Effects of large computational time steps on the computed turbulence were investigated using a fully implicit method. In turbulent channel flow computations the largest computational time step in wall units which led to accurate prediction of turbulence statistics was determined. Turbulence fluctuations could not be sustained if the computational time step was near or larger than the Kolmogorov time scale.

Effects of the computational time step on numerical solutions for turbulent flow

On centrifugal pumps

Tip clearance on pressure fluctuation intensity and vortex characteristic of a mixed flow pump as turbine at pump mode

Tip clearance in pump induces tip leakage vortex (TLV), which interacts with the main flow and leads to instability of flow pattern and decrease of pump performance. In this work, the characteristics of TLV in a mixed-flow pump are investigated by the numerical simulation using shear stress transport (SST) k–ω turbulence model with experimental validation. The trajectory of the primary tip leakage vortex (PTLV) is determined, and a power function law is proposed to describe the intensity of the PTLV core along the trajectory. Spatial–temporal evolution of the TLV in an impeller revolution period T can be classified into three stages: splitting stage, developing stage, and merging stage. The TLV oscillation period TT is found as 19/160 T, corresponding to the frequency 8.4 fi (fi is impeller rotating frequency). Results reveal that the TLV oscillation is intensified by the sudden pressure variation at the junction of two adjacent blades. On analysis of the relative vorticity transport equation, it is revealed that the relative vortex stretching item in Z direction is the major source of the splitting and shedding of the PTLV. The dominant frequency of pressure and vorticity fluctuations on the PTLV trajectory is 8.4 fi, same as the TLV oscillation frequency. This result reveals that the flow instability in the PTLV trajectory is dominated by the oscillation of the TLV. The blade number has significant effect on pressure fluctuation in tip clearance and on blade pressure side, because the TLV oscillation period varies with the circumferential length of flow passage.

Spatial-temporal evolution of tip leakage vortex in a mixed flow pump with tip clearance

Tip clearance between blade end and shroud is inevitable in pump operation and of great importance on pump energy performance and pressure fluctuation. As the tip clearance size increases, the head and efficiency of the mixed flow pump drop accordingly. The simulation results show that the development of a leakage vortex is observed as the tip clearance increases, and the trajectory of this leakage vortex remains in the same direction along the blade suction side for different tip clearances. With the increase in tip clearance size, the intensity of the leakage vortex is enhanced, and the separation between the main leakage vortex and the secondary leakage vortex is also strengthened. The leakage separation angle near the blade tip remains at the same value of 10° for different tip clearance sizes. As for the spectrum analysis, the maximum amplitudes of pressure fluctuations dramatically increase in the impeller when the tip clearance increases from 0.0 mm to 1.0 mm, and the dominant frequencies go from 145 Hz to 184 Hz due to the considerable leakage flow.

https://www.mdpi.com/1996-1073/10/2/191/pdf

Energy Performance and Flow Patterns of a Mixed-Flow Pump with Different Tip Clearance Sizes

Rotor-stator interaction in axial pumps can produce pressure fluctuations and further vibrations even damage to the pump system in some extreme case. In this paper, the influence of tip clearance on pressure fluctuations in an axial flow water pump has been investigated by numerical method. Three-dimensional unsteady flow in the axial flow water pump has been simulated with different tip clearances between the impeller blade tip and the casing wall. In addition to monitoring pressure fluctuations at some typical points, a new method based on pressure statistics was proposed to determine pressure fluctuations at all grid nodes inside the whole pump. The comparison shows that the existence of impeller tip clearance magnifies the pressure fluctuations in the impeller region, from the hub to shroud. However, the effect on pressure fluctuation in the diffuser region is not evident. Furthermore, the tip clearance vortex has also been examined under different tip clearances.

Influence of tip clearance on pressure fluctuations in an axial flow pump

Numerical investigation on characteristics of transient process in centrifugal pumps during power failure

In this article, computational fluid dynamics technique has been applied to examine the disk friction loss and leakage effect on performance of a Francis model turbine, and a model test for obtaini...

Investigation on disk friction loss and leakage effect on performance in a Francis model turbine

In this paper, the dynamic stall characteristics of the S809 airfoil have been numerically predicted, and the flow under dynamic stall condition has been controlled by introducing synthetic jet for enhancing the airfoil aerodynamic performance. In addition, experimental results have been adopted to validate the obtained numerical ones. The results show that the oscillation of the airfoil can delay the occurring of stall, and the stall angle of attack is increased from 9.5° to 13.1°. The introduction of synthetic jet can improve the lift coefficient of airfoil under dynamic stall conditions: The mean lift coefficient in a period of oscillation has been increased by 33%, and the drag coefficient has been reduced by 39%. Moreover, the stall angle of attack is increased from 13.1° to 16.2°. Therefore, the airfoil performance under dynamic stall condition could be greatly improved by the introduction of synthetic jet technology.

Guangkuan Wu

Papers

Influence of tip clearance on pressure fluctuations in an axial flow pump

Numerical investigation on characteristics of transient process in centrifugal pumps during power failure

Investigation on disk friction loss and leakage effect on performance in a Francis model turbine

Control of Dynamic Stall of an Airfoil by Using Synthetic Jet Technology

Multi-condition optimisation design of a hydrofoil based on deep belief network