Towing Tank Facility - nptel - Indian Institute of Technology Madras

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Hydraulics Prof. B.S. Thandaveswara They are held by vertical rigid rods fixed with the axis of the current meters parallel to the tank wall. Threshold Velocity The current meter is towed at very low speed range from 0.02 ms -1 in steps of 0.010 ms - 1 till the current meter begins rotating after overcoming the inertia and the corresponding velocity is taken as threshold velocity. Rating Run The rating of the current meter is carried out in still water. The rating run length at constant velocities are (i) 10 m for velocities less than 0.60 ms -1 and (ii) 20 m to 30 m for velocities above 0.60 ms -1 . The carriage runs are made in the forward direction at selected velocities. The velocity of the carriage is changed from 0.20 to 3.60 ms -1 in steps of 0.2 ms -1 initially and then 0.3 ms -1 so as to obtain about 14 readings. The accuracy of rating is of the order of ±0.5%. Evaluation of the Data From the readings, the actual value of the velocity of the carriage, V (ms -1 ) and the corresponding revolution per second of the cup/propeller 'N' are calculated and plotted. One or more straight lines are manually fitted through the data points as necessitated and the corresponding equation(s) are determined. The graphical plot will also be provided with the report for reference. Tabulation Using the rating equation(s) determined as above, the current meter rating tables are prepared on a digital computer. Special remarks if any are noted in the rating chart (Table 1). Indian Institute of Technology Madras
Hydraulics Prof. B.S. Thandaveswara Typical Chart Environmental and Water Resource Engineering Division, Dept. of Civil Engineering, I.I.T. Madras, Chennai – 600036 Table 1: Ready Reckoner for Velocity, given for the number of revolution Time in secs 40 41 42 43 44 45 65 66 67 68 Indian Institute of Technology Madras Meter No. Report No. CIE /EWRE/ Date of Calibration Spin Test secs V max ms -1 Threshold velocity ms -1 Velocity Equation(s) V = m*n+c Velocity in m/s Velocity in m/s 5 Revs 40 Revs 50 Revs 60 Revs 250 Revs 300 Revs Salient Technical Data Towing Tank Length 82.0m Width 3.2m Maximum Water Depth 2.8m Gauge of Rails Towing Carriage 3.5m Net Weight 3.9 tones Length 5.3m Speed Range 0.02 to 5.5 ms -1 Test length for 5.5 m/s 50.0m Velocity Precision ± 0.1% Maximum Acceleration/Deceleration 3 ms -2 Drag Balance Maximum drag 100 N Least Count 0.01 N Error range ± 1% Ratio of Force Transmission 2 : 1 or 4 :1 Time in secs 40 41 42 43 44 45 65 66 67 68
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Towing Tank Facility - nptel - Indian Institute of Technology Madras

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