Name: drag forces in fluids and surfaces
Uploaded: 2025-03-26T15:00:05.000Z
Duration: 1 min 23 s
Channel: JoVE

Drag is a force that resists an object's motion through a fluid caused by pressure and shear forces on its surface. It has two main components: friction drag and pressure drag.Friction drag results from shear stress on the surface, depending on wall shear stress and the surface's alignment with the flow. For surfaces parallel to the flow, all shear force contributes to friction drag.&nbsp;The friction drag coefficient varies with the Reynolds number and surface roughness. In laminar flow, it decreases with the Reynolds number, while in turbulent flow, roughness increases the drag coefficient.&nbsp;Pressure drag results from pressure forces perpendicular to the surface and depends on the object's shape and orientation.&nbsp;At high Reynolds numbers, the pressure drag coefficient is generally unaffected by the Reynolds number. However, at low Reynolds numbers, viscosity influences the drag coefficient, and boundary layer separation can increase drag.The drag coefficient, a dimensionless value, reflects the influence of shape, Reynolds number, and surface roughness. It allows findings from scale models to be applied to full-scale designs across various shapes and flow conditions.

drag

Drag is a resistive force opposing an object’s motion through a fluid, resulting from surface pressure and shear forces. It comprises two components: a perpendicular one from pressure and a tangential one from shear stress. Accurate drag calculations use pressure and wall shear stress distributions, often determined through Computational Fluid Dynamics (CFD) or wind tunnel testing. The drag coefficient, a dimensionless measure, depends on factors like shape, Reynolds number, Mach number, Froude number, and surface roughness, allowing results from scale models to inform full-scale designs.Friction drag arises directly from shear stress on a surface and depends on wall shear stress and surface orientation relative to flow. For a flat plate aligned parallel to flow, all shear force contributes to friction drag, while for a perpendicular surface, shear stress has minimal effect. Friction drag on a flat plate can be calculated using the friction drag coefficient, which is influenced by the Reynolds number and surface roughness. In laminar flows, friction drag is independent of roughness, but in turbulent flows, surface roughness significantly impacts friction drag as roughness elements disrupt the laminar sublayer.Pressure or form drag results from pressure differences across an object’s surface, primarily influenced by shape and orientation. For example, a flat plate parallel to flow has minimal pressure drag as forces act equally on both sides. When the plate is perpendicular, however, pressure forces create significant drag. The pressure drag coefficient helps calculate this force, and in high Reynolds number flows, it is mainly independent of Reynolds number as pressure scales with dynamic pressure.Understanding friction and pressure drag contributions aids in optimizing structures for reduced resistance- improving aerodynamic and hydrodynamic efficiency.

Drag

Drag is a resistive force opposing an object’s motion through a fluid, resulting from surface pressure and shear forces. It comprises two components: a ...

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Civil Engineering

Unit 1

Flow over Immersed Bodies

KEY TERMS AND CONCEPTS

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 roughness. Because of this, results from scale models can be used to help design full-size objects.

Friction drag comes directly from shear stress on a surface. It depends on wall shear stress and on how the surface is oriented to the flow. For a flat plate parallel to the flow, the shear force becomes friction drag. For a surface perpendicular to the flow, shear stress has very little effect.

A flat plate can also be used to calculate friction drag with a friction drag coefficient. This coefficient depends on Reynolds number and surface roughness. In laminar flow, roughness does not change friction drag. In turbulent flow, roughness has a much larger effect because roughness elements disturb the laminar sublayer.

Pressure drag, also called form drag, comes from pressure differences across an object. Shape and orientation control this force. A flat plate parallel to the flow has little pressure drag because the forces are balanced on both sides. When the plate is perpendicular, pressure forces create much larger drag.

The pressure drag coefficient helps calculate this force. At high Reynolds number, pressure drag is mainly independent of Reynolds number because pressure scales with dynamic pressure. Understanding how friction drag and pressure drag work helps engineers reduce resistance and improve aerodynamic and hydrodynamic efficiency.

Drag Forces in Fluids and Surfaces

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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...

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key terms and concepts

Explore drag as a force from pressure and shear stress, plus friction drag and pressure drag. Learn how shape, roughness, and flow affect resistance.

Drag Forces in Fluids and Surfaces | 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 Forces in Fluids and Surfaces

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