Search

The paper examines the flow through a highly porous canopy patch made of streamwise-oriented thin plates arranged in a staggered configuration and placed in a rough-bed open channel. This patch geometry contrasts with the patches made of bluff bodies, which are nearly exclusively used in the literature. Particle Image Velocimetry was used to measure the flow upstream, within and downstream of the patch. The canopy patch has the effect of drastically reducing the turbulence level of the incoming flow, especially the turbulence shear stress, which is reduced by 85%. Spectral analysis of the velocity shows that the reduction in turbulent kinetic energy occurs at all length scales. Yet, at the entrance of the patch, the energy from the smallest scales up to the scale of the water surface increases. This suggests a spectral shortcut mechanism by which the large-scale structures of the incoming flow are disintegrated by the group of plates instead of decaying through the energy cascade. The increased small-scale turbulent energy then dissipates through the patch. [ABSTRACT FROM AUTHOR]

Flow experiments are conducted in a two-stage compound open-channel, with varying intensity of the velocity difference between the main channel (deep part) and the floodplain (shallower part), using a large-scale free surface PIV technique (LS-PIV). For all investigated flows, a shear layer develops at the interface between main channel and floodplain, characterised by a peak of turbulent shear stress. Yet, two different kinds of shear layer could be identified. The first kind is characterised by the presence of large-scale quasi-periodic structures of Kelvin-Helmholtz type which are growing in downstream direction, whereas the second kind is characterised by smaller-scale vortical structures without quasi-periodicity and which do not grow in downstream direction. The shear parameter λ = (U 2 - U 1) / (U 2 + U 1) , where U 1 and U 2 are defined as the velocities outside the shear layer, is identified as a key parameter to distinguish between these two types of shear layers, supporting a result from Proust et al. (Water Resour Res 53: 3387–3406, 2017). A physical interpretation of the λ -criterion is proposed, based on the inhibiting effect of ambient turbulence (the turbulence level outside the shear layer) on the emergence of Kelvin-Helmholtz structures. Accordingly, the threshold value of λ , above which large-scale structures can develop, is dependent on the level of the ambient turbulence. Despite their very different behaviours, the two types of shear layer have the same efficiency to generate turbulent shear stress for a given velocity difference across the shear layer, except for λ -values close to the threshold value. [ABSTRACT FROM AUTHOR]

In steep rivers, sediment is often transported over immobile cobbles and boulders. Previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains. Here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine‐sediment bed, modeled as a patch of hemispheres, modify the local hydrodynamics near the fine‐sediment bed. Five protrusion levels defined by P=k/R= {0%, 20%, 40%, 60%, 80%} were investigated, where k is the protrusion height and R the radius. For small protrusion (P=20%), enhanced shear stress and turbulence intensity on the fine‐sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine‐sediment bed. Moreover, sweep events generated near the top of the hemispheres reach the fine‐sediment bed. For large protrusions (P ≥ 60%), the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops. The remaining turbulence near the fine‐sediment bed, although enhanced by the wakes generated by the hemispherical caps, is quasi‐isotropic. The transition between these two distinct near‐bed flow regimes is found to be around P=40%, corresponding to the protrusion levels observed by Grams and Wilcock, https://doi.org/10.1002/2013JF002925) above which the erosion of fine sediment ceases. Key Points: The local hydrodynamics over fixed small grains for varying protrusion of immobile coarse grains, modeled by hemispheres, is investigated via particle image velocimetryA strong increase of the local shear stress for small protrusion levels around 20% is observed, which Raupach's drag partition model cannot predictFor protrusion levels higher than about 50%, a reduction of the turbulent normal and shear stresses as well as the latter's quadrant distribution is observed, all suggesting reduced grain mobility [ABSTRACT FROM AUTHOR]

The roughness sublayer in a turbulent open-channel flow over a very rough wall is investigated experimentally both within the canopy and above using particle image velocimetry by gaining complete optical access with new methodologies without disturbing the flow. This enabled reliable estimates of the double-averaged mean and turbulence profiles to be obtained by minimizing and quantifying the usual errors introduced by limited temporal and spatial sampling. It is shown, for example, that poor spatial sampling can lead to erroneous vertical profiles in the roughness sublayer. Then, in order to better define and determine the roughness sublayer height, a methodology based on the measured spatial dispersion is proposed which takes into account temporal sampling errors. The results reveal values well below the usual more ad hoc estimates for all statistics. Finally, the double-averaged mean and turbulence statistics in the roughness sublayer are discussed. [ABSTRACT FROM AUTHOR]

