WHY? When air encounters a bluff body, such as a apple, the air divides into two streams which flow around either side of the apple. However, the flow cannot negotiate the turn around the rear of the body. This causes the flow to separate and form a wake behind the apple. If the candle is close enough to the apple, the candle will be blown out. As the distance increases, the wake dissipates. As the wake dissipates, the velocity of the air decreases. Therefore, if the candle is farther away from the apple, the candle will not be blown out, but merely flicker a little bit. When the apple is sliced in half, the air is still split into two streams, but the air is directed to either side. Since there is a sharp edge, instead of a smooth surface, the air mainly continues in the same direction and the wake is no longer formed behind the apple. However, when the candle is moved to the side of the apple, the candle will be blown out. ASSESSMENT: Can the student communicate what they observed?

The student is able to communicate that the air flow splits into two streams around the bottle. The airflow then separates to form a wake behind the bottle. If the candle is close enough to the bottle it will blow out. However, if the candle is further away, it will not blow out since the wake dissipates. Also, the student recognizes that when the apple is sliced in half, the air can't flow around the apple so the air goes off to either side of the apple. The student is able to communicate that the air flow separates to form a wake behind the apple. Furthermore, the candle is close enough to be blown out, however as the candle is moved downstream, the wake is no longer strong enough to blow the candle out. Also, the student recognizes that when the apple is sliced in half, the air moves to either side of the apple. The student is able to communicate that the air flow blew out the candle, however as the candle is moved downstream, the air flow was no longer strong enough to blow the candle out. Also, when the apple is sliced in half the air goes off to the side. The student is able to communicate that the candle went out the first time, but when the candle was further away from the apple, the candle remained lit. The student also recognizes that the candle was blown out when placed to the side of the apple which was cut in half.

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Last modified: Fri Feb 27 15:15:29 PST 1998

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