CFD for propeller analysis

aperturaA marine propeller is normally fitted to the stern of the vessel where it operates in water that has been disturbed by the boat as it moves ahead. This zone is named “wake”.

Daniele Bruno

A propeller that revolves in the clockwise direction (viewed from aft) when propelling the vessel forward is called a right hand propeller.  A propeller can be realized in several ways, the geometry can change according to the engine power and torque, it’s usual to see light and slow speed boat with three blades propeller and fast boat with four or five blades.
From a geometric point of view, some important characteristics are: diameter, pitch, skew angle, rake angle, pitch ratio, expanded area. The blade’s shape is designed to create an high pressure zone on the front and a low pressure one on the back of the surface during the revolution movement. Imagine to cut a single blade in a cylindrical section it is possible to recognize the section of an airfoil. “L” is the lift generated by differential pressure, “D” is the resistance inducted, both under a fluid flow coming with velocity “v” and angle “a”.
When a propeller is moved rapidly in the water then the pressure in the liquid adjacent to body drops in proportion to the square of local flow velocity. If the local pressure drops below the vapor pressure of surrounding liquid, small pockets or cavities of vapor are formed. Then the flow slows down behind the object and these little cavities are collapsed with very high explosive force. If the cavitation area is sufficiently large, it will change the propeller characteristics such as decrease in thrust, alteration of torque, damage of propeller material (corrosion and erosion) and strong vibration excitation and noise. During 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. While these studies have shown great advancement in the technology, some issues still need to be addressed for more practicable procedures. These include mesh generation strategies and turbulence model selection.

Geometric modeling
Geometric modeling is carried out using 3D modeling software starting from one blade to build the entire propeller. The non-dimensional geometry data of the propeller were firstly converted into point co-ordinate data to generate the expanded sections, these sections stacked according to their radial distance along stack line these sections were rotated according to pitch angle.

Grid generation
The flow domain is required to be discretized to convert the partial differential equations into series of algebraic equations. This process is called grid generation. A solid model of the propeller was created in 3d software as a first step of grid generation. An example of the complexity of the complete domain is shown. The inlet was considered at a distance of 3D (where D is diameter of the propeller) from mid of the chord of the root section. Outlet is considered at a distance of 4D from same point at downstream. In radial direction domain was considered up to a distance of 4D from the axis of the hub. This peripheral plane is called far-field boundary. The mesh was generated in such a way that cell sizes near the blade wall were small and increased towards outer boundary.

Boundary Condition
The continuum was chosen as fluid and the properties of water were assigned to it. A moving reference frame is assigned to fluid with a rotational velocity. The wall forming the propeller blade and hub were assigned a relative rotational velocity of zero with respect to adjacent cell zone. A uniform velocity of flow was prescribed at inlet. At outlet outflow boundary condition was set.

Non-Cavitation condition
The performance of propeller is conventionally represented in terms of non-dimensional coefficients, i.e., thrust coefficient (KT), torque coefficient (KQ) and efficiency and their variation with advance coefficients (J). The software also estimated thrust and torque from the computational solutions for different rotational speeds (rps) of the propeller.
Shows the pressure distribution on surface of impeller blades in terms of pressure at advance coefficient assigned. The face and back are experiencing high pressure and low pressure respectively. However when propeller was operating at very low rpm it is not able to generate thrust, so a reverse trend in pressure was observed. This explains the development of thrust by propeller at high rotations whereas the propeller is contributing to resistance. It is evident that there is a concentration of high-pressure region near the leading edge of the propeller.

Propeller under Cavitation
When the operating pressure was lowered below the vapor pressure of surrounding liquid it simulates cavitating condition. In this condition two phases, water and water vapour are considered in simulations.  The pressure is expected to remain constant over the cavitating part of the blade. But some change in pressure distribution is observed when propeller started cavitating. This may be because of the fact that cavitation has just initiated or the computational solution could not capture the phenomenon properly. From this it is observed that the volume fraction is varying from around 0.7 to 1.0. Shows the development of cavities on propeller blade. It clearly shows that water got vaporized in particular area and this particular portion of the propeller blade is made to cavitate. Thus it reduces the thrust generated by the propeller and slight increase the torque demand.

Outlook
From a scientific point of view, CFD simulation software permit to obtain consistent results in term of absolute values and repeatability of the trials, mathematical models applied to solve analytically the Navier-Stokes equations are widely considered as standard. However many software hide some model adopted due to commercial and industrial secrets especially about the meshing algorithms and the technique to perform mesh movement and force calculation. Anyway CFD can help any naval architect to perform optimization campaigns to follow the goals of their project. About propeller study, my opinion is that it’s definitely possible replace trials in towing tank in.

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