Short Answer:

In fluid mechanics, drag is the resistance force that acts opposite to the direction of fluid flow over a solid object. It occurs due to the interaction between the moving fluid and the surface of the object. The faster the object or fluid moves, the more drag it experiences.

Drag is important in civil engineering because it affects the design of structures like bridges, buildings, poles, and chimneys. Engineers must consider drag forces to ensure safety, stability, and energy efficiency, especially for objects exposed to wind or water flow.

Detailed Explanation:

Concept of Drag in Fluid Mechanics

Drag in fluid mechanics refers to the opposing force exerted by a fluid (like air or water) when it flows over or around a solid body. It resists the motion of the body and can either slow it down or increase the force needed to move it. Drag is a key factor in the design of structures and machines that interact with flowing fluids.

The total drag force depends on the shape of the object, the fluid speed, the fluid’s properties, and the surface texture of the object. In civil engineering, drag is considered while designing towers, bridges, vehicles, pipelines, and wind-sensitive structures.

How Drag Is Generated

When a fluid flows over a body, two main things happen:

1. Frictional resistance (skin friction):
   The fluid particles close to the object’s surface stick to it and slow down due to viscosity. This creates a thin layer called the boundary layer, and the resistance caused by it is known as skin friction drag.
2. Pressure difference (form drag):
   As fluid flows over the front and back of the object, a pressure difference is created. If the flow separates from the surface, a low-pressure zone forms behind the object, pulling it backward. This resistance is known as pressure drag or form drag.

Total drag is the sum of skin friction drag and pressure drag.

Factors Affecting Drag

1. Shape of the Object:
   • Streamlined shapes reduce drag.
   • Blunt or flat surfaces increase drag due to flow separation.
2. Flow Velocity:
   • Drag increases rapidly with speed (proportional to the square of velocity).
3. Surface Roughness:
   • Rough surfaces increase friction and drag.
   • Polished or smooth surfaces help reduce drag.
4. Fluid Properties:
   • Higher viscosity and density increase drag.
5. Reynolds Number:
   • It determines whether the flow is laminar or turbulent, which influences drag behavior.

Importance of Drag in Civil Engineering

• Building Design: Tall buildings are exposed to wind. Drag forces must be calculated to ensure they can withstand strong winds without swaying or collapsing.
• Bridge Piers and Towers: Structures in rivers or oceans face water flow. Drag affects their stability, foundation, and resistance to current loads.
• Chimneys and Poles: These vertical structures experience drag due to air flow. Engineers must design proper foundations and support systems.
• Wind Tunnels and Model Testing: Engineers test scaled models to measure drag and improve design efficiency, especially for structures like dams, cooling towers, and high-speed trains.

Conclusion:

Drag in fluid mechanics is the resistance force caused by a fluid flowing over a solid surface. It affects both the fluid and the object it moves past. Understanding drag helps civil engineers design safer and more efficient structures by considering fluid forces due to air or water. It is a crucial factor in structural design, environmental flow studies, and model testing.