(81g) The Ball-in-Tube Device: A New Approach to Introductory Fluid Dynamics Education

The ability to apply methods from a course to a real world problem is an essential goal of engineering courses. A few simple experiments are often used to illustrate the theoretical principles behind undergraduate-level fluid dynamics. It is not uncommon for students to be tasked with determining the pressure distribution across a Venturi meter, the flow rate out of a draining tank, or the pressure drop across a fluidized bed. In this work we demonstrate the use of the traditional “ball-in-tube” control experiment as another means to introduce a fluid-dynamics teaching module.

The module is constructed by placing a computer fan below a vertical acrylic pipe and securing an ultrasonic rangefinder in the center of the tube. By controlling the velocity of the air in the tube a ball can be made to float while its distance from the bottom of the pipe is recorded by the rangefinder. This data is then fed into a PID controller which adjusts the velocity of the fan in order to keep the ball at a set height. A differential pressure sensor measures the pressure drop across the tube, and is used in the analysis of the fluid dynamics of the system.

The ball-in-tube experiment has often been used in the classroom to illustrate principles of process control. The traditional goal of this teaching module is to precisely maintain a ball at a height set-point within a tube by controlling the velocity of air flowing past the ball. The ball in tube experiment is an integrating system and experiences significant turbulent noise over small time scales. Despite that behavior, analysis of this system shows that it conforms to traditional undergraduate fluid dynamic theory over extended time intervals, making it amenable to simultaneous illustration of control and fluid dynamic principles.