Application of NN-ARX Model to Predict Groundwater Levels in the Neishaboor Plain, Iran

There is no doubt that groundwater is an important and vital source of water supply in arid and semi-arid areas. Therefore, prediction of groundwater level fluctuations is necessary for planning conjunctive use in these areas. This research was aimed to predict groundwater levels in the Neishaboor plain using Neural Network – AutoRegressive eXtra input (NN-ARX) and Static-NN models. The NN-ARX model determines a nonlinear ARX model of a dynamic system by training a hidden layer neural network with the Levenberg-Marquardt algorithm. In this model the current outputs depend not only on the current inputs, but also on the inputs and outputs at the pervious time periods. The available observation wells in the study area were clustered according to their fluctuation behavior using the “Ward” method, which resulted in six areal zones. Then, for each cluster, an observation well was selected as its representative, and for each zone, values of monthly precipitation, temperature and groundwater extraction were estimated. The best input of the Static-NN model was identified using combination of Gamma Test and Genetic Algorithm. Also, Gamma Test is applied to identify the length of the training dataset. The results showed that the NN-ARX model was suitable and more practical. The performance indicators (R ²?=?0.97, RMSE?=?0.03 m, ME?=?--0.07 m and R ²?=?0.81, RMSE?=?0.35 m, ME?=?0.60 m, respectively for the best and worst performance of model) reveals the effectiveness of this model. Moreover, these results were compared with the results of a static-NN model using t-test, which showed the superiority of the NN-ARX over the static-NN.     

Dead-time optimisation with reducing voltage distortion for nine-switch inverter

Nine-switch inverter with two sets of three-phase outputs is an improved topology proposed in place of the 12-switch back-to-back converters and has therefore attracted much attention in recent years. This inverter can be used with two conventional pulse width modulation approaches: different frequency and the constant frequency. One disadvantage of using this modulation method is the possibility of short-circuits in the legs (shoot-through), which decreases the reliability of converter and system. This paper presents a new modulation technique, in which switching pulses of nine-switch inverter are produced by not only the original carrier signals but also through two auxiliary carrier signals. In this method, adjustable three-phase voltages are produced in the inverter’s terminals, and so there is no possibility of any shoot-through in the inverter’s legs. The suggested reliable modulation approach does not rely on any information about the load polarity, as switching is performed by a simple and reliable algorithm. The result is the considerably better waveform quality of the output voltages in comparison with other methods. To verify the analysis, an experimental platform based on DSP is built. The simulation and experimental results are given to demonstrate the effectiveness and feasibility of this new approach.     

Characterization of the rheological and mechanical properties of shear thickening fluid-coated Twaron® composite

The effects of the weight fraction (concentration) and silica particle size on steady and dynamic rheological responses of shear thickening fluids (STFs) and the quasi-static (QS) penetration resistance of Twaron®/STF composites have been investigated. The STFs have been made by mechanically dispersing 12 or 60 nm silica particles in polyethylene glycol with molecular weight of 400 gr/mol at three different concentrations. Rheological results indicate that with the increase in the concentration of silica in the STF, the critical shear rate (onset of shear thickening) diminish, the initial, critical, and ultimate viscosities increase and the slope of the shear thinning and shear thickening regions tend to be more steeper. Regarding STFs with larger particle sizes, critical shear rate, the initial, critical, and ultimate viscosities as well as the viscoelastic modules diminish. Addressing the Twaron®/STF composite, the increase in STF concentration and reduction in the silica particles size contribute to considerable increase in the puncture characteristics. The most noticeable puncture characteristics turn out in the case of 35 wt. % STF/Twaron® composites containing 12 nm particles in which the maximum bearable load and the maximum absorption energy are nearly 3.6 and 2.3 times larger than those of the neat Twaron®.