These elements may be used in 2-D/3-D dynamic analysis in cartesian coordinates only. In case of 2-D elements, the number of nodes is variable (4-9). If node 4 is specified to be zero then the element is assumed triangular and node 7 also must be specified to be zero. Inside the fluid domain, these elements has one degree of freedom per node. This degree of freedom is the value of the potential function 
at the nodes. At the free surface the element has extra NSD translational degrees of freedom to accommodate the free surface motion. This element enforces the continuity (equilibrium in solids) equation along the mesh domain. The applied forces represent the discharge (unit volume per second) at this node. The positive discharge is in-discharge and the negative one is out-discharge. For global equilibrium the in-discharge must equal to the out discharge. At the boundaries, only the normal velocity may be specified because the tangential velocity does not affect the discharge. The nodal equilibrium is satisfied if the sum of discharge at the node equals the applied discharge. The boundary nodes that have unknown in/out discharge must have essential boundary condition, either or 
specified at the nodes. The corresponding reaction will be the unknown discharge at the node.