Examination of Entrance Length in Open Channels by CFD Modelling

In the present paper, an attempt has been made to investigate the length required for development of velocity profiles in open channels using computational fluid dynamics approach. The water gets retarded after entering into a channel due to boundary friction effects. A number of parameters such as roughness, Froude’s number (Fr), Reynolds number (Re) etc. may have their impact on the flow development in open channels. In the present study, a three dimensional numerical code FLUENT has been used to explore the impact of roughness and Froude’s number on entrance length. This CFD code solves Reynolds-Averaged Navior Stokes equations using finite volume method. A straight rectangular channel has been considered in this work. Four different roughness values and three Fr values were considered. Only sub-critical flow condition was considered in this study. The velocity contours and velocity profiles at various locations along the length of the channel were plotted to show the flow development. Once the flow distribution reached a stabilized state where no further change occurred along the length, it was concluded that the flow has fully developed. This study showed that the roughness affects the length of flow development considerably and flow establishment length decreases gradually as the roughness increases, however Froude’s number has no impact on entrance length.