A solid object is bounded by a fluid in external flow. Contrast this with internal flow in which a fluid is bounded on all sides by a solid. Simply put, external flow refers to flow around objects whereas internal flow refers to flow inside objects. Air flowing up a flat wall or around a horizontal steam pipe are both examples of external flows encounterd in natural convection.

In natural convection external flows are caused by buoyancy forces.Applied pressure gradients cause external flow in forced convection.