Analysis of the Effect of Changes in Pitch Ratio and Number of Blades on Cavitation on CPP

⎯ cavitation is a detrimental phenomenon to ship operations because it causes many losses. It caused some effects i.e decreased propeller efficiency, damaged propeller material, lower ship speed, vibration, and extreme noises. In that regard, this research conducts cavitation analysis on controllable pitch propeller (CPP) by varying number of blade i.e. 3, 4 and 5 blades; diameter i.e. 30, 40 cm and 50 cm; also pitch i.e 0.4, 0.6 and 0.8. The research method is carried out by the author in this study by conducting a simulation method based on the CFD approach. The simulation process consists of 3 stage-post processor, solver manager, and post-processor. From the simulation based on the CFD approach result, it was found that propeller rotation has an effect on the pressure ratio value. As the propeller rotation increase, the value of the pressure ratio will increase as well. The value of the pressure ratio in propeller design affects the cavitation area that occurs in the propeller. The percentage of the cavitation area on the propeller has an increasing tendency with the number of blades, rotation, and pitch. On the propeller with diameter 300 mm, 3 blades, pitch 0.8 at rotation 125 rpm no indication of cavitation, then it increases to 1.41% at rotation 175 rpm and keeps getting higher at rotation 225 to be 4.22% from total propeller expanding area. Whereas at rotation 225 rpm and pitch 0.4 is 3.38 %, then it becomes 3.85 % at pitch 0.6, which is getting bigger at pitch 0.8 that is 4.22 %. Keywords⎯ ambient temperature, cavitation, CFD approach, controllable pitch propeller (CPP), propeller design, simulation.

I. INTRODUCTION 1 Duri ng recent year's great advancement of computer performance, Computational Fluid Dynamics (CFD) methods for solving the Reynolds Averaged Navier-Stokes (RANS) equation have been increasingly applied to various marine propeller geometries, and more and more research articles have been published [1].
While these studies have shown great advancement in technology, some issues still need to be addressed for more practicable procedures. These include mesh generation strategies and turbulence model selection. With the availability of superior hardware, it becomes possible to model complex fluid flow problems like propeller flow and cavitation [2].
For many years, propellers were predicted using the lifting line theory, where the blade was represented by a vortex line and the wake by a system of helicoidal vortices. With the advent of computers, numerical methods developed rapidly from the 1960s onwards. The first numerical methods were based on the lifting line theory, and later the lifting surface model was developed. Salvatore et al. [3] presented the theoretical basis of the lifting-line theory based on perturbation methods.
Fujiyama [4] has analyzed the unsteady cavitating flow of HSP-II and CP-II propeller at behind-hull condition both in the model and full scale, using commercial software SC/Tetra v13. The results show that the unsteady propeller cavitation phenomena can be captured in the numerical calculation.
Kawakita et al. [5] has developed energy-saving devices that improve the propulsive performance and fuel consumption of ships, including reaction fins for low-speed full ships and stator fins for high-speed slender ships by developed computational fluid dynamics (CFD) technologies that analyze and evaluate the cavitation occurrence characteristics of propellers equipped with energy-saving devices as a unit, including the hull and rudder.
Long [6] has researched the propeller cavitating flow behind the hull, analyzed the vorticity distribution and particle tracks as well, using commercial software CFX and Zwart cavitation model. The cavitation patterns predicted resemble well with the experimental observations, with some over-prediction of the cavitation area. Pereira et al. [7] presented an experimental and theoretical investigation on a cavitating propeller in uniform inflow. Flow field investigations by advanced imaging techniques are used to extract quantitative information on the cavity extension. Pereira and Sequeira [8] developed a turbulent vorticity-confinement strategy for RANSbased industrial propeller-flow simulations. The methodology aims at an improved prediction of tip vortices, which are the origin of cavitation.
Arifin et al. [9] [10] analyzed the cavitation on the propeller by using the simulation based on the CFD approach in order to get the best configuration for the effectiveness of the propeller. The numerical or experimental analysis and comparison of results highlight the peculiarities of propellers, the possibility to increase efficiency and reduce cavitation risk, in order to exploit the design approaches already well-proven for conventional propellers also in the case of unconventional geometries. The simulated flow pattern agrees with the experimental data in most cases.
Controllable Pitch Propeller (CPP) is one of the developments of the propeller. CPP is a type of propeller that can change the pitch or angle of its blade. This angle of the blade will be adjusted to the need of the ship [11] [12]. CPP has several advantages compared to other types of propellers. The use of CPP (by modifying the pitch) will help us change the engine rotation easily to reduce vibration and noise in the engine, just like the pitch that can be modified to reduce cavitation in various engine rotations [13] [14].
So in this present paper, the simulation method based on the CFD approach is conducted to analyze the cavitation on the CPP against changes in working pitch and number of blades, and compare the cavitation on changes in working pitch and amount angles on the CPP through the simulation results. By varying number of blade i.e. 3, 4 and 5 blades; diameter i.e. 30, 40 cm and 50 cm; also pitch i.e 0.4, 0.6 and 0.8, the simulation results are compared.

