Jan A. Szantyr

An Experimental and Numerical Study of Tip Vortex Cavitation

An Experimental and Numerical Study of Tip Vortex Cavitation The article presents the results of the research project concerning tip vortex cavitation. This form of cavitation is very important in operation of many types of rotary hydraulic machines, including pumps, turbines and marine propellers. Tip vortex cavitation generates noise, vibration and erosion. It should be eliminated or significantly limited during the design of these types of machines. The objective of the project was to develop an accurate and reliable method for numerical prediction of tip vortex cavitation, which could serve this purpose. The project consisted of the laboratory experiments and numerical calculations. In the laboratory experiments tip vortex cavitation was generated behind a hydrofoil in the cavitation tunnel and the velocity field around the cavitating kernel was measured using the Particle Image Velocimetry method. Measurements were conducted in three cross-sections of the cavitating tip vortex for a number of angles of attack of the hydrofoil and for several values of the cavitation index. In the course of numerical calculations two commercial CFD codes were used: Fluent and CFX. Several available approaches to numerical modeling of tip vortex cavitation were applied and tested, attempting to reproduce the experimental conditions. The results of calculations were compared with the collected experimental data. The most promising computational approach was identified.

A complete design of ship propellers using the new computer system

A complete design of ship propellers using the new computer system The computer system presented in this article is composed of several program blocks for the complete design of ship propellers. The design calculations are based on a combination of the modified lifting line theory and on the vortex lifting surface theory. The system enables solution of the following design problems: - calculation of the scale effect on the ship wake velocity field, including the influence of the propeller and rudder on this field at the propeller location - maximization of the propulsive efficiency - optimization of the propeller blade geometry on the basis of the compromise between the cavitation and blade strength requirements - optimization of the number of propeller blades and blade geometry on the basis of the acceptable level of induced pressure pulses and unsteady shaft bearing forces - calculation of the blade spindle torque for the controllable pitch propellers. The computer system is equipped with many numerical options for graphical visualization of the input data, including an easy possibility of their correction and control of the intermediate and final results of calculations.

A computer system for the complete design of ship propellers

The integrated computer system presented in this paper is capable of conducting the complete design calculations of ship propellers, including their analysis in the conditions of a real inflow velocity field behind the ship hull. The system enables solution of the following tasks: • calculation of the scale effect on the velocity field in the area of propeller operation, • correction of this velocity field due to the presence of the rudder, • maximization of the propeller efficiency, • optimization of the propeller blade geometry on the basis of the cavitation and strength requirements, • optimization of the propeller number of blades and of the blade geometry on the basis of the level of induced pressure pulses and unsteady bearing forces. The computer system contains numerous options for visualization of the input data and the results of calculations.

Mutual hydrodynamic interaction between the operating propeller and the rudder

The paper presents a description of the algorithm and computer program developed for numerical analysis of the hydrodynamic interaction between operating propeller and ship rudder. The program is based on unsteady lifting surface model representing the propeller and on boundary element model representing the rudder. The program accepts arbitrary geometry of both the rudder and the propeller as the input data. Interaction with the ship hull is taken into account in the form of given non-uniform velocity field of the ship wake. The mutual hydrodynamic interaction is taken into account by simultaneous solution of the unsteady kinematic boundary condition on both objects. The results of calculation include time-dependent pressure distribution on the propeller and on the rudder together with fluctuating hydrodynamic forces on both objects. Apart from that the program is capable of detecting and describing the dynamic development of different forms of the cavitation phenomena on the propeller and on the rudder. The paper includes the results of calculation of the hydrodynamic characteristics and cavitation phenomena of different propeller-rudder configurations. These results are compared with experimental data wherever available. This comparison confirms the effectiveness of the described computation method as the tool for design and analysis of the propeller-rudder configurations.

Comparative analysis of the theoretical models of ideal propulsor, ideal fluid brake, ideal screw propeller and ideal axial wind turbine

Abstract The article presents a detailed discussion of the theoretical models of four different fluid dynamic devices: an ideal propulsor, an ideal fluid brake, an ideal screw propeller and an ideal turbine. The four models are presented with all relevant mathematical formulae regarding the forces, the power and the efficiency. It is demonstrated that the application of the model of an ideal optimum fluid brake according to the Betz theorem for determination of the maximum effectiveness coefficient of an axial wind turbine is not correct. In the case of a turbine the inclusion of important rotational flow losses may increase the maximum value of the turbine effectiveness coefficient above the level defined by Betz. Therefore the model of an ideal turbine should be an inversion of the model of an ideal screw propeller. This conclusion is supported by numerical calculations. It may influence the design procedures of wind turbines and may lead to increase in their efficiency.

