Experimental Investigation of a Cavitating Water Flow With the Addition of Drag-
R. Azadi, D. S. Nobes
Dept. of Mechanical Engineering, University of Alberta, Canada
Local reduction of the pressure in a flow field to levels lower than the saturation pressure triggers the production of tiny vaporous bubbles and causes the cavitation phenomenon. Higher pressure reductions significantly increase the cavitation bubbles’ population, which can coalesce and generate large-scale cavitation structures such as cloud cavitation that shed downstream of the flow channel. The chaotic collapse of cavities produces strong shockwaves in regions with a recovered pressure which causes serious erosion on solid surfaces and high levels of noise. Hence, there is a growing interest in cavitation control methods. In this study, drag-reducing polymer additives are utilized as cavitation reducing (CR) agents in a converging-diverging mesoscale nozzle to verify the applicability of these agents in the control of the cavitation process. Analysis of high-speed images of the cavitating flow fields reveals that the viscoelastic flow of a 400 ppm polymer solution reduced the cavitation intensity by nearly 60 % relative to the pure water flow at a similar Reynolds number. Ultra-high-speed imaging of single cavitating bubbles at the inception showed that in a viscoelastic flow, the collapse period of cavities is longer, and their sizes are shrunk at a lower rate relative to their counterparts in the puer water. Particle image velocimetry (PIV) was used to study the near-wall turbulent flow fields at the flow conditions close to the cavitation inception, at different flow locations with non-zero pressure gradients present on the curved surfaces. Preliminary analysis of the results reveals that viscoelasticity alters the near-wall turbulence and distribution of the pressure gradient fluctuations, which might link to the significant reduction of cavitation intensity in polymeric flows.