II. METHOD
In this research, the simulation method by using a CFD approach is used by varying number of blade i.e. 3, 4 and 5 blades; diameter i.e. 30, 40 cm and 50 cm; also pitch i.e 0.4, 0.6 and 0.8. Geometric modeling of propeller is carried out using PropCad software. This study was conducted by considering the following aspects:

A. Basic Equation of Computational Fluid Dynamic
Implementation on used software is Computational Fluid Dynamic (CFD) covering inputs of fluid condition as flowing media, i.e: 1) Boundary Condition Inlet Is path fluid flowing input in normal condition without any phenomenon occurred. a) Mass and Momentum The momentum that occurs in fluid flow is influenced by mass and velocity with velocity vectors U, V, and W.
The direction engaged in treating the boundary is the normal direction to the domain. Component of flow velocity (Cartesian Velocity Vector) is by resultant: The total pressure, the plot for fluid is defined as: c) Speed of Mass Flow Rate The mass flow rate limit, determined through the direction of the component where the influx is mass, is calculated using the formula: 2) The boundary of Outlet Condition a) Outlet Speed The outlet boundary velocity component is a cartesian velocity vector component U outlet = U spec i + Vspec j + Wspec k (4) b) The outlet of Fluid Pressure The outlet fluid pressure is static inlet pressure plus the occurred changing pressure P tot = P static + ρU 2 3) Boundary of Wall Condition a) Walk Relative Static Pressure is: The mass distribution in the wall area is determined by the mass flow: where the value of F is calculated so that M tot = ∑all m, and the force is a total mass flow in the boundary wall. Therefore F can be used as follows :

B. Cavitation
Cavitation is defined as the forming process of the vapor phase of a liquid when the liquid is getting reduced pressure at a fixed ambient temperature. Generally, the liquid is considered to get cavitation if there is a bubble inside the liquid formed due to reduced pressure [9] [10] [15]. There are many causes that can lead to cavitation. The most common example is boiling water. In boiling water, the vapor pressure increases due to increasing water temperature.
In marine hydrodynamic cavitation is generally caused by fluid flow. That flowing is two-phase flowing consisting of liquid and its liquid steam. Phase transition is created due to changes in hydrodynamic pressure. Figure 1 shows the cavitation mechanism when a foil is put at a small hitting angle in the steady twodimensional flow without viscosity. Far ahead from this cross-section.
International Journal of Marine Engineering Innovation and Research, Vol. 5 (4), Dec. 2020. 255-264 (pISSN: 2541-5972, eISSN: 2548-1479 257 The velocity is steady and uniform are considered U0 and total pressure p0. For a particular flow line, the Bernoulli theory provides: Therefore, at any point of the flow line, the following equations apply p1 and U1 is pressure and velocity at that point: The change in pressure at that point is If U1 is faster than U0 so p1 will be smaller than p0 so ∆p will get the more negative points. At point S in front of the nose, the flow will be split. The fluid that follows the flow line will rotate at 90• and loses the entire speed of its momentum in the direction according to its movement along the flow line. Thus, at that S point (stagnation point) the velocity U1 is zero. q is the stagnant flow. The pressure at the point on the backside of foil is: and q is stagnation of that flow pressure on the back of the blade is So, p1 will be zero if, This means that flow will break at that point considering that water cannot withstand tension. Bubbles and cavity in cavitation will appear if, Pv is the water vapor pressure when water starts to boil. Because of that cavitation will occur if and ∆p is the pressure change and the geometry characteristic of flowing σv is called vapor cavitation rate. In this figure number p0 is static pressure which is the sum of the pressure hydrostatic and atmosphere. Pv vapor pressure is not affected by temperature. stagnation pressure q depends on the mass of fluid type and velocity. C. CFD Approach Simulation 1) Initial Stage This initial stage determines formula and problem identification to deal with. Furthermore, it will be a reference to formulate the implemented method. The discussed problem is how to analyze cavitation on changing of pitch system and number of blades in controllable pitch propeller (CPP)

