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Chapter 6: Problem 5

On a windy day in a city with many tall buildings, leaves and papers can be seen swirling about in the air or on the sidewalks. What causes these whirling air currents?

Short Answer

Expert verified
Tall buildings disrupt wind flow, creating turbulent air and whirling currents.

Step by step solution

01

Understand the Influence of Tall Buildings

Tall buildings create obstacles that interrupt the smooth flow of air. When the wind hits these buildings, it is forced to move around them, causing variations in wind speed and direction.
02

Analyze Air Movement Around Obstacles

As wind travels along the sides of tall buildings, it undergoes changes in speed and can create areas of low and high pressure. This causes the air to swirl as it moves around the corners and edges of the buildings.
03

Consider Lift and Turbulence

Wind moving over the tops of buildings can create lift, drawing more air upwards. This, combined with the turbulent flow around the building, contributes to swirling air patterns.
04

Conclusion: Cause of Whirling Air Currents

The combined effects of obstacles created by tall buildings and the resulting turbulent airflows cause the whirling air currents observed on windy days.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Turbulence
Turbulence happens when smooth airflow gets disrupted and starts moving chaotically. When wind hits buildings, it can't move in a straight line anymore. It gets pushed around corners and edges, creating random, swirling movements.

These chaotic airflows are called turbulent flows. Turbulent air is responsible for the leaves and papers you see spinning in different directions.

Unlike smooth, laminar flow, which is predictable, turbulence is chaotic and unpredictable. It can carry objects into spirals and can cause sudden gusts of wind that we often feel on city streets.
Wind Patterns
Wind patterns describe how wind moves across an area. Around tall buildings, these patterns change dramatically.

When wind flows towards a building, it splits and goes around and over it. The wind that goes around the sides can speed up due to the narrow space. This is known as the Venturi effect.

Over the top of the building, wind may also speed up, creating a low-pressure zone. The combined effect of these patterns creates complex wind behaviors, including swirling and eddy currents.
  • Wind speeds up when compressed.
  • Wind slows down in the lee of buildings.
  • Wind direction can change, creating vortices.
Aerodynamics
Aerodynamics is the study of how air moves around objects. When wind encounters a building, it is forced to change direction, which influences how the air flows.

Buildings are not aerodynamic like airplanes, so the airflow around them becomes disturbed. This disturbance in the airflow causes the wind to swirl unpredictably.

Good aerodynamic design can reduce turbulence and make the environment more comfortable. Features like rounded edges, airflow channels, and wind barriers can help manage wind movement around buildings.
Pressure Variations
Pressure variations occur when different areas experience different air pressures. Around tall buildings, wind movement causes some areas to have higher pressure and others lower pressure.

High-pressure zones form where the wind hits the building directly. Low-pressure zones can form on the sides and behind the buildings where the air is drawn away.

These differences in pressure cause the swirling motion of the air as it moves from high-pressure zones to low-pressure zones. This helps explain why papers and leaves lift off the ground, carried by the sudden changes in airflow.
  • High pressure pushes air away.
  • Low pressure draws air in.
  • Differences in pressure create wind movement.

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