Based on wind-speed records of Alaska’s 19 first-order weather stations, we analyzed the near-surface wind-speed stilling for January 1, 1984 to December 31, 2016. With exception of Big Delta that indicates an increa...Based on wind-speed records of Alaska’s 19 first-order weather stations, we analyzed the near-surface wind-speed stilling for January 1, 1984 to December 31, 2016. With exception of Big Delta that indicates an increase of 0.0157 m·s–1·a–1, on average, all other first-order weather stations show declining trends in the near-surface wind speeds. In most cases, the average trends are less then?–0.0300?m·s–1·a–1. The strongest average trend of?–0.0500?m·s–1·a–1 occurred at Homer, followed by?–0.0492?m·s–1·a–1 at Bettles, and?–0.0453?m·s–1·a–1 at Yakutat, while the declining trend at Barrow is marginal. The impact of the near-surface wind-speed stilling on the wind-power potential expressed by the wind-power density was predicted and compared with the wind-power classification of the National Renewable Energy Laboratory and the Alaska Energy Authority. This wind-power potential is, however, of subordinate importance because wind turbines only extract a fraction of the kinetic energy from the wind field characterized by the power efficiency. Since wind turbine technology has notably improved during the past 35 years, we hypothetically used seven currently available wind turbines of different rated power and three different shear exponents to assess the wind-power sustainability under changing wind regimes. The shear exponents 1/10, 1/7, and 1/5 served to examine the range of wind power for various conditions of thermal stratification. Based on our analysis for January 1, 1984 to December 31, 2016, Cold Bay, St. Paul Island, Kotzebue, and Bethel would be very good candidates for wind farms. To quantify the impact of a changing wind regime on wind-power sustainability, we predicted wind power for the periods January 1, 1984 to December 31, 1994 and January 1, 2006 to December 31, 2016 as well. Besides Big Delta that suggests an increase in wind power of up to 12% for 1/7, predicted wind power decreased at all sites with the highest decline at Annette (≈38%), Kodiak (≈30%), King Salmon (≈26%), and Kotzebue (≈24%), where the effect of the shear exponents was marginal. Bethel (up to 20%) and Cold Bay (up to 14%) also show remarkable decreases in predicted wind power.展开更多
Declines in wind speed(u)(termed as“stilling”)has been reported in many regions of the world.To explore the temporal trends of u and its aerodynamic effects is vital to understand the changes in water resources.This...Declines in wind speed(u)(termed as“stilling”)has been reported in many regions of the world.To explore the temporal trends of u and its aerodynamic effects is vital to understand the changes in water resources.This study analyzed the changes of temporal trends for u and its aerodynamic effects using the data during 1959-2000 at 266 stations across China.The improved PenPan model was used to estimate pan evaporation(Epan)and quantify the contribution of radiative and aerodynamic components(aerodynamic component separated into wind speed u,vapour pressure deficit D,and air temperature Ta).Climate factors include Epan measured with the standard Chinese 20 cm diameter pan,u,Ta,relative humidity(rh)and sunshine hours(sh).The results showed:stilling occurred in most of stations(206 among 266)and 105 stations presented significant decreasing trends at 99%confidence level;stilling was the main cause for controlling the trends in Epan in most part of China,especially in the west and north of China.The results indicated that decreasing trends in Epan due to stilling would inevitably alter water resources,and should be put further investigation incorporation other factors.展开更多
3-D numerical simulation was carried out for the water flow in a stilling basin with multi-horizontal submerged jets by using two different turbulence models, namely, the VOF RNG k-ε and Mixture RNG k-ε turbulence m...3-D numerical simulation was carried out for the water flow in a stilling basin with multi-horizontal submerged jets by using two different turbulence models, namely, the VOF RNG k-ε and Mixture RNG k-ε turbulence models. The calculated water depth, velocity profile and pressure distribution are in good agreement with the data obtained in experiments. It indicates that the numerical simulation can effectively be used to study the water flow movement and the energy dissipation mechanism. The numerical simulation results show that the turbulent kinetic energy distribution obtained by using the Mixture turbulence model covers a region about 18% larger than that calculated by using the VOF turbulence model, and is in better agreement with the actual situation. Furthermore, the Mixture turbulence model is better than the VOF turbulence model in calculating the air entrainment.展开更多
