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Lockdowns imposed in most of the countries were lifted following a decline in the COVID-19 cases towards May-June 2020. A recent surge (second wave) in the COVID-19 cases in Europe and other temperate countries as compared to the tropical regions suggests the likely role of solar radiation. We hypothesized that ultraviolet radiation's effect might be a significant factor moderating the spread of the COVID-19 across countries. Regression analysis was done for the UV radiation data for seven hotspot cities (New Delhi, Mumbai, Milan, Madrid, New York, Melbourne and Sydney) with the daily COVID-19 cases. Global erythemal UV radiation values were lower during winter and higher during summer. In general, the daily new COVID-19 cases registered were higher during the winter months having low UV radiation dose (0.5-3.7 kJ m-2). Cases began to decline with summer onset that corresponded to increased UV radiation (2.5-6.7 kJ m-2). Our studies suggest that the natural UV radiation could be a strong determining factor moderating the spread of COVID-19 cases. The importance of UV radiation in natural sunlight as a disinfectant for SARS-CoV-2 cannot be ignored since the lockdowns were lifted; further, it can be considered as a factor for COVID management., Comment: 17 PAGES

During the past two decades, there has been an increased reliance on the use of computational fluid dynamics methods for modeling rotors in high speed forward flight. Computational methods are being developed for modeling the shock induced loads on the advancing side, first-principles based modeling of the trailing wake evolution, and for retreating blade stall. The retreating blade dynamic stall problem has received particular attention, because the large variations in lift and pitching moments encountered in dynamic stall can lead to blade vibrations and pitch link fatigue. Restricting to aerodynamics, the numerical prediction of dynamic stall is still a complex and challenging CFD problem, that, even in two dimensions at low speed, gathers the major difficulties of aerodynamics, such as the grid resolution requirements for the viscous phenomena at leading-edge bubbles or in mixing-layers, the bias of the numerical viscosity, and the major difficulties of the physical modeling, such as the turbulence models, the transition models, whose both determinant influences, already present in static maximal-lift or stall computations, are emphasized by the dynamic aspect of the phenomena.