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HISTORIEK - HISTORIQUE |
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La propulsion "Voith"
Les pales à incidence réglable du propulseur attaquant l'eau verticalement sont montées dans un rotor circulaire arasant le fond du bâtiment et tournant à vitesse constante autour d'un axe vertical. Les tourillons des pales dans le rotor sont dotés de biellettes reliées au point N. Si ce point N est excentré vis-à-vis de 0, les pales exécutant un mouvement d'oscillation sur leur axe. il se produit un sillage sur un côté et une poussée de l'autre côté.
Les bielles sur les croquis ne sont qu'explicatives. Dans les propulseurs, l'incidence des pales est commandée par un attelage. (Source : document Voith - Schneider) HYDRODYNAMIC PRINCIPALS OF CYCLOIDAL PROPULSION SYSTEMS The idea of this unique propulsion and manoeuvring system was born by the Austrian engineer Mr. Schneider in 1926. In the following a short explanation of the hydrodynamic principle will be given. The physical principle of the thrust generation by a VSP is comparable to a fish’s fin or a bird’s wing action. They are also producing simultaneously thrust and steering forces. Animals with such movements have the optimal adoption to their living environment.
On a cycloidal propulsor (VSP and VCR) the blades project below the ship's hull and rotate on a rotor casing about a vertical axis, having an oscillatory motion about its own axis superimposed on this uniform motion. The blade’s oscillating movement - a non-stationary process in hydrodynamic theory - determines the magnitude of thrust through variation of the amplitude, the phase correlation determines the thrust between 0 and 360 degrees. Therefore an identical thrust can be generated in any direction. Both variables - thrust magnitude and thrust direction - are controlled by the hydraulically activated kinematics of the propeller, with a minimum of power consumption. Consideration of the processes on each blade position during one revolution provides the simplest explanation of the blades velocities and the resultant hydrodynamic forces. 1.1 ACTUAL PATH OF ONE CYCLOIDAL PROPULSOR BLADE (CYCLOID)
Fig 1.1. Cycloidal path By superimposing the rotary movement of the rotor casing on a straight line perpendicular to the rotational axis (to represent the movement of the vessel), the blade of the cycloidal propulsor follows a cycloid. The rolling radius of the cycloid is equal to ? x D/2 and the forward motion of the propeller during one revolution is therefore ? x D x p. 1.2 VELOCITIES ON CYCLOIDAL PROPULSOR BLADE
Fig 1.2. Velocities on cycloidal propulsor blade for no thrust condition For the no thrust condition of the propulsor (the hydrodynamic lift is zero) the blades are set in such a manner that at each point the velocity w, resulting from the circumferential velocity u and the forward velocity ve, is directed towards the profile axis (zero lift). This basic law governs the motion of the blades: The geometric triangle NOPn is similar to the velocity triangle uve w for all blade positions. The perpendiculars to the profile axes for all blade positions during one revolution must meet at one point, “the steering centre N”. During thrust generation the steering centre N is always displaced at the right angles to the resultant thrust direction by the dimension ON from the centre of rotation O (eccentricity). For the no thrust condition N’ coincides with N. (See Fig. 1.3.) The ratio of the distance ON to D/2 corresponds to the ratio of forward velocity ve to the circumferential velocity u, “the advance coefficient ?”. As long as the propulsor generates no thrust the advance coefficient is identical to the pitch ratio.
1.3 FORCES ON THE CYCLOIDAL PROPULSOR BLADE
Fig 1.3. To generate thrust the propulsor blade profile has to be turned against the blade path by the angle a by moving the steering centre from N to N’. The ratio ON’ to D/2= ?o is the pitch ratio of a cycloidal propulsor. Through this angle of attack a hydrodynamic lift will be generated at right angles to the resultant velocity w, i.e. perpendicular to the cycloidal path. The magnitude of the hydrodynamic lift depends on the angle of attack a and the resultant velocity w.
1.4 THRUST GENERATION BY THE CYCLOIDAL PROPULSOR
Fig 1.4. The hydrodynamic lift varies during the blade’s revolution due to the “non-stationary” condition of the blades. Integration of the components of the lift forces created over the entire propulsor circumference shows: - the lift components acting in the direction of motion result in the propulsor thrust - the lift components acting at right angles to the direction of motion cancel each other out. Consequently only the lift forces acting in the direction of motion generate thrust. Since the thrust is always perpendicular to line ON’ (moored condition) or NN’ (free-running condition) thrust can be produced in any direction merely through movement of the steering centre N’. Due to the rotational symmetry of the cycloidal propulsor identical thrust can be generated in all directions. For moored conditions a circular thrust diagram is achieved through the possible movement of ON’ through 360 °. However, as thrust is perpendicular to NN’ for free-running conditions, a steering force can be produced additionally to longitudinal force up to available pitch limits. The basis of thrust generation is the hydrodynamic lift acting on the blades. Unlike screw propellers, the speed through the water over the whole blade is constant. The effective propeller area of a cycloidal propeller is about 60% bigger than the area of a screw propeller. Therefore the VSP works with a very low speed of rotation. Rotation at speeds of about 20 % of those used in screw propellers for comparable thrust are common. The hydrodynamic principle of the cycloidal propulsor is the basis that allows the control of thrust in magnitude and direction steplessly, precisely and quickly. 2 CONSTRUCTION OF CYCLOIDAL PROPULSION SYSTEMS
Fig 2.1 Kinematics for 5-bladed VSP The hydrodynamic principle of the blade action is produced mechanically by the kinematics (Fig 2.1.) inside VSP and VCR. For reasons of compact construction the kinematics must produce the correct angular movement of the blades through an eccentricity smaller than the steering centre eccentricity ?o x D/2o. On a modern VOITH SCHNEIDER Propeller this is achieved using crank type kinematics. The links of each blade actuating system are directly supported by the lower spherical bush of the control rod, which can be displaced eccentrically and connected to the crank, which pivots around the bearing pin fitted to the rotor casing. A connecting rod transfers this movement to the blade through the blade actuating lever. This crank type kinematics will be modified for the VCR to two blades. The rotor casing of a VSP carries 4 to 6 blades, and of a VOITH CYCLOIDAL Rudder, two blades around its circumference. The blade axes lie parallel to the propeller’s main vertical axis. The rotor casing is axially supported by the thrust plate and radially by a roller bearing. The roller bearing centres the rotor casing and transmits the thrust through the propeller housing to the ship’s hull, while the thrust bearing supports the weight of the rotating parts and the tilting forces generated by propeller thrust and gear tooth pressure. A reduction gear flanged to the propeller housing and a bevel gear drive the rotor casing. The crown wheel is connected to the rotor casing through the thrust plate and the driving sleeve. The control of the kinematics is achieved by the control rod, which is actuated by two hydraulic servomotors arranged at 90 degrees to each other. The speed servomotor controls the pitch component for longitudinal thrust (ahead and astern). The steering servomotor controls the pitch component for the transverse thrust (port and starboard). Based on the success of the more than 3700 VSP delivered to date, which have been the hallmark of manoeuvrability and reliability for 75 years in the shipping industry, the VOITH CYCLOIDAL Rudder is now under development.