Since the 1960 s,the global land surface wind speed(SWS)has significantly weakened,a phenomenon known as global terrestrial stilling.The latest research found that the stilling reversed around 2010,and since then the ...Since the 1960 s,the global land surface wind speed(SWS)has significantly weakened,a phenomenon known as global terrestrial stilling.The latest research found that the stilling reversed around 2010,and since then the global SWS has been strengthening.However,there is still a lack of systematic quantitative analysis in China.We analyzed the transition and regional differences in the long-term trends of the SWS in China based on observational SWS data from 1971 to 2019.The results showed that annual mean SWS in China underwent a reversal from a continuously weakening trend to a significantly strengthening trend around 2014 and implying that stilling may have ended in 2014.The reversal had obvious regional and seasonal variations.In Northeast China,Western Xinjiang as well as on the Tibetan Plateau,the years with both annual and seasonal mean SWS changing from weakening to strengthening were around 2013/2014,1993/1994,and 2000.However,in the west of North China,SWS showed an obviously strengthening trend only in autumn and winter after 2007;while only autumn mean SWS showed a strengthening trend after 2012 in South China.It should be noted that stilling is ongoing in the eastern and southern coastal areas,North China and Eastern Xinjiang.展开更多
Measurements of turbulent flow fields in a stilling basin of multi-horizontal submerged jets were made with the single- camera Particle Image Velocimetry （PIV）. The particle images were captured, processed, and subs...Measurements of turbulent flow fields in a stilling basin of multi-horizontal submerged jets were made with the single- camera Particle Image Velocimetry （PIV）. The particle images were captured, processed, and subsequently used to characterize the flow in terms of the 2-D velocity and vorticity distributions. This study shows that the maximum close-to-bed velocity in the stilling basin is approximately reduced by 60%, comparing to the jet velocity at the outlet of orifices. The jet velocity is distributed evenly at the latter half of the stilling basin and the time-averaged velocity of the cross section is reduced by 77%-85%, comparing to the jet velocity at the outlet of orifices. These results show that the vortices with horizontal axes are continuously repeated during the form-merge-split-disappear process. The vertical vortices are continuously formed and disappeared, they appear randomly near the slab and intermittently reach the slab of the stilling basin. The range of these vortices is small. Vortices with horizontal axes and ver- tical vortices do not coincide in space and the vortices with horizontal axes only affect the position of the tail of the vertical vortices attached to the slab of the stilling basin.展开更多
Energy dissipations induced by the hydraulic jump and the trajectory jet are the most widely known as the two dissipation modes at the downstream of flood discharging structures, which are often considered quite diffe...Energy dissipations induced by the hydraulic jump and the trajectory jet are the most widely known as the two dissipation modes at the downstream of flood discharging structures, which are often considered quite different even contradictory. However, such two energy dissipators can be used jointly and harmonically. In this paper, a new type of stilling basin with a shallow-water cushion and a triangular bottom deflector is proposed based on two different scale physical model tests of the flood discharging tunnel No.2 of Luding hydropower project. The experimental results show that the flow regime of the hydraulic jump in the presented stilling basin with bottom deflector enjoys a good and stable performance within a large range of flow rates and the energy dissipation rate is considerably high as compared to the conventional stilling basin even at a low Froude number. The results also indicate that the stilling basin with triangular bottom deflector has a better performance in improving the potential cavitation erosion according to the analysis of the pressure and the cavitation number compared to the trapezoidal one. The proposed new type of shallow-cushion stilling basin with a shallow-water cushion can be applied in similar energy dissipation projects with low Froude number and large range of flow rates.展开更多
Experiments were conducted to characterize forced hydraulic jumps in stilling basins for enforced cases due to tail water level or dam site arrangement and construction. The case with a single tall sill was simulated ...Experiments were conducted to characterize forced hydraulic jumps in stilling basins for enforced cases due to tail water level or dam site arrangement and construction. The case with a single tall sill was simulated in a horizontal flume downstream of a sluice gate. Results of experiments are compared with the classical hydraulic jump, and significant effect of tall sill on dissipation of energy in shorter distance was confirmed. Furthermore, the generated jumps were classified based on the ratio of sill height to basin length, and a simple design criterion was proposed to estimate the basin length for a desired jump and particular inflow.展开更多
