Propellers and propulsion

Propellers and propulsion Kul-24.3200 Introduction of Marine Hydrodynamics Aalto University 02/11/2015 Introduction of Marine Hydrodynamics 1

Content of the course Resistance Propulsion Introduction, Momentum theory on propeller action Screw propeller Propeller-hull interaction Early design of a propeller Propeller main engine interaction Stopping, accelerating and backing properties Propeller cavitation Special types of propulsors Afterbody form of a ship Ship dynamics Introduction of Marine Hydrodynamics 2

30 Oct: Introduction of Cavitation Introduction of Marine Hydrodynamics 3

Propeller cavitation: Summary of the previous lecture What does cavitation mean? When does it develops? What is cavitation number? Introduction of Marine Hydrodynamics Aalto University 6

Propeller caviation Additional reading Matusiak J (2010) Laivan propulsio. M-176. Chapter 8 Matusiak J (2008) Short introduction to Ship Resistance and Propulsion. Section 5.10 Lewis E.V., editor (1988) Principles of Naval Architecture, Second revision. Volume II. SNAME. Chapter 7. Available in Knovel. Introduction of Marine Hydrodynamics Aalto University 7

Propeller cavitation Related exercises Exercise 5: Task 3 (deadline: 09 Nov.) Estimate maximum thrust of the propeller using Burrill s curves Exercise 8 Select a propeller for the ship that you design Introduction of Marine Hydrodynamics Aalto University 8

Outline: Propeller cavitation Types of propeller cavitation Effect of propeller geometry on the cavitation Blade area ratio Pitch Shape of the hydrofoil Effect of the cavitation on the performance of the propeller Secondary effects of a cavitating propeller Evaluation Introduction of Marine Hydrodynamics 9

Outline: Propeller cavitation Types of propeller cavitation Effect of propeller geometry on the cavitation Blade area ratio Pitch Shape of the hydrofoil Effect of the cavitation on the performance of the propeller Secondary effects of a cavitating propeller Evaluation Introduction of Marine Hydrodynamics 10

Types of propeller cavitation PNA Categories of hydrodynamic cavitation: Travelling Fixed Vortex Vibratory Classification according to the physical nature of the propeller cavitation Sheet* Bubble * Cloud* Tip vortex * Hub-vortex * * * Lecture notes * Introduction of Marine Hydrodynamics Aalto University 11

Types of propeller cavitation Sheet cavitation Glassy thin layer of vapour attached to blade. If it not changing rapidly with blade angle Ѳ, it does not cause much harm. Unsteady volume variations cause varying pressures and vibration problems. Starts normally at the leading edge, where the pressure as the minimum value. Introduction of Marine Hydrodynamics Aalto University 12

Types of propeller cavitation Sheet cavitation Sheet cavitation on the suction side Super-cavitation Introduction of Marine Hydrodynamics Aalto University 13

Types of propeller cavitation Bubble or cloud cavitation Is created as a result of unsteadiness of sheet cavitation or of a strong turbulence. Cavitation bubbles pass into a high ambient pressure region where they disintegrate This results in high valued and rapid pressure peaks that cause noise and erosion of the blade material Introduction of Marine Hydrodynamics Aalto University 14

Types of propeller cavitation Bubble or cloud cavitation Introduction of Marine Hydrodynamics Aalto University 15

Types of propeller cavitation Tip-vortex cavitation At the tip and at the root of propeller blade, tip vortices are formed If they are sufficiently strong, they start to cavitate, especially for a blade close to top position Root cavitation occurs seldom because of deep submergence (high hydrostatic pressure) If it occurs it causes much harm in a form of erosion Introduction of Marine Hydrodynamics Aalto University 16

Types of propeller cavitation Tip-vortex and hub cavitation Introduction of Marine Hydrodynamics Aalto University 17

Types of propeller cavitation Pressure side cavitation Note: Cavitation occurs normally at suction side CPP When revolutions are kept high and pitch is low. Angles of attack may get negative values, especially in a tip region As a result flow accelerates strongly Strong and narrow low pressure peak at the pressure side Unsteady cavitation and bubble cavitation occur. Introduction of Marine Hydrodynamics Aalto University 18

Types of propeller cavitation Types of cavitation and flow parameters Cavitation depends strongly on hydrofoil loading and on hydrostatic pressure Hydrofoil loading is well represent by angle of attack α e Propeller loading: advance number J Static pressure related to stagnation pressure: cavitation number σ. Suction side cavitation Pressure side cavitation Cavitation free region Cavitation free region Tip-vortex cavitation Suction side cavitation Pressure side cavitation Collapse of thrust Introduction of Marine Hydrodynamics Aalto University 19

Outline: Propeller cavitation Types of propeller cavitation Effect of propeller geometry on the cavitation Blade area ratio Pitch Shape of the hydrofoil Effect of the cavitation on the performance of the propeller Secondary effects of a cavitating propeller Evaluation Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 20

Effect of propeller geometry Parameters, that affect the dynamic pressure Dynamic pressure is related directly to V r2. Rule of thumb for maximum tip vortex velocity: 35 m/s Biggest effect Propeller blade area Pitch Shape of the hydrofoil Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 21

Effect of propeller geometry Blade area ratio Initial values can be obtained from Criteria of Burrill Keller s equation Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 22

