Wen-xin Huai

NUMERICAL SIMULATION AND ANALYSIS OF PRESSURE PULSATION IN FRANCIS HYDRAULIC TURBINE WITH AIR ADMISSION

In this article, the three-dimensional unsteady multiphase flow is simulated in the whole passage of Francis hydraulic turbine. The pressure pulsation is predicted and compared with experimental data at positions in the draft tube, in front of runner, guide vanes and at the inlet of the spiral case. The relationship between pressure pulsation in the whole passage and air admission is analyzed. The computational results show: air admission from spindle hole decreases the pressure difference in the horizontal section of draft tube, which in turn decreases the amplitude of low-frequency pressure pulsation in the draft tube; the rotor-stator interaction between the air inlet and the runner increases the blade-frequency pressure pulsation in front of the runner.

Mathematical Model for The Flow with Submerged and Emerged Rigid Vegetation

The article summarizes previous studies on the flow in open channels with rigid vegetation, and constructs a mathematical model for submerged and emerged rigid vegetation. The model involves the forces balance in the control volume in one-dimensional steady uniform flow. For submerged vegetation, the whole flow is divided into four regions: external region, upper vegetated region, transition region and viscous region. According to the Karman similarity theory, the article improves the mixing length expression, and then gives an analytical solution to predict the vertical distribution of stream-wise velocity in the external region. For emerged vegetation, the flow is divided into two region: outer region and viscous region. In the two circumstances, the thicknesses of each region are determined respectively. The comparison between the calculated results and our experimental data and other researchers’ data proves that the proposed model is effective.

Two-dimensional analytical solution for compound channel flows with vegetated floodplains

This paper presents a two-dimensional analytical solution for compound channel flows with vegetated floodplains. The depth-integrated N-S equation is used for analyzing the steady uniform flow. The effects of the vegetation are considered as the drag force item. The secondary currents are also taken into account in the governing equations, and the preliminary estimation of the secondary current intensity coefficient K is discussed. The predicted results for the straight channels and the apex cross-section of meandering channels agree well with experimental data, which shows that the analytical model presented here can be applied to predict the flow in compound channels with vegetated floodplains.

Verification and validation of Urans simulations of the turbulent cavitating flow around the hydrofoil

In this paper, we investigate the verification and validation (V&V) procedures for the Urans simulations of the turbulent cavitating flow around a Clark-Y hydrofoil. The main focus is on the feasibility of various Richardson extrapolation-based uncertainty estimators in the cavitating flow simulation. The unsteady cavitating flow is simulated by a density corrected model (DCM) coupled with the Zwart cavitation model. The estimated uncertainty is used to evaluate the applicability of various uncertainty estimation methods for the cavitating flow simulation. It is shown that the preferred uncertainty estimators include the modified Factor of Safety (FS1), the Factor of Safety (FS) and the Grid Convergence Index (GCI). The distribution of the area without achieving the validation at the Uv level shows a strong relationship with the cavitation. Further analysis indicates that the predicted velocity distributions, the transient cavitation patterns and the effects of the vortex stretching are highly influenced by the mesh resolution.

Large eddy simulation of the interaction between wall jet and offset jet

The interaction between a plane wall jet and a parallel offset jet is studied through the Large Eddy Simulation (LES). In order to compare with the related experimental data, the offset ratio is set to be 1.0 and the Reynolds number Re is 1.0×104 with respect to the jet height L and the exit velocity U0. The Finite Volume Method (FVM) with orthogonal-mesh (6.17×106 nodes) is used to discretize governing equations. The large eddies are obtained directly, while the small eddies are simulated by using the Dynamic Smagorinsky-Lily Model (DSLM) and the Dynamic Kinetic energy Subgrid-scale Model (DKSM). Comparisons between computational results and experimental data show that the DKSM is especially effective in predicting the mean stream-wise velocity, the half-width of the velocity and the decay of the maximum velocity. The variations of the mean stream-wise velocity and the turbulent intensity at several positions are also obtained, and their distributions agree well with the measurements. The further analysis of dilute characteristics focuses on the tracer concentration, such as the distributions of the concentration (i.e., C/C0 or C/Cm), the boundary layer thickness δc and the half-width of the concentration bc, the decay of the maximum concentration (Cm/C0) along the downstream direction. The turbulence mechanism is also analyzed in some aspects, such as the coherent structure, the correlation function and the Probability Density Function (PDF) of the fluctuating velocity. The results show that the interaction between the two jets is strong near the jet exit and they are fully merged after a certain distance.

