drag simulations and experiments

project vision

teaching principle

how it will help students

By Matt BallIn cooperation with Dr. Meredith Metzger

This project aimed to provide the first step in a comprehensive learning module capable of bridging the gap between conceptual knowledge and practical application. A firm conceptual understanding of fluid mechanics is essential in order to efficiently design objects that move through a fluid such as water or air. For this reason, the project combined experimen-tal, computational, and laboratory exercises that can be incorporated di-rectly into the fluid mechanics course, and subsequent fluid mechanics classes.

Sometimes called fluid resistance, drag is a force that opposes the rel-ative motion of an object. Understanding drag is vital to innovation in all fields that involve movement, for example Aerospace. Drag can be taught using the following simple equation, where the most difficult part of conceptual drag understanding is the drag coefficient, which depends on the relative speed and shape of the object.

Fd = ½ρν²CdA Where ρ = density of the fluid ν = speed of the object Cd = drag coefficient A = objects cross sectional area

Perhaps the most difficult part of understanding drag, is visualizing its effects on different shapes and sizes. An effective mode of teaching stu-dents to visualize the affect drag force has on an object is by having the student personally perform the numerical simulations, and experiments.

design iterationsThe Idea was to make a fully mechanical system to measure finite drag forces on objects. The difficulty is that the mechanical system needs to be extremely sensitive. Several paths to achieve a mechanical advan-tage were investigated. Three of the best ideas are shown in the 3-D drawings below. The final design was chosen to be a weighted lever arm that creates an opposition to the drag force by using gravity and is the top of the three pictures below.

I see and I forget. I hear and I remember. I do and I understand

—Confucius

The Cone of Learning

Source: Edgar Dale (1969)

In the class room, students will learn to calculate the drag over simple objects such as the sphere shown in the picture below. The students will then be asked to use this same process to model an object of their choosing, and calculate the drag forces on that object. The hands on part of this learning module will be where the student builds that same object and tests it in the water channel shown.

With modern technologies, students can build their objects in a 3-D mod-eling program such as SolidWorks. Then those objects can be printed on a 3-D printer that will give the object exact dimensions chosen by the student.

Edgar Dale was an American educationist who developed the Cone of Expe-rience that was published in his book ‘Audio visual methods in teaching’ that was published in 1946. Dale believed that learning becomes more meaningful when abstract learning and concrete experience are related.

Showing the reaction of a fluid flowing past a sphere. (media.efluids.com)

A graph showing the relationship between the Coefficient of friction and Reynolds Number

3-D drawings of design considerations including the chosen final design with the cart and water chan-nel. (Top: weighted bar hinged at the top that will oppose the drag force using gravity. The object below the cart is what the students will design. Right: a long lever arm that uses the mechanical advantage gained to displace a spring. Left: a pulley system set up such that the force can be amplified)

3-D drawings of possible simple objects that students could design them-selves. (top: a glider, bottom: a submarine).

Department of Mechanical Engineering

April 2013

SET