There is still discrepancy regarding the verification of CFD U-RANS simulations of vertical-axis wind turbines (VAWTs). In this work, the applicability of the Richardson extrapolation method to assess mesh convergence is studied for several points in the power curve of a VAWT. A 2D domain of the rotor is simulated with three different meshes, monitoring the turbine power coefficient as the convergence parameter. This method proves to be a straightforward procedure to assess convergence of VAWT simulations. Guidelines regarding the required mesh and temporal discretization levels are provided. Once the simulations are validated, the flow field at three characteristic tip-speed ratio values (2.5-low, 4-nominal and 5-high) is analyzed, studying pressure, velocity, turbulent kinetic energy and vorticity fields. The results have revealed two main vortex shedding mechanisms, blade-and rotor-related. Vortex convection develops differently depending on the rotor zone (upwind, downwind, windward or leeward). Finally, insight into the loss of performance at off-design conditions is provided. Vortex shedding phenomena at the low tip-speed ratio explains the loss of performance of the turbine, whereas at the high tip-speed ratio, this performance loss may be ascribed to viscous effects and the rapid interaction between successive blade passings.

In this paper, the aerodynamic field around a FX 63-137 airfoil for four angles of attack and low Reynolds numbers was simulated with a Large Eddy Simulation (LES). Following, an acoustic analogy method was employed to calculate the airfoil trailing edge (TE) noise. In this second scheme step, the far-field acoustic pressure was predicted from the LES source terms using two different methods based on Lighthill's analogy: Curle's surface approach and Ffowcs-Williams and Hall's volumetric analogy (FW-Hall). Numerical results have been validated with hot-wire anemometry for the aerodynamic fields, thus verifying the accuracy of the CFD simulation for the prediction of noise propagation to the far field. Additionally, aeroacoustic results were validated with experimental measurements carried out in an anechoic wind tunnel using a frequency analyzer. The FW-Hall formulation shows a better agreement with the experiments, especially in the range of frequencies corresponding to the trailing edge, whereas Curle's analogy overpredicts airfoil sound. An exhaustive analysis of the aerodynamic flow field has been performed in order to better understand the generation mechanisms of the TE noise. The aeroacoustic calculations presented in this work contribute to develop a more reliable and efficient prediction methodology based on the Computational Aeroacoustics Approach (CAA).

Many studies have tried to give insight into the optimal values of solidity and the airfoil geometry that maximize the performance and self-starting capability of vertical axis wind turbines, but there is still no consensus. In addition, most studies focus on one particular airfoil or airfoil family, which makes the generalization of the results difficult. In this work, these research gaps are intended to be assessed. An exhaustive analysis of the influence of solidity, blade Reynolds number and airfoil geometry on the performance of a straight-bladed vertical axis wind turbine has been performed using a methodology based on streamtube models. An airfoil database of 34 airfoils has been generated, developing a practical and cost-effective tool for the quick comparison of turbine designs (70 different configurations were analyzed). This tool, validated with results from the literature and computational fluid dynamics simulations performed by the authors, has allowed to propose an optimal solidity range from 0.25 to 0.5 and the use of almost symmetrical airfoils (camber < 3%). Finally, this tool has been applied to design two vertical axis wind turbines optimized for low and medium wind speeds.

An exhaustive investigation of the structure of the turbulence around an asymmetric FX 63-137 wind turbine airfoil is carried out in this paper. Reliable hot-wire velocity measurements, made at the Xixon Aeroacoustic Wind Tunnel, are presented with the aim of analyzing the turbulent flow features. The probe was placed at two different positions along the streamwise direction, one over the airfoil and the other at the wake, both on the suction and pressure side. These measurements were performed in order to capture the evolution of the flow and its behavior at the wake. The experimental data were collected at a Reynolds number of 350000 for several incidence angles to explore their influence in the turbulence characteristics. The data processing from the dual hot-wire, capable of measuring two velocity components, allowed to achieve half set of the Reynolds stresses, the turbulence intensity and the degree of anisotropy. The boundary layer and wake size were estimated from the Reynolds stress components. In addition, the production term of the turbulence kinetic energy budget is calculated to visualize the unsteadiness energy inside the boundary layer. As a result of these analyses, it was observed that the transversal fluctuations were higher than the longitudinal ones. Besides, an alternative description of the turbulence structure is obtained when a frequency analysis of the motion is provided, disclosing a clear change in the spectra tendencies in the wake and boundary layer regions. This analysis, combined with the degree of anisotropy analysis, was helpful to define a transition zone between the clearly distinguishable instability zone and the free-stream zone. Finally, the integral length scale of turbulence was estimated from the area under the autocorrelation function of the velocity fluctuations. The combination of the results of this work have provided a wide description of the turbulent behavior of the flow around the airfoil and present a clearer physical picture of the phenomena.

This paper presents the unsteady numerical methodology for the CFD simulation of Air-Operated Diaphragm Pumps. The model reproduces the unsteady displacement of the diaphragm using dynamic mesh techniques and fully resolves the Fluid Structure Interaction (FSI) responsible for the motion of the check valves. The governing parameters have been modified with User Defined Functions (UDFs), using an implicit scheme for the grid motion that guarantees the stability and realizability of the twodimensional model adopted. The analysis of the instantaneous delivered flow rate, as a function of the discharged outlet pressure, has provided interesting and useful information for the future design of new prototypes. The comparison between numerical results and the experimental performance curves has confirmed the accuracy of the model and the correct mesh selection in the small gaps and passages of the pump internal geometry. The leakage flows, especially in the exhausting valve during the forward stroke, and the ball tapping responsible for instabilities and high-frequency noise during oscillations of the valves has been accurately simulated. At high-delivered pressures, it has been observed a characteristic ripple in the instantaneous flow rate during the deceleration of the diaphragm towards its top-dead-center, associated to a partial reopening of the exhausting valve. A closer look to the dynamics of the balls has revealed a strong coupling between inlet and outlet check valves. In addition, despite of the remarkable level of accuracy (less than 9% of deviation), the recirculating cells found in the flow fields inside the pump suggest the convenience of the development of a full-unsteady 3D model in the near future.