Computing streamfunction and velocity potential in a limited domain of arbitrary shape. Part I: Theory and integral formulae

The non-uniqueness of solution and compatibility between the coupled boundary conditions in computing velocity potential and streamfunction from horizontal velocity in a limited domain of arbitrary shape are revisited theoretically with rigorous mathematic treatments. Classic integral formulas and their variants are used to formulate solutions for the coupled problems. In the absence of data holes, the total solution is the sum of two integral solutions. One is the internally induced solution produced purely and uniquely by the domain internal divergence and vorticity, and its two components (velocity potential and streamfunction) can be constructed by applying Green’s function for Poisson equation in unbounded domain to the divergence and vorticity inside the domain. The other is the externally induced solution produced purely but non-uniquely by the domain external divergence and vorticity, and the non-uniqueness is caused by the harmonic nature of the solution and the unknown divergence and vorticity distributions outside the domain. By setting either the velocity potential (or streamfunction) component to zero, the other component of the externally induced solution can be expressed by the imaginary (or real) part of the Cauchy integral constructed using the coupled boundary conditions and solvability conditions that exclude the internally induced solution. The streamfunction (or velocity potential) for the externally induced solution can also be expressed by the boundary integral of a double-layer (or single-layer) density function. In the presence of data holes, the total solution includes a data-hole-induced solution in addition to the above internally and externally induced solutions.