Water is likely the most limiting factor in increasing agricultural production in large parts of Africa. Reference evaporation (ET0) is a key hydrological parameter to use efficiently the scarce supply. Several method...Water is likely the most limiting factor in increasing agricultural production in large parts of Africa. Reference evaporation (ET0) is a key hydrological parameter to use efficiently the scarce supply. Several methods are available for predicting reference evaporation, but the accuracy of any of the methods has not been established for the Ethiopian highlands. The objective of this study is, therefore, to select the best methods for calculating the reference evaporation ET0. For the section, meteorological data of the Bahir Dar station were used, because all data needed for this study including the Class A pan Evaporation were recorded on a daily basis. Pan evaporation was considered as the best estimator of the reference evaporation. The results showed that the FAO-Penman Monteith (using solar radiation, wind speed, temperature and relative humidity) and Enku method (using only maximum daily temperatures) have acceptable daily ET0 ranges and predicted to Class A pan evaporation with correlation coefficients greater than 90% in a monthly basis. Next best was the Thornthwaite’s method with correlation coefficient of 89% with pan evaporation. Piche methods performed relatively well with correlation coefficient of greater than 70%. Blaney-Criddle, Priestley & Taylor, and Hargreaves performed the poorest in predicting pan evaporation. These methods should be recalibrated for local condition and therefore not recommended for use in the Ethiopian highlands. In summary, the FAO-Penman Monteith is recommended for locations where the input data are available;otherwise, the Enku method using maximum daily temperature is best for estimating the reference evaporation.展开更多
Smart structure with active materials embedded in a rotating composite thin-walled beam is a class of typical structure which is using in study of vibration control of helicopter blades and wind turbine blades. The dy...Smart structure with active materials embedded in a rotating composite thin-walled beam is a class of typical structure which is using in study of vibration control of helicopter blades and wind turbine blades. The dynamic behavior investigation of these structures has significance in theory and practice. However, so far dynamic study on the above-mentioned structures is limited only the rotating composite beams with piezoelectric actuation. The free vibration of the rotating composite thin-walled beams with shape memory alloy（SMA） fiber actuation is studied. SMA fiber actuators are embedded into the walls of the composite beam. The equations of motion are derived based on Hamilton＇s principle and the asymptotically correct constitutive relation of single-cell cross-section accounting for SMA fiber actuation. The partial differential equations of motion are reduced to the ordinary differential equations of motion by using the Galerkin＇s method. The formulation for free vibration analysis includes anisotropy, pitch and precone angle, centrifugal force and SMA actuation effect. Numerical results of natural frequency are obtained for two configuration composite beams. It is shown that natural frequencies of the composite thin-walled beam decrease as SMA fiber volume and initial strain increase and the decrease in natural frequency becomes more significant as SMA fiber volume increases. The actuation performance of SMA fibers is found to be closely related to the rotational speeds and ply-angle. In addition, the effect of the pitch angle appears to be more significant for the lower-bending mode ones. Finally, in all cases, the precone angle appears to have marginal effect on free vibration frequencies. The developed model can be capable of describing natural vibration behaviors of rotating composite thin-walled beam with active SMA fiber actuation. The present work extends the previous analysis done for modeling passive rotating composite thin-walled beam.展开更多
文摘Water is likely the most limiting factor in increasing agricultural production in large parts of Africa. Reference evaporation (ET0) is a key hydrological parameter to use efficiently the scarce supply. Several methods are available for predicting reference evaporation, but the accuracy of any of the methods has not been established for the Ethiopian highlands. The objective of this study is, therefore, to select the best methods for calculating the reference evaporation ET0. For the section, meteorological data of the Bahir Dar station were used, because all data needed for this study including the Class A pan Evaporation were recorded on a daily basis. Pan evaporation was considered as the best estimator of the reference evaporation. The results showed that the FAO-Penman Monteith (using solar radiation, wind speed, temperature and relative humidity) and Enku method (using only maximum daily temperatures) have acceptable daily ET0 ranges and predicted to Class A pan evaporation with correlation coefficients greater than 90% in a monthly basis. Next best was the Thornthwaite’s method with correlation coefficient of 89% with pan evaporation. Piche methods performed relatively well with correlation coefficient of greater than 70%. Blaney-Criddle, Priestley & Taylor, and Hargreaves performed the poorest in predicting pan evaporation. These methods should be recalibrated for local condition and therefore not recommended for use in the Ethiopian highlands. In summary, the FAO-Penman Monteith is recommended for locations where the input data are available;otherwise, the Enku method using maximum daily temperature is best for estimating the reference evaporation.
基金supported by National Natural Science Foundation of China (Grant No. 10972124)Shandong Provincial Natural Science Foundation of China (Grant Nos. Y2006F37, ZR2011EEM031)Science & Technology Project of Shandong Provincial Education Department of China (Grant No. J08LB04)
文摘Smart structure with active materials embedded in a rotating composite thin-walled beam is a class of typical structure which is using in study of vibration control of helicopter blades and wind turbine blades. The dynamic behavior investigation of these structures has significance in theory and practice. However, so far dynamic study on the above-mentioned structures is limited only the rotating composite beams with piezoelectric actuation. The free vibration of the rotating composite thin-walled beams with shape memory alloy（SMA） fiber actuation is studied. SMA fiber actuators are embedded into the walls of the composite beam. The equations of motion are derived based on Hamilton＇s principle and the asymptotically correct constitutive relation of single-cell cross-section accounting for SMA fiber actuation. The partial differential equations of motion are reduced to the ordinary differential equations of motion by using the Galerkin＇s method. The formulation for free vibration analysis includes anisotropy, pitch and precone angle, centrifugal force and SMA actuation effect. Numerical results of natural frequency are obtained for two configuration composite beams. It is shown that natural frequencies of the composite thin-walled beam decrease as SMA fiber volume and initial strain increase and the decrease in natural frequency becomes more significant as SMA fiber volume increases. The actuation performance of SMA fibers is found to be closely related to the rotational speeds and ply-angle. In addition, the effect of the pitch angle appears to be more significant for the lower-bending mode ones. Finally, in all cases, the precone angle appears to have marginal effect on free vibration frequencies. The developed model can be capable of describing natural vibration behaviors of rotating composite thin-walled beam with active SMA fiber actuation. The present work extends the previous analysis done for modeling passive rotating composite thin-walled beam.