Critical review of propeller performance scaling methods, based on model experiments and numerical calculations

ABSTRACT The article presents the results of experimental and numerical investigation of propeller scale effects, undertaken in co-operation of the Hamburg Ship Model Basin (HSVA), Germany, and Ship Design and Research Centre (CTO SA), Poland. The objective of the investigation was to test the adequacy of the methods currently used to account for the propeller scale effect and to develop possible improvement of the methods. HSVA has conducted model experiments in the large cavitation tunnel together with panel method and CFD calculations. CTO SA has performed model experiments in the towing tank, together with lifting surface and CFD calculations. Both institutions have suggested different new approaches to the problem and different new procedures to account for the propeller scale effects. In the article the procedures are presented together with the description of the underlying experimental and theoretical research.

A complete design of tandem co-rotating propellers using the new computer system

A complete design of tandem co-rotating propellers using the new computer system The computer system for the complete design of the tandem co-rotating propellers, presented in this article, has several common blocks and procedures with the computer system for the design of conventional single propellers, presented in detail in Polish Maritime Research No. 1 Vol. 16 (2009). In this article only these blocks and procedures are described, which are different in both systems. The comparative analysis of the designed tandem propeller and a conventional propeller is also included.

Numerical Analysis of the Unsteady Propeller Performance in the Ship Wake Modified By Different Wake Improvement Devices

Abstract The paper presents the summary of results of the numerical analysis of the unsteady propeller performance in the non-uniform ship wake modified by the different wake improvement devices. This analysis is performed using the lifting surface program DUNCAN for unsteady propeller analysis. Te object of the analysis is a 7000 ton chemical tanker, for which four different types of the wake improvement devices have been designed: two vortex generators, a pre-swirl stator, and a boundary layer alignment device. These produced five different cases of the ship wake structure: the original hull and hull equipped alternatively with four wake improvement devices. Two different propellers were analyzed in these five wake fields, one being the original reference propeller P0 and the other - a specially designed, optimized propeller P3. Te analyzed parameters were the pictures of unsteady cavitation on propeller blades, harmonics of pressure pulses generated by the cavitating propellers in the selected points and the fluctuating bearing forces on the propeller shaft. Some of the calculated cavitation phenomena were confronted with the experimental. Te objective of the calculations was to demonstrate the differences in the calculated unsteady propeller performance resulting from the application of different wake improvement devices. Te analysis and discussion of the results, together with the appropriate conclusions, are included in the paper.

Numerical prediction of steady and unsteady tip vortex cavitation on hydrofoils

ABSTRACT The article presents the numerical method for prediction of tip vortex cavitation generated on hydrofoils. This method has been developed in the course of numerical and experimental research described in earlier publications. The objective of the research was to design the optimum discrete grid structure for this specific computational task and to select the best turbulence model for such an application The article includes a short description of the method and a computational example demonstrating its performance. In this example the results of numerical prediction of the cavitating tip vortex obtained from two commercial CFD codes are compared with experimental photographs taken in the cavitation tunnel in the corresponding flow conditions. Altogether nine different flow conditions are tested and analyzed, but only selected results are included. The accuracy of the numerical predictions is discussed and the reasons for minor existing discrepancies are identified. The unsteady tip vortex calculations are also presented, showing the fluctuations of the transverse velocity components predicted for three cross-sections of the cavitating vortex kernel.

The crucial contemporary problems of the computational methods for ship propulsor hydrodynamics

The paper presents the state-of-the-art review of the contemporary computational methods in ship propulsor hydrodynamics, pointing out the crucial problems on which the international researchers community attention is, or should be, focused. The review is based on papers presented at important international conferences within the last four years. At the beginning four basic categories of the computational fluid mechanics methods are briefly presented: traditional lifting line and lifting surface, Boundary Element Methods, Reynolds Averaged Navier Stokes Equations and Large Eddy Simulation or Direct Numerical Simulation methods. Then the problems with application of these methods to specific propulsor hydrodynamic problems are discussed in greater detail. This presentation starts with the narrowly defined design procedures, i.e. determination of propeller geometry fulfilling the required dynamic parameters: ship speed, propeller thrust and rpm with maximum efficiency. Then the computations of the complicated flow around a propeller of given geometry are presented, including the complicated cases of manoeuvring propellers or hydro-elastic effects. Subsequently the important problems of determination of propellergenerated vortex wakes and prediction of the different forms of cavitation and their hydrodynamic consequences are discussed. A separate section is devoted to unconventional propulsors, concentrating on the most important: pod propulsors and waterjets. The paper ends with a summary pointing out the directions for future research and applications of computational methods in propulsor hydrodynamics.