A review of open channel turbulence, focusing especially on certain features stemming from the presence of the free surface and the bed of a river. Part one presents the statistical theory of turbulence; Part two addresses the coherent structures in open-channel flows and boundary layers.

Turbulence in Open-Channel Flows

This study presents the results of laboratory experiments on depth-limited open channel flow with submerged vegetation. To investigate the impact of stem flexibility on the mean flow and turbulence structures, two flow types with similar drags but with flexible and rigid stems are compared. It is found that the stem flexibility hardly affects the mean flow but increases the peak value of the Reynolds shear stress. In addition, the stem flexibility increases and decreases the streamwise component of the turbulent intensity in the upper and vegetation layers, respectively. However, the stem flexibility increases the vertical component of the turbulent intensity over the whole depth. General profiles of the mean flow, the Reynolds shear stress, and the turbulent intensity are presented. The results of a quadrant analysis and a budget analysis are also provided.

Impact of stem flexibility on mean flow and turbulence structure in depth-limited open channel flows with submerged vegetation

This study presents the results of laboratory experiments on depth-limited open channel flow with submerged vegetation. To investigate the impact of stem flexibility on the mean flow and turbulence structures, two flow types with similar drags but with flexible and rigid stems are compared. It is found that the stem flexibility hardly affects the mean flow but increases the peak value of the Reynolds shear stress. In addition, the stem flexibility increases and decreases the streamwise component of the turbulent intensity in the upper and vegetation layers, respectively. However, the stem flexibility increases the vertical component of the turbulent intensity over the whole depth. General profiles of the mean flow, the Reynolds shear stress, and the turbulent intensity are presented. The results of a quadrant analysis and a budget analysis are also provided. RESUME

Impact of stem flexibility on mean flow and turbulence structure in depth-limited open channel flows with submerged vegetation Impact de la flexibilité de tige sur l'écoulement moyen et la structure de la turbulence dans un canal de profondeur limitée avec de la végétation submergée

This study evaluated a typical nonlinear fourth-order k-ω model and the associated roughness boundary condition for their suitability for simulating secondary current of the second kind in open-channels induced by roughness non-uniformity. Three test cases were simulated: square duct flow, open-channel flow with uniformly roughened bed and smooth side walls, and open-channel flow over roughness patches. The nonlinear model and the roughness boundary condition performed relatively well for the first two cases where the corner effect was dominant. For the third case, with alternate roughness patches, strong pressure gradient was generated at the edge between adjacent patches and the secondary current was greatly over-predicted. Two remedial options were tested: increasing the roughness height for smooth walls to a reasonable value and smoothing the roughness transition with a hyperbolic tangent function. We found the first option works better and the second only marginally improves the results.

Numerical investigation on the suitability of a fourth-order nonlinear k-ω model for secondary current of second type in open-channels

This paper reports a numerical investigation of the characteristics of mean flow and turbulence statistics for a depth-limited flow with submerged vegetation in a rectangular channel. To achieve this, the double-averaged Navier–Stokes equations were solved using the Reynolds stress closure model. A zone with increased velocity was observed at the corner between sidewall and free surface, thus forming two velocity maxima, one at the centre of the channel and one at the corner. This is consistent with previous experimental observations. The velocity maximum at the corner can be attributed to the enlarged bottom vortex caused by submerged vegetation, which transports high-momentum fluids from the centre to the corner of the channel. The velocity maximum at the corner was found to occur in the flow with submerged vegetation regardless of roughness density and relative submergence. The velocity maximum at the corner affects the mean flow, but has a negligible effect on turbulence statistics.

Characteristics of mean flow and turbulence statistics of depth-limited flows with submerged vegetation in a rectangular open-channel

The flow and transport of suspended sediment in open-channel flows over smooth-rough bed strips were simulated numerically. The flow equations were solved with the aid of the Reynolds stress model. Simulated flow structures are provided and compared with measured data available in the literature. The comparisons indicate that the numerical model successfully predicts the mean flow and turbulence statistics. The sediment distribution of suspended particles in such flows was also simulated. The transport equation for suspended sediment was solved using the eddy diffusivity concept. The computed results show that flows with higher concentration occur over the smooth strips as observed in a river flow during the floods. The eddy diffusivity profile was also obtained and compared with that from an analytical expression reported byWang and Cheng [Advances inWater Resources. 28 (2005) 441–450]

Numerical simulations of cellular secondary currents and suspended sediment transport in open-channel flows over smooth-rough bed strips

