| The cavitation erosion damages the materials. For a long time, the cavitation erosion has seriously impacted the property and service life of pumps, turbines, valves and ship propellers, so anti-cavitation erosion has currently become a key technical issue needed to be addressed in the whole world. Cavitation is an origin of cavitation erosion, which is influenced greatly by the pressure distribution and flow field of the near wall region. It has been shown that both the surface structure and material have influence on the flow field, which in turn generates influence on the occurrence of cavitation. Nowadays, most researching about anti-cavitation erosion focuses on the utilization of coating to improve the mechanical property of material surface, such as hardness, adhesive force, and so on. While the influence of surface structure and material on cavitating flow has been ignored, the present thesis is devoted to the influence of surface characteristics on cavitating flow over hydrofoils.In the thesis, a set of experimental research method has been built up. A set of experimental models with different surface roughness and material property and a brace device have been designed. The high speed visualization and PIV have been combined together to research on the flow pattern and kinetics of cavitating flow field.The results show that the surface roughness generates great influence on cavitating flow structure. The roughness always induce to local flow separation and the attachment of spot cavity, while the cavity attached on the smooth hydrofoil is in form of finger-like one. The difference of the cavitating flow structure leads to variety of developing process of cavitating flow. The surface scratch and distributed roughness have great effect on the flow patterns and developing process of cloud cavitating flow over hydrofoils.The surface materials have influence on the cavitating flow patterns and the developing process. The comparison between cavitation patterns over hydrofoils with aluminum, stainless steel, and epoxy coating on the surface verifies that the surface materials really have effect on the incipient and attached sheet cavitating flow, while have little effect on cloud cavitating flow. The stainless steel and epoxy coating restrains the occurrence and development of cavitation, and the stainless steel performs better.Kinetics of cloud cavitating flow field is studied by employing PIV and the developing process of re-entrant jet is also discussed. During the process of cavity increasing, the velocity in the near-wall region is relatively lower than in the mainstream field. The accumulation and rotation of cloud bubbles near the trailing region of hydrofoils generate influence on the flow field near trailing region, which induces to the interaction of cloud bubbles with the shear flow from pressure side and suction side. The interaction and movement of flows brings liquid water from the pressure side into the suction side of the hydrofoil, which finally becomes the re-entrant jet. The jet keeps moving along the foil’s surface and toward the leading edge of hydrofoils, resulting to the large scale shedding of cloud bubbles. Several whirlpools appear and dissipate during the process. |
