Affordable and student-safe fluid experiment: Bent-straw vortex ring generator*

Vortex is a fundamental way nature organizes flow. Thus, a simple vortex ring generator in a classroom can provide opportunities to grasp this fundamental concept intuitively. Here, we present an affordable and student-safe experiment that can demonstrate vortex formation phenomena at various vortex formation times. After filling a bent straw with a column of food-dyed water, the thumb closes the top opening to maintain the pressure head. Then, the dyed water is transported to the water bath, and upon stabilizing the straw, the thumb is released. A beautiful vortex ring is created, and its propagation can be easily observed. This simple setup can be made more sophisticated by graduating the straw and thus quantifying the water-level height. This allows students to explore the effect of vortex formation time, L/D, where L is the column height and D is the diameter.