Abstract
Large-eddy simulations (LES) of the flow over a non-uniformly roughened channel bed are carried out to study the effect of non-uniform bed roughness on turbulence driven secondary currents and turbulence statistics. The channel bed is comprised of alternating rough and smooth strips, the width of which corresponds to the water depth. The Reynolds number based on hydraulic radius and bulk velocity is 34,000. The LES are successfully validated using experimental data. The secondary flow and bed roughness have a significant effect on the streamwise velocity and second order turbulence statistics. Turbulence is enhanced over rough strips and suppressed over smooth strips. Significant lateral momentum transfer takes place due to both advection and turbulence. The bed shear stresses over the smooth strips are approximately four times less than over the rough strips a result of near bed low momentum fluid being transported from the rough strips to the smooth strips and high momentum fluid being convected from the surface towards the bed. The most significant terms in the streamwise momentum equation are quantified and discussed with regard to momentum transfer.
Highlights
Secondary currents are considered an important aspect in river engineering because they affect the primary mean flow field, the spanwise variation of bed shear stresses, the transport of bed-load and suspended sediments as well as the conveyance and mixing of dissolved matters
Good agreement between Large-eddy simulations (LES) and experimental data was found and additional turbulence statistics were extracted from the LES, presented and discussed
Turbulence is enhanced over rough strips and suppressed over smooth strips
Summary
Secondary currents are considered an important aspect in river engineering because they affect the primary mean flow field, the spanwise variation of bed shear stresses (and the bed and banks erosion), the transport of bed-load and suspended sediments as well as the conveyance and mixing of dissolved matters. Secondary currents transport low momentum fluid from the side walls towards the center of the channel, where high-momentum fluid is suppressed below the free-surface. This triggers a downflow of near surface fluid towards the bed, which results in the velocity dip observed in natural channels (a good summary of early work on this topic is found in Nezu and Nakagawa’s 1993 textbook [2]). The distribution of wall shear stresses along the wetted perimeter in an open-channel is affected by these secondary currents, resulting in local bed shear maxima wherever there is downflow of high momentum fluid and in local bed shear minima wherever there is upward movement (Figure 1). With Laser Doppler Anemometry (LDA) and an ultrasonic bedform instrument, Nezu et al [8] and Onitsuka and Nezu [9] found that the organized fluid motions and the associated sediment transport occurred intermittently on a movable plane sand bed and, after the sand ridges were formed, the secondary-current cells appeared stable in the entire channel cross section
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