An hourly-resolution dataset from observations at the automatic weather stations(AWSs) is developed and applied to study the characteristics of weakening surface wind in the urban areas of Beijing City in 2008–2017. ...An hourly-resolution dataset from observations at the automatic weather stations(AWSs) is developed and applied to study the characteristics of weakening surface wind in the urban areas of Beijing City in 2008–2017. The "Urban Stilling Island(USI)" is defined and quantified to depict the surface wind speed(WS) differences between rural and urban regions. The urban(rural) sites are represented by 45(6) stations within(outside) the 6th Ring Road(RR). The results demonstrate remarkable smaller annual and seasonal average WS values in urban areas than in rural areas, indicating significant USI, especially in the central urban areas(within the 4th RR) in spring and winter. Further analysis reveals that the surface roughness effect dominates and enhances the USI intensity(USII) under the stronger large-scale background wind in spring and winter, whereas the Urban Heat Island(UHI) effect may dominate and decrease the USII under weaker large-scale wind in summer and autumn. The diurnal USII variations are characterized by a steady low-value phase from 1900 to 0800 Beijing Time(BT) and a high-value phase from 1100 to 1500 BT,with rapid shifts of USII in between. Long-term variation of hourly USII shows large mean USII in 2008–2012 but decreased USII in 2013–2017, possibly attributed to the change of urbanization level around the rural observation sites.展开更多
Using four types of stilling basin models,this paper studies frequency characteris- tics of the fluctuating pressure acting on the stilling basin apron when Froude number is not greater than 4.5.The testing results sh...Using four types of stilling basin models,this paper studies frequency characteris- tics of the fluctuating pressure acting on the stilling basin apron when Froude number is not greater than 4.5.The testing results show that the frequency of fluctuating pressure,f_s in the ranges of 0. 4Hz≤f_s≤1.1Hz.0.2Hz≤f_s≤2.5Hz and 0.4Hz≤f_s≤3.3Hz.can be used as the design frequency corresponding to three patterns of hydraulic jump.The reliability of f_s for three jump patterns is all nearly equal to 90%.展开更多
“Stillness”,as the Vimoksha state of Buddhism’s Nirvana state,is the ultimate experience of religious practice.It has rich connotations.Firstly,“Stillness”is an ultimate aesthetic experience;secondly,“Stillness...“Stillness”,as the Vimoksha state of Buddhism’s Nirvana state,is the ultimate experience of religious practice.It has rich connotations.Firstly,“Stillness”is an ultimate aesthetic experience;secondly,“Stillness”means transcending real society,the secular world and utility.In Lao-tzu’s thought,“To the Ultimate,You Yourself Empty”,“Remain in Silence and Clarity”,“Anātman”,etc.,are presented,showing that Buddhism and Lao-tzu have some commonalities.Based on the text,this article will make a simple comparative analysis,after separately discussing Lao-tzu’s“Xu Jing”and Buddhism’s“Silence”,in order to show their similarities and differences more fully and objectively.展开更多
文摘Based on wind-speed records of Alaska’s 19 first-order weather stations, we analyzed the near-surface wind-speed stilling for January 1, 1984 to December 31, 2016. With exception of Big Delta that indicates an increase of 0.0157 m·s–1·a–1, on average, all other first-order weather stations show declining trends in the near-surface wind speeds. In most cases, the average trends are less then?–0.0300?m·s–1·a–1. The strongest average trend of?–0.0500?m·s–1·a–1 occurred at Homer, followed by?–0.0492?m·s–1·a–1 at Bettles, and?–0.0453?m·s–1·a–1 at Yakutat, while the declining trend at Barrow is marginal. The impact of the near-surface wind-speed stilling on the wind-power potential expressed by the wind-power density was predicted and compared with the wind-power classification of the National Renewable Energy Laboratory and the Alaska Energy Authority. This wind-power potential is, however, of subordinate importance because wind turbines only extract a fraction of the kinetic energy from the wind field characterized by the power efficiency. Since wind turbine technology has notably improved during the past 35 years, we hypothetically used seven currently available wind turbines of different rated power and three different shear exponents to assess the wind-power sustainability under changing wind regimes. The shear exponents 1/10, 1/7, and 1/5 served to examine the range of wind power for various conditions of thermal stratification. Based on our analysis for January 1, 1984 to December 31, 2016, Cold Bay, St. Paul Island, Kotzebue, and Bethel would be very good candidates for wind farms. To quantify the impact of a changing wind regime on wind-power sustainability, we predicted wind power for the periods January 1, 1984 to December 31, 1994 and January 1, 2006 to December 31, 2016 as well. Besides Big Delta that suggests an increase in wind power of up to 12% for 1/7, predicted wind power decreased at all sites with the highest decline at Annette (≈38%), Kodiak (≈30%), King Salmon (≈26%), and Kotzebue (≈24%), where the effect of the shear exponents was marginal. Bethel (up to 20%) and Cold Bay (up to 14%) also show remarkable decreases in predicted wind power.
