This paper assesses, through an extensive literature review, the use of ventilation and High-Efficiency Particulate Air (HEPA) purifiers as practical mitigation strategies for reducing the spread of aerosolized COVID-...This paper assesses, through an extensive literature review, the use of ventilation and High-Efficiency Particulate Air (HEPA) purifiers as practical mitigation strategies for reducing the spread of aerosolized COVID-19<sup>1</sup> virus. HEPA is a well-defined standard by the U.S. Department of Energy for filters. The focus of the literature review was on indoor air quality (IAQ) and COVID-19, with a particular emphasis on classroom settings. The start of the review, January 2020, was chosen to coincide with the first cases of COVID-19 in North America. Although children under the age of 12 are currently not yet vaccinated, there is mounting pressure for a return to normal by the start of the new school year, 2021. Also, many classrooms lack pre-installed mechanical ventilation systems (<a href="#ref79">Olsiewski et al., 2021</a>);therefore, mitigation in classrooms often falls solely in the hands of teachers and students. Research shows that ventilation and air purification are essential tools to counter aerosolized transmission (<5 μm) of the COVID-19 virus. According to <a href="#ref25">Curtius et al. (2020)</a>, the inhaled dose of particles containing virus RNA is six times lower when using air purifiers with an ACH (air changes per hour) of 5.7. However, ventilation and air purifiers are not replacements for masks, which remain vital for countering droplet (>5 μm) transmission. In addition, occupancy (i.e., number and proximity of people present in a given area) and group activity levels (e.g., talking, shouting, singing) play a critical role in viral transmission. Although natural ventilation by opening windows can be an essential strategy to help counter the spread of the virus, the level of ventilation offered by opening windows is largely uncontrollable as it is subject to weather <span style="font-family:Verdana;">conditions and building design. One must also consider the energy implications (i.e., loss of heat) that this strategy carries. Scientific evidence shows that varying levels of continuous and/or intermittent ventilation, either mechanical or natural, combined with the use of HEPA air purifiers, can provide a higher degree of protection than window access alone (<a href="#ref25">Curtius et al., 2020</a>). Systematic deployment of a hybrid mitigation strategy incorporating both ventilation and HEPA air purification in schools, offices, or other facilities offers a practical way to establish a safe re-opening of society in Canada.</span>展开更多
Gulf Air has become the first Middle Eastcustomer for the A330,with an order forsix aircrafts.The contract,which also makesGulf Air the largest Airbus Industrie customerin the Middle East,was signed in Bahrain on13th ...Gulf Air has become the first Middle Eastcustomer for the A330,with an order forsix aircrafts.The contract,which also makesGulf Air the largest Airbus Industrie customerin the Middle East,was signed in Bahrain on13th November by His Excellency Salim binAli Assiyabi,President and Chief ExecutiveOfficer of Gulf Air,and Airbus展开更多
The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson＇s ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digit...The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson＇s ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digital samples of asphalt mixture with different shapes of single air void were built using discrete element software PFC^（3D）. The aggregate gradation, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the three cubic samples loaded in three different directions. Dynamic modulus in three directions and Poisson＇s ratio in six directions were calculated from the compression stress-strain responses. Results show that both the modulus and Poisson＇s ratio are dependent on the preferential orientation of air voids. The anisotropy of the modulus and Poisson＇s ratio increases as the pressure loading on the asphalt mixture increases. Compared to the modulus, Poisson＇s ratio due to air voids has been shown to be more anisotropic. The maximum of Poisson＇s ratio and modulus is shown to be up to 80% and 11% higher than the minimum, respectively.展开更多
It is presented that there is a pumping effect at its base in the development process of a cumulus.In the strongest stage of cumulus development,the pumping is mainly produced by the buoyance at the base,and may be ta...It is presented that there is a pumping effect at its base in the development process of a cumulus.In the strongest stage of cumulus development,the pumping is mainly produced by the buoyance at the base,and may be taken as the first approximation of the ascending speed at the base. The results of numerical calculations and simulations of four observed radar echoes show that as the first approximation,the height of air-mass cumulus may be simulated by Haltiner model in the absence of ob- served ascending speeds at the base,and the Haltiner model can be characterized by the sensitivity of the cumulus development to the virtual temperature excess over environment at the base.展开更多