Abstract In general, the flow in a wide open-channel is two-dimensional in the region away from the sidewall. However, the same flow over a mobile bed is three-dimensional, showing a series of pairs of counter-rotating vortices over longitudinal bedforms. This is due to the cellular secondary currents formed over the entire cross section. The initiation mechanism of such cellular secondary currents has not yet been clearly demonstrated. The interaction between the pre-existing vortex created by the sidewall and the bottom sediment is thought to be related to the initiation of those secondary currents. The presence of the free surface and sidewall as well as the non-uniformity of sediment particles are also known to strengthen the cellular secondary currents. In the present paper, turbulent open-channel flows over sand ridges and troughs are numerically simulated. The Reynolds averaged Navier-Stokes equations are solved with the non-linear k-ε model. This turbulence model was selected mainly due to its speed in computation. Mean flows and turbulence statistics are presented. The simulated secondary currents clearly showed upflows and downflows over the ridges and troughs, respectively, and the simulated results are compared with experimental data sets available in the literature. The modeling presented here is an important step in investigating the initiation mechanism of cellular secondary currents in a wide open-channel.

Non-linear k-ε model for open-channel flows over sand ridges and troughs

The eddy diffusivity of suspended sediment is a key element in determining the concentration profile of suspended sediment in open-channel flows at equilibrium state. Traditionally, the eddy diffusivity has been related to the eddy viscosity by using the empirical coefficient β. However, the literature study reveals that the coefficient β is quite uncertain. This paper presents applications of the turbulent bursting-based model by Cao et al. (1996) to the prediction of suspended sediment concentration. The model predicts the concentration profile using the vertical turbulent intensity, mean duration of turbulent bursts, and particle relaxation time. The model is applied to two sets of experimental data: open-channel flow over a rough bed and open-channel flow with submerged vegetation. In the open-channel flow over a rough bed, it is found that the eddy diffusivity and concentration profiles by the turbulent bursting-based model are very similar to the results by the modified parabolic formula by Lyn (1993). In the open-channel flow with submerged vegetation, the eddy diffusivity profile is predicted by the turbulent bursting-based model, and the sensitivity of particle size is investigated. The distribution of suspended sediment is obtained and discussed through comparisons with the result by the numerical model and with the Rousean

Turbulent Bursting-Based Model Applied to Prediction of Suspended Sediment Concentration

In this study, laboratory experiments are conducted to explore the turbulent and the coherent structures of the open-channel flows with the submerged vegetation. Mean and fluctuation velocities are measured using the acoustic Doppler velocimeter. From the measured data, the vertical profiles of mean velocity, turbulent intensity, Reynolds stress, drag coefficients, and turbulent energy budget are obtained. It is shown that the mean velocity profile of open-channel flows with submerged vegetation is approximated to be a hyperbolic tangent function like the velocity distribution in the mixing layer. The zero plane displacement is estimated and used as a reference level of the flow. The turbulent intensity and Reynolds stress, increasing from the bottom, peak around the vegetation height, and they decay. This appears to be consistent with the previous experimental results. The estimated eddy viscosity profile is different from the parabolic distribution. In the turbulent energy budget, it is shown that turbulent production by shear is dominant above the vegetation height while turbulent production by vortex is below the vegetation height. As for the coherent structure, it turns out that the ejection and sweep are dominant above and below the vegetation height, respectively.

Turbulent and Coherent Structures of Vegetated Open-Channel Flows Affected by Water Depth

In general, the flow in a wide open-channel is two-dimensional in the region away from the sidewall. However, the same flow over a mobile bed is three-dimensional, showing a series of pairs of counter-rotating vortices over longitudinal bedforms. This is due to the cellular secondary currents formed over the entire cross section. The initiation mechanism of such cellular secondary currents has not yet been clearly demonstrated. The interaction between the pre-existing vortex created by the sidewall and the bottom sediment is thought to be related to the initiation of those secondary currents. The presence of the free surface and sidewall as well as the non-uniformity of sediment particles are also known to strengthen the cellular secondary currents. In the present paper, turbulent open-channel flows over sand ridges and troughs are numerically simulated. The Reynolds averaged Navier-Stokes equations are solved with the non-linear k-ϵ model. This turbulence model was selected mainly due to its spe...

https://bioone.org/journals/Journal-of-Coastal-Research/volume-2008/issue-10052/1551-5036-52.sp1.33/Non-Linear-k-%ce%b5-Model-for-Open-Channel-Flows-over/10.2112/1551-5036-52.sp1.33.pdf

Moonhyeong Park

Papers

Numerical simulations of cellular secondary currents and suspended sediment transport in open-channel flows over smooth-rough bed strips

Non-linear k-ε model for open-channel flows over sand ridges and troughs

Turbulent Bursting-Based Model Applied to Prediction of Suspended Sediment Concentration

Turbulent and Coherent Structures of Vegetated Open-Channel Flows Affected by Water Depth

Non-Linear k-ϵ Model for Open-Channel Flows over Sand Ridges and Trough