Investigating the impact of local and convective term suppression in numerical solution of unsteady supperitical free surface flow

The accurate computer simulation of flow in man-made natural channels is of great importance in design of hydraulic structures. The numerical simulation of unsteady super-critical flow are quite different due to the required boundary conditions. As a result, one the resort to great simplifications if he or she to make the numerical algorithm invariant with response to required boundary conditions. Gradual suppression of either local or convective (spatial) acceleration and / or both is considered one possible remedy to utilize the Same algorithm for both sub-and supercritical regimes. Two well-established softwares namely; MILKE11 and HEC-RAS use the cited simplifications to be able to utilize the same algorithm foe noth regimes . while suppression of convective acceleration (implemented in MIKLE11) reserves the hyperbolicity of original partial differential equations. Suppression of local as well as covective acceleration (implemented in HEC-RAS) converts the original PDEs into a parabolic one. The consequences of such suppression on numerical solution of unsteady open channel flow is a poorly understood subject. In this paper , the impact pf suppessing either convective term and / or both local and convective acceleration terms is investigated in some detail. For this purpose, a few numerical experiments are designed and the results of a fully dynamic wave model so called FLDWAV are compared and contrasted with the results obtained from both MIKE11 and HEC-RAS for a variety of scenarios including prismatic, non-prismatic channel cross sections with or discharge hydrographs obtained from various algorithms and scenarios. The results of numerical experiments show that the cited simplification introduces a range of solutions bounding fully dynamic solution with HEC-RAS results as the lower limit and MIKE11 results as the upper limit. In conclusion , one has to be every cautious when using such simplification in numerical modeling of river flow.

Keywords:

Local term , Convective term , Flow regime , de Saint-Venant equations , MIKE11 , HEC-RAS , FLDWAV