جستجوی مقالات مرتبط با کلیدواژه "مدل ریاضی" در نشریات گروه "اکولوژی"

The population growth along with the rise in essential needs, has caused creating the concept of water-energy-food NEXUS and the increase in attention to this framework in order to apply it to optimal use of resources. One of the most important issues in the optimal use of resources that has received less attention is environmental damages which has not been addressed as much as the cost and benefit debate. Beside the optimal use of resources, another point is to attention to sustainable supply needs, so that different parts of the system do not be disrupted during operation. The effectiveness of the NEXUS is also important for decisions as in case of minor effects, there is no need to complicate the decision model. In this study, a mathematical model of water-energy-food NEXUS was implemented in a study area for the purpose of minimizing costs and maximizing reliability, and using ant colony optimization algorithm, optimal solutions were obtained. The effect of considering and not considering the NEXUS between resources and its effect on costs was also examined. The results showed that considering the environmental costs without adopting intensified policies does not affect decisions and only increases the costs of the system. NEXUS impact analysis showed that NEXUS considerations are optimal to make decisions under minimization of costs and it does a better performance of maximizing the overall system reliability index, without negatively affecting other parts.

In this paper a mathematical model is developed to investigate the effects of tidal river fluctuations on the groundwater head of confined aquifer. To simulate the interaction between groundwater flow of the confined aquifer near tidal river in unsteady flow, we used separation method. This model is for simulating groundwater level and it requires estimation of hydrogeological parameters of transmissivity and storage coefficient. Two things falling and rising water levels have been studied in this paper. To verify the analytical solution of the model used GMS software Modflow and the results of the survey conducted comparing the effectiveness of analytical and numerical model solved by separation of variables and methods proposed model for the hydraulic confined aquifer and river tidal showed. Compared with an analytical solution based on a sinusoidal assumption and a numerical solution generated by MODFLOW, this solution provided better performance in groundwater-level prediction in a confined aquifer According to the investigation done if the coefficient of transmissibility increases water level rises. Evaluations of analytical solutions show that the effect of changes in levels of transmission capability between 1 and 2 km from the border tidal was remarkable. And with increased transmission capability, head of the underground water level rises also away from the border reduced and it create wave whit lesser amplitude and need more time to create it.

Repair and maintenance expenses as one of the variable costs, are necessary for improving the quality and quantity of machine performance. Records of such costs, can be affect predicting repair and maintenance expenditures. Mathematical modeling reduces the volume of these data and provide a better and more percision economical planning for managers. In order to fit those data and obtaining the relationship between accumulated hours of work and repair and maintenance costs, this study was conducted on selected 16 crawler tractors which are active in Earthmoving operations in the co-operative of Jahad Nasr in East Azarbaijan Province, Iran. Required information including operation hours and all costs by considering inflation rates were gathered over the period of ten years from 1997 to 2006. The regression analysis was done on four mathematical models (linear, quadratic, exponential and power), based on significance of coefficients for these models and maximum of. Therefore, the best and final model was quadratic which predicts lower costs than the other models mentioned previously. The reason may be atributed to the nature of quadratic model as well as the lower wages of repair and maintenance personnels in the region.

This research was conducted in order to determine a relationship between accumulated hours and R&M costs, for 27 conventional tractors in the Misagh Sabz Agri-Industrial. Company that were divided into two models including Massy Ferguson 285 and Universal 650. Required data including the R&M costs and hours of annual usage over the fifteen year period from 1991 to 2005 was collected. By performing regression analysis of the data, mathematical prediction models were estimated for both models of tractors separately and these estimations were performed again for all of the tractors. In the first period of life (about 2000 hours) power model as y = 0.024x1.600 and for the rest of its life quadratic model in the form of y = 0.005x2 – 0220x + 5.659 showed the best fitted models. Results revealed that accumulated R&M costs in percent of purchase price were higher in Universal 650 than the Massy Ferguson 285. To verify the results obtained from this study, a comparison was made between the model presented in the current study and some other national and international studies. Finally, it was shown that accumulated R&M costs as a percent of purchase price were nearly less than that of other previous studies. The reasons are discussed in this paper.