The exchange of dissolved matter between a groyne field and a main stream influences the transport and distribution of a pollutant cloud in a river. In forecasting models, groyne fields are represented as dead zones with effective properties like exchange coefficients and exchanging volume. Despite its relevance for such practical applications, little research has been done on the exchange process between a groyne field and the main stream itself. Therefore, this study is aimed at examining this exchange process and validating the dead-zone prediction model, which treats the exchange process as a first order system. A schematized physical model of a river with groynes was built in a laboratory flume. The exchange process was visualized quantitatively with dye in adjacent groyne fields. In order to couple the exchange process to the velocity field, particle tracking velocimetry measurements were performed. Two different types of exchange were observed. First, exchange takes place via the mixing layer that ...

The motion of dense particles in a turbulent gas flow has been studied by means of numerical simulations. The single‐phase turbulent pipe flow was modelled using Direct Numerical Simulation and Large Eddy Simulation. At tube Reynolds numbers of 5300, 18300 and 42000 particles with dimensionless relaxation times ranging from 5 to 10⁴ were released. Assuming the system to be dilute, the characteristics of particle dispersion, deposition and concentration distribution were studied under various conditions of gravity and lift. This study shows that for small particles the deposition process is governed by the properties of the near‐wall layer where the wall‐normal turbulence intensity is low, while for large inertial particles turbulent dispersion determines the chances for particles to hit the tube wall. The motion of the latter particles appears to scale properly with the Lagrangian integral time scale of the turbulence. Furthermore we demonstrated the segregation of particles towards the wall, as a result of particle‐turbulence interaction.

This research is aimed at finding efficient alternative designs, in the physical, economical, and ecological sense, for the standard groynes as they are found in the large rivers of Europe. In order to test the effects of various groyne shapes on the flow in a groyne field, experiments were performed in a physical model of a schematized river reach, geometrically scaled 1:40. Four different types of schematized groynes were tested, all arranged in an array of five identical groyne fields, i.e., standard reference groynes, groynes with a head having a gentle slope and extending into the main channel, permeable groynes consisting of pile rows, and hybrid groynes consisting of a lowered impermeable groyne with a pile row on top. Flow velocities were measured using particle tracking velocimetry. The design of the experiment was such that the cross-sectional area blocked by the groyne was the same in all cases. Depending on the groyne head shape and the extent of submergence variations in the intensity of vort...

Transverse exchange of momentum between the channel and the floodplain in straight uniform compound channel flow is considered in this paper. This process results in the so-called “kinematic effect,” a lowering of the total discharge capacity of a compound channel compared to the case where the channel and the floodplain are considered separately. The mechanisms responsible for the momentum exchange are considered. The transverse shear stress in the mixing region is modeled using a newly developed effective eddy viscosity concept, that contains: (1) the effects of horizontal coherent structures moving on an uneven bottom, taking compression and stretching of the vortices into account and (2) the effects of the three-dimensional bottom turbulence. The model gives a good prediction of the transverse profiles of the streamwise velocity and the transverse shear stress of the flood channel facility experiments. Characteristic features of the lateral profile of the eddy viscosity are also well predicted qualita...

The development of two shallow mixing layers with different water depths is analyzed experimentally by means of laser Doppler anemometry. The experiments show that bottom friction plays an important role in the growth of the mixing layer width and in the strength and dimensions of the large quasi two-dimensional turbulence structures therein. It is found in this study that the initial growth rate of both mixing layers is similar to what has been found for deep water plane mixing layers. Further downstream the reduction of the growth rate can be ascribed to the decrease of the velocity difference between the two ambient streams in combination with the suppression of the growth of the large turbulence structures. In the most shallow mixing layer considered, the influence of the bottom friction is dominant, impeding the further growth of the mixing layer width. It is demonstrated that the reduced mixing layer growth is related to a loss of coherence in the large turbulence structures. This loss of coherence also reduces the characteristic length-scale that establishes the lateral mixing of matter and momentum in the mixing layer. Eventually the water depth becomes the dominant length scale that determines the characteristic motion of the turbulence in that case. From the energy density spectra of the turbulence fluctuations and from the phase relation between the two velocity components in the horizontal plane it is concluded that large structures contribute most to the exchange of momentum in the mixing layer and thus to the Reynolds-stresses.