Turbulent flow over a boundary is a complex phenomenon for which there

Question

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 water’s 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 average winds.    1 
 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 which reduces the force exerted by the high wind
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 6 times the force of a 10-mile-per-hour wind
E. directly affect water current

2 
 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 
 3. The author’s 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 wind speed

RC Butler 2022  -  Practice Two RC Passages Everyday.   
Passage # 245 Date: 20-Jun-2022
 This question is a part of RC Butler 2022 .  Click here for Details

PrateekVermaPV · Answer

3. The author’s 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 wind speed

Confusion between option B & C pls explain?

Sajjad1994 · Answer

Official Explanation

3. The author’s 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:

Explanation

The suggestion that the occasional storm will make a large contribution to momentum relies on the main argument that high winds can contribute more to the momentum of water than light winds (see paragraphs two and four, specifically). High winds can increase the intensity of waves, or increase the apparent roughness of the surface, which allows the exertion of more force. With the increased roughness, the force increases even more than the square of the wind speed (which is the usual estimation of the force exerted by wind on a surface).

Choices A  and  D  are not relevant to the statement, and thus would not weaken it if they were true.

Choice C , if true, would affect the transfer of momentum from both high and low winds to water, so this is not a good choice.

Choice E  actually is true, and does not weaken the argument.

Answer: B

Suvankar8250 · Answer

Question 1.

This is a direct question from the passage and an easy one. As the passage says, "as five hours of 60-knot storm winds will put more momentum into the water than a week of 10-knot breezes."

Therefore, the only option to mention this build up of momentum is Option (C).

Question 2.

The passage primarily discusses about turbulent flow over a boundary and if required, can be extrapolated to turbulent flow of wind over water (a boundary).
A. oceanic momentum and current ---- Not at all, since it ignores the primary concern of the passage, how wind flows over a boundary.
B. turbulent flow of wind over water -----  seems to be correct, when extrapolated and the boundary is considered water. We can consider the first paragraph as background information. 
C. wind blowing over water as related to causing tidal flow ----- The passage is more about how wind behaves rather than causing tidal flow
D. the significance of high wind conditions on ocean momentum ----- This separates out only a specific portion of passage. Too NARROW.
E. experiments in wind force ---- "Experiments" is too BROAD.

Question 3.

A. air momentum is converted directly into increased ocean current ----- therefore results in storms, STRENGTHENS
B. high speed winds slip over waves as easily as low speed winds ----  seems a WEAKENING statement, since if high wind slips as easily as a low one, then winds might not result into storms by transferring considerable momentum. 
C. waves do not move in the direction of wind ---- might help the high and low winds both to gain momentum, partially strengthens
D. the force exerted on a wheat field was the same as on mown grass ---- does not affect much, implies both high and low winds will act the same way
E. the force of wind under normal conditions increased as the square of wind speed ---- already mentioned in the passage

Kudos will be appreciated!

Turbulent flow over a boundary is a complex phenomenon for which there

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Turbulent flow over a boundary is a complex phenomenon for which there

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