Rigorous exact solutions in engineering are essentially non existent In all engineering problems one has to resort to some kind of approximations in which the physics of the problem is approximated and formulated in a format tractable by analytical or numerical means. This kind of approximation is termed MODELING.
In the quest for more accurate, but yet, sufficiently simple approaches to the solution of two phase flow problems MODELING is of utmost importance. The term "Mechanistic Modeling" was adopted for modeling where the physical phenomenon Is approximated by taking into consideration the most important processes, neglect other less important effects that can complicate the problem but do not add considerably to the accuracy of the solution. "Mechanistic Modeling" should be sufficiently close to the natural phenomenon as the flow pattern involved should not be overlooked. Yet it will be sufficiently simple so that the solution is amenable with reasonable analytical or numerical efforts.
Specific examples which relate to recent developments in Mechanistic Models for two phase flow such as the prediction of flow pattern, stratified flow and the stability of stratified flow, slug behavior in pipes, slug length distribution, severe slugging phenomenon and splitting in a tee junction will be presented.
In the solution of engineering problems one can identify four major levels of approaches:
Exact or rigorous approach.
Approach via Modeling
Approach via Mechanistic modeling
The division among these four options is not always clearly defined and it may be a relative definition depending on the specific terminology used in any specific area.
The term rigorous or exact solution in fluid mechanics is used for the exact solution of the Navier-Stokes equation.