相位鎖定螺槳空化影像之處理與分析

Phase-Locked Images of Cavitation on a Marine Propeller:Processing and Analysis

船舶推進以螺槳推進為主，當船用螺槳處於高負載以及高轉速，使得螺槳表面的水流壓力低於水的飽和蒸汽壓時，導致水中的氣體以微氣泡為核心生成水蒸汽汽泡，這個現象稱為空化（cavitation）。螺槳發生空化時，因推力的損失會造成螺槳效率降低，也會導致船體振動和產生噪音，甚至可能產生浸蝕而破壞螺槳的結構！因此，螺槳空化一直是船舶推進器領域中至為重要的研究課題。
本文以相位鎖定數位拍攝（phase-locked digital imaging）系統，在空蝕水槽（cavitation tunnel）的船舶螺槳單獨試驗（open-water test）中，獲取一系列螺槳空化影像（propeller cavitation images），並開發適用於螺槳空化影像的處理與分析（image processing and analysis）演算法，用以提取出參數化的空化特徵，包括：由於螺槳葉片表面不同區域會產生不同的空化型態，本文將影像進行切割，再針對不同區域以不同次數的形態學（morphology）之膨脹（dilation）與侵蝕（erosion）進行處理。最終結果有望用於驗證相關的數值模擬並進一步改善螺槳的設計。

Ship propulsion is commonly achieved by marine propellers. Due to their usual working conditions of high load and high rotational speed, the water pressure on the surfaces of marine propellers easily drops below the saturated vapor pressure to form vapor bubbles with micro gas bubbles in the water as nuclei. This phenomenon is known as cavitation, which may result in a loss of thrust for marine propellers and hence reduce their efficiencies, induce vibrations on ship hulls and generate noise, and cause erosion that damages structures of marine propellers. As a result, propeller cavitation has long been a key field of research in marine propulsion systems.
This study presents the processing and analysis of a series of propeller cavitation images acquired by a phase-locked digital imaging system, which was integrated with a cavitation tunnel for open-water tests. Effective procedures of image processing and analysis suitable for propeller’s cavitation were developed to extract parameterized characteristics of cavitation. Since different regions of the propeller blade surface generate different types of cavitation, the images were segmented accordingly. Each region was then processed with a varying number of morphological operations, including dilation and erosion, to enhance and accurately capture the cavitation characteristics specific to each area. The final results can be expected to be used for validating corresponding numerical simulations and further advancing the propeller’s design.