Lab Demonstration: Flow of Water at the Seafloor

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, Pressure1Velocity2, Pressure2

Bernoulli found that the velocity and pressure in the different parts of the pipe were related as . 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.