FLOW STRUCTURE IN PARTIALLY VEGETATED RECTANGULAR CHANNELS

This article discusses the transverse distributions of the depth averaged velocity and the Reynolds stress in a steady uniform flow in partially vegetated rectangular channels. The momentum equation is expressed in dimensionless form and solved to obtain the depth averaged velocity. The analytical solution of the velocity in dimensionless form shows that the depth-averaged velocity is determined by gravity and its distribution is mainly determined by the frictions due to water or vegetations. The analytical solution of the Reynolds stress is also obtained. A relationship between the second flow and the inertia is established and it is assumed that the former is proportional to the square of the depth averaged velocity. The Acoustic Doppler Velocimeter (Micro ADV) was used to measure the steady uniform flow with emergent artificial rigid vegetation. Comparisons between the measured data and the computed results show that our method does well in predicting the transverse distributions of the stream-wise velocity and the Reynolds stress in rectangular channels with partially vegetations.

Large Eddy Simulation of open channel flows with non-submerged vegetation

Results of several Large Eddy Simulations (LES) of open channel flows with non-submerged vegetation are presented in this article. It is shown that the vegetation can make the flow structure in the mainstream direction uniform for both supercritical and subcritical flows. For subcritical flows, the LES results of the ensemble-average of time-averaged velocity distributions at four vertical sections around a single plant are in good agreement with measurements. The velocity sees double peaks at the upper and lower positions of flows. For supercritical flows, the ensemble-average velocities see some discrepancy between LES and measurement results. Some secondary flow eddies appear near the single plant, and they just locate in the positions of the double peaks in stream-wise velocity profiles. It is also found that the vegetation drag coefficient deceases as the Froude number increases.

Numerical investigation of flow through vegetated multi-stage compound channel

This paper addresses the problem of the renormalization group k - ε turbulence modeling of a vegetated multi-stage compound channel. Results from Micro acoustic Doppler velocimeter (ADV) tests are used with time and spatial averaging (doubleaveraging method) in the analysis of the flow field and the characterization. Comparisons of the mean velocity, the Reynolds stress, and the turbulent energy distribution show the validity of the computational method. The mean velocity profile sees an obvious deceleration in the terraces because of vegetation. Secondary flow exists mainly at the junction of the main channel and the vegetation region on the first terrace. The bed shear stress in the main channel is much greater than that in the terraces. The difference of the bed shear stress between two terraces is insignificant, and the presence of vegetation can effectively reduce the bed shear stress.

Curved Open Channel Flow on Vegetation Roughened Inner Bank

A RNG k − ε numerical model together with a laboratory measurement with Micro ADV are adopted to investigate the flow through a 180o curved open channel (a 4 m straight inflow section, a 180° curved section, and a 4m straight outflow section) partially covered with rigid vegetations on its inner bank. Under the combined action of the vegetation and the bend flow, the flow structure is complex. The stream-wise velocities in the vegetation region are much smaller than those in the non-vegetation region due to the retardation caused by the vegetation. For the same reason, no clear circulation is found in the vegetated region, while in the non-vegetation region, a slight counter-rotating circulation is found near the outer bank at both 90° and downstream curved cross-sections. A comparison between the numerical prediction and the laboratory measurement shows that the RNG k − ε model can well predict the flow structure of the bend flow with vegetation. Furthermore, the shear stress is analyzed based on the numerical prediction. The much smaller value in the inner vegetated region indicates that the vegetation can effectively protect the river bank from scouring and erosion, in other words, the sediment is more likely to be deposited in the vegetation region.

Analytical solution for vertical profile of streamwise velocity in open-channel flow with submerged vegetation

A three-zone model was constructed and applied to study vertical profiles of streamwise velocity in steady uniform, open-channel flows with submerged vegetation. Three zones are examined—lower vegetation, upper vegetation and non-vegetated. Dominant forces acting on the water body were mainly gravity, vegetation drag and Reynolds stress. The latter was estimated by mixing length theory. A power series method was used to solve the governing differential equation of the upper vegetation zone. Other governing equations for the remaining two zones were directly solved analytically, deriving formulas for calculating the streamwise velocities. Values calculated with the formulas agreed well with measured experimental data, which demonstrates the practical applicability of the model.