The Clean Plate

SCIENCE CONCEPT:

The laminar flow of water will remain undisturbed until the water is turned up. At that point the laminar flow becomes turbulent. Because of the change in the speed, the water turns turbulent.

STUDENT OBJECTIVE:

The student will observe what happens when he places a plate in a sink and turns on the water slowly. The student will then observe what happens when the speed of the water is turned up. What happens to the water and what happens to the plate?

OVERVIEW:

Each student will be given an opportunity to try this experiment with laminar vs. turbulent flow. Have the student place a plate in the sink and then turn on the water slowly. Have the student observe that the water is laminar. When the student turns up the water there is a larger flow rate which will become turbulent. The shear stress (drag) increases with the turbulent flow causing the plate to be more easily cleaned off.

PREPARATION TIME:

5 minutes.

LESSON TIME:

30 minutes.

TEACHER PREP:

Bring in a few large plastic plates

WORDS TO KNOW:

laminar

turbulent

stress(drag)

flow

attached

separation

pressure

TEACHER TEXT:

To explain the laminar vs. turbulent flow we will discuss how this flow changes the speed of a golf ball and the speed of a swimmer. With the shear stress (drag) increasing when the water is turned up the laminar flow changes into a turbulent flow. The same principle applies when we change the laminar flow around a golf ball. A smooth golf ball experiences laminar flow just as the slow water experiences laminar flow. But when you apply dimples to the golf ball it experiences turbulent flow. The surface roughness causes the flow to transition from laminar to turbulent, just like when the water's speed is increased it also becomes turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer. That is why the plate gets cleaner faster when the water is turbulent. The water is able to exert greater pressure on the plate and remove the dirt.

Some swimmers are faster than others. The difference lies in the ability to use the fundamental principles of fluid dynamics to one's advantage. The same principles which makes the best swimmers so fast are employed by anyone who has ever treaded water. In order to understand how these principles apply, we must first understand the forces acting on a swimmer. There is a pressure drag that is on the swimmer and can be divided into two components: pressure drag and skin friction drag. The pressure drag comes from the frontal area exposed to the water and the separation that occurs behind the swimmer. This is similar to the pressure drag of the smooth golf ball. In order to reduce the drag on the golf ball, dimples were introduced which changed the nature of the flow from laminar to turbulent, causing the golf ball to travel further.

The turbulent flow delays separation and therefore reduces the pressure drag. However, the flow around a swimmer is already turbulent. Therefore, a swimmer must streamline his body to reduce the amount of separation. If the turbulent flow is too great around the swimmer the water becomes a force working against the swimmer. In the same way, if the turbulent flow of water on the plate is too great the plate will be moved away from the force of the water and remain dirty.

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Last modified: Sat Dec 20 12:52:43 PST 1997