On September 5,the journalist learned from the Shaanxi Province State-owned Assets Supervision and Administration Commission that,as the key R&D production base of Titanium alloy material for China’s air and spac...On September 5,the journalist learned from the Shaanxi Province State-owned Assets Supervision and Administration Commission that,as the key R&D production base of Titanium alloy material for China’s air and space industry,Shaanxi Nonferrous Baoti Group supplied 90%Titanium alloy material usage to China’s air and space industry,making it the main supplier of Titanium alloy for China’s new fighters and big planes.展开更多
文摘This paper assesses, through an extensive literature review, the use of ventilation and High-Efficiency Particulate Air (HEPA) purifiers as practical mitigation strategies for reducing the spread of aerosolized COVID-19<sup>1</sup> virus. HEPA is a well-defined standard by the U.S. Department of Energy for filters. The focus of the literature review was on indoor air quality (IAQ) and COVID-19, with a particular emphasis on classroom settings. The start of the review, January 2020, was chosen to coincide with the first cases of COVID-19 in North America. Although children under the age of 12 are currently not yet vaccinated, there is mounting pressure for a return to normal by the start of the new school year, 2021. Also, many classrooms lack pre-installed mechanical ventilation systems (<a href="#ref79">Olsiewski et al., 2021</a>);therefore, mitigation in classrooms often falls solely in the hands of teachers and students. Research shows that ventilation and air purification are essential tools to counter aerosolized transmission (<5 μm) of the COVID-19 virus. According to <a href="#ref25">Curtius et al. (2020)</a>, the inhaled dose of particles containing virus RNA is six times lower when using air purifiers with an ACH (air changes per hour) of 5.7. However, ventilation and air purifiers are not replacements for masks, which remain vital for countering droplet (>5 μm) transmission. In addition, occupancy (i.e., number and proximity of people present in a given area) and group activity levels (e.g., talking, shouting, singing) play a critical role in viral transmission. Although natural ventilation by opening windows can be an essential strategy to help counter the spread of the virus, the level of ventilation offered by opening windows is largely uncontrollable as it is subject to weather <span style="font-family:Verdana;">conditions and building design. One must also consider the energy implications (i.e., loss of heat) that this strategy carries. Scientific evidence shows that varying levels of continuous and/or intermittent ventilation, either mechanical or natural, combined with the use of HEPA air purifiers, can provide a higher degree of protection than window access alone (<a href="#ref25">Curtius et al., 2020</a>). Systematic deployment of a hybrid mitigation strategy incorporating both ventilation and HEPA air purification in schools, offices, or other facilities offers a practical way to establish a safe re-opening of society in Canada.</span>
文摘Gulf Air has become the first Middle Eastcustomer for the A330,with an order forsix aircrafts.The contract,which also makesGulf Air the largest Airbus Industrie customerin the Middle East,was signed in Bahrain on13th November by His Excellency Salim binAli Assiyabi,President and Chief ExecutiveOfficer of Gulf Air,and Airbus
基金Funded by the National Natural Science Foundation of China(No.51208178)the Fundamental Research Funds for the Central Universities(No.2015B17014)
文摘The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson＇s ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digital samples of asphalt mixture with different shapes of single air void were built using discrete element software PFC^（3D）. The aggregate gradation, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the three cubic samples loaded in three different directions. Dynamic modulus in three directions and Poisson＇s ratio in six directions were calculated from the compression stress-strain responses. Results show that both the modulus and Poisson＇s ratio are dependent on the preferential orientation of air voids. The anisotropy of the modulus and Poisson＇s ratio increases as the pressure loading on the asphalt mixture increases. Compared to the modulus, Poisson＇s ratio due to air voids has been shown to be more anisotropic. The maximum of Poisson＇s ratio and modulus is shown to be up to 80% and 11% higher than the minimum, respectively.
文摘It is presented that there is a pumping effect at its base in the development process of a cumulus.In the strongest stage of cumulus development,the pumping is mainly produced by the buoyance at the base,and may be taken as the first approximation of the ascending speed at the base. The results of numerical calculations and simulations of four observed radar echoes show that as the first approximation,the height of air-mass cumulus may be simulated by Haltiner model in the absence of ob- served ascending speeds at the base,and the Haltiner model can be characterized by the sensitivity of the cumulus development to the virtual temperature excess over environment at the base.
文摘On September 5,the journalist learned from the Shaanxi Province State-owned Assets Supervision and Administration Commission that,as the key R&D production base of Titanium alloy material for China’s air and space industry,Shaanxi Nonferrous Baoti Group supplied 90%Titanium alloy material usage to China’s air and space industry,making it the main supplier of Titanium alloy for China’s new fighters and big planes.