文摘Declines in wind speed(u)(termed as“stilling”)has been reported in many regions of the world.To explore the temporal trends of u and its aerodynamic effects is vital to understand the changes in water resources.This study analyzed the changes of temporal trends for u and its aerodynamic effects using the data during 1959-2000 at 266 stations across China.The improved PenPan model was used to estimate pan evaporation(Epan)and quantify the contribution of radiative and aerodynamic components(aerodynamic component separated into wind speed u,vapour pressure deficit D,and air temperature Ta).Climate factors include Epan measured with the standard Chinese 20 cm diameter pan,u,Ta,relative humidity(rh)and sunshine hours(sh).The results showed:stilling occurred in most of stations(206 among 266)and 105 stations presented significant decreasing trends at 99%confidence level;stilling was the main cause for controlling the trends in Epan in most part of China,especially in the west and north of China.The results indicated that decreasing trends in Epan due to stilling would inevitably alter water resources,and should be put further investigation incorporation other factors.
基金Project supported by the National Key Basic Research Program of China (973 Program, Grant No. 2007CB714105)the Science Foundation of Ministry of Education of China (Grant No. 2008108111)the Program for New Century Excellent Talents in University (Grant No. NCET-08-0378)
文摘3-D numerical simulation was carried out for the water flow in a stilling basin with multi-horizontal submerged jets by using two different turbulence models, namely, the VOF RNG k-ε and Mixture RNG k-ε turbulence models. The calculated water depth, velocity profile and pressure distribution are in good agreement with the data obtained in experiments. It indicates that the numerical simulation can effectively be used to study the water flow movement and the energy dissipation mechanism. The numerical simulation results show that the turbulent kinetic energy distribution obtained by using the Mixture turbulence model covers a region about 18% larger than that calculated by using the VOF turbulence model, and is in better agreement with the actual situation. Furthermore, the Mixture turbulence model is better than the VOF turbulence model in calculating the air entrainment.
基金supported by the National Key R&D Program of China(Grant No.2016YFA0600404)the National Natural Science Foundation of China(Grant Nos.41705073,41530532,41506040)the Jiangsu Collaborative Innovation Center for Climate Change。
文摘Since the 1960 s,the global land surface wind speed(SWS)has significantly weakened,a phenomenon known as global terrestrial stilling.The latest research found that the stilling reversed around 2010,and since then the global SWS has been strengthening.However,there is still a lack of systematic quantitative analysis in China.We analyzed the transition and regional differences in the long-term trends of the SWS in China based on observational SWS data from 1971 to 2019.The results showed that annual mean SWS in China underwent a reversal from a continuously weakening trend to a significantly strengthening trend around 2014 and implying that stilling may have ended in 2014.The reversal had obvious regional and seasonal variations.In Northeast China,Western Xinjiang as well as on the Tibetan Plateau,the years with both annual and seasonal mean SWS changing from weakening to strengthening were around 2013/2014,1993/1994,and 2000.However,in the west of North China,SWS showed an obviously strengthening trend only in autumn and winter after 2007;while only autumn mean SWS showed a strengthening trend after 2012 in South China.It should be noted that stilling is ongoing in the eastern and southern coastal areas,North China and Eastern Xinjiang.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 51279118, 50479062)the Key Projects in the National Science and Technology Pillar Program (Grant No. 2008BAB29B04)
文摘Measurements of turbulent flow fields in a stilling basin of multi-horizontal submerged jets were made with the single- camera Particle Image Velocimetry （PIV）. The particle images were captured, processed, and subsequently used to characterize the flow in terms of the 2-D velocity and vorticity distributions. This study shows that the maximum close-to-bed velocity in the stilling basin is approximately reduced by 60%, comparing to the jet velocity at the outlet of orifices. The jet velocity is distributed evenly at the latter half of the stilling basin and the time-averaged velocity of the cross section is reduced by 77%-85%, comparing to the jet velocity at the outlet of orifices. These results show that the vortices with horizontal axes are continuously repeated during the form-merge-split-disappear process. The vertical vortices are continuously formed and disappeared, they appear randomly near the slab and intermittently reach the slab of the stilling basin. The range of these vortices is small. Vortices with horizontal axes and ver- tical vortices do not coincide in space and the vortices with horizontal axes only affect the position of the tail of the vertical vortices attached to the slab of the stilling basin.