Effect of propeller geometry Blade area ratio: Burrill Non-dimensional thrust compared to the stagnation pressure If the projected surface area is not known Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 23

Effect of propeller geometry Blade area ratio Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 24

Effect of propeller geometry Pitch Propeller pitch selected on the basis of model test series characteristics These were conducted in atmospheric condition with an aim of maximum efficiency As a result pitch spanwise distribution is constant These kind of propellers are used very seldom In practice In ship propellers pitch is reduced by 15% at root (P hub /P 0.7 = 0.85), that is the region of decelerated flow To secure good vibration and noise properties, pitch is reduced at the tip as well Big reduction of pitch at tip reduces propeller efficiency Final propeller design is ensured by lifting-line and/or surface computations CPP Easily noise and erosion due to cavitation if the pitch is significantly reduced and revolutions high Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 25

Effect of propeller geometry Shape of the hydrofoil In old times Hydrofoil was of the same shape as airofoil Typically used shape was NACA-four digit airfoil which had poor cavitation properties Strong low pressure peak at the leading edge Better option So-called NACA a = 0.8 mean line (modified) airfoil Pressure is distributed more evenly Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 26

Effect on the performance of propeller Beginning of the cavitation Not much effect on the open water characteristics of the propeller When substantial part of the blades are covered by sheet cavitation First: effect on the thrust Later: effect also on the torque As thrust decreases faster, the efficiency decreases Cavitation does not affect significantly the maximum efficiency of the propeller The effect depends upon propeller loading (J) and cavitation number σ Strong cavitation an increase of revolutions results in a decrease of thrust and torque Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 28

Secondary effects of cavitation Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 30

Secondary effects of cavitation Power spectrum of the pressure caused by the propeller Up to tens of khz Blade frequency f b = Z n Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 32

Rotation of the blades Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 33

Changing thickness of the hydrofoil Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 34

Changes in the volume of the sheet cavitation Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 35

Collapse of the cavitation bubbles Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 36

Secondary effects of cavitation Effect of propeller skew on vibration excitation The propeller blade with no skew (on the left) meets wake peak simultaneously at each radius Skew (right figure) smoothens the blade entrance in the decelerated flow region This decreases the time derivate of cavitation volume and decreases induced pressures Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 39

Secondary effects of cavitation Effect of propeller skew on vibration excitation Cavitation model tests: how the introduction of blade skew reduce the propeller excitation Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 40

Evaluation Relevant for the evaluation of the propeller-induced pressure Apart pressures, structural properties of a ship matter when judging vibration level Pressure is at maximum in the very vicinity of a propeller Different pressure expected and required for different ship types The presented values should be treated cautiously based on literature and Matusiak s experience (his own words: old and somewhat obsolete) Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 44

Evaluation Three categories of methods Model tests Theoretical methods Approximate methods Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 45

Evaluation Model tests The most reliable Conducted in a cavitation tunnel or in a underpressurized towing tank. Inaccuracy of the blade freq. component approx. 30-50% Noise predicted with a 5 db accuracy Expensive and slow to conduct Pressures measured in abt 20 points. Not always sufficient for vibration analysis Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 46

Evaluation Theoretical models of propeller induced pressures Difficult to predict by the theoretical means Difficult model of an unsteady cavitation Despite of it inaccuracy is usually satisfactory The best of the methods predict pressures with 30% - 100 % inaccuracy. Noise evaluation within 10 db Theoretical computations are cheap and fast to conduct Pressures can be evaluated on a large area Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 47

Evaluation Approximate methods to evaluate induced pressures Based on regression analysis Same type as Holtrop s method for resistance/propulsion evaluation Quite inaccurate Inaccuracy of 500% is not an exception Easy to use Combined with a reference technique may be a powerful tool Ducted propellers difficult to judge Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 48

Evaluation Approximate methods to evaluate induced pressures Holden s method The most popular one. Based on the measured data of 72 ships. Enables approximating the magnitude and the distribution of the pressure caused by the propeller. Frequencies: Blade frequency and 2 * blade frequency Vibratory level of the ship. Given: Parameters that the method needs and the limits on the allowed values of these parameters. Do not use it the case does not fulfil the limits! Does not take into account the skew. If skew is significant, apply another method for its effect (see earlier). Final pressure: the result of Holden multiplied by FS. Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 49

Evaluation When is the wake smooth enough? (BSRA criteria) Criterion 1 Maximum value of the wave when and Criterion 2 Maximum value of the whole wake Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 50

Evaluation When is the wake smooth enough? (BSRA criteria) Criterion 3 The width of the top of the wake: Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 51

Evaluation When is the wake smooth enough? (BSRA criteria) Criterion 4 Criterion 5 Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 52

Summary List and describe types of cavitation How do you take into account the cavitation in the design of the propeller? How does the cavitation affect the performance of the propeller? What are the secondary effects of the cavitation? How can you evaluate the cavitation? Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 53

References Matusiak J (2010) Laivan kulkuvastus. M-289. Available in Noppa Matusiak J (2013) Slides Propulsion ENG 3. Available in Noppa Principles of Naval Architecture, Volume II, Chapter 7. Available in Knovel Aalto University 5/11/2014 Introduction of Marine Hydrodynamics 54