Cavitation control using passive flow control techniques

Abstract

Passive flow control techniques, and particularly vortex generators have been used successfully in a broad range of aero-and hydrodynamics applications to alter the characteristics of boundary layer separation. This study aims to review how such techniques can mitigate the extent and impact of cavitation in incompressible flows. This review focuses first on vortex generators to characterize key physical principles. It then considers the complete range of passive flow control technologies, including surface conditioning and roughness, geometry modification, grooves, discharge, injection, obstacles, vortex generators, and bubble generators. The passive flow control techniques reviewed typically delay and suppress boundary layer separation by decreasing the pressure gradient at the separation point. The literature also identifies streamwise vortices that result in the transfer of momentum from the free stream to near-wall low energy flow regions. The area of interest concerns hydraulic machinery, whose performance and life span are particularly susceptible to cavitation. The impact on performance includes a reduction in efficiency and fluctuations in discharge pressure and flow, while cavitation can greatly increase wear of bearings, wearing rings, seals, and impeller surfaces due to excessive vibration and surface erosion. In that context, few studies have also shown the positive effects that passive controls can have on the hydraulic performance of centrifugal pumps, such as total head and efficiency. It is conceivable that a new generation of design in hydraulic systems may be possible if simple design features can be conceived to maximize power transfer and minimize losses and cavitation. There are still, however, significant research gaps in understanding a range of impact factors such as manufacturing processes, lifetime, and durability, and essentially how a static design can be optimized to deliver improved performance over a realistic range of operating conditions.

Cavitation suppression and transformation of turbulence structure in the cross flow around a circular cylinder: Surface morphology and wettability effects

“…Besides, comparing the results with those from Churkin et al [11] obtained for similar flow conditions and wall morphology, the small-scale roughness is much more effective on the cylinder than on the NACA0015 hydrofoil due presumably to the significant surface curvature and faster separation of the boundary layer on the bluff body. Zaresharif et al [74] reported that the intensity of cavitation can be reduced with the aid of some laser-made textures as compared to polished surfaces. This is also consistent with our measurements.…”

“…Various passive methods of cavitation control based on these principles were discussed in detail in the review by Zaresharif et al [74] , including those employing the concept of variation of the contact angle. Numerous papers have been published so far, where the effect of wall irregularities on flow structure was investigated in detail for both a cylinder (e.g., [71] , [63] , [55] , [8] , [77] , [45] ) and other objects of different shapes (e.g., [53] , [39] , [7] , [33] , [11] , [34] ).…”

Bubble Growth on Hydrophobic Rough Surfaces in the Shear Flow

“…It should be noted that here the bubble formation due to the pressure reduction is different from cavitation, which happens when the local pressure of the liquid falls below its saturated vapor pressure. 37,38 For flowing water at 20 °C, cavitation bubbles typically generate when the local pressure falls below its vapor pressure of ∼2.73 kPa, which indicates that a severe pressure reduction of ∼100 kPa is needed to induce cavitation. However, in this experiment, bubbles are formed when the local pressure reduction in the crevice is less than 1 kPa, which is much too small to induce cavitation.…”

Control mechanisms of different bionic structures for hydrofoil cavitation

“…Podnar et al [20] studied the influence of spherical blade shape on flow characteristics, and found that modifying the design parameters of hydrofoil can reduce the occurrence and development of cavitation. In addition, setting surface roughness [21] , [22] , [23] , vortex generators [24] , [25] , [26] and jet holes [27] can also realize the passive control of hydrofoil cavitation. It was found that the passive control methods of cavitation have a low cost, easy operation, high flexibility, and good development prospects.…”