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.
Highlights
Cavitation is defined as the appearance of vapor cavities due to the phase change in a liquid medium.[1]
Vortex generators have been used successfully in a broad range of aero- and hydrodynamics applications to alter the characteristics of boundary layer separation
The results indicated that micro-vortex generators (mVGs) should be located closer to the adverse pressure gradients region than traditional vortex generators (VGs)
Summary
Cavitation is defined as the appearance of vapor cavities due to the phase change in a liquid medium.[1]. It was found that suppressing or eliminating this separation can effectively delay or suppress the formation of an attached cavity.[20] The higher momentum of the turbulent flow improves its ability to resist adverse pressure gradient over convex surfaces and, limit the incidence of separation.[1,21] Compared to turbulent boundary conditions, a laminar boundary flow is more likely to separate, resulting in a higher drag penalty. Passive solutions include devices that do not rely on the controller or energy sources needed for active control.[23] Passive and active can be effective techniques to manipulate and change wall-bounded or free-shear flows. This change can be made by delaying or inducing advanced transition, suppressing or boosting turbulence, and provoking or suppressing separation.
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