Towing Tank Facility - nptel - Indian Institute of Technology Madras

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