ANALYTICAL SOLUTIONS OF SAINT VENANT EQUATIONS DECOMPOSED IN FREQUENCY DOMAIN

摘要

The Saint Venant equations are often merged into a single equation for being easily solvable. By doing so, the most general form of the single equation is formulated in this study if all terms are preserved. As a result, the generalized model (GM) results and contains several unexpected nonlinear terms that may impose a great limitation on model analyses. In order to identify these redundant terms, this paper discusses the employment of the linearized Saint Venant equations (LSVE) governing subcritical flow in prismatic channels. The LSVE is solved by a new procedure that separates, in the Laplace frequency domain, the governing equation of water depth from that of flow velocity and thus enables us to consider two independent equations rather than two coupled ones. This allows us to obtain analytical solutions in a much easier way. Comparisons of the response functions of LSVE and the linearized generalized model (LGM) show that the two equations provide identical solutions if the redundant terms embedded in LGM are neglected. It then follows that the response function of LGM can be utilized as a replacement for solving the analytical solution of LSVE that is valid for prismatic channels of any shape. Validity of the analytical solution is verified by repeatedly comparing with the corresponding numerical solutions of finite difference method or Crump's algorithm, depending on whether the flow domain is finite or semi-infinite. It is clearly demonstrated in this paper that LSVE serves as an excellent substitution for LGM whose variants have been employed for quite a few years.