Induction of Internal Flow by Ambient Currents

Animals are sometimes able to take advantage of ambient air currents (winds) or water currents in such ways that the currents induce fluid flow through their bodies or through structures they build. Prairie dogs, for example, construct their burrow systems in such a way that wind flowing parallel to the ground surface is forced to rise and fall as it blows across some burrow openings but not others. Because of Bernoulli’s principle, the pressure exerted at some burrow openings is lower than at others as a result. Wind blowing parallel to the ground surface thus induces flow of air through the burrows—renewing O₂ supplies far underground. Here we offer a more thorough discussion of this effect and other, similar cases of internal flow induced by ambient currents.

The most obvious way for internal flow to be induced by an ambient current occurs when the opening of a tube-shaped structure is positioned so that it faces partly or fully into the path of an ambient current, as in Part 1 of the figure. But what if a structure is oriented at a right angle to the current, as in Part 2? Is it then possible for an ambient current to induce internal flow?

Part 3 of the figure illustrates one principle by which an ambient current can induce internal flow when flowing across (rather than into) the orifices of a tubular structure. This principle explains internal flow through prairie dog burrow systems. In this case, a burrow or other tubular system opens at two spots on the ground or some other surface. One of the openings is higher than the other. As a wind or water current flows along the surface, the fluid stream accelerates as it passes over the elevation, and Bernoulli’s principle (familiar in the study of airfoils) comes into play. According to this principle, the lateral pressure exerted by a fluid stream decreases as the speed of the stream increases. Thus the ambient pressure at the elevated orifice of the tube is lower than that at the lower orifice, and consequently, fluid is forced to flow through the tubular system. One of the remarkable features of this type of induction of internal flow is that, unlike the situation in Part 1, it does not depend on the direction of the ambient wind or water current. Provided the ambient current is homogeneous, the acceleration of the fluid stream as it passes over the elevation will cause the pressure at the upper orifice to be lower than that at the lower orifice, regardless of the direction. As already noted, the burrow systems of prairie dogs are believed to be ventilated by a mechanism such as this. The mounds the prairie dogs build at some of their burrow-system openings differ in height and shape, and experiments using tracer gases indicate that even light winds along the surface of the ground induce internal flow of fresh air through prairie dog burrow systems, thereby helping to refresh O₂ and purge CO₂.

Mechanisms by which ambient currents can induce flow through tubular structures (Vogel 1994 provides a more complete discussion.)

Another mechanism by which ambient currents can potentially induce internal flow depends on the principle of viscous entrainment or viscous suction. When a current flows across the opening of a tube, there is a tendency for fluid to be drawn from the tube into the current because of the viscosity of the fluid (the fluid’s “internal stickiness,” or resistance to shear forces). Other things being equal, this force of suction is greater, the greater the velocity of the current. Consequently, if a tubular structure is open at two places and the two orifices are exposed to different ambient current velocities, as in Part 4, fluid will tend to flow through the tube toward the end exposed to the greater velocity. This principle can lead to flow through a tubular system in situations where Bernoulli’s principle does not apply. When a fluid flows in laminar fashion along a substrate, as illustrated in Part 5, there is a gradient of fluid velocity within a short distance of the substrate surface; fluid relatively far from the substrate flows more rapidly than that near the substrate. In this commonplace situation, there is no difference in pressure associated with the velocity gradient; Bernoulli’s principle does not say that any rapidly moving stream exerts less pressure than any more slowly moving stream, but instead, it applies only when a particular fluid stream is accelerated or decelerated. As illustrated, however, the velocity gradient itself can induce flow through a tubular system because of differential viscous suction at orifices located different distances from the substrate surface. Again, the direction of the ambient current does not matter. This type of mechanism is believed to help move water through the tubular systems of bottom-attached sponges, because higher and lower openings to the tubular system are at different distances from the substrate to which the sponges are attached.

References

Vogel, S. 1994. Life in Moving Fluids: The Physical Biology of Flow, 2nd ed. Princeton University Press, Princeton, NJ.