D9 Pressure/Flow Sensors

Topic

Pressure and flow sensors enable direct measurement of the pressure field on the surface of a fish's body—or in the surrounding fluid—as well as the local flow velocity at specific points. In hydrodynamic studies, miniaturized pressure sensors implanted in or attached to the fish's surface record the pressure distribution acting on its body; from this, one can directly infer the drag force the fish experiences and the lift distribution it generates. Flow sensors—whether electromagnetic or hot-wire—measure the velocity of the fluid itself, offering spatial and temporal resolution at a very fine scale.

Implantable pressure sensors, constructed as tiny piezoresistive elements, measure the difference between the pressure at the implantation site and a reference pressure, or alternatively, the absolute pressure at that point. The pressure recorded by these sensors on the fish's surface comprises two distinct components: hydrostatic pressure, which depends solely on the fish's depth, and dynamic pressure, generated by the water flow passing over the fish's body—a pressure that increases in proportion to water density and the square of the flow velocity.

Precisely because dynamic pressure depends on flow velocity, the dynamic pressure distribution recorded around the fish's body can be used to reconstruct the external flow velocity field by applying the principle of energy conservation that describes the relationship between pressure and velocity in a fluid; thus, the fish's body itself, when covered with pressure sensors, acts as its own instrument for measuring the velocity of the surrounding flow. Among the flow sensors mentioned at the beginning of this model, hot-wire anemometers measure fluid velocity based on the heat loss from a metal wire maintained at a constant temperature; according to King's law, the electric current required to maintain the wire's temperature against the cooling effect of the flow is a calibrated function of that flow's velocity.

When the objective is to measure the total flow rate in a single direction—such as within a swim tunnel—rather than the flow velocity at a specific point, electromagnetic or Doppler ultrasonic flowmeters are used instead, as they measure flow rate with high precision.

The most revealing behavior observed with these pressure sensors is that, when placed on the fish's body, they detect the pressure wave generated by the beating of its own caudal fin—the very pressure wave described earlier in this model as part of the pressure field on the body's surface. This finding makes it possible to study how the fish itself could, in principle, use its lateral line to detect the flow field it generates while swimming.

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