Bladed Vertical-Axis Water Turbine Operating in a 2 m/s Current

The present work proposes a full campaign of simulation of a Darrieus-type Vertical-Axis Water Turbine (VAWaterT) operating in an open flow-field. After describing the computational model and the relative validation procedure, a complete campaign of simulations based on full RANS unsteady calculations is presented for a three-bladed rotor architecture, characterized by a NACA 0025 blade profile. Flow field characteristics are investigated for several values of tip speed ratio and for a constant unperturbed free-stream water velocity of 2 m/s. Finally, the torque coefficient generated from the three blades is determined for each simulated angular velocity, allowing the calculation of the rotor power-curve. Keywords: Vertical-Axis Water Turbine, hydrokinetic technology, CFD, NACA 0025.

Contrasting the Parasitic Torque on a Drag-Driven VAWT through a Variable-Blade Rotor Architecture

The present work exhibits a further development of a mean for furthering conventional drag-driven VAWT technology by contrasting the counter-rotating (parasitic) torque generated from drag-driven vertical-axis wind turbine (VAWT) blades travelling into the wind through the use of a variable rotor blade geometry. A former research on water tank experimental tests on a small rotor characterized by three horizontal blades having variable geometry has been presented [. Each blade was obtained by connecting a horizontal flat plate to a tilting one swinging over a horizontal axis, so that the tilt angle between the two plates could vary depending on flow field conditions. In this paper several open-field measurements at different free-stream wind velocities have been carried out, in order to characterize the energy performance of the rotor for two different blade profile architectures: first experimental investigations were performed on a fixed-angle blade configuration of 41.3 degs between the horizontal and the inclined flat plates. Then, attention mainly focused on tilting blades, achieving a quantification of the influence of blade swinging on overall rotor performance. A full campaign of analysis was completed for both blade configurations and overall rotor torque and power were analyzed as a function of incoming wind velocity and rotor angular speed, resulting the variable-blade configuration maximum power coefficients much higher with respect to the corresponding classical drag-type fixed-blade architecture.

Numerical performance and stress prediction for a vertical-axis wind turbine as a function of the aerodynamic control strategy

A combined performance/stress numerical analysis is proposed for a three-bladed vertical-axis wind turbine (VAWT) adopting a Blade Element-Momentum (BE-M) algorithm as a simulation tool. After determining the operational curves of the turbine, the aerodynamic forces acting on rotor blades, as well as the global thrust acting on the whole machine, are evaluated as a function of the aerodynamic control strategy of the rotor, obtaining a first estimation to be used for the structural sizing of both rotor blades and shaft.