VOITH CYCLOIDAL RUDDER
Fig. 4. VCR arrangement principle scheme and 3D - view As with the VOITH SCHNEIDER Propeller, the VOITH CYCLOIDAL Rudder has a rotor casing with a vertical axis of rotation. Two rudder blades lying parallel to the axis of the rotor casing project from it below the vessel's hull. This rotor is turned via a reduction gear by diesel, gas turbine or electric motor. The main characteristic of the VCR is that it has two different modes of operation: Passive and active. These two modes enables the VCR to give the ship very unique manoeuvring and propulsion features.
4.1 PASSIVE MODE OF OPERATION
In passive mode, the rotor casing does not continuously rotate but instead is slightly rotated from the longitudinal to produce steering forces much like a conventional rudder. Thus the locked rudder blades are adjusted relative to the inflow and transverse forces for steering are generated. The passive mode of operation of the VOITH CYCLOIDAL Rudder is identical in principal to a conventional ship’s rudder and is used at cruising speeds. But conventional rudders are designed for producing sufficient rudder forces with small inflow forces and at high vessel speeds the rudder area is oversized because of the squared dependence of rudder force to speed and produces additional drag resistance. But as this passive mode for VCR is used only for high speed operation, rudder area may be designed much smaller and appendage losses will be greatly reduced. Due to the reduction of rudder area, acoustic noise radiation will also be influenced positively.
4.2 ACTIVE MODE OF OPERATION
Fig 4.2 Active mode of operation In active mode of operation, the VCR rotor casing is rotated and the system functions like a VOITH SCHNEIDER Propeller as described earlier in this article. Controllable thrust, stepless in direction (0-360°) and magnitude is produced. Therefore an identical thrust can be generated in all directions. Both variables - thrust magnitude and thrust direction - are controlled by the hydraulically activated kinematics of the VOITH CYCLOIDAL Rudder with a minimum of power consumption. Main propulsion can be reduced to stand-by condition, CP-propellers may be in sailing mode while FPpropellers can be windmilling. This mode of operation is selected for slow speed operation when high manoeuvrability is needed, e.g. during man overboard, search and rescue, going alongside or in the harbour, both in narrow channels and while mooring and getting underway. Further with excellent manoeuvrability crossing of mine fields in clean corridors is possible. Manoeuvring inside harbours without infrastructure and tug assistance will be possible. In emergency situations including loss of main propulsion, the VOITH CYCLOIDAL Rudder guarantees take home capability. Unlike fin-stabilisers, VOITH CYCLOIDAL Rudders allow roll stabilisation even without vessel forward speed. The thrust direction of active VCR may be electronically controlled to oppose roll motion. As thrust direction can be varied quickly and precisely, excellent station keeping allows ROV operation and helicopter landing in sea-states much higher than today’s operational limits. 4.3 STATE OF DEVELOPMENT OF VCR
Fig 4.3.a Results of CFD-Calculation for VCR To get a deeper insight into the physics of the VCR Voith Schiffstechnik is using the CFD technology and experimental techniques. The use of the modern Computational Fluid Dynamics (CFD) Technology enables the calculation of the forces acting on the VCR. The solution of the Reynolds Average Navier Stokes Equation (RANSE) is possible due to the application of the parallel CFD-code (COMET). Only the parallel CFD code makes the calculation of the non-stationary flow field of the VCR in an acceptable time possible. The results of the CFD-calculation are used for the design of the VCR and for the prediction of the active and passive propulsion and steering forces
Fig 4.3.b VCR model test performed in VOITH circulation tank. VOITH has performed detailed model experiments with VOITH CYCLOIDAL Rudder in its own circulation tank for active as well as passive mode of operation (Fig. 4.3b). Blade profile, blade shaft position as well as scale effects have been varied. Based on the model experimental results and the CFD calculation a program for predicting forces/thrust in project stage was developed. As the mechanical construction of the VOITH CYCLOIDAL Rudder will be based on several thousand practical approved VOITH SCHNEIDER Propellers, even the prototype can be seen as proven technology. Detailed discussions with classification societies of the concept signals principal approval. At this moment control and interface is the next focus of the development.
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u circumferential velocity ve speed of advance w resultant velocity a angle of attack O propeller centre N steering centre NN´ displacement of steering centre A hydrodynamic lift W induced and profile drag R resultant hydro. force |