A Practical Approach for Design of Marine Propellers with Systematic Propeller Series
Original scientific paper Although there have been important developments in marine propellers since their first use in 1850, the main concept has been conserved. Parallel to the developments in the computer technologies in the past 50 years, methods based on the circulation theory are often used for the design and analysis of propellers. Due to the ability to predict the propulsion performance with just a few design parameters, the use of systematic propeller series based on open water model experiments is still widespread. The design with standard propeller series is usually made with diagrams developed from model experiments. Reading errors during the use of these diagrams are inevitable. In the presented study a practical approach is developed for preventing reading errors and time loss during the design with standard propeller series. A practical approach based on empirical formulas for the design and performance prediction for four-bladed Wageningen B propeller series is presented, and design applications for three propellers with different loading conditions are realized. The results obtained from the presented method are compared with those of open water diagram data and a good agreement between the results is observed.
Numerical Simulation of the Flow around Marine Propeller Series
The objective of marine propeller design optimization study is to obtain a propeller with minimum power absorption, maximum efficiency and good materials resistance. In this study, results of numerical simulation carried out on the flow around a conventional marine propeller are presented. The investigation focused on the aspects related to the influence of skew magnitude, thickness and blade number on the propeller performances. First, open water performances of a conventional propeller model DTMB 4148 was estimated using RANS (Reynolds Averaged Navier-Stokes) method. The flow around rotating propeller model was analyzed in the steady state using RANS approach of the commercial CFD (computational fluid dynamics) code fluent. The results provide good agreement with literature data. Numerical results show that the number of blades has an influence on the open water performances of marine propellers. It’s noticed that the best propeller has four or five blades from only the hydrodynamic aspect. The thickness blade effect has been studied for the same propeller model and compared to the blade with three different thickness values. Results of the calculation show that the blade thickness increases moderately the propeller efficiency. Finally, numerical simulation is performed to study the magnitude skew effect on the propeller blade performance, so three different models were generated. The results of the simulation show that the skew distribution has a positive effect on the open water performances of the marine propellers.
Inertia controlled instability and small scale structures of sheet and cloud cavitation
The present investigation focuses on the numerical simulation of inertia driven dynamics of 3-D sheet and cloud cavitation on a 2-D NACA 0015 hydrofoil. Special emphasis is put on the numerical analysis of the re-entrant flow, the break-up of the sheet cavity and the formation of clouds. We demonstrate that our CFD-Tool CATUM (CAvitation Technische Universität München) is able to predict even delicate 3-D flow features such as irregular break-up patterns, cavitating hairpin and horseshoe vortices, 3-D instabilities in spanwise direction and the formation and propagation of shocks due to collapsing clouds close to the trailing edge of the hydrofoil. The numerically predicted flow features agree well with the experimental observations of Kawanami et al [1]. INTRODUCTION Kawanami et al [1] investigate the 3-D structure of the cavitating flow around a NACA 0015 hydrofoil with cord length lcord=0.08 m and span lspan=0.15 m. The hydrofoil is placed within a rectangular test section, the angle of attack is 8.36°. They observe that the dimensionless spanwise length lcav,s/lcord of the clouds is roughly proportional to the dimensionless length lcav,c/lcord of the sheet cavity along the cord of the hydrofoil. Furthermore, they show the formation and shedding of multiple clouds if lcav,c<<lspan (Fig. 1). An interesting situation arises at the region of lcav,c/lcord>0.5, where Kawanami et al observe irregular break-up patterns. If lcav,c/lcord≈1, then the irregular pattern is replaced by the shedding of one single cloud that covers the full width of the test section. Franc [2] refers the previously stated 3-D aspects of cloud shedding as being intrinsic instabilities often referred as self-excited instabilities. Although the width of the test section might influence the characteristics of multiple shedding and irregular break-up, the physical origin of these instabilities seems to be determined by the cavity itself. It is known that the basic shedding mechanisms the formation of re-entrant flow, the resulting separation of the sheet cavity, the formation of a downstream traveling spanwise vortex and the subsequent break-up into numerous cloudy structures are mainly inertia controlled [3]. The aim of this investigation is to demonstrate that the occurrence of irregular break-up patterns and of 3-D spanwise instabilities is essentially inertia controlled as well. The key idea is to focus exclusively on the inviscid dynamics of cavitating flows by means of numerical simulation. This allows distinguishing inertia driven instabilities from instabilities due to viscosity/turbulence. PHYSICAL MODEL It is known that highly purified water can reach significant metastable thermodynamic states including tension [4]. The classical nucleation theory predicts that homogeneous nucleation is negligible for the typically arising flow conditions within hydraulic machinery. It is widely accepted that flow induced evaporation cavitation is dominated by heterogeneous processes such as the growth of liquid embedded gas bubbles [5]. Provided that the number density and the average size of heterogeneous nuclei are sufficiently large, the onset of evaporation occurs close to the saturation conditions defined by the Clausius-Clapeyron relation. Concerning natural (unpurified) water as working fluid within large scale hydraulic machinery, it is reasonable to neglect metastable thermodynamic conditions and to assume stable thermodynamic conditions exclusively. This assumption is questionable for operating conditions close to cavitation inception, yet it is sophisticated for sufficiently developed cavitating flow such as the investigated sheet and cloud cavitation. If we consider an open thermodynamic system at one instant in time t=t1 with given total mass m(t1), total internal energy U(t1) and fixed volume V, then the fundamental laws of thermodynamics imply that the only stable thermodynamic state is the equilibrium state, denoted by the subscript “eq”. It follows that the pressure peq(t1), the temperature Teq(t1) and the mass specific Gibbs energy geq(t1) are spatially and temporally constant within the specified thermodynamic system. Moreover, the system has minimal total Gibbs energy Geq(t1) and maximal total entropy Seq(t1). This information provides a unique and complete specification of the thermodynamic state including the phase properties (e.g. the mass fractions of liquid and vapor) of the system at time instant t=t1. The previous consideration is the thermodynamic basis for modeling and simulation of cavitating flows due to the following aspects.