Drag Forces

Drag forces were tested in a flume at the Human Performance Centre, Dunedin. The dimensions of the flume — length, width, and depth — were 10, 2.5, and 1.4 m, respectively. The tests were conducted at flow velocities of 0.5, 1.0, 2.0, and 2.8 m s-1, the latter being the maximum velocity of the flume. The forces and concurrent flow velocities of each test run were logged by an online data recorder for at least 2 min at a logging frequency of 10 Hz. To see if high-frequency events occurred, three individuals were logged at a frequency of 1000 Hz. As the flume at the "Human Performance Centre" could not be run with highly corrosive sea water, the drag tests were conducted in freshwater. Since Durvillaea is an intertidal seaweed and frequently experiences rain water, a temporary exposure to freshwater of 10-15 minutes was not considered to change the seaweed's mechanical performance, and no obvious signs of changes in appearance were observed.

FIGURE 3.4 Schematic drawing of the experimental setup of the flume experiments: (1) test specimen of Durvillaea antarctica, (2) pump, (3) attachment, (4) homogenizer, (5) force transducer, and (6) connection to online PC. Not to scale. For details, see text.

Prior to testing, the seaweeds were cut just above the holdfast and prepared for testing as shown in Figure 3.4. The stipe was fastened with a hose clamp (also called "jubilee clip"), which was fixed to a swivel by four pieces of low-strain yachting rope of 4 mm diameter. The swivel was connected to another piece of low-strain rope, which was redirected via a pulley and attached to a force transducer (RDP Group, Model 41, maximum load 250 lb) outside the water. The pulley was screwed to a wing spar, which had only a small influence on the flow in the flume and was therefore considered negligible.

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