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11:628:200 Marine Science
Lab demonstration: flow of water at the seafloor
Animals and plants continually interact with the living and non-living features of their
surroundings. In benthic ecology, water flow is an especially important part of the physical environment.
Due to physical properties of fluids in contact with ‘solid’ boundaries (such as the seabed), there is a
strong vertical gradient in the velocity of the fluid. Near the seabed, flow speed is very low — in fact, it
is zero at the sediment-water interface, a phenomenon known as the no slip condition. With only small
increments in distance away from a boundary, however, flow speed rapidly increases. The region where
water flow velocity is changing with distance from the seafloor is referred to as the boundary layer.
Experiments to examine the effects of water flow on organisms require the use of specialized
laboratory equipment. Today, we are using a flume to create different flow speeds. A flume (from the
Latin fluere, to flow) is simply an artificial channel for a stream of water. While the concept of generating
different water flow speeds in the laboratory is simple enough, in practice it takes considerable effort to
ensure that the flow does not have undesirable and unnatural properties. For example, wind tunnels can be
thought of as ‘flumes for air’ and it is clearly essential that such devices accurately simulate real-world
conditions (think of testing airplane models). In addition to accurately reproducing real world flow
conditions in the laboratory, we need to be able to measure the water velocity in such experiments. Here
we are using a laser Doppler velocimeter (LDV) to measure flow speeds. This instrument relies on the
Doppler effect, defined as “an apparent change in the frequency of waves, as of sound or light, occurring
when the source and the observer are in motion relative to one another” and was named for its discoverer,
the Austrian physicist Christian Doppler (1803-1853). Since velocity is a vector, it has both magnitude
and direction. This instrument measures the shift in the frequencies of blue and green laser light beams to
compute the magnitude of the downstream and vertical components of the velocity.
An LDV is a great tool for measuring flow velocity on very fine scales, but is not good for
visualizing flow on larger scales. To get a “big picture” of what the flow is, the introduction of tracers
into the fluid is a time-tested method. A tracer can be as simple as a dye. Today, we will use food coloring
to demonstrate Bernoulli’s Principle.
Daniel Bernoulli was a Swiss scientist who discovered the general relationship between the
velocity of a fluid and fluid pressure. Consider the flow of water through a pipe with a constriction:
Velocity1, Pressure1
Velocity2, Pressure2
Bernoulli found that the velocity and pressure in the different parts of the pipe were related as
Velocity12 +Pressure1 =Velocity22 +Pressure2 . Since Velocity 1 is less than Velocity 2, this means that
Pressure 1 must be greater than Pressure 2. Furthermore, since we know that in a boundary layer there is a
gradient in flow velocity, we can predict that there should also be a gradient in fluid pressure. Animals
can take advantage of this to passively ventilate their burrows, as we will demonstrate.
Further reading
Nowell, A.R.M., Jumars, P.A., 1984. Flow environments of aquatic benthos. Annual Review of Ecology
and Systematics 15: 303-328. [One of the first papers focused on the effects of boundary layer
flow on benthic organisms.]
Nowell, A.R.M., Jumars, P.A., 1987. Flumes: theoretical and experimental considerations for simulation
of benthic environments. Oceanography and Marine Biology Annual Review 25: 91-112.
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