Mixing In Open Channel Flow Research Articles

Longitudinal dispersion coefficient for mixing in open channel flows with submerged vegetation

An experimental investigation was conducted to analyze the flow and pollutant mixing characteristics in the vegetated channel, in which the velocity data was collected by micro acoustic Doppler velocimeter, and pollutant dispersion data was obtained through tracer tests in an open channel with submerged vegetation. The effects of submergence ratio (Sr = 2.0–3.5), and stem density (M = 0.4–5.9) on the vertical distribution of the stream-wise velocity and the turbulent shear stress were analyzed. The results of the flow experiments showed that the intensity of velocity deviation became larger with increasing stem density and decreasing submergence ratio. The turbulent shear stress at the exchange zone also increased with submergence ratio, as well as with increasing stem density. The calculation results of the longitudinal dispersion coefficient based on the vertical velocity profile data showed that as the submergence ratio and stem density increased, the dispersion coefficients increased linearly. This monotonic increase of the velocity-based dispersion coefficient differed from former research, in which concentration-based dispersion coefficient values approached a constant value in a higher submergence ratio of over 2. Comparison of the velocity-based longitudinal dispersion coefficients with concentration-based coefficients in this research demonstrated that the velocity-driven coefficient had a linear relation with the concentration-driven coefficients, with smaller values than the concentration-based dispersion. This difference can be explained by the fact that concentration-driven dispersion coefficient included the storage effects due to the submerged vegetation, while the velocity-driven coefficient only accounted for shear flow effects.

Modeling non-Fickian pollutant mixing in open channel flows using two-dimensional particle dispersion model

The non-Fickian particle dispersion model was developed in this study to model two-dimensional pollutant mixing in open channel flows. The proposed model represents shear dispersion using step-by-step arithmetic calculations, which consist of horizontal transport and vertical mixing steps, instead of using Fick's law. In the sequential calculations, the model directly applied the effect of vertical variations of both longitudinal and transverse velocities, whereas the Fickian dispersion model incorporates the effect of shear flow in the dispersion coefficients. Furthermore, in order to avoid the numerical diffusion errors induced by the grid tracking method of previously developed non-Fickian dispersion models, this model adopted the particle tracking technique to trace each particle. The simulation results in the straight channel show that the proposed model reproduced the anomalous mixing, which shows a non-linear increase of variance with time and large skewness coefficient in the initial period. However, in the Taylor period, the variance and skewness of the concentration curves approached the Fickian mixing. The simulation results in the meandering channel reveal that the proposed model adequately reproduced the skewed concentration–time curves of the experimental results whereas the Fickian dispersion model, CTM-2D, generated symmetrical curves. Further comparison between the simulation results and the tracer test results conducted in the Hongcheon River shows that the proposed model properly demonstrated the two-dimensional mixing without adopting Fick's law.

Effects of emergent and submerged natural vegetation on longitudinal mixing in open channel flow

An awareness of mixing processes is imperative in understanding the transport of pollutants in open channel flows, important for environmental impact studies. To date, controlled laboratory studies of the effects of vegetation on mixing processes have used simulated plants. This may neglect some of the important variables introduced by the presence of natural vegetation. In this study natural vegetation was planted within a laboratory channel, and a series of experiments quantifying velocity, turbulence, and longitudinal mixing were conducted over a time period sufficient to allow growth of the vegetation to impact on the mixing processes. In emergent conditions the results generally confirmed previous artificial vegetation and modeling studies, showing that vegetation reduces the magnitude of longitudinal shear dispersion. Additionally, measureable change in longitudinal mixing was observed primarily as a function of flow depth but also of plant age. Normalization using previously suggested parameter combinations failed to yield predictive trends. Submerged tests uniquely covered natural vegetation with a significant wake zone, and from this it was observed that longitudinal mixing is primarily a function of the degree of submergence. Overall, this paper presents a new data set quantifying the effects of natural vegetation on longitudinal mixing processes and illustrating deficiencies in previous understanding and predictive expressions based on idealized artificial vegetation.

Discussion of “ Transverse Mixing in Open Channel Flow ” by Gisbert Webel and Michael Schatzmann (April, 1984)

Discussion of “ <i>Transverse Mixing in Open Channel Flow</i> ” by Gisbert Webel and Michael Schatzmann (April, 1984)

Closure to “ Transverse Mixing in Open Channel Flow ” by Gisbert Webel and Michael Schatzmann (April, 1984)

Closure to “ <i>Transverse Mixing in Open Channel Flow</i> ” by Gisbert Webel and Michael Schatzmann (April, 1984)

Transverse Mixing in Open Channel Flow

An experimental study motivated by dimensional analysis was carried out to investigate the variation in the transverse mixing coefficient in straight, rectangular, smooth, and rough open channel flows. The findings have led to a number of suggestions relating to the future laboratory simulations of prototype cooling-water or wastewater dispersion in a river.

Longitudinal Dispersion and Turbulent Mixing in Open-Channel Flow

Longitudinal Dispersion and Turbulent Mixing in Open-Channel Flow

Predicting Effects of Dead Zones on Stream Mixing

Longitudinal mixing in open-channel flow is described by two coupled partial differential equations, the solution of which, in the frequency domain, is a function of four parameters: mean residence time; Peclet number or dimensionless dispersion coefficient; fraction of stream volume which is dead zone; and mean residence time in a dead zone. The solution is given and methods of estimating the parameters from hydraulic data are described. The fraction of stream volume which is dead zone is a function of the friction factor, and the ratio of mean dead zone residence time to mean residence time in a reach is essentially a constant. The effects of errors in estimating values of the parameters are discussed and illustrated.