The turbulent characteristics of the neutral boundary layer developing over rough surfaces are not well predicted with operational weather-forecasting models. The problem is attributed to inadequate mixing-length models, to the anisotropy of the flow and to a lack of controlled experimental data against which to validate numerical studies. Therefore, in order to address directly the modelling difficulties for the development of a neutral boundary layer over rough surfaces, and to investigate the turbulent momentum transfer of such a layer, a set of hydraulic flume experiments were carried out. In the experiments, the mean and turbulent quantities were measured by a particle image velocimetry (PIV) technique. The measured velocity variances and fluxes $${(\overline{{u_{i}^{\prime}}{u_{j}^{\prime}}})}$$ in longitudinal vertical planes allowed the vertical and longitudinal gradients (∂/∂ z and ∂/∂ x) of the mean and turbulent quantities (fluxes, variances and third-order moments) to be evaluated and the terms of the evolution equations for ∂ e/∂ t, $${\partial \overline{u^{\prime 2}}/\partial t}$$, $${\partial \overline{w^{\prime 2}}/\partial t}$$ and $${\partial \overline{{u^{\prime}}{w^{\prime}}}/\partial t}$$ to be quantified, where e is the turbulent kinetic energy. The results show that the pressure-correlation terms allow the turbulent energy to be transferred equitably from $${\overline{{u^{\prime}}^{2}}}$$ to $${\overline{{w^{\prime}}^{2}}}$$. It appears that the repartition between the constitutive terms of the budget of e, $${\overline{{u^{\prime}}^{2}}}$$, $${\overline{{w^{\prime}}^{2}}}$$ and $${\overline{{u^{\prime}}{w^{\prime}}}}$$ is not significantly affected by the development of the rough neutral boundary layer. For the whole evolution, the transfers of energy are governed by the same terms that are also very similar to the smooth-wall case. The PIV measurements also allowed the spatial integral scales to be computed directly and to be compared with the dissipative and mixing length scales, which were also computed from the data. [ABSTRACT FROM AUTHOR]

The biomass dynamics of epilithic biofilm, a collective term for a complex microorganism community that grows on gravel bed rivers, was investigated by coupling experimental and numerical approaches focusing on epilithic biofilm-flow interactions. The experiment was conducted during 65 days in an artificial rough open-channel flow, where filtered river water circulated at a constant discharge. To characterize the effect of near-bed turbulence on the chronic detachment process in the dynamics of epilithic biofilm, local hydrodynamic conditions were measured by laser Doppler anemometry and turbulent boundary layer parameters inferred from double-averaged quantities. Numerical simulations of the EB biomass dynamics were performed using three different models of chronic detachment based upon three different descriptors for the flow conditions: Discharge Q, friction velocity u*, and roughness Reynolds number k +. Comparisons of numerical simulation results with experimental data revealed chronic detachment to be better simulated by taking the roughness Reynolds number as the external physical variable forcing chronic detachment. Indeed, the loss of epilithic matter through the chronic detachment process is related not only to hydrodynamic conditions, but also to change in bottom roughness. This suggests that changes in the behavior and dimensions of river bed roughness must be considered when checking the dynamics of epilithic biofilm in running waters. [ABSTRACT FROM AUTHOR]

This paper is devoted to an experimental characterization of the collapse under buoyancy forces of the turbulent motions in the wake of a sphere in a linearly stable saline stratification. This phenomenon is part of the transition between two commonly recognized regions of the wake, the near wake and the far wake. It is shown here that the wake evolves in four distinct stages: the three-dimensional near wake, the collapse, a transition region, and finally the far wake. Hot-film measurements in the near wake and the region beyond show that initially the mean defect velocity decreases as in a nonstratified fluid, but then increases briefly before continuing to decrease again. The short period of mean defect-velocity increase or mean-flow acceleration characterizes the collapse and results in the higher levels of kinetic energy found in stratified wakes as compared to nonstratified wakes. The far-wake region of the flow was studied by a digital particle image velocimetry technique yielding decay laws for the defect velocity as well as the vertical component of vorticity for both laminar and turbulent wake regimes. The decay of these far-wake quantities was found not to depend on the regime considered, implying that the decay rates are independent of the collapse. However, it was found that the characteristic longitudinal scales do depend on the Froude number, implying a selective memory of the initial conditions in spite of collapse. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]