From June 11, 2020, a surge in new cases of coronavirus disease 2019 (COVID-19) in the largest wholesale market of Beijing, the Xinfadi Market, leading to a second wave of COVID-19 in Beijing, China. Understanding the transmission modes of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the personal behaviors and environmental factors contributing to viral transmission is of utmost important to curb COVID-19 rise. However, currently these are largely unknown in food markets. To this end, we completed field investigations and on-site simulations in areas with relatively high infection rates of COVID-19 at Xinfadi Market. We found that if goods were tainted or personnel in market was infected, normal transaction behaviors between sellers and customers, daily physiological activities, and marketing activities could lead to viral contamination and spread to the surroundings
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fomite, droplet or aerosol routes. Environmental factors such as low temperature and high humidity, poor ventilation, and insufficient hygiene facilities and disinfection practices may contribute to viral transmission in Xinfadi Market. In addition, precautionary control strategies were also proposed to effectively reduce the clustering cases of COVID-19 in large-scale wholesale markets.
There has been an unresolved question of whether there is any significant degree of aerodynamic braking during wing deceleration in the flapping flight of birds, with direct analogies existing with flapping micro air vehicles. Some authors have assumed a complete conversion of kinetic energy into (useful) aerodynamic work during wing deceleration. Other authors have assumed no aerodynamic braking. The different assumptions have led to predictions of inertial power requirements in birds differing by a factor of 2. Our work is the first to model the aerodynamic braking forces on the wing during wing deceleration. A model has been developed that integrates the aerodynamic forces along the length of the wing and also throughout the wing beat cycle. A ring-billed gull was used in a case study and an adult specimen was used to gather morphometric data including a steady state measurement of the lift coefficient. The model estimates that there is a 50% conversion of kinetic energy into useful aerodynamic work during wing deceleration for minimum power speed. This aerodynamic braking reduces the inertial power requirement from 11.3% to 8.5% of the total power. The analysis shows that energy conversion is sensitive to wing inertia, amplitude of flapping, lift coefficient and wing length. The aerodynamic braking in flapping micro air vehicles can be maximised by maximising flap angle, maximising wing length (for a given inertia), minimising inertia and maximising lift coefficient.
INTRODUCTIONThere are four main contributions to energy demand in level flapping flight of birds and FMAVs (flapping micro air vehicles): induced drag, body drag, wing profile drag and wing inertial drag. The inertial power requirement of birds and FMAVs is significant because the wings need to be accelerated and decelerated twice during the wing beat cycle. In addition, flapping occurs at high frequencies, typically between 3Hz to 30Hz for birds (Pennycuick 1996). The wings of birds and FMAVs have a low inertia in order to minimise the inertial power requirements of flapping flight. In this paper we develop a theoretical model of inertial power for flapping flight that takes into account conversion of kinetic energy into useful aerodynamic work that occurs during wing deceleration. Previous researchers have been split between those assuming complete conversion of kinetic energy into useful aerodynamic work (Norberg 1990) and those assuming zero conversion (Berg and Rayner 1995). Our study enables the validity of these two approaches to be assessed for the case of the ring-billed gull.Weis-Fogh studied the hummingbird Amazilia fumbriata fluviatilis and estimated that 19% of the kinetic energy was converted into useful aerodynamic work (Weis-Fogh, 1972). However this was not based on modeling of aerodynamic braking but by estimating wasted energy during the wing beat cycle. In our work we make a direct calculation of aerodynamic braking by calculating the aerodynamic forces along the length of the wing and throughout the wingbeat cycle.A...
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