Coupling of U-Tube and Shear Layer Oscillations in an Interconnected Flow Compartment

Results of experiments conducted in a 2?m high flume at large Reynolds numbers are reported in this paper. The flume was partitioned into two compartments. Flow entered the bottom of the upstream test compartment as a wall jet, at jet Reynolds number ranging from 11,000 to 170,000. Periodic oscillations of the free surface in the two compartments resembling the oscillatory flow in a liquid-filled U-tube, and large coherent structures formed above the potential core of the wall jet were observed. Coupling of the U-tube oscillations and vortex shedding is attributed to fluid-dynamic and fluid-resonant feedback processes. For test compartment length, Lc = 0.8?m, fluid-resonant feedback was found to be dominant, and the shear layer was observed to oscillate at the natural frequency of the two-compartment, U-tube system. The observed U-tube oscillations are initiated by the oscillations of the shear layer at a frequency equal to the subharmonic component for the U-tube. The flow oscillations were generally weaker for Lc = 1.2 and 2.0?m with oscillation frequencies governed by fluid-dynamic feedback, verified from a comparison with the results from a previously reported study.     

Laboratory Experiments on the Failure of Coarse Homogeneous Sediment Natural Dams on a Sloping Bed

We present the results of a systematic series of 168 laboratory experiments that examine the critical conditions for the failure of landslide dams, which obstruct the course of mountain rivers. The experiments were carried out by using three different sediment materials with a quasiuniform grain size distribution and with a flume bed slope angles that ranged between 0° and 5.71° (0–10%). Three main typologies of dam failure were observed for increasing values of the dam’s downstream-face angle: (1) overtopping; (2) headcutting, which led to the formation of an erosion channel on the dam’s downstream face that progressively migrated up to the dam crest; and (3) initial slide of large part of superficial layer of the dam’s downstream face, which was followed by headcutting. The experiments focused on the second type of failure to provide a safety criterion based on the upstream reservoir level. The quantities that govern this phenomenon have been identified and a functional relationship is proposed based on the dimensional analysis and curve fitting of the minimum level of the upstream reservoir leading to dam failure. A comparison of both the experimental findings and field data that are available in literature shows that the proposed relationship generally provides a conservative estimate for landslide dams in which comminution effects (due to fragmentation of material in the landslide process) are negligible (dam volume lower than 106?m3). Finally, the applicability to larger volume landslide dams or to geometric configurations other than those investigated is discussed.