The tip-leakage vortex (TLV) cavitation is a challenging issue for a variety of axial hydraulic turbines and pumps from both technical and scientific viewpoints. The flow characteristics of the TLV cavitation were widely studied in the past decades, but the knowledge about the tip-leakage cavitating flow is still limited. The present paper reviews the progresses in the researches of the TLV cavitation, including the numerical methods for the TLV cavitation, the flow characteristics of the TLV, the influences of the TLV cavitation on the local flow field and the control strategies of the TLV cavitation. It is indicated that the non-condensable gas may play an important role in the development of the TLV cavitation, and this fact should be considered during a careful simulation of the TLV cavitation. It is also suggested that the development of the TLV cavitation will significantly influence the distributions of the vorticity and the turbulence kinetic energy. Due to the complexity of the TLV cavitation, it is still an open question how to suppress the TLV cavitation in a simple but effective way. Finally, based on these understandings, some advanced topics for the future work are suggested to further promote the study of the TLV cavitation, for a deeper knowledge about the TLV cavitation.

A review of cavitation in tip-leakage flow and its control

With the increasing adoption of renewable energy sources globally, hydropower contributes significantly to energy generation through various schemes ranging from big to small-scale plants. In small-scale hydropower plants, the preference for reverse-operated pumps (known as pump as turbines or PATs) over small-scale hydroturbines has increased. However, apart from the associated economic advantages, PATs, like any other hydraulic machinery, are not free from common problems such as cavitation. Cavitation is a phenomenon in which air bubbles are formed within the fluid medium due to substantial local pressure drop and their eventual collapse causes material erosion and degrades the overall machine efficiency. Several studies have focused on PAT conventional operating mode, while its reverse mode just begun to gain research interest. Nevertheless, cavitation remains a common problem in PATs at various hydro-sites. Therefore, to analyze PAT cavitation performance and highlight the differences between its two operating modes in terms of their development mechanisms, this article presents a thorough review of PAT cavitation dynamics and influencing parameters, as well as the future research directions. It is found that PAT reverse mode is more prone to cavitation, but more damages would occur in the conventional mode. Nevertheless, modifying the PAT geometric design parameters can considerably improve its cavitation performance. However, this approach has not been sufficiently investigated for PAT reverse operating mode and hence requires further research. Note that the terms “PAT conventional mode,” “PAT pumping mode,” and “pump” are equally used throughout this paper. • Studies on cavitation performance in hydraulic pumps as turbines (PATs) are reviewed. • Both pump and turbine modes are considered in the review. • PATs are found to have gained importance due to their applicability in remote areas. • PAT reverse mode operations require high heads and flows; prone to cavitation. • Geometric design modification is widely used for PAT cavitation performance improvement. 

Pump as turbine cavitation performance for both conventional and reverse operating modes: A review

Propeller–duct interaction on the wake dynamics of a ducted propeller

The large eddy simulation model coupled with the modified Schnerr-Sauer cavitation model has been used to numerically simulate the unsteady cavitation and noncavitation flow of the three-dimensional NACA66 hydrofoil under different operating conditions. The resulted show that the magnitude of the cavitation number plays a decisive role in the hydrofoil cavitation quasiperiodic phenomenon. The cavitation number 1.25 is used as a typical working condition for analysis. Using the Ffowcs Williams-Hawkings acoustic analogy approach accompanied with the vorticity transport equation splitting, the growth and shedding of cavitation also lead to the growth and shedding of the vortex structure. The cavitation-vortex interaction is mainly influenced by the vortex stretching term and vortex dilatation term and amplitude of them are larger than 500. The baroclinic torque term may be responsible for generating vorticity during the cloud cavitation collapse and has a lower amplitude about 200. The cavity volume acceleration is the main influencing factor of the low-frequency pressure fluctuation around the cavitating hydrofoil. Moreover, the NACA66 hydrofoil surface-pressure data are collected for dynamic mode decomposition to locate the hydrofoil surface noise sources. The alternate high and low amplitude regions in the mode results overlap highly with the cavitation transformation regions. The cavity transformation and pressure fluctuations are the main reason for the generation of periodic low-frequency noise source regions on the hydrofoil surface. Moreover, the corresponding frequencies of each order mode are linearly correlated with the cavitation shedding frequency of 5.70 Hz. Combined with the results of the multiple mode comparisons.

Numerical study on cavitation-vortex-noise correlation mechanism and dynamic mode decomposition of a hydrofoil

Cavitation is a limiting phenomenon in many domains of fluid mechanics. Instabilities of a partial cavity developed on an hydrofoil, a converging-diverging step or in an inter-blade channel in turbomachinery, have already been investigated and described in many previous works. The aim of this study is to evaluate a passive control method of the sheet cavity. According to operating conditions, cavitation can be described by two different regimes: an unstable regime with a cloud cavitation shedding and a stable regime with only a pulsating sheet cavity. Avoiding cloud cavitation can limit structure damages since a pulsating sheet cavity is less agressive. The surface condition of a converging-diverging step, like a Venturi-type obstacle, is here studied as a solution for a passive control of the cavitation. This study discusses the effect of an organized roughness, in the shape of longitudinal grooves, on the developed sheet cavity. Analyzes conducted with Laser Doppler Velocimetry, visualisations and pressure measurements show that the grooves geometry, and especially the groove depth, acts on the sheet cavity dynamics. Results show that modifying the surface condition, by varying the grooves geometry, can reduce cavity sheet length and even suppress the cloud cavitation shedding.

Study of the cavitating instability on a grooved Venturi profile

Cavitation is a process of liquid evaporation, bubble or vapor sheet formation, and further collapse of vapor structures, which plays a destructive role in many industrial applications. In marine transport and hydraulic machinery, cavitation usually occurs nearby the surface of a ship propeller and rudder, impeller blades in a pump, and distributor vanes and runner blades in a hydroturbine and causes various undesirable effects such as vibrations of frameworks and/or moving parts, material erosion, and noise enhancement. Based on an extensive literature review, this research is aimed at an experimental investigation of a passive approach to control cavitation on a benchmark hydrofoil using a wedge-type vortex generator in different flow regimes with a high Reynolds number. In this study, we employed a high-speed imaging method to explore the spatial patterns and time evolutions of cavitation structures and utilized a hydroacoustic pressure transducer to record and analyze local pressure pulsations due to the collapse of the cavities in the hydrofoil wake region. The results show that the examined control technique is quite effective and capable of hindering the formation of cloud cavities and reducing the amplitude of pressure pulsations associated with unsteady cavitation dynamics. This study provides important experimental information, which can be useful for improving industrial technologies and for promoting new developments in this particular research field.

Control of unsteady partial cavitation and cloud cavitation in marine engineering and hydraulic systems

https://iopscience.iop.org/article/10.1088/1755-1315/405/1/012030/pdf

Mikhail Yu. Nichik

Papers

Control of unsteady partial cavitation and cloud cavitation in marine engineering and hydraulic systems

Experimental verification of the effect of gap geometry on the tip-leakage flow pattern and tip-clearance cavitation

Flow dynamics of an axisymmetric impinging jet under two-frequency external forcing. A study by time-resolved PIV and DMD

Effect of an end-clearance width on the gap cavitation structure: Experiments on a wall-bounded axis-equipped hydrofoil