Pressure fluctuation is the primary reason for unstable operations of double-suction centrifugal pumps. By using flush mounted pressure transducers in the semispiral suction chamber and the volute casing of a double-suction pump, the pressure fluctuation signals were obtained and recorded at various operating conditions. Spectral analyses were performed on the pressure fluctuation signals in both frequency domain and time-frequency domain based on fast Fourier transform (FFT) and an adaptive optimal-kernel time-frequency representation (AOK TFR). The results show that pressure fluctuations at the impeller rotating frequency and some lower frequencies dominated in the semispiral suction chamber. Pressure fluctuations at the blade passing frequency, the impeller rotating frequency, and their harmonic frequencies were identified in the volute casing. The amplitude of pressure fluctuation at the blade passing frequency significantly increased when the flow rate deviated from the design flow rate. At 107% of the design flow rate, the amplitude increased more than 254% than that at the design flow rate. The time-frequency characteristics of these pressure fluctuations were affected greatly by both operating conditions and measurement locations. At partial flow rates the pulsation had a great irregularity and the amplitudes at the investigated frequencies were much larger than ones at the design flow rate. An asymmetrical pressure fluctuation structure in the volute casing was observed at all flow rates. The pulsation behavior at the blade passing frequency was the most prominent near the volute tongue zone, and the pressure waves propagated in both the radial and circumferential directions.

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This paper aims to reveal the influence of a rigid wall with a gas entrapping hole on the characteristics of the dynamic behavior of a laser-induced bubble collapse. A high-speed camera system was used to record the oscillation process of the laser-induced bubble on a rigid wall with a gas entrapping hole. When a bubble is generated by a laser above the wall with a gas entrapping hole, the entire bubble collapse stays away from the wall or splits into two bubbles because of a radial jet induced by bubble contraction. These two distinctive collapse modes are dependent on the distance between the wall and the bubble. The focus of this study is the quantitative analysis of the jet formation, bubble migration, and oscillation period, and compared with the behavior of the bubble near a rigid wall. The results show that unlike the generation of the bubble near a rigid wall, a rigid wall with a gas entrapping hole affects the morphology of the jet and changes the direction of migration of the bubble, and decreases the oscillation period. Thus, the rigid wall with a gas entrapping hole could be effective for reducing cavitation erosion on the wall surface, which is supported by our experiment results.

The objective of this paper is to observe and investigate the early evolution of the shock wave, induced by a nanosecond pulsed laser in still water. A numerical method is performed to calculate the propagation of the shock wave within 1µs, after optical breakdown, based on the Gilmore model and the Kirkwood-Bethe hypothesis. The input parameters of the numerical method include the laser pulse duration, the size of the plasma and the maximally extended cavitation bubble, which are measured utilizing a high time-resolved shadowgraph system. The calculation results are verified by shock wave observation experiments at the cavitation bubble expansion stage. The relative errors of the radiuses and the velocity of the shock wave front, reach the maximum value of 45% at 5 ns after breakdown and decrease to less than 20% within 20 ns. The high attenuation characteristics of the shock wave after the optical breakdown, are predicted by the numerical method. The quick time and space evolution of the shock wave are carefully analyzed. The normalized shock wave width is found to be independent of the laser energy and duration, and the energy partitions ratio is around 2.0 using the nanosecond pulsed laser.

Pressure fluctuation may cause high amplitude of vibration of double-suction centrifugal pumps, but the impact of impeller stagger angles is still not well understood. In this paper, pressure fluctuation experiments are carried out for five impeller configurations with different stagger angles by using the same test rig system. Results show that the stagger angles exert negligible effects on the characteristics of head and efficiency. The distributions of pressure fluctuations are relatively uniform along the suction chamber wall, and the maximum pressure fluctuation amplitude is reached near the suction inlet tongue region. The pressure fluctuation characteristics are affected largely by impeller rotation, whose dominant frequencies include impeller rotation frequency and its harmonic frequencies, and half blade passage frequency. The stagger angle exerts a small effect on the pressure fluctuations in the suction chamber while a great effect on the pressure fluctuation in volute casing, especially on the aspect of decreasing the amplitude on blade passage frequency. Among the tested cases, the distribution of pressure fluctuations in the volute becomes more uniform than the other impeller configurations and the level of pressure fluctuation may be reduced by up to 50% when the impeller stagger angle is close to 24° or 36°. The impeller structure pattern needs to be taken into consideration during the design period, and the halfway staggered impeller is strongly recommended. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

a b s t r a c tFlow induced vibration on a hydrofoil may be significantly reduced with a slight modification of the trailing edge without alteration of the hydrodynamic performance. Particularly, the so called Donaldson trailing edge shape gave remarkable results and is being used in a variety of industrial applications. Nevertheless, the physics behind vibration reduction is still not understood. In the present study, we have investigated the hydrodynamic damping of a 2D hydrofoil with Donaldson trailing edge shape. The results are compared with the same hydrofoil with blunt trailing edge. The tests are carried out in EPFL high speed cavitation tunnel and two piezoelectric patches are used for the hydrofoil excitation in non-intrusive way. It was found that the hydrodynamic damping is significantly increased with the Donaldson cut. Besides, as the flow velocity is increased, the hydrodynamic damping is found to remain almost constant up to the hydrofoil resonance and then increases linearly, for both tested trailing edge shapes and for both first bending and torsion modes.

Added mass and hydrodynamic damping play significant roles in fluid-structure interaction (FSI) in hydraulic turbines. Added mass can reduce natural frequencies, while hydrodynamic damping could result in a higher amplitude decay speed of the vibration. In order to quantify the added mass and hydrodynamic damping of a three-dimensional (3D) NACA 0009 hydrofoil with a blunt trailing edge, a two-way FSI simulation method was employed. The effects of grid scale, time-step, turbulence model, exciting force, and numerical damping on the calculation accuracy of the two-way FSI numerical simulation were analyzed in great detail through comparison with the previously published experimental data. Hydraulic force was obtained by using a transitional shear stress transport model at the flow region of the Reynolds number ReL = 0.2 × 106–2 × 106. The vortex shedding frequency, the natural frequency of the first-order bending mode in water, and the hydrodynamic damping ratio obtained from the numerical simulations agree well with the experimental data, with maximum deviations in 6.12%, 4.53%, and 8.82%, respectively. As the flow velocity increases, the natural frequency may not significantly change, while the added mass coefficient gradually increases, considering the effect of added stiffness. Above the first-order bending mode lock-in region, the results indicate that the first-order bending mode hydrodynamic damping ratio increases linearly with velocity. The present numerical achievements offer a higher level of accuracy for predicting the added mass and hydrodynamic damping characteristics of a hydrofoil.

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