Zhongdong Qian

Vibration-Based Condition Monitoring

Condition monitoring is the process of monitoring a condition parameter in machinery, so that a significant change is indicative of a developing failure. The use of conditional monitoring allows maintenance to be scheduled, or other actions taken to avoid the consequences of failure before it actually occurs.

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.

Large eddy simulation of turbulent attached cavitating flow with special emphasis on large scale structures of the hydrofoil wake and turbulence-cavitation interactions

In this paper, the turbulent attached cavitating flow around a Clark-Y hydrofoil is investigated by the large eddy simulation (LES) method coupled with a homogeneous cavitation model. The predicted lift coefficient and the cavity volume show a distinctly quasi-periodic process with cavitation shedding and the results agree fairly well with the available experimental data. The present simulation accurately captures the main features of the unsteady cavitation transient behavior including the attached cavity growth, the sheet/cloud cavitation transition and the cloud cavitation collapse. The vortex shedding structure from a hydrofoil cavitating wake is identified by the Q-criterion, which implies that the large scale structures might slide and roll down along the suction side of the hydrofoil while being further developed at the downstream. Further analysis demonstrates that the turbulence level of the flow is clearly related to the cavitation and the turbulence velocity fluctuation is much influenced by the cavity shedding.

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 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.

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.

Influence of dynamic seals on silt abrasion of the impeller ring in a centrifugal pump

The water flow and movement of silt in a prototype double suction centrifugal pump was simulated using an Euler–Lagrange multiphase flow model. Back-blade and J-groove configurations were adopted to protect the impeller ring from silt abrasion. Four back-blades and four J-grooves were considered. The results show that the relative velocity of water around the impeller ring is too low to move silt out of the spacing between the impeller and the casing, which results in a high silt concentration around the impeller ring. The high silt concentration around the impeller ring is the major contributor to silt abrasion of the ring. Back-blade and J-groove configurations are effective in reducing the silt concentration around the ring but extra friction loss is also introduced and the pump efficiency is decreased. Optimization of the length, position and number of back-blades, and the shape and number of J-grooves, decreases losses in pump efficiency and effectively protects the impeller ring. Case 4 for the back-blade and Case 8 for the J-groove were the most effective configurations in this study.

NUMERICAL STUDY OF FLOW AND DILUTION BEHAVIOR OF RADIAL WALL JET

The radial wall jet is a flow configuration that combines the radial jet and the wall jet. This article presents a simulation of the radial wall jet by applying the transition Shear-Stress Transport (SST) model. Tanaka’s experimental data are used for validation. The computed velocity profiles agree well with the experimental ones. The distributions of the velocity on cross-sections show a similarity in the main region and the profiles are different with those of the free radial jet or the wall jet, because the presence of the wall limits the expansion of the jet. By introducing the equivalent nozzle width, the maximum velocity decays and the half-width distributions are normalized, respectively. In addition to compare the flow field with experiments, this paper also analyzes the dilution effect of radial wall jets in terms of the concentration distributions. The concentrations on the wall keep constant within a certain distance from the nozzle. And the concentration distributions also show a similarity in the main region. Both the decays of the maximum concentration and the distributions of the concentration half-width fall into a single curve, respectively. The dilution effect of radial wall jets is thus verified.

Numerical simulation of sediment erosion by submerged plane turbulent jets

Erosion of loose beds acted by submerged plane turbulent jets was simulated with the Eulerian two-phase model, which implements Euler-Euler coupled governing equations for fluid and solid phases. A modified κ−ε turbulence model was chosen to enclose the fluid phase. Both flow-particle and particle-particle interactions were considered in this model. The computational results were in a good agreement with previous laboratory measurements. The characteristics of the flow field in the two phases and the influences of hydraulic and geometric parameters on eroded bed profiles were analyzed based on the computational results. The calculational results reveal that: the Densimetric Froude number is the most important factor that influences the computational results of the eroded bed; sands may keep movable or still under the interaction among particles gravity, seepage force of pore water and friction between particles. And sands on the upwind side of the dune may be promoted along the water-sand interface to the sand mound by the local shear stress, which mainly influences the scoured shape; by adjusting the computational steps and methods, two problems in the Eulerian two-phase simulation are solved.

Fundamentals of cavitation and bubble dynamics with engineering applications

Cavitation and bubble dynamics are important topics of fluid mechanics. Generally, cavitation phenomenon plays a negative role in the hydraulic machineries (e.g. hydroturbines, pumps, space engines, and marine propellers) through various kinds of mechanisms. First, the cavitation could cause serious damage on the surfaces of the materials (e.g. patterned erosions with pits) during its final collapse near the boundary. Second, flow inside/outside the machines will be further deteriorated by the vortices generated by the cavitating flows. Third, significant unstable flow could persist for a long time due to the generation of cavitation. As a result, both the efficiency and stable operation of the machines will be challenged by the cavitation.