Alberto Pedro Blasetti

Vortices and dynamic stall play a critical role in the performance of vertical-axis wind turbines, often leading to significant energy losses. Active and passive control strategies can be employed to delay the dynamic stall, particularly at low tip-speed ratios below 0.5. In this study, three-dimensional Computational Fluid Dynamics simulations of a Darrieus H turbine equipped with a NACA 0018 airfoil are performed using a sinusoidal pitch control method. A vorticity analysis framework is developed to evaluate vortex transport, growth, and detachment, enabling a rigorous assessment approach. The simulations, conducted at 8 m/s wind speed, are validated against experimental data, showing less than 4% deviation. The analysis examines the correlation between vorticity dynamics and turbine performance across both the span and chord of the airfoil. At a tip-speed ratio of 0.5, where dynamic stall is dominant, the pitch control delays flow separation and increases performance by 238%. At a tip-speed ratio of 1.4, performance improves by 57%, although the flow shifts toward a drag-dominated regime in which vortex detachment plays a reduced role. This demonstrates that quantitative vorticity analysis remains effective under variable pitch and can identify stages of the dynamic stall, including imminent vortex separation, offering a basis for optimizing vertical-axis wind turbines.

Vortices play a critical role in the operation of VAWTs (Vertical-Axis Wind Turbines). In spite of this, most studies have approached these matters via the qualitative analysis of vortex shedding, and torque-extraction data. These approaches rely only on the visual observation of vortices that can lead to subjective interpretations. In our work, a 3D framework is employed to address this issue. On this basis, the present study establishes a relationship between vorticity, dynamic stall and turbine performance, by examining various locations along the span and the chord of the airfoil. To conduct this analysis, a 3D-CFD (Computational Fluid Dynamics) simulation of an H-Darrieus with a symmetrical NACA 0018, powered by 8 m/s winds, is considered. The CFD simulations are validated based on the agreement of calculated power coefficients, with those obtained from experimental data, reported in the technical literature, with deviations being lower than 4%. The simulation results for various TSRs (Tip Speed Ratios) report new findings concerning the critical stages of VAWT operation. This shows that there is a link between the maximum vorticity, the imminent vortex-separation condition and the dynamic stall, with this being a function of the various axial positions in the VAWT.

Large quantities of pumpkins, rich in valuable nutrients, are lost due to superficial imperfections or size variations. This study explores a solution: transforming this unused resource into a highly functional food ingredient-pumpkin powder obtained from dehydration. This study emphasizes the importance of a detailed particle-level mathematical model in dehydrator design and operation, particularly for drying conditions using air at temperatures between 333 K and 353 K. The model investigates the effect of sample geometry on the moisture reduction rate and the product quality. Here, a model considers mass and energy transport, including the shrinkage ratio of the samples. The results effectively demonstrate the deformation, moisture content, and temperature evolution within the samples throughout the drying process. The findings reveal that both the drying temperature and initial sample geometry significantly influence the moisture loss rate, the final product texture, and the powder’s absorption capacity. Notably, the nutritional composition (except for lipids) remains largely unaffected by the drying process. Additionally, the bulk and compacted densities of the powders decrease with increasing temperature. These insights not only illuminate the performance of the drying process but also provide valuable knowledge regarding the dehydrated product’s technological behavior and potential functionalities within various food applications.

This study introduces a Vorticity Index (VI) and an Imminent Vortex Separation Condition (IVSC), which are considered valuable indicators to quantify the vorticity impact on vertical axis wind turbines (VAWTs) operation. The VI and IVSC are specifically applied to a H-Darrieus vertical axis wind turbine (VAWT). Findings show that these two parameters display a direct relationship with the aerodynamic forces that govern the performance of this type of VAWT. This analysis is accomplished via 2D-CFD simulations of a H-Darrieus with a symmetrical NACA 0018, powered by high winds (8 and 20 m/s), by using a Shear Stress Transport SST k-ω model. The 2D model used is validated for Class II winds (8 m/s), for tip speed ratios (λ) ranging from 0.4 to 0.9. Power coefficients (C_p) predictions are close to those obtained with both 3D simulations and with experimental data, reported in the technical literature. It is found with the numerical simulations developed, that despite the significant increase of the average rotor overall torque values, when the wind speed is augmented from 8 m/s to 20 m/s, the energy extracted by the rotor seems to be moderately lessened by the amplified turbulence and vorticity.