ABSTRACT
The effect of free surface perturbation on a fully developed smooth open channel flow is investigated using particle image velocimetry (PIV). The perturbation is generated by using light floater boards placed on the free surface at the inlet section of the flume and the measurement is taken at a downstream location of thirty-five depths from the disturbance. As a base case, a fully developed condition is achieved in the flume at a constant Reynolds number, Re_H = 3.7×104 or an aspect ratio (width/depth, b/H) of 7 without any perturbation. The same experiment is then repeated with the floater boards of two different lengths. The results indicate that the floaters have a significant influence in the flow characteristics adjacent to the free surface at the far-stream measurement location. The characteristics of the outer layer varies with the level of perturbation. A dip in mean velocity profile is observed in the presence of the floater boards and the magnitude of the dip tends to increase with an increase in the floater length. The boundary layer wake parameter and the Reynolds shear stress become negative in the case of the longer floater. Joint probability distribution of velocity fluctuations and the contours of covariance integrand near the free surface (y/H = 0.95) shows a shift of Q2 and Q4 events to Q1 and Q3 with the inclusion of the floaters. It implies that the inverted boundary layer developed initially on the floaters are shed as shear layers that sustain their characteristics even in a far downstream location and influence the flow at the measuring station.
