항력

Drag consists of friction drag and pressure drag. Friction drag results from shear stress acting tangentially on a surface, while pressure drag arises from pressure forces acting perpendicular to the surface. Together, these components determine the total resistive force opposing an object's motion through a fluid.

When a flat plate is parallel to flow, all shear force contributes to friction drag. Conversely, when the plate is perpendicular to flow, shear stress has minimal effect on drag. Surface orientation relative to flow direction directly determines how much shear stress translates into friction drag force.

In laminar flow, friction drag is independent of surface roughness because the smooth laminar sublayer dominates. In turbulent flow, roughness elements disrupt this sublayer, significantly increasing friction drag. The friction drag coefficient therefore varies with roughness only under turbulent conditions.

The drag coefficient is a dimensionless value reflecting the influence of shape, Reynolds number, and surface roughness. It allows findings from scale models tested in wind tunnels or using computational fluid dynamics to be applied to full-scale designs across various shapes and flow conditions.

At low Reynolds numbers, viscosity influences the drag coefficient significantly, and boundary layer separation can increase drag substantially. At high Reynolds numbers, the pressure drag coefficient is generally unaffected by Reynolds number changes. In laminar flow, the friction drag coefficient decreases with increasing Reynolds number.

When a flat plate is parallel to flow, pressure forces act equally on both sides of the surface, resulting in minimal net pressure drag. Pressure drag becomes significant only when the plate is perpendicular to flow, where pressure differences across the surface create substantial resistive forces.

Accurate drag calculations use pressure and wall shear stress distributions determined through computational fluid dynamics or wind tunnel testing. These methods capture how both pressure and shear forces vary across the object's surface, enabling engineers to predict total drag for complex geometries and optimize structures for reduced resistance.