MODEL TESTS AT GLASGOW UNIVERSITY
7.4 Layout of Manoeuvring Experiments
The model towing tests were conducted within the scope of the rectangular open area in the centre of the Glasgow towing carriage with the fixed tow post mounted at the fore-end of the space near the centre of the tank width. This restricted the scope of the towing line length but, as the estimated required tow length was within this limitation this was not significant. The opening also restricted the lateral excursion of the model which effectively limited the maximum lateral and angular velocities of the model during the tests.
7.4.1 Test on MARINER Model
Along the central axis, the model was fitted with a light alloy channel bar extending over the bow which could be adjusted to give different tow positions ahead of or within the bow. Instead of a simple flat section, a concave shape section was chosen to increase the bending stiffness of the bar and there were twenty two numbered hole positions on the bar two inches apart. At the top end (two inches ahead of hole No. 1) of the tow channel bar, a small pin was fixed and through which the tow wire was linked. For these tests a light fishing line was used for the tow line, strong enough to carry the towing load of approximately four Newtons. At station 1 and two inches behind (between station 1 and station 1.5), there were a pair of screws permanently fixed on the hull of the model. By positioning both screws through any two holes on the tow bar and fastening them firmly, the model was equipped with the tow bar and a certain tow point position was achieved (Figures 7.5 and 7.6). Different longitudinal tow positions ( X j) on the model could be readily obtained by shifting to various hole positions on the channel bar.
Two independent sets of equipment were adopted to measure the motion of the model during the tests (a general description of these facilities is given in Section 7.5):
(a) Two potentiometers were provided by UCL. One, at the end of the tow post, measured the angle of the tow line relative to the axis of the tank. The second was placed on the model tow bar to measure the angle of the tow line relative to the model's longitudinal axis. These angles were recorded via an OASIS 4 data logger linked to a portable lap top computer TOSHIBA T4400sx as described in Chapter 4. The on-line circular strain gauge load cell prepared by UCL was also
found to be limited up to 1 Newton which was far below the average towing load of 400 grams. Therefore this online circular load cell was not used later in the actual test runs.
(b) A tow post force gauge and a SELSPOT Video tracking system using two
cameras were provided by Glasgow University. For this system two LED's were fitted to the model on the tow bar: one at the first hole position near the towing point where the potentiometer was also fitted; another at the other end of the bar (Figure 7.5 & Figure 7.6). The output from this system gave the x and y coordinates of each of the LED's during the test run from which a time record of lateral displacement and heading angle could be derived. This system is described in detail in Section 7.5.
7.4.2 Test on M V BRITISH BOMBARDIER Model
The SELSPOT video tracking system and the tow post force gauge were the same as those used in the tests with MARINER model. In the light of that experience there were several improved arrangements for the MV BRITISH BOMBARDIER model tests:
(a) The fishing line was replaced by a flexible light wound wire as the tow line in order to avoid the former’s elastic behaviour and reduce the high frequency noise this produced.
(b) The start arrangements were also modified by holding the model to one side by a single line attached at the model tow point and of fixed length to the carriage structure (Figure 7.7). This provide a more consistent initial start condition and ensured that the model was at a natural streaming angle prior to release.
(c) For the video tracking system, two LEDs were also fitted on the model along the longitudinal axis: one remained at the first hole position on the tow channel bar near the tow point but another was moved to the LCG on the hull (Figure 7.8). This gave more direct and accurate measurements of the displacements at the LCG and improved the values of heading angle (j3) as well as those for the distance benveen the two LEDs track. An attempt was also made to use additional track points of LEDs to reduce the measurement and calibration errors. By the starboard side of the two main LED sensors, another pair of LEDs were fitted as a backup; along the central axis one more LED was positioned near station 4.5 behind of the LCG as shown in Figure 7.9 and Figure 7.10. Unfortunately some
Chapter 7 Model Tests At GLASGOW University
instrumentation problem developed during the setting up period and as the time available in the tank was running out the experiments had to be carried out with a smaller camera system whose tracking scope was sufficient just for the two primary LEDs but not wide enough to cover all the additional LEDs.
(d) As the SELSPOT system was adopted as the primary method of track recording, only one of the UCL rotary potentiometers was fitted on the model at the towing point and used as a monitor. As in the MARINER tests, it measured the angle of the tow line relative to the model's longitudinal axis, but these signals were then fed through the Glasgow University's Data Collecting System, together with the all the other data from LEDs, strain gauge etc., and finally this data was recorded by the Glasgow carriage computer system. Therefore the sample rate of signals from the potentiometer could be controlled to ensure consistency with the rest of the experimental data for comparison purposes.
(e) The towing post was equipped with an LVDT angle gauge and a Novatech load cell. Unlike the MARINER tests, the force gauge was connected with the towing post through a pulley, so that it could rotate relative to the tow wire angle during the model runs. The LVDT on the pulley at the bottom of the tow post bar tracked the angle of the tow wire relative to central axis of the tank. The drawings of the Novatech aluminium load cell is given in Figure 7.11, while the rotating tow system with LVDT and force gauge is shown in Figure 7.12.