PSI - Issue 27

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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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These relations are affected significantly for the process of transition from sheet to cloud cavitation. At higher Reynolds number with constant cavitation number, it may result in cloud cavitation instead of sheet cavitation. Afterward, for the same cavitation number, erosion rates are proportional to the higher flow of velocity. As the case results, Peters et al. (2018) had performed a simulation to investigate the effect of the scale size of the propeller model and full scale, respectively. To compare the flow conditions between these scales, they defined a reference velocity, u 0 (see Eq. 4). The time step was adapted to obtain the same rotational speed per time step with the model scale (Peters et al., 2018). u 0 = √ u 2 + (0.7 Dπn ) 2 (4) Based on the results for the simulation full scale and model scales, the torque and thrust produced are slightly different. Table 1 shows the lists of several differences that can be conducted by Peters et al. From the table, known that the reference velocity was higher for a full scale. It may result that in a higher Reynolds number, the values of the thrust and torque coefficient also have little differences. The torque coefficient decreased by 1.7%, and the thrust coefficient increased by 2.6%. Furthermore, as shown in the table of the simulation results, we can conclude that the model scale can be considered as a validation model to design a full-scale propeller. With the model scale of the propeller, it allowed needs less effort and fund resources to set up experimental research as well as for numerical simulation. 3. Skew configuration on performance characteristics With the shipping requirement in the recent decade that needs high speeds and high loads vessels, it will be challenging to avoid cavitation. Few researchers have considered to understanding the influence skew angle on the cavitation flow on the propeller. The studies focused on how to reduce the cavitation effect with a redesigned propeller skew pattern and skew angle. Skew configuration angle at different propeller allowed the differences for the vortex pattern in the cavitation flow. Gaggero et al. had revealed that although tip vortex cavitation does not affect the propeller propulsion efficiency, it had been found that tip vortex cavitation increases the noise produced (Gaggero et al., 2014). Liu had studied the effect of propeller skew on pressure fluctuation with the different skew angles of four propellers in the non-uniform inflow (Liu, 2012). The result shows that for given flow conditions, the 20 degrees skew angle is the best design, which allowed minimum pressure fluctuations. To achieve a deeper understanding of the effect of the skew angle and skew pattern to the cavitation behavior on the propeller, Feng and Lu have performed numerical simulations to study the difference of cavitation patterns and pressure fluctuations on the propeller (Feng and Lu, 2019). Two types of the propeller with the same skew angle but the different skew pattern was applied. The differences between these types depend on skew distribution along the radius of the propeller blades, r/R. An advanced simulation was used to measure the pressure fluctuations characteristics in several points of the propeller. These propellers are simulated under unsteady flow conditions. The calculation time step is set to the time needed when the propeller rotates 1.5 degrees, so the time step is 0.000238095 s. operating conditions for cavitating flow is set by J = 0.725 and σ n = 1.6 at rotational speed 1050 rpm. The physical properties of water used for calculation are water at 20 °C. The results show a cavitation pattern on the suction side of a balanced and biased propeller at the conditions stated. It can be known that the cavitation is of the biased propeller is larger than the balanced propeller. Fig. 1 presents the comparison pattern of the vapor volume fraction of the propeller. The numbers 1,2, and 3 represent the cavitation pattern of the propeller at 90°, 150°, and 240° for the biased and balanced propeller, respectively. Table 1. Model scale and full-scale simulations of the propeller (Peters et al., 2018). u 0 (m/s) K T 10 K Model-scale 16.823 0.2331 0.4621 α ̅ (10 7.218 7.747 -6 ) n pit rev 25388 375450 Full-scale 38.546 0.2391 0.4541