INFLUENCE OF PERMEABILITY ON NEAR-BED TURBULENT FLOW CHARACTERISTICS IN GRAVEL BED STREAM

摘要

This work aims at quantifying experimentally the influence of stream bed permeability on near-bed turbulent flow characteristics in open channel flows. Experiments were carried out in a 15 m long, 0.90 m wide and 0.80 m deep open channel flume. Two types of bed were prepared by laying uniform gravels of median diameter,d_(50) = 40 mm over the flume bottom. The rough impermeable-bed was composed of single layer of gravels, whereas the permeable bed was prepared by laying five layers of gravels on the flume bottom uniformly. Each test consisted of two runs at two different Reynolds number (Re. - 67854 and Re_2 = 84817). A non-dimensional parameter namely roughness geometry function is used as a multiplier of intrinsic time-averaged flow quantities. Nortek's Vectrino four-receiver Acoustic Doppler velocimeter (ADV) probe was used to capture the time-averaged velocity components. Data filtration were performed by acceleration threshold method ensuring velocity power spectra to a satisfactory fit with the Kolmogorov "-5/3 scaling-law " in the inertial sub-range. Experimental results revealed significant effects of stream bed permeability on near-bed flow resistance. The velocity distributions followed the log-law of the wall above the crest level, interestingly below the crest level it deviated and followed polynomial law in permeable bed. The near-bed RSS distributions exhibit an apparent increase due to increasing level of turbulence fluctuations in permeable bed and well in agreement with the results obtained by Manes et.al. (2009). No significant difference is observed in turbulence intensity in stream wise and vertical directions .A close observation in rough permeable bed shows the increment in near-bed eddy size. This increment in near-bed eddy size is due to the increasing level of turbulence fluctuations close to the stream bed in permeable bed.