Effects of boundary roughness on turbulence in uniform open channel flow

Boundary Roughness;Turbulence;Open Channel Flow;Vertical Turbulent Flux;Wall Similarity;Turbulent Kinetic Energy

Moller, Nicholas ; Garcia ; Marcelo H.

关键词: Boundary Roughness;    Turbulence;    Open Channel Flow;    Vertical Turbulent Flux;    Wall Similarity;    Turbulent Kinetic Energy;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/72823/Nicholas_Moller.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Bed composition plays an important role in natural streams. However, it is hard to quantify these effects in the field. Effects on turbulence and shear stresses are of interest, since these forces greatly impact the process of sediment transport and mixing in open-channel flows. In this study, using highly controlled laboratory experiments, the effects of various bed configurations on turbulence, shear stresses, and other parameters have been analyzed and quantified and the results compared to previous research. A series of experiments performed in a uniform open-channel flow across various flow and bed conditions provided the data analyzed. The Laser Doppler Velocimetry (LDV) measurement system was used in the “Wood’s Hole” flume which was configured with a smooth plastic, rough-sanded PVC and with 14mm glass marbles as the varying bed conditions. The flume was originally built at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts. The flume is 17m long and sufficient for a fully developed turbulent flow. The aim was to achieve conditions that can be defined as smooth, transitional, and fully rough. The LDV system is capable of measuring the velocity of a fluid with a very small measuring volume and a high sampling rate. Due to the fine spatial resolution of the LDV, measurements were possible not only in the near wall region but also within the viscous sublayer. Using the data from both the log law and linear viscous sublayer fits allowed for more reliable computations of shear stresses as well as a better understanding of how turbulence behaves over the full range of the flow. Previous studies have documented a phenomenon known as wall similarity: at a certain normalized distance from the bed the normalized vertical turbulent flux is relatively constant (~0.33) for any wall conditions. This phenomenon is re-examined with the current set-up, allowing for measurement improvements over the Acoustic Doppler Velocimetry (ADV) system used in the past. The results show that bed configuration plays a major role in the resulting flow characterization. However, at the same time there is a similarity in normalized vertical turbulent flux independent of the wall design.