Pressure, Pressure Head, and Hydraulic Head Formulation
Within COMSOL Multiphysics, all equations are formulated in terms of pressure. When working with the Subsurface Flow Module, you have the flexibility to define pressure, hydraulic head, or pressure head as boundary conditions for your model. Similarly, you can employ any of these three variables throughout your analysis and postprocessing. It is important to note that these formulations are interchangeable, and the software internally handles the conversion between them. For instance, if you specify a boundary condition in terms of pressure or hydraulic head, the software seamlessly translates it into the corresponding pressure variable for computation.
Hydraulic head (also known as piezometric head), H(m), and pressure head, Hp(m), relate to pressure p as
where ρ is the fluid density, g denotes the acceleration of gravity, and D is the elevation or elevation head. When defined as vertical elevation, the horizontal gradients in D equal zero and vertical gradients in D equal one. Turning off gravity effects sets the elevation D to zero, and the hydraulic head equals the pressure head.