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Session 3268

A Couple of Fluid Mechanics Brainteasers

Alan Mironer University of Massachusetts Lowell

Abstract

This paper describes two fluid mechanics demonstrations which are presented to the class in the form of puzzles or paradoxes. Both demonstrations use very simple apparatus to dramatically show that considering only changes in a flow’s linear momentum may not be sufficient to determine the flow forces acting on a body; i.e., pressure forces may be important. In the first demonstration pressure forces are significant because of the deceleration of the flow. Pressure forces in the second demonstration arise from both the flow’s acceleration and mechanical energy dissipation and very nearly balance the change in linear momentum.

Introduction

Most students consider fluid mechanics to be one of their more difficult courses, finding it too abstract and mathematical. It becomes for many a dry exercise in manipulating series of equations. In an effort to counteract this mindset and at the same time offer a much-welcome respite from the normal class routine I introduce a limited number of demonstrations and puzzles.

My procedure is to describe the puzzle using a sketch on the blackboard, and then ask the class to think about what should occur. I tell the students not to guess based on a gut feeling, but to anchor their prediction on fundamental principles of fluid mechanics. After some class discussion I demonstrate the puzzle using the simplest of apparatus. Invariably, the outcome is at odds with the students’s predictions. I then guide the students to resolve the discrepancy by asking certain pertinent questions. This results in a lively exchange of ideas and a deeper understanding of fundamentals.

Blowing can cause attraction

This puzzle is given to the class after the fundamental equations of continuity, force/linear momentum, and Bernoulli have been covered.

The demonstration uses a large sewing machine-thread spool, 1 1/2 inches in diameter at the bottom and 3/4 inches in diameter at the top, with a 3/8 inch diameter center hole and two paper disks, 1 1/2 inches and 3/4 inches in diameter. See Figure 1.

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Mironer, A. (1999, June), A Couple Of Fluid Mechanics Brainteasers Paper presented at 1999 Annual Conference, Charlotte, North Carolina. 10.18260/1-2--8124