6 research outputs found
Flow characteristics in gravel bed rivers: an experimental investigation
No full textThe characterization of the kinematic characteristics of flow in gravel-bed rivers is one of the most important problems. In the present paper the streamwise velocity and turbulence intensity profiles are investigated on the basis of data collected in a straight laboratory gravel-bed flume. In order to take into account spatial heterogeneities, the double-averaged velocities are considered for the anal-ysis. Results show that, because of the presence of gravels, all the streamwise velocity profiles show an inflection point. Thus, the experimental profiles can be interpreted by the mixing layer scheme and considered as a composition of two parts of constant velocity separated by a confined interme-diate region (mixing layer) containing the inflection point. The thickness of the mixing layer varies passing from the banks to the channel axis
Gravel-bed rivers: roughness surface identification and effect on flow characteristics
No full textRiverbeds are often covered with gravels. The characterization of the gravel bed is very complex, depending on many factors such as the gravels’ shape, morphology and spatial distribution. The presence of gravels determines a significant impact on flow field (Ferraro et al., 2016). The evaluation of flow velocity and turbulence characteristics in natural rough beds is a complex subject, still poorly understood despite the effort of several researchers, requiring both theoretical and experimental analyses. On one hand the bed roughness exhibits a significant spatial variability, on the other hand the presence of gravels determines a significant effect on flow velocity distribution, especially affecting it in the near-bed flow zone and determining the formation of coherent turbulence structures. How to reasonably quantify gravel bed surface roughness is still an open question and different methods have been proposed in literature (among others Nikora et al., 1998; Aberle and Nikora, 2006). It is important to understand the turbulent flow characteristics by resolving the spatial heterogeneities
Influence of Bed Roughness on Flow and Turbulence Structure Around a Partially-Buried, Isolated Freshwater Mussel
No full textThe present study uses eddy-resolving numerical simulations to investigate how bed roughness affects flow and turbulence structure around an isolated, partially-buried mussel (Unio elongatulus) aligned with the incoming flow. The rough-bed simulations resolve the flow past the exposed part of a gravel bed, whose surface is obtained from a laboratory experiment that also provides some additional data for validation of the numerical model. Results are also discussed for the limiting case of a horizontal smooth bed. Additionally, the effects of varying the level of burial of the mussel inside the substrate and the discharge through the two mussel siphons are investigated via a set of simulations in which the ratio between the median diameter of the (gravel) particles forming the rough bed, d50, and the height of the exposed part of the mussel, h, varies between 0.10 and 0.22. The increase of the bed roughness is associated with a strong amplification of the turbulence kinetic energy in the near-wake region. Increasing the bed roughness and/or reducing h intensifies the interactions of the eddies generated by the bed particles with the base and tip vortices induced by the active filtering and by the mussel shell, respectively, which, in turn, induces a more rapid dissipation of these vortices. Increasing the bed roughness also reduces the strength of the main downwelling flow region forming in the wake. The strong downwelling near the symmetry plane is the main reason why the symmetric wake shedding mode dominates in the smooth bed simulations with negligible active filtering. By contrast, the anti-symmetric wake shedding mode dominates in the simulations conduced with a high value of the bed roughness. The mean streamwise drag force coefficient for the emerged part of the shell and the dilution of the excurrent siphon jet increase with increasing bed roughness
Influence of Bed Roughness on Flow and Turbulence Structure Around a Partially‐Buried, Isolated Freshwater Mussel
No full textThe present study uses eddy-resolving numerical simulations to investigate how bed roughness affects flow and turbulence structure around an isolated, partially-buried mussel (Unio elongatulus) aligned with the incoming flow. The rough-bed simulations resolve the flow past the exposed part of a gravel bed, whose surface is obtained from a laboratory experiment that also provides some additional data for validation of the numerical model. Results are also discussed for the limiting case of a horizontal smooth bed. Additionally, the effects of varying the level of burial of the mussel inside the substrate and the discharge through the two mussel siphons are investigated via a set of simulations in which the ratio between the median diameter of the (gravel) particles forming the rough bed, d50, and the height of the exposed part of the mussel, h, varies between 0.10 and 0.22. The increase of the bed roughness is associated with a strong amplification of the turbulence kinetic energy in the near-wake region. Increasing the bed roughness and/or reducing h intensifies the interactions of the eddies generated by the bed particles with the base and tip vortices induced by the active filtering and by the mussel shell, respectively, which, in turn, induces a more rapid dissipation of these vortices. Increasing the bed roughness also reduces the strength of the main downwelling flow region forming in the wake. The strong downwelling near the symmetry plane is the main reason why the symmetric wake shedding mode dominates in the smooth bed simulations with negligible active filtering. By contrast, the anti-symmetric wake shedding mode dominates in the simulations conduced with a high value of the bed roughness. The mean streamwise drag force coefficient for the emerged part of the shell and the dilution of the excurrent siphon jet increase with increasing bed roughness
