STEADY GRADUALLY ACCELERATING FLOW IN A SMOOTH OPEN CHANNEL

Laboratory experiments on the structure of spatially accelerating flows in a smooth open channel are reported. The influences of acceleration (i) on the velocity profiles; (ii) on the turbulence intensity profiles; and (iii) on the longitudinal evolution of the bottom shear stress are investigated. The most important conclusions of the study can be summarized as follows: a. The velocity distributions cannot be represented entirely by the universal log-law. However, this law seems to follow the data very close to the wall, up to about y/d = 0.02 approximately 0.05. Higher in the flow, the data increasing deviate from this law. In the external layer, the equilibrium state of the flow was tested: some near-equilibrium flow situations exist, but mostly the flow was out of equilibrium; b. The generalization of Coles' model was tried, resulting in correlation of the wake parameter, PI, and the dimensionless pressure gradient parameter, beta, for beta < -0.5; c. The acceleration of the mean velocity field leads to reduction of the turbulence intensity over the entire depth; d. Clauser's method for the calculation of the friction velocity is applicable only to the measurements in the layer close to the wall (y/d < 0.02 approximately 0.05); Truckenbrodt's relation predicts reasonably well the friction velocity; e. The bottom stress intensity ratio decreases in the flow direction.