For reasons of efficiency and accuracy, water wave propagation is often simulated with potential or inviscid models rather than Navier-Stokes solvers, but for wave-induced flows, such as wave-structure interaction, viscous effects are important under certain conditions. Alternatively, general purpose (CFD) can have limitations when applied to free-surface problems dealing large amplitude waves,...

Integrating factors and adjoint equations are determined for linear and nonlinear differential equations of an arbitrary order. The new concept of an adjoint equation is used for construction of a Lagrangian for an arbitrary differential equation and for any system of differential equations where the number of equations is equal to the number of dependent variables. The method is illustrated by...

Metric independent σ models are constructed. These are field theories which generalise the membrane idea to situations where the target space has fewer dimensions than the base manifold. Instead of reparametrisation invariance of the independent variables, one has invariance of solutions of the field equations under arbitrary functional redefinitions of the field quantities. Among the many inte...

Many integrable hierarchies of differential equations allow a variational description, called Lagrangian multiform or pluri-Lagrangian structure. The fundamental object in this theory is not Lagrange function but $d$-form that integrated over arbitrary $d$-dimensional submanifolds. All such action integrals must be stationary for field to solution the problem. In paper we present procedure obta...