Apple Blow

SCIENCE CONCEPT:

Air flows around various objects and tries to conform to the shape of the object. For flow around a apple, the flow will try to follow the contour of the apple. However, the flow will separate and form a wake on the leeward side (downstream) side of the apple. As the wake travels downstream away from the apple, the wake dissipates. Therefore, if the candle is close enough to the apple, it will be blown out, however, if it is too far downstream, the candle flame will simply flicker but not blow out.

STUDENT OBJECTIVE:

The student will use a lighted candle and a apple to study the airflow around a cylindrical object.

OVERVIEW:

In this activity, the student will use a simple experiment to observe fluid dynamics in action. The student will place a lit candle behind a apple and try to blow out the candle. If the candle is close enough to the apple, the student will be able to blow out the candle.

PREPARATION TIME:

negligible.

LESSON TIME:

10 minutes.

TEACHER PREP:

Gather the materials required for the experiment.

WORDS TO KNOW:

windward

leeward

separation

wake

TEACHER TEXT:

Fluid dynamics is a complex phenomena which is difficult to solve numerically. The idea of an idealized fluid has been one method to solve simple fluid problems. Ideal fluids do not exist in nature, but can be used to gain insight into many practical problems. However, the concept breaks down in many problems which appear quite simple at first.

One such situation is the concept of potential flow around a sphere or sphere. Potential flow is simple an idealized situation were the fluid is inviscid (no frictional forces between the body and the fluid, i.e. no skin friction) and irrotational (can't produce lift). If the flow around the sphere was truly a case of potential flow, the streamlines would be symmetric above and below the sphere as well as upstream and downstream of the sphere. This would make for a simple explanation of how the candle was blown out. Unfortunately for us, this is not reality.

The flow does behave as a potential fluid upstream of the sphere and around the first half of the sphere. However, as the sphere begins to curve back around the leeward face, the flow encounters an adverse pressure gradient. Fluids generally like to move from regions of high pressure to regions of low pressure. This is in fact the case for the windward side of the sphere. However, on the leeward side, the fluid is forced to move from a low pressure region to a high pressure region. Due to this and the interaction with the boundary layer, the flow separates to form a wake behind the apple.

As the distance behind the apple increases, the wake dissipates. This is due to a number of factors including shear stresses, and existing air currents.

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Last modified: Fri Feb 27 16:46:43 PST 1998