Short Answer:

Kármán vortex street is a repeating pattern of swirling vortices (small circular flows) that forms behind a bluff body when a fluid like air or water flows past it. These vortices are shed alternately from each side of the object, creating an oscillating flow in the wake.

It is commonly observed behind cylindrical poles, bridge piers, chimneys, or towers when wind or water flows past them. This phenomenon can cause vibrations and must be considered in civil engineering design to avoid structural damage due to continuous oscillations.

Detailed Explanation:

Kármán Vortex Street 

Kármán vortex street is a famous concept in fluid mechanics that describes a specific flow pattern of alternating vortices forming behind a non-streamlined object (usually a bluff body) when fluid flows past it. This was first explained by Theodore von Kármán, a Hungarian-American physicist and engineer.

This flow pattern occurs in a specific range of Reynolds numbers, typically between 47 and 10,000, where the fluid separates from the surface of the body and rolls into rotating vortices. These vortices are shed alternately from opposite sides, forming a zigzag or street-like pattern in the downstream flow.

Where Kármán Vortex Street is Observed

Kármán vortex streets can be seen in both natural environments and man-made structures, especially where cylindrical or bluff bodies are exposed to steady fluid flow. Some common places where it is observed:

1. Bridge Piers in Rivers

• When water flows around a bridge pier or pillar, vortices are created alternately on either side of the pier.
• This can cause vibrations known as vortex-induced vibrations (VIV).

2. Tall Chimneys and Towers

• Wind flowing past slender vertical structures creates Kármán vortex streets.
• These can lead to oscillations, which may result in fatigue failure over time.

3. Electric Poles and Cables

• In strong winds, cables and poles may vibrate due to vortex shedding, producing humming sounds and physical motion.

4. Cooling Towers

• Tall cylindrical structures like cooling towers in power plants may also exhibit vortex shedding when exposed to crosswind.

5. Smoke Trails and Clouds

• In the sky, aircraft or rising smoke columns sometimes show visible vortex patterns due to temperature and velocity differences.

Why It Matters in Civil Engineering

The alternate shedding of vortices causes unsteady lift forces on the object, which leads to periodic vibrations. If the frequency of these forces matches the natural frequency of the structure, resonance can occur. This can cause dangerous structural oscillations and even collapse if not controlled.

Solutions in Design:

• Use aerodynamic fairings or modify the shape to prevent vortex formation.
• Helical strakes are added to chimneys or towers to break up vortex formation.
• Increase damping in structures to absorb the vibration energy.

These preventive steps help protect civil engineering structures from damage due to Kármán vortex streets.

Conclusion:

Kármán vortex street is a repeated swirling flow pattern created behind bluff bodies due to alternating vortex shedding. It is observed in many real-world scenarios like around bridge piers, chimneys, or poles exposed to steady fluid flow. In civil engineering, this phenomenon is critical because it can cause unwanted vibrations and potential structural failure. Understanding and addressing it ensures the safety and durability of structures affected by fluid motion.