Influence of inflow tripping intensity on stimulating a fully developed state in open channel flow experiments

Open channel flow is fully developed when there is no streamwise variation of flow variables and boundary layer thickness is equal to flow depth. Planar particle image velocimetry is used to study the variation of flow characteristics with different tripping intensities, keeping Reynolds number, flow depth and upstream development length constant. Trip intensity is gradually increased from zero up to a critical value at which a fully developed state is achieved, beyond which distribution of flow variables deviates from the fully developed state.

In previous studies, fully developed flow is ensured based on validation of log-law and velocity defect law. These criteria are not adequate since they only demonstrate a well-behaved boundary layer which is possible for a properly normalized developing flow. Due to uncertainty in evaluating free stream velocity, in this study the boundary layer thickness is estimated using the wall-normal distribution of Reynolds stresses and fully developed flow is achieved at the highest trip intensity that ensures boundary layer thickness equals flow depth, while retaining properties of a well-behaved boundary layer. Quadrant analysis shows a difference in boundary layer characteristics of the developing and developed flow due to the free surface.