Which sensor detects low-pressure areas around an obstruction in a fluid flow?

The correct answer is a piezoelectric sensor. This type of sensor is capable of detecting changes in pressure by converting mechanical strain due to pressure changes into an electrical signal. When fluid flows around an obstruction, low-pressure areas can develop due to the fluid's velocity changes, and piezoelectric sensors are adept at detecting these variations in pressure through their sensitivity to the mechanical deformation (strain) caused by the fluid dynamics.

Piezoelectric sensors are particularly useful in various applications related to fluid flow because they can respond quickly to changes and can be calibrated for specific pressure ranges. Their ability to detect dynamic pressure variations makes them a valuable choice in scenarios where precise readings of low-pressure areas around obstructions in fluid flows are necessary.