In hydrodynamic problems that include a fluid free-surface boundary, it is common practice to define a system of axes with the origin in the mean free surface of the fluid, as shown in Figure 1.1: Definition of Axis Systems. In Aqwa this is referred to as the fixed reference axes (FRA), global axes, or OXYZ, which is a fixed right handed axis system with the origin in the mean free surface and Z-axis pointing vertically upwards.