基金Project supported by the National Science Foundation of China(Grand Nos.51079091,51209154)
文摘Energy dissipations induced by the hydraulic jump and the trajectory jet are the most widely known as the two dissipation modes at the downstream of flood discharging structures, which are often considered quite different even contradictory. However, such two energy dissipators can be used jointly and harmonically. In this paper, a new type of stilling basin with a shallow-water cushion and a triangular bottom deflector is proposed based on two different scale physical model tests of the flood discharging tunnel No.2 of Luding hydropower project. The experimental results show that the flow regime of the hydraulic jump in the presented stilling basin with bottom deflector enjoys a good and stable performance within a large range of flow rates and the energy dissipation rate is considerably high as compared to the conventional stilling basin even at a low Froude number. The results also indicate that the stilling basin with triangular bottom deflector has a better performance in improving the potential cavitation erosion according to the analysis of the pressure and the cavitation number compared to the trapezoidal one. The proposed new type of shallow-cushion stilling basin with a shallow-water cushion can be applied in similar energy dissipation projects with low Froude number and large range of flow rates.
基金the Chamran University of Ahwaz,Jundishapur University of Technology,and the Centre of Shahid Excellence on Operation Management of Irrigation and Drainage Networks for financial support and facilitation of the experiments
文摘Experiments were conducted to characterize forced hydraulic jumps in stilling basins for enforced cases due to tail water level or dam site arrangement and construction. The case with a single tall sill was simulated in a horizontal flume downstream of a sluice gate. Results of experiments are compared with the classical hydraulic jump, and significant effect of tall sill on dissipation of energy in shorter distance was confirmed. Furthermore, the generated jumps were classified based on the ratio of sill height to basin length, and a simple design criterion was proposed to estimate the basin length for a desired jump and particular inflow.
基金Supported by the National Key Research and Development Program of China(2018YFA0606302)National Natural Science Foundation of China(41775078,41675092,and 41575003)。
文摘An hourly-resolution dataset from observations at the automatic weather stations(AWSs) is developed and applied to study the characteristics of weakening surface wind in the urban areas of Beijing City in 2008–2017. The "Urban Stilling Island(USI)" is defined and quantified to depict the surface wind speed(WS) differences between rural and urban regions. The urban(rural) sites are represented by 45(6) stations within(outside) the 6th Ring Road(RR). The results demonstrate remarkable smaller annual and seasonal average WS values in urban areas than in rural areas, indicating significant USI, especially in the central urban areas(within the 4th RR) in spring and winter. Further analysis reveals that the surface roughness effect dominates and enhances the USI intensity(USII) under the stronger large-scale background wind in spring and winter, whereas the Urban Heat Island(UHI) effect may dominate and decrease the USII under weaker large-scale wind in summer and autumn. The diurnal USII variations are characterized by a steady low-value phase from 1900 to 0800 Beijing Time(BT) and a high-value phase from 1100 to 1500 BT,with rapid shifts of USII in between. Long-term variation of hourly USII shows large mean USII in 2008–2012 but decreased USII in 2013–2017, possibly attributed to the change of urbanization level around the rural observation sites.
文摘Using four types of stilling basin models,this paper studies frequency characteris- tics of the fluctuating pressure acting on the stilling basin apron when Froude number is not greater than 4.5.The testing results show that the frequency of fluctuating pressure,f_s in the ranges of 0. 4Hz≤f_s≤1.1Hz.0.2Hz≤f_s≤2.5Hz and 0.4Hz≤f_s≤3.3Hz.can be used as the design frequency corresponding to three patterns of hydraulic jump.The reliability of f_s for three jump patterns is all nearly equal to 90%.
文摘“Stillness”,as the Vimoksha state of Buddhism’s Nirvana state,is the ultimate experience of religious practice.It has rich connotations.Firstly,“Stillness”is an ultimate aesthetic experience;secondly,“Stillness”means transcending real society,the secular world and utility.In Lao-tzu’s thought,“To the Ultimate,You Yourself Empty”,“Remain in Silence and Clarity”,“Anātman”,etc.,are presented,showing that Buddhism and Lao-tzu have some commonalities.Based on the text,this article will make a simple comparative analysis,after separately discussing Lao-tzu’s“Xu Jing”and Buddhism’s“Silence”,in order to show their similarities and differences more fully and objectively.
文摘本文采用Stillinger-Weber势函数,对液态锗的快速凝固过程进行了分子动力学模拟,运用均方位移(mean square displacement,MSD),对相关函数(pair correlation function PCF),配位数(coordination number)和Wendt-Abraham关系式分析锗快速凝固过程中微观结构的变化。模拟得到液态锗的对相关函数和实验基本一致,说明Stillinger-Weber势准确反映了锗原子之间的受力情况,Stillinger-Weber势模拟锗的熔点在2100 K左右。冷却过程中,对相关函数第一峰随温度的降低而逐渐变尖锐,第二峰逐渐分裂为前低后高的两个次峰,由此表明快速凝固过程中得到的是非晶态锗。模拟结果表明,锗凝固的温度范围是1200-600 K,玻璃转化温度Tg为646 K.
