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ADV flow sensor |
Laser scanner |
ADV principle of operation:
The ADV emits an acoustic signal at 10MHz into the water and calculates the flow vectors along the three directions (U, V, W = longitudinal, transversal and vertical) from the Doppler frequency shift at the three receivers. These non-intrusive measurements are recorded in a sampling cell of approximately 0.25ccm situated 5cm below the sensor head. The resolution ranges from 0.1cm/s to 250cm/s and the maximum data recording rate is 25Hz. More details can be found at www.nortek-as.com.
Basics of the laser scanner:
The scanner consists of two laser sources, mounted along the stem of the ADV and diffracted through lenses to project a cross-stream line on the bottom, combined with a CMOS camera. Because of the inclination angle of the camera, height differences are seen as a vertical shift in the resulting image. This picture is automatically analysed with a brightness detection routine and a conversion factor (pixel - millimetre) in the controlling PC. Depending on selected settings, groups of pixels are bundled to obtain robust average height values and standard deviations. The vertical resolution is 0.3mm and the field of view of the camera covers a maximal range of height differences of 42mm.
Left: software during a combined measurement of flow (top and lower right) and relief (lower left)
Centre: combined measurement of bottom relief and flow around a gastropod (Aporrhais pespelecani
)
Right: example of the resolution capacity of the laser system
The sensors are mounted on a PC-controlled positioning system ontop of the main flume channel and they can therefore exactly be positioned. This is achieved with a special software which allows either to maintain a given position or to move along complex patterns. The bottom relief of a structure can thus be recorded step by step while the flow speed is simultaneously measured at the same place. The resulting data can be used to assemble a clear picture of the flow around a single animal or for an entire community. Dynamic processes like sediment transports (erosion, deposition) interacting with biogenic structures can also be monitored this way.
Despite all the technical details, the flow channel can also be used to simply observe the behaviour of marine organisms under different flow conditions. Wide windows made of acrylic glass allow a good overview and the use of optical devices such as cameras.
Left: a shrimp (Palaemon) swimming in the flume, Right: barnacle (Balanus) capturing food particles
