Victory for the ACT Student Text 15e

354 • S CIENCE

Passage IX The blast wave of compressed air created by an aerial explosion propagates in a spherical pattern from the center of the detonation and loses force with the distance traveled from the center. Overpressure is the difference between the air pressure at the leading edge of the blast wave and the ambient air pressure before the arrival of the blast wave. Figure 1 shows the typical overpressure history of a blast wave at locations distant from the center of the explosion. Passage Perfect To answer an assumption question, you can take several different routes. The fastest would be to see if you can automatically recognize what part of the argument has a logical gap. If you cannot automatically see the gap in reasoning, you can substitute each answer into the argument and see which strengthens the line of reasoning most. This will show you which answer choice is the argument’s major assumption. After the passage of the blast front, air pressure drops abruptly and eventually comes back into equilibrium with the ambient pressure. Figure 2 shows the overpressure and dynamic pressure (a measure of the wind gust force per unit area) at the blast front as a function of time following an explosion. Distance from Explosion Overpressure t 1 t 2 t 3 t 4 t 5 t 6 Peak Overpressure Blast Front Figure 1

For a ground level explosion , a portion ‘ˆ –Š‡ „Žƒ•– ™ƒ˜‡ ‹• ”‡ϐŽ‡…–‡† „ƒ… „› ƒ”–Šǯ• •—”ˆƒ…‡Ǥ Š‡ –Š‡ ”‡ϐŽ‡…–‡† „Žƒ•– ™ƒ˜‡ merges with the initial wave impulse, the result is a shock wave with overpressure that may be 12 times more than twice that of the initial impulse. The various aspects of the blast wave are a function of the peak overpressure (see Figure 3), which is dependent on the amount and type of explosive. 10 3 10 4

100

70

7

7

40

4

4

2

2

20

Shock Velocity

10

10 2

10 3

7

7

7

4

4

4

2

2

2

Velocity (FT/SEC)

Peak Dynamic Pressure (PSI)

10 2

10

1.0

Particle Velocity

7

0.7

7

4

0.4

4

Reϐlected Pressure

Peak Reϐlected Pressure for Normal Incidence (PSI)

Dynamic Pressure

2

0.2

2

10

1

0.1

1

2

4 7 10 20 40 70 100

Figure 3

Peak Overpressure (PSI)

+

Restored

Time

Atmospheric Pressure

Ambient Pressure

Compression

0

Suction

Overpressure

Positive Phase

-

Negative Phase

Overpressure Dynamic Pressure

Blast Front Arrives

Weak Wind Toward Explosion

Feeble Wind Away from Explosion

Strong Wind Away from Explosion (Decreasing to Zero)

Air Flow Ceases

Wind Reversal

Wind Reversal

Time

Dynamic Pressure Positive Phase

Figure 2

Negative Phase