MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

82Most early tympanums were of the easy-to-construct design labeledHRES-D, in Figure 3.24. However, early in the 17th century, La Fayenoted that this design kept the water at the periphery of the compartment.The water exerted the maximum resistingmoment about the axle due to thepull of gravity on the water. This would obviously require more inputpower to overcome. Therefore, La Faye suggested the spiral-wheel designin Figure 3.33 which kept the water almost vertically in line, beneaththe axle and thus created a smaller moment arm (Ewbank, 1876). Severalother designs, such as the logarithmic and Archimede’s spiral curve(Figure 3.24) are also widely used today.Recognizing that compartment shape effects the performance of atympanum, the Hydraulic Research and Experiment Station (HRESj of theUnited Arab Republic’s Ministry of Irrigation conducted a program fortesting various designs. Using 6 x 75 cm plastic models of the shapes inFigure 3.24, they determined that the D2 design was capable of higherdischarges at a given head then the others and that its simple, “straightline”design afforded easy construction by users in developing countries.They also found that utilizing individual outlet compartments, such asare shown in the D 1 ’ DZ ’and Fathi designs in Figure 3.24, improve theefficiency of a tympanum by preventing’discharge from one compartmentleaking back into the compartment(s) below it. This separation of dischargescan also be accomplished by having the discharge outlets in thesidewalls, as in the enclosed noria. The term tablia is used to specifythistype of tympanum.Tympanums, are commonly constructed of galvanized sheet metal andutilize second-hand roller bearings in the axle. Typical performances ofsuch tympanums are given in Table 3.6. No data is available on constructioncosts of tympanum devices.