Author(s): Stephan Spiller; Nils Ruther; Benjamin Baumann
Linked Author(s): Nils Ruther
Keywords: Experiment; Particle Image Velocimetry; Artificial armor layer; Coherent flow structures
Abstract: The understanding of flow structures over static armor layers is essential for the development of modern sediment transport models, including functions for the influence of particle hiding and exposure. Coherent turbulent flow structures affect the incipient motion of single particles in a way that is still difficult to estimate. Furthermore, they can cause suspension of fine sediments that deposit elsewhere. Therefore it is important to acquire additional experimental data of flow structures close to the streambed and to draw conclusions on morphodynamic effects of those processes. The authors of the present study earlier developed a new technique of producing artificial copies of a gravel streambed. Such a channel bed is manufactured in a way that the surface structure is hardly distinguishable from the original. At the same time, it can be produced and colored in a way that is highly advantageous for Particle Image Velocimetry (PIV) measurements. This artificial streambed in combination with special camera optics enables high quality stereoscopic PIV measurements and excellent picture quality. Disturbing laser reflections from the channel surface can be reduced to a minimum. Thus, the recorded field of view provides data very close to the streambed. Experiments were performed in a straight hydraulic flume under free surface conditions. The unique outlet geometry of the flume allows positioning the laser optics on the downstream end, outside the flume. The vertical laser sheet is pointing upstream through a glass window. Therefore, the free water surface did not have to be penetrated in any kind to perform the velocity measurements. This experimental setup has therefore multiple advantages for the measurement of different flow situations, both steady and unsteady. The present study however concentrates on the turbulent flow field over an artificial static armor layer for several different steady discharges. A closer look at the double averaged velocity profiles, as well as the distribution of form-induced stress give an insight in the flow characteristics over the armor layer. Quadrant maps can be drawn and directly compared. The results show that similar patterns in the distribution of form induced stresses appear for a wide spectrum of discharges.