The effect of bed roughness on scalar fluctuations in turbulent boundary layers

The effect of bed roughness on an iso-kinetic chemical plume released into the logarithmic region of the bed boundary layer of an open channel flow is examined. The bed roughness is varied systematically to generate flows in the smooth (painted polyethylene sheets), transitional rough (2.5-mm and 11.5-mm gravel beds), and fully rough (21.0-mm gravel bed) regimes. Concentration fields are measured in horizontal planes using the planar laser-induced fluorescence (PLIF) technique. The time-averaged concentration follows a self-similar Gaussian transverse profile for each bed roughness. The plume width grows as a power law function of the streamwise distance (i.e., r / xn; with n 0.75 near the source and n 0.5 farther downstream). As the bed roughness increases, the variance of the fluctuations is smaller and decreases faster. Near the plume source, the transverse profiles of the standard deviation of the scalar fluctuations are self-similar and Gaussian. For the two rougher beds, the transverse distribution of the standard deviation transitions downstream to a bi-modal profile. The probability density functions (PDFs) of the concentration fluctuations near the plume source are highly skewed distributions. Increased bed roughness accelerates the approach of the PDF towards a Gaussian distribution.