External Flow, Boundary Layers and Forces

Fluid immediately adjacent to a fixed wall satisfies the no-slip condition. Viscosity transfers momentum between this slow fluid and the faster outer stream, generating a boundary layer. Its thickness generally increases downstream.

For a smooth flat plate in zero streamwise pressure gradient, standard laminar formulas give approximate thickness and average skin-friction coefficient. They require flow that remains laminar over the modelled length. State the leading-edge reference and the selected characteristic length.

Skin friction acts tangentially because of wall shear. Pressure drag arises from the pressure distribution, often strongly affected by flow separation. Their sum is the overall drag, but the appropriate coefficient must correspond to the drag mechanism and reference area.

Forces can be expressed with dimensionless coefficients multiplying dynamic pressure and a reference area. Lift is conventionally perpendicular to the oncoming velocity and drag is parallel and opposite to it. A coefficient depends on geometry and flow condition, so do not assume an arbitrary universal value.

Boundary-layer transition depends on disturbances, pressure gradients and surface conditions as well as Reynolds number. Laminar correlations should not be applied beyond their assumed regime without discussion. A provided turbulent correlation has its own restrictions.

The dynamic pressure $\frac{}{}$ makes aerodynamic or hydrodynamic loads particularly sensitive to speed. Doubling speed roughly quadruples coefficient-based force only if the coefficient is unchanged. Check whether the reference area means frontal projection, planform area or wetted surface.