Racetrack Flume
The racetrack flume is modeled after one at the University of Washington's Friday Harbor Laboratory (Nowell et al. 1989). The flume is oval in shape, with a working channel length of 620 cm and width of 70 cm. The total bottom area of the working channel, return channel and turning sections is 157,000 cm2. The maximum working water depth is 25 cm (channel depth is 30.5 cm). At a water depth of 10 cm, the water volume in the flume is approximately 1570 liters. All construction materials are chemically inert: the side walls in the working channel are optical-quality glass, the side walls in the return channel are titanium (part of the heat exchange system), and the floor and turning section are PVC plastic. It is possible to use seawater (raw or filtered) or freshwater in the flume.
Two deep boxes are located along the working channel at 2.9 and 4.8 meters from the beginning of the channel. A typical use of these boxes is as containers for sediments and associated benthic organisms. Each box is 53-cm wide, 79-cm long and has a maximum depth of 17 cm. A shallower depth can be achieved by adjustments to a false bottom. In addition, plates over the two deep boxes can be modified to accept different shallow (on the order of 1 to 2 cm) or specialized treatment trays.
The racetrack flume is ideal for larval settlement experiments, infauna and epifauna feeding experiments, and experiments involving the general effects of flow on organisms or sediment transport. Water temperature is controlled by titanium heat exchangers along the return channel. Temperatures can be controlled easily between 10 and 20 oC with a precision of ± 0.3 oC. Four adjustable vanes located in each turning section are used to minimize secondary flows created as the water moves around the bends. Flow is driven by a set of 28 paddles distributed along an endless chain spanning the entire length of the return channel. Each paddle is articulated to enter, move through, and leave the water in a perpendicular orientation, minimizing shearing forces. The chain is connected to a DC electric motor and can be driven in two possible modes; a direct-drive or a 60:18 gear drive depending on the range of flow speeds desired. With a 10-cm water depth, near-surface flow speeds up to 48 cm sec-1 can be attained. Faster flow speeds are possible with a deeper water depth, for example, with a 14-cm water depth, a speed of 64 cm sec-1 has been achieved. Flow speeds can be programmed into the Camile Data Acquisition and Control system for experiments that involve changes in flow speeds.
Flow speed is measured with a 2-axis (measuring the downstream horizontal and vertical components of the velocity) laser Doppler velocimeter (LDV) operating in the backscatter mode. Velocity measurements are completely non-intrusive, with no disruption of the flow field. The fiber-optic probe is mounted on a 3-axis traverse that enables precise positioning of the measurement volume at any point in the water column.