Spindle torque
by Rasmus Carstens, project manager at FORCE Technology
The spindle torque on controllable pitch propellers is the torque needed to adjust the propeller blades, i.e. pitch, during operation of the ship.
Knowledge about the size of the required spindle torque is crucial in the design phase, because if the torque is underestimated there is a risk that the CP system used to turn the blades is dimensioned to weak and therefore cannot deliver the required torque. Consequently, the propeller blades cannot be adjusted to the desired pitch and the propeller cannot deliver the required power. An example could be a condition where maximum ship speed cannot be reached, because the blade cannot be turned to full pitch. Failure to deliver the required pitch setting is typically not discovered until the ship has been built and is put in operation and the solution often requires that the CP system is re-dimensioned and replaced, which is time consuming and very costly.
Therefore, to minimize the above problem reliable and accurate tools are required to minimize the occurrence of the problem. Looking at the problem from a technical point of view, the spindle torque consists of three components. These are: the frictional component originating from the mechanical parts sliding on each other, while the blade is turned to the desired pitch, the inertia component originating from inertial forces and moment of the rotating blade and finally, the hydrodynamic component originating from the impact of the water on the blade.
Concerning the frictional and inertia components there are currently different methods to estimate or measure them, but when it comes to the hydrodynamic part there is only methods based on potential theory available. The potential theory based methods do not include viscosity or turbulent effect, so it is interesting to if other methods, without these limitations, are available and accurate enough to be used for determination of the hydrodynamic component. RANS based CFD solvers includes both viscosity and turbulence and they have shown promising results for propeller computations. Therefore, it is very interesting to see if this method can used to be calculate the hydrodynamic component of the spindle torque and hereby help solving the problem described above.
Objective
In connection with CFD based spindle torque calculations, the ultimate goal is to come up with a complete model including hull, rudder and propeller.
For CFD calculations where the goal is to determine propeller thrust and torque for propellers working at self-propulsion point behind ships the CFD tools have shown promising results, even though such a model requires many CPU resources. However, when it comes to calculation of spindle torque, which requires a quite precise prediction of the pressure distribution on the blades, the propeller flow becomes much more complicated, because the considered conditions may contain strongly separated flows. This happens for instance when the blades are set in reversed position while the ship is still moving forward. For this type of flow the experience with the CFD tools is limited, so while the experience is built up it is necessary to work with a simpler model, which does not contain the hull, but still contains the physics of a typical condition used for spindle torque calculation. Therefore, the objective of this project is to model the propeller in an open-water condition in order to investigate how the CFD tool performs for conditions typically considered in connection with spindle torque calculation. In order to quantify how well the computation performs compared to reality validation against experimental data must be done.
Project activities
In the present project the RANS code STAR-CCM+ will be applied to model propellers in openwater conditions in order to calculate the spindle torque. To model the propeller in conditions where the flow separates on the blades is complicated, and added to the relatively scanty experience in this form of application, it will be necessary to consider a simple model, which only contains the propeller. A simpler (smaller) model has a shorter turn-around time, which allows for thorough testing of the method and gives more flexibility in terms of for instance mesh optimization while experience is built up.
The below-mentioned activities state how the numerical model can be built up, so that at the end of the project it can be demonstrated if it is possible to calculate spindle torque by means of CFD:
- Planning and definition of procedure
- Model setup and validation
- Computation of spindle torque for representative conditions and flow study
- Documentation and knowledge sharing between the parties
Initially the computations will be done for one propeller geometry, but depending on how well the CFD tool performs the idea is to considered additional geometries with different blade types. Due to limited experience with this type of flow simulation it is too early to say how many different geometries it will be possible to evaluate.
All computations will be made in model scale as it is common practice with RANS-based CFD.
This project will be formed by the following three partners:
• MAN Diesel
• Korsør Propeller A/S
• FORCE Technology
For further information on this project, please contact Rasmus Carstens (rac@force.dk), project manager, FORCE Technology.