In fluid mechanics and related fields, the term "free surface" refers to the boundary between a fluid (such as water or air) and another medium (such as air or a solid container) where the fluid is subjected to atmospheric pressure or pressure from the surrounding medium. This free surface is not constrained by any solid walls or surfaces and can move or deform freely.
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A free surface is a fluid boundary whose position is part of the solution and whose traction is prescribed by the adjoining medium. Its motion obeys a kinematic boundary condition, while its pressure or stress supplies a dynamic boundary condition.