Surface deformation and friction properties of free surface flows over a train of cylindrical ribs*

We report experimental measures about the surface deformation and the friction factor of turbulent, subcritical & quasi-uniform flows facing an array of half-cylinders at the bottom of a horizontal flume. A train of undular jumps appears on the free surface, whose relative amplitude depends on both, the Froude (Fr) and Reynolds (Re) number. From this relationship, an interesting decreasing scaling emerges between this amplitude and the relative depth of the flow, involving a characteristic length-scale. This length scale suggests that the amplitude of the jumps is the result of a competition between the inertial and viscous forces of the flow. Surprisingly, the wavelength of this train of jumps does not depend on Re, nor the Fr number. The friction factor resulting from this study, follows a form similar to the classical Nikuradse's law. This coefficient it is also related with the amplitude of these jumps, suggesting that the deformations at the surface seems to contribute to the overall friction of the flow.