1. Candida Oancea Institute, Polytechnic Univ. of Bucharest, Spl. Independentei 313, Bucharest 060042, Romania; Romanian Academy, Calea Victoriei 125, Bucharest 010071, Romania (corresponding author).
2. Dept. of Hydraulics, Hydraulic Machinery and Environmental Engineering, Polytechnic Univ. of Bucharest, Spl. Independentei 313, Bucharest 060042, Romania.
3. Dept. of Physics, West Univ. of Timisoara, V. Parvan 4, Timisoara 300223, Romania.