2) Model Variation
Making of model propeller uses PropCad software. Propeller design is conducted by varying numbers of blades, diameter, and pitch propeller. The number of blades in this propeller design is 3, 4, and 5 blades by varying diameters of 30, 40, and 50 cm also by the varying pitch of propeller that is 0.4, 0.6, and 0.8. Below is the result of geometry design visualization in ANSYS software as shown in Figure 2. The number of blades from propeller design is 3, 4, and 5 with varying diameter is 30, 40, and 50 cm, also by varying propeller pitch around 0.4, 0.6, dan 0.8.
The angle of attack of the propeller is calculated by using the following equation: So, the angle of attack of the varying model is shown as follows: 3) Simulation using CFD The model of the ship and propeller developed in the previous chapter is simulated by using CFD software. Data gathered from the simulation process will be used as validation by using other software. There are several steps to conduct the simulation process based on the CFD approach.

a) Pre-Processor
Pre-Processor is step is early-stage where programming language of model design will be translated by Solver Manager. The model will be formed in such a way that several parts can limit conducted fluid flow and make the model as an object flowed by fluid. In this case, there are 2 parts in modeling as an object and to make a boundary for fluid. From the two parts object and the boundary as shown in Figure 3(a), then make fluid flow direction, they are inlet dan outlet flow so that fluid flow will touch the object.
The object is a wall CFX language. The developed model then will be imported into CFX software, whereas the previous model is the only surface but after imported into CFX it becomes solid.
The next step is meshing. In CFX, the developed model will be conditioned based on the real situation. For analysis purposes, it needs to enter the domain or conditioned model as shown in Figure 3 Examples of domains are type, temperature, velocity, and a number of iterations.

b) Solver Manager
Solver manager is the second step to CFX. This step is to function as a file translator from .def format to be res format. The next step can be translated by post-processor [8] [9].

c) Post-Processor
At post-processor step will show the calculation result conducted in the Solver Manager phase. The result is numerical data visualizing fluid flow on the model. Numerical data taken is the characters of fluid and its variables.       2) Discussion Based on the conducted simulation, it is identified that the pressure ratio on the propeller tends to increase at higher rotation as shown in Tables 2 to  4. For example, at rotation 125 rpm, the ratio of the pressure value between the face and backside is 11984 Pa. At rotation 175 rpm the ratio of the pressure value of the face and the backside is 16000 Pa. While at rotation 225 rpm, the value is 22459 Pa. Moreover, the pressure ratio on the propeller tends to increase on the higher pitch at the constant rotation. Whereas at rotation 125 rpm and pitch 0.4 the ratio between the face and backside is 11984 Pa. At pitch 0.6 the value is Pa, so it is concluded the trend is increasing.

Propeller Cavitation 1) Propeller Cavitation Analysis
Based on the CFD simulation conducted, can be easily known that the characteristic propensity of each propeller design of each rotation variation. The cavitation that occurred in each propeller can be identified by using the available menu in ANSYS using isosurface CFD. So that the cavitation area can be easily seen and calculated based on the simulation results. The simulation results for design propeller variation (rotation, number of blades, and pitch) on occurred cavitation is represented by the following Figure below.     2) Discussion Based on the simulation results as shown in Figure 13 to 21, it can be seen that the cavitation area that occurred on the propeller blade tends to increase on higher rotation. It proved by the calculation results shown in Tables 5 to 7. For example, on the propeller with 3 blades, diameter 300 mm, pitch 0.4 at rotation 125 rpm no indication of cavitation that is 0, then it increases to 1.44% at rotation 175 rpm, and getting higher at rotation 225 rpm to be 4.22% from the total propeller expanding area. Furthermore, on the propeller with 4 blades, diameter 300 mm, pitch 0.4 at rotation 125 rpm no indication of cavitation that is 0, then it increases to 1.68% at rotation 175 rpm, and getting higher at rotation 225 rpm to be 5.02% from the total propeller expanding area. Moreover, on the propeller with 5 blades, diameter 300 mm, pitch 0.4 at rotation 125 rpm no indication of cavitation that is 0, then it increases to 1.58% at rotation 175 rpm and getting higher at rotation 225 rpm into 3.20% from the total propeller expanding area. Besides that, the cavitation area percentage that occurred in the propeller blade tends to increase while the pitch is increased at constant rotation. Whereas at rotation 225 rpm and pitch 0.4 is 3.38 %, then it becomes 3.85 % at pitch 0.6, which is getting bigger at pitch 0.8 that is 4.22 %.