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.