A tube that gradually decreases to a minimum diameter in its center section is known as a __________ tube.

A tube that gradually decreases to a minimum diameter in its center section is known as a Venturi tube. This design is utilized in fluid dynamics to create a change in fluid velocity and pressure. As the fluid flows through the tube and encounters the narrowing section, its velocity increases while the pressure decreases, according to the principles of Bernoulli's equation.

The Venturi effect is commonly used in various applications, including carburetors in internal combustion engines, where the vacuum created by the increasing fluid velocity draws fuel into the airflow. The unique shape of the Venturi tube, with its tapered design leading to a narrow throat, is what makes it effective in creating this pressure differential.

In contrast, the other choices refer to different concepts. A converging tube would merely imply any tube that decreases in diameter, but it doesn't specifically describe the function related to fluid dynamics. A diverging tube would increase in diameter, which would not match the description provided. Meanwhile, an expansion tube generally refers to a tube that allows fluid flow to slow down and pressure to recover after passing through a restriction, which again doesn't align with the characteristics of the tube in question. Therefore, the defining features and practical uses of a Venturi tube support its selection as the correct