Flow over Immersed Bodies

External flow describes how moving air or water interacts with objects such as buildings, bridges, and towers. These objects feel two main forces. Drag acts along the flow and opposes motion, while lift acts at right angles to the flow. The shape, size, and orientation of the object all affect these forces.

Objects in external flow are often described as streamlined or blunt. Streamlined bodies, such as bridge cables and slender towers, let the fluid move around them more smoothly. This lowers...

Laminar and turbulent flow near a solid surface helps explain how boundary layers form in fluids. When a fluid meets a stationary wall, a thin region develops next to the surface where viscous forces dominate. In this region, the fluid speed changes from zero at the wall to the free-stream velocity farther away.

The boundary layer starts at the leading edge of the surface and grows as the fluid moves downstream. Friction between the fluid and the wall slows the particles closest to the surface.

Drag is the resistive force that slows an object moving through a fluid. It comes from pressure acting on the surface and from shear forces along the surface. To calculate drag accurately, scientists often use pressure and wall shear stress distributions from Computational Fluid Dynamics (CFD) or wind tunnel tests.

The drag coefficient is a dimensionless number that helps compare drag across different cases. It depends on shape, Reynolds number, Mach number, Froude number, and surface...

Lift is the upward aerodynamic force that acts perpendicular to the airflow. It is essential for flight and also helps stabilize many vehicles. On an airfoil, lift comes from pressure differences. Air moving over the wing creates lower pressure above the surface and higher pressure below it. That imbalance produces an upward force.

The lift force can be calculated with the lift equation. In the equation, L is lift, ρ is air density, U is airflow velocity, A is wing surface area, and CL is the...

Wind load check for a facade disc shows how engineers test whether a mounted shape can stay safe in strong air flow. In this example, a circular disc on a building facade faced wind at 25 meters per second. The air density was 1.25 kilograms per cubic meter, and the disc diameter was first set at 4 meters.

The drag force on the disc was calculated using a drag coefficient of 1.1. The drag coefficient describes how much resistance an object experiences in a fluid such as air. The result was...