Wonjun Yang

A two-layer approach for depth-limited open-channel flows with submerged vegetation

A two-layer approach for depth-limited open-channel flow with submerged vegetation is described. A momentum balance is applied to each layer and expressions for the mean velocities are proposed. The velocity is assumed to be uniform in the vegetation layer and logarithmic in the upper layer. The proposed relationship successfully predicts the mean velocity distribution when compared with the measured data. Using the velocity formula, the layer-averaged mean velocities in the upper layer and over the entire layer are derived. An expression for the roughness coefficient increased by vegetation is also presented, performing better for the roughness coefficient than other formulas. Another relationship is proposed for predicting the distribution of suspended sediment in depth-limited flow with submerged vegetation by using an eddy-viscosity profile. The predicted profiles moderately agree with the measured data. Comparisons with simulated data from the Reynolds-averaged Navier–Stokes equations with the k–ϵ model suggest that these proposals successfully predict suspended sediment transport in a depth-limited flow with submerged vegetation.

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.

Evolution of crystal structures in GeTe during phase transition

We investigated changes in the crystal structure of GeTe during its phase transition. Using density functional theory (DFT) calculations, four possible crystal structures were identified: R3m, P1, Cm, and Fm3m. Among these, P1 and Cm were examined here for the first time. By calculating the internal energy of the crystal volume change, we verified that P1, R3m, and Cm can coexist in crystalline GeTe. The X-ray diffraction spectra of annealed and laser-irradiated GeTe films revealed coexisting P1 or R3m and Cm. In addition, we confirmed that Cm transforms into P1 or R3m after laser irradiation. The presence of these new structures was revealed in the crystal Raman spectra. Many of the Raman peaks in the crystalized GeTe could be explained by the coexistence of various structures. By calculating the band gaps of these structures, we also found that a structural transformation induces a change in the crystal resistance, owing to differences in the band gaps of individual structures. The generation of new crystal structures suggests a facile phase change and instability during the structural transformation.

Effects of resonant bonding and structural distortion on the phase change properties of Sn2Sb2Se5

The phase-change characteristics of Sn2Sb2Se5 (SSS), a pseudo-binary chalcogenide material, were investigated for use in phase-change random access memory applications. Although an analysis of the power and phase-change speed using laser static test equipment showed superior phase-change properties, several instabilities existed during the phase-change process. It was also found that the difference in resistivity between the crystalline and amorphous structures was high, as compared to conventional Ge2Sb2Te5 (GST). The SSS material also required a higher set/reset switching power than GST in electrical pulse switching tests. Based on extended X-ray absorption fine structure measurements and ab initio calculations of the charge distribution, short and long bonds were not found to co-exist around the Sn atoms, unlike the Ge atoms in GST. This evidence leads to enhanced resonant bonding in SSS, which prevents the Sn atoms from participating in the Ge-like phase-change mechanism. While the Ge atoms in crystalline GST tend to occupy defective octahedral sites, the Sn atoms in SSS prefer a tightly bonded resonant bonding state with a six-fold geometry. This strong resonant bonding results in a lack of Peierls-like distortion in the SSS structure. As a result, the competition between Peierls-like distortion and resonant bonding significantly affects the phase-change characteristics such as the SSS instability and switching process.

Characteristics of Turbulent Flows and Suspended Sediment Transport in Open-channel with Submerged Vegetation

The open-channel flow with submerged vegetation shows distinct features in two separate regions, namely upper and vegetation layers. In the upper layer, the flow is akin to the open-channel flow, while the flow in the vegetation layer is relatively uniform with suppressed turbulence due to vegetation stems. This paper presents laboratory experiments to investigate the characteristics of turbulent flows and suspended sediment transport in open-channel flows with submerged vegetation. An open-channel facility, 0.5 m wide and 12 m long, was used for laboratory experiments. Various discharges were employed with depth ratios of 2~3, and wooden cylinders were used for vegetation. To make equilibrium suspension, sediment particles of median diameter of 75 were fed until capacity condition. Laser Doppler velocimeter was used to measure instantaneous velocity, and direct sampling with vinyl tube was used to measure the concentration of suspended sediment. Using the sampled data, the mean flow and turbulence structures were provided and characteristics of suspended sediment concentration with Rouse number were presented.

Development of mean flow model for depth-limited vegetated open-channel flows.

Open-channel flows with submerged vegetation show two distinct flow structures in the vegetation and upper layers. That is, the flow in the vegetation layer is featured by relatively uniform mean velocity with suppressed turbulence from shear, while the flow in the upper layer is akin to that in the plain open-channel. Due to this dual characteristics, the flow has drawn many hydraulic engineers` attentions. This study compares layer-averaged models for flows with submerged vegetation. The models are, in general, classified into two-layer and three-layer models. The two-layer model divides the flow depth into vegetation and upper layers, while the three-layer model further divides the vegetation layer into inner and outer vegetation layers depending on the influence of the bottom roughness. This study compares the two-layer model and the three layer-model. It is found that the two-layer model predicts better the average value of the velocity and the prediction by the three-layer model is sensitive to Reynolds shear stress. In the three-layer model, the mean flow in the inner vegetation layer does not affect the flow seriously, which motivates the proposal of the modified two-layer model. The two-layer model, capable of predicting non-uniform mean velocity, is based on the Reynolds stress which is linear and of power form in the upper and vegetation layers, respectively. Application results reveal that the modified two-layer model predicts the mean velocity at an accuracy similar to the two- and three-layer models, but it predicts poorly in the case of very low vegetation density.

Mean and Turbulence Strutures of Open-Channel Flows with Submerged Vegetation

In this study, the mean flow and turbulent structures of the open-channel flows with the submerged vegetation was explored. For this, laboratory flume experiments are conducted with model vegetation without foliage. Wooden dowel is used for rigid vegetation, and polyethylene film is used for flexible vegetation. Other properties of model vegetation are made the same. Mean and fluctuation velocities are measured using the laser Doppler anemometer. From the measured data, the vertical profiles of mean velocity, turbulent intensity, Reynolds shear stress are obtained. The general profiles for mean velocity and turbulent intensity is suggested, and the profiles of Reynolds shear stress were investigated. The impact of stem flexibility was also investigated. It was shown that mean velocity and Reynolds shear stress can hardly distinguished from that with rigid vegetation. However, turbulent intensity profiles were affected by stem flexibility.

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

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