PSI - Issue 27

Muhammad Yusvika et al. / Procedia Structural Integrity 27 (2020) 109–116 Yusvika et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Cavitation inception is the preliminary process of cavitation erosion. In general, cavitation generates from a local decrease in hydrostatic pressure of the fluid — the pressure drop produced by the motions of the propeller. The formation of a bubble occurred when the local pressure reached the saturation vapor pressure of the ambient fluid. In this case, the pressure is fall to a lower pressure level, while the temperature of the liquid is kept constant. Cavitation erosion will not occur if the bubble cavity does not explode near the propeller body, therefore it still detrimental in terms of performance loss. When the traveling bubble cavities reach the collapse point in the trailing edge of the fixed cavity, the local pressure will recover as a higher pressure region. The collapse mechanism of the traveling bubble cavities generates a shock pressure, and a microjet is formed (Brennen, 1995). Present research in propeller cavitation has achieved a large understanding of the mechanism of cavitation phenomenon. However, it is far from being complete because of the complexity. Cavitation is a localized phenomenon that involves the interaction of hydro-dynamical, metallurgical, thermo-dynamical, and chemical effects. Understanding such complex phenomena requires all clarification of the cavitation mechanism and parameters that affect it. So, the cavitation effect can be prevented or reduced. It will be achieved after full information, and massive data can be collected from different conditions. This paper addresses several validation studies and assume that despite the researchers have succeeded in aiding improvement in the design, the reliability of predictions is not yet such enough to avoided problems at all time and all conditions. Therefore, this paper presents the discussion of several previous research that have performed observation and prediction of the mechanism and damage of cavitation phenomena on material damage or known as cavitation erosion. 2. Model scaling in cavitation erosions Scale effects are evident in different types of flows. The fundamental factor that affects cavitation inception is the sudden change in pressure, the birth of nuclei, and time factor. Times play an important role in the growth of a nucleus until reaching the critical radius, then explode into vaporous bubbles (Peters et al., 2018). The period of bubble growth may lead to scale effect, where the time needed for a single bubble on a model scale is smaller than on a full scale. The scale factor, λ , for Froude scaling is 1 √ λ ⁄ and 1 λ 2 ⁄ for Reynolds scale. Other scale parameters are usually using the assumption that both models of full scale and model scale have the same advance ratio. The scale factor λ=D S D M ⁄ is the ratio of the diameter D of the full scale and the model scale propeller. The inlet velocity is scaled with √ λ= u S u M ⁄ and the propeller rotational speed scaled with √ λ= n M n S ⁄ . The similarity of cavitation is obtained by applying for the same cavitation number σ n (see Eq. 1) to compare model and full-scale propellers (Peters et al., 2018). σ n = P Ref - P v 1 2 ⁄ ρ (n D) 2 (1) Generally, by fulfilling these parameters, the behavior of cavitation is qualitatively similar. Peters et al. stated that at higher reference velocity and a larger scale in the same cavitation number, cavitation inception leads earlier and generates a higher vapor phase (Peters et al., 2018). The differences in cavitation behavior may occur due to the Reynolds number, and local Mach number at a larger scale or full scale is higher than at model scale. It is known from the literature that propeller blade appendages are composed of hydrofoil section arrangement in a particular reference line. The differences between model and full scale are also changing the Reynold number and Mach number over the solid body. Schnerr et al. (2006) and Ganesh et al. (2017) had proven these differences. Schneer et al. have performed simulations for cavitating flow around hydrofoil with neglect viscosity using Euler flow solver. The results show an agreement between simulations and experimental results. On the results, they stated that cavitation might depend on the Reynolds Number, Re (see Eq. 2). While Ganesh et al. reported that relating the flow velocity, v , with the speed of sound of the mixture, c , is known as Mach number, Ma (see Eq. 3) (Ganesh et al., 2017).

ρ u L μ

Re =

(2)

u c

Ma =

(3)