Hyeongsik Kang

Reynolds stress modeling of vegetated open-channel flows

The Reynolds stress model is applied to open-channel flows with vegetation. For the computation of pressure-strain term, the Speziale, Sarkar, and Gatskis model is employed. Mellor and Herrings model and Rottas model are used for diffusion and dissipation rate of Reynolds stress, respectively. Flow structures of open-channels under two vegetative conditions are simulated, namely submerged and emergent plants. Plain open-channel flows are also simulated for comparisons. Computed profiles are compared with the results from the κ-ε model and the algebraic stress model as well as measured data available in the literature. For the plain open-channel flow and the open-channel flow with emergent vegetation, the Reynolds stress model is observed to simulate the non-isotropic nature of the flows better than the algebraic stress model and the κ-ε model. For the open-channel flow with submerged vegetation, it is found that the Reynolds stress model predicts the mean flow and turbulence quantities best compared with the other models. Sediment transport capacity of vegetated open-channel flows is also investigated by using the computed profiles. It is shown that the isotropic turbulence model underestimates the suspended load seriously.

Numerical investigations of mean flow and turbulence structures of partly-vegetated open-channel flows using the Reynolds stress model

A Reynolds stress model for the numerical simulation of partly-vegetated flows is presented. The model uses Speziale, Sarkar, and Gatskis model for the pressure–strain correlation, Mellor and Herrings model and Rottas model for the diffusion and the dissipation rate of the Reynolds stress, respectively. The model is applied to partly-vegetated rectangular open-channel flows, and simulated results are compared with experimental data. The model satisfactorily predicts mean flow and turbulence statistics. Through numerical experiments, the evolution of secondary current patterns and mean flow structure are presented for different densities of vegetation. A budget analysis of the streamwise vorticity equation is also performed to investigate the mechanism by which secondary currents in a partly-vegetated open-channel flow are generated. In the vegetated zone, the production by anisotropy is important in generating secondary currents over the entire depth, except for regions close to the free surface and the bottom where Reynolds shear stress plays a key role. This is different from the vortical structure of plain open-channel flows.

Numerical Estimations of Nakdong River Flows Through Linking of Watershed and River Flow Models

In this study, the watershed and water body models were linked for the simulation of the Nakding river flow. This is a pre-step study for the estimation of the effect of the flow and water quality on the climate change. For models of watershed and river flow, the SWAT and EFDC were used, respectively. The runoff discharge at each boundary points for the simulation of the river flow was provided from the drainage basin model. The calculated runoff discharge by the SWAT model was compared with the measured data of the Ministry of Environment at 13 locations along the Nakdong river and 30 locations along the tributary streams. The computed water discharge was shown to be similar with the measured data. For the model calibration and verification, % difference, NSE, and were computed. The computed % difference was within 15% except of a few points. The NSE and were also within a fair level. The Nakdong river flow of 2007 was simulated by using the EFDC model. The comparison with the measured data showed that the model reflected the actual values of low and high flow well. Also, it was confirmed that the acceleration and deceleration in the curved areas were appropriately simulated. The movement of dye injected at the upstream boundary was simulated. The result showed that the arrival time up to the estuary dam was computed to be about 65 days.

A Study on Flood Storage Plans of Farmlands for Extreme Flood Reduction

Extreme water events such as heavy rainfalls due to recent climate change are continually increasing and their scale has also shown an increasing trend. To overcome these natural disasters, this policy study suggests securing lateral river space as an effective method for extreme flood. To support the importance of restoration and expansion of lateral river space, Gumi upstream region of the Nakdong River basin was chosen as a target area and flood reduction analysis of the washland by using LISFLOOD model have been examined. The 500-year frequency flood was simulated for the estimation of possibly occurable flood level and it turns out that the secured lateral river space on the selected site effectively lowers about 0.53 m flood level and reduces the flood damage of the city on the lower reaches of the river. In addition, based on this result, multilateral river space securing plans were compared, and conservation easement and natural disaster insurance were suggested for sustainable and cost-effective alternatives. The costs of land purchase and conservation easement for securing the river space were also compared.

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

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]

3D Numerical Simulation of Compound Open-channel Flow with Vegetated Floodplains by Reynolds Stress Model

This paper presents a Reynolds stress modeling of compound open-channel flows with vegetation on the floodplain. In the Reynolds stress model, we use the SSG model by Spezialeet al., for the pressure-strain correlation term, Mellor and Herrings model for the turbulent diffusion term, and Hanjalic and Launders model for the dissipation term. In order to take into account the anisotropy of turbulence due to the free surface, the combination of Shirs model and Gibson and Launders model is included in the pressure-strain correlation model. Model validations are carried out for the compound open-channel flows without vegetation. Then, the model is applied to the compound open-channel flows with vegetated floodplains. The mean flow and turbulence structures are simulated and the impact of vegetation on the floodplains is investigated.

Effect of Climate Change on Fish Habitat in the Nakdong River Watershed

In this study, the potential effects of increased water temperature on fish habitat were analysed in the streams of Nakdong River watershed. The changes in suitable habitats for each fish species and in species number at a habitat site were predicted, based on the maximum thermal tolerances of 22 fish species. The estimated maximum thermal tolerance ranged between and . Then, the increase of water temperature in 78-sites of Nakdong River watershed by 2100 was predicted by using the estimated air temperature data by 2100 in the literature and the regression analysis between air-temperature and water-temperature at each sites. The water temperature was estimated to have increased by , , and in 2011~2040 (period S1), 2041~2070 (S2), and 2071~2100 (S3), respectively. With such increases in water temperature, the averaged suitable habitats for all 22 fish species would be influenced by 21.9%, 36.3%, and 51.4% in periods S1, S2, and S3, respectively.

Turbulence Modeling of Solute Transport in Open-Channel Flows Over Submerged Vegetation

A model for numerical simulations of solute transport in vegetated open-channel flows is proposed. The Reynolds-Averaged Navier-Stokes model is used for the flow analysis. For the turbulence closure, the Reynolds stress model is used, and the generalized gradient diffusive hypothesis is used to close the Reynolds-averaged advection/diffusion equation. The developed model is applied to an experimental case of solute transport in turbulent open-channel flows over submerged vegetation reported by Ghisalberti and Nepf (2005). The simulated distributions of mean concentration along the streamwise direction are compared with measured data, showing a good agreement. In addition, numerical simulations reveal that the pattern of secondary currents in vegetated open-channel flows is significantly different from that in plain open-channel flows. Using the simulated results, the vertical turbulent Schmidt number for the vegetated open-channel flow is estimated and a value of 0.58 is obtained. This value can be compared to 0.47, which was obtained by Ghisalberti and Nepf (2005) using laboratory data.

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

ABSTRACT 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.

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

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.