3 questions/ 7 minutes
Turbulent flow over a boundary is a complex phenomenon for which
there is no complete theory. Nevertheless, much experimental data
has been collected on flows over solid surfaces, both in the laboratory
and in nature, so that from an engineering perspective, the situation
is well understood. The force exerted on a surface varies with
the roughness of that surface and approximately with the square
of the wind speed at a fixed height above it. A wind of 10 meters
per second (about 20 knots, or 22 miles per hour) measured at a
height of 10 meters will produce a force of some 30 tons per square
kilometer on a field of mown grass, or of about 70 tons per square
kilometer on a ripe wheat field. On a really smooth surface, such
as glass, the force is only about 10 tons per square kilometer.
When wind blows over water, it is more complicated. The roughness of the
water is not a given characteristic of the surface but depends on the wind
itself. Not only that, the elements that constitute the roughness, the waves,
themselves are mostly in the direction of the wind. Recent evidence indicates
that a large portion of the momentum transferred from the air into the water
goes into waves rather than directly into making water currents; only as
the waves break or otherwise lose energy does their momentum become available
to generate currents. Waves carry a substantial amount of both energy and
momentum (about as much as is carried by the wind in a layer about one wavelength
thick), and so the wave-generation process is far from negligible.
A violently wavy surface belies its appearance by acting, as far as the wind
is concerned, as though it were very smooth. At a wind of 10 meters per second,
the force on a wavy surface is much less than the force would be over mown
grass and scarcely more than over glass; in light winds (2 or 3 meters per
second) the force on a wavy surface is even less than it would be on glass.
The waves motion seems to modify the airflow so that air slips over
the surface more freely than if it were smooth. This is not the case at higher
wind speeds (above about 5 meters per second), but the force remains quite
low relative to other surfaces.
Unfortunately, there are no direct observations under conditions when high
winds, greater than about 12 meters per second, have had time and fetch (the
distance over water) enough to raise substantial waves. A few indirect studies,
however, suggest that the waters apparent roughness may increase under
high wind conditions, so that the force on the surface increases more rapidly
than the square of the wind speed.
If the force increases at least as the square of the wind speed, high-wind
conditions will produce effects far more important than their frequency of
occurrence would suggest, as five hours of 60-knot storm winds will put more
momentum into the water than a week of 10-knot breezes. If it should be shown
that, for high winds, the force on the surface increases even more than the
square of the wind speed, then the transfer of momentum to the ocean will
turn out to be dominated by the occasional storm rather than by the long-term
1. According to the passage, several
hours of storm winds (60 miles per hour) over water would:
(A) be similar to the force exerted by light winds for several
hours over glass.
(B) create a surface roughness that reduces the force exerted by the high
(C) be more significant in increasing the momentum of the water than constant
light winds over a period of a few days.
(D) create a force not greater than six times the force of a 10-mile-per-hour
(E) directly affect water current.
2. The main purpose of the passage
is to discuss:
(A) oceanic momentum and current.
(B) turbulent flow of wind over water.
(C) wind blowing over water as related to causing tidal flow.
(D) the significance of high wind conditions on ocean momentum.
(E) experiments in wind force.
3. The authors suggestion that
the transfer of momentum to the ocean is dominated by the occasional
storm would be most weakened if which of the following were true:
(A) Air momentum is converted directly into increased ocean current.
(B) High speed winds slip over waves as easily as low speed winds.
(C) Waves do not move in the direction of wind.
(D) The force exerted on a wheat field was the same as on mown grass.
(E) The force of wind under normal conditions increased as the square of