Aspects of the Laplace Transform Isotherm Migration Method
Radford, Linda Elisabeth
There are many different methods available for the solution of the heat equation and the choice of which to use is dependent upon the nature of the problem and the specific regions of the domain where the temperature is required. In the case of melting or freezing problems it is usual for the position of the boundary, at which change of physical state (phase change) occurs, to be of greater interest than the temperature at particular points. Again there are several solution methods enabling the tracking of the moving interface between the physical states of the material. For this work we begin with the isotherm migration method, which first appeared in the 1970s but is less frequently cited now. We first solve problems in one dimension with no phase change using the isotherm migration method, which is in itself new work, since all references we have found allude to it as a tool for the solution of phase change problems. We test the method using a variety of examples to explore the difficulties and challenges it produces, and we find it to be robust and tolerant of errors. We then combine it with the Laplace transform method, a well-established technique for solving ordinary and partial differential equations, in which the number of independent variables is reduced by one. The solution is then transformed back into the time domain using a suitable numerical process. The Laplace transform isotherm migration method is a new process, not mentioned previously to our knowledge, and it produces results which are comparable with the isotherm migration method. The new process is applied to one-dimensional phase change problems,where we find that due to the mathematics at the phase change boundary, we are required to make a modification to the usual manner of operating the Laplace transform. This is novel as far as we are aware. Our method is applied to a variety of problems and produces satisfactory results. We then move on to a two-dimensional setting where we find the situation to be much more complex and challenging, as it requires interpolation and curve-fitting processes. Finally we examine the possiblity of speeding up the calculation time using the Laplace transform isotherm migration method by setting problems in a parallel environment and using an MPI platform. This has not been previously attempted and we are able to show a measure of success in our objective.