Name: drag, lift, and flow around objects
Uploaded: 2025-03-26T14:59:54.000Z
Duration: 1 min 26 s
Channel: JoVE

Understanding external flow characteristics is essential for designing objects like vehicles and buildings that interact with moving fluids.&nbsp;In fluid mechanics, the study of external flows explores how objects affect surrounding flow patterns.Bodies immersed in a moving fluid experience two main forces: drag, which opposes their motion, and lift, a force perpendicular to the flow direction. Both are influenced by the object’s shape, size, and orientation.Objects in a moving fluid can be classified as streamlined or blunt, each impacting flow differently.&nbsp;Streamlined structures, like bridge cables or slim towers, allow wind to flow smoothly, reducing drag and structural stress.&nbsp;Blunt structures, like building walls, create turbulence, increasing drag and requiring additional reinforcement to withstand these forces.The Reynolds number, representing the ratio of inertial to viscous forces, is crucial for determining flow behavior.&nbsp;At low Reynolds numbers, viscous forces dominate, while higher values lead to inertia-driven flow, thin boundary layers, and flow separation.Analyzing these characteristics helps optimize the design and enhance the stability, safety, and efficiency of structures like bridges, towers, and high-rise buildings.

general external flow characteristics

The study of external flow is essential for creating structures and objects that interact efficiently and safely with moving fluids, such as air or water. When a body is immersed in a flowing fluid, it experiences two primary forces: drag, which opposes motion along the flow direction, and lift, which acts perpendicular to the flow. The shape, size, and orientation of the object influence these forces.<h3 style="margin-left:0in;">Streamlined and Blunt Bodies in External Flow</h3>Objects in fluid flow are classified as streamlined or blunt based on their impact on the surrounding flow. Streamlined bodies, such as bridge cables or slender towers, allow the fluid to flow smoothly around them, reducing drag and minimizing structural stress. In contrast, blunt bodies, like building walls, generate turbulence and wake regions behind them. This turbulence increases drag and often necessitates stronger reinforcements to withstand the additional forces created by the flow.<h3 style="margin-left:0in;">Importance of the Reynolds Number</h3>The Reynolds number is a key parameter that indicates the relative importance of inertial and viscous forces in a fluid flow. It provides insight into whether the flow will be smooth, laminar, or turbulent. At low Reynolds numbers, viscous forces dominate, resulting in steady, smooth flow patterns. At high Reynolds numbers, inertial forces become more significant, leading to thin boundary layers, flow separation, and turbulence.<h3 style="margin-left:0in;">Applications in Design</h3>Understanding external flow characteristics helps improve the design of structures and objects, ensuring stability, safety, and efficiency. Streamlined shapes reduce resistance and vibrations caused by moving fluids, while blunt structures are reinforced to handle the additional stresses from turbulence. This knowledge is critical for optimizing the performance and durability of buildings, bridges, and other structures exposed to fluid flow.

General External Flow Characteristics

The study of external flow is essential for creating structures and objects that interact efficiently and safely with moving fluids, such as air or water. ...

Education

JoVE Core

Civil Engineering

Unit 1

Flow over Immersed Bodies

KEY TERMS AND CONCEPTS

drag, lift, and flow around objects

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 drag and reduces stress on the structure. Blunt bodies, such as building walls, disturb the flow more strongly. They create turbulence and a wake region behind the object, which increases drag and can require stronger reinforcement.

The Reynolds number is another important idea in external flow. It compares inertial forces and viscous forces in a fluid. At low Reynolds numbers, viscous forces are more important, and the flow is steady and smooth. At high Reynolds numbers, inertial forces become more important. This can lead to thin boundary layers, flow separation, and turbulence.

Knowing these flow patterns helps engineers design safer and more efficient structures. Streamlined shapes can reduce resistance and the vibrations caused by moving fluids. Blunt structures must often be reinforced to handle extra stress from turbulence. This is important for buildings, bridges, and other structures exposed to moving air or water.

Drag, Lift, and Flow Around Objects

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...

boundary layer characteristics

Laminar and Turbulent Flow Near Surfaces

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

Drag Forces in Fluids and Surfaces

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

How Wings Generate Lift

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...

design example: calculating safe diameter for wind-exposed disc

design-example-calculating-safe-diameter-for-wind-exposed-disc

Wind Load Check for a Facade Disc

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...

key terms and concepts

Learn how drag, lift, streamlined and blunt shapes, and Reynolds number affect external flow around buildings, bridges, and other structures.

Drag, Lift, and Flow Around Objects | JoVE K12

JoVE Core Civil Engineering offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

Drag, Lift, and Flow Around Objects

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