Correlation Strategy for Propeller Excited Pressure Fluctuations

Market demands have lead to a significant improvement of the vibration and comfort level, especially for complex ship types such as RoRo and RoPax ships. The need to optimise the design for efficiency does not allow to spend extra material with the single purpose to keep the vibration levels at the required values. The better strategy is to minimize the exciting forces as such, where the propeller is the most important source for discomfort, noise and vibration. Whenever a new ship design had vibration problems, the propeller could be identified as the main reason. Changes related to the propeller are a substantial cost factor. To minimize the propeller impact on the hull, the wake field has to be optimised on one hand and the design of the wake adapted final propeller on the other. The demand for low pressure pulses has lead to propeller designs characterized by low cavitation volumes, leading to the situation that in some cases the non-cavitating part becomes dominating for blade rate. Furthermore, the designs are characterized by the fact that the higher harmonics do not necessarily decrease, but some of them can take comparable values to blade rate. This makes it difficult to decide whether a specific propeller design is acceptable or not when the decision is only based on the judgement of the pressure pulses, which are typically measured in appropriate testing facilities. Therefore, two major development needs are obvious: Firstly, methods are needed to evaluate propeller designs and their impact on the hull structure before a blade is tested. This requires a combination of numerical methods and correlation techniques. The propeller induced pressure fluctuations have to be calculated with sufficient accuracy, taking into account the effective wake on one hand as well as the interaction with the rudder. These loads, which have to be generated during the initial design stage before the contract is made have then to be transferred to an FEM model of the steel structure which then allows to optimize the steel design to fulfill contract specification values. The paper describes the essentials of these methods and strategies. 1 Problem A prediction of propulsor induced pressure pulses is difficult because of the complex conditions within the flow in the field of the aftbody. The interactions between the wake field, the propulsor and the aftbody require a simulation of a fluid with high Reynolds numbers. The flow around a real ship has Reynolds numbers round about 109. Even, one profile of a propeller blade is working at a Reynolds number round about 107. Furthermore, cavitation is a problem which cannot be ignored. High Reynolds numbers cause in difficult turbulent flows, which are not easy to simulate with RANSE methods. Furthermore such calculations are very time expensive. For a prediction of pressure pulses within the project state such methods are not practical, yet. Methods of less numerical effort are of less physical accuracy. But it has to be analysed how big this error is and whether it is acceptable or not. An analysis of modern propellers shows that the non-cavitating part is one main cause of the pressure pulses. Because of low cavitation volumes of such propeller designs, the cavitating part of propeller are not any longer the primary reason. This effect can be recognised by comparing cavitating and non-cavitating measurements of the same design. There are measurements where a design has pressure pulses of 2kPa in the non-cavitating case and only 2.2kPa in the cavitating case. This shows that it is first of all important to focus more on the non-cavitating effects and to handle afterwards the cavitating part of the pressure pulses. Therefore, this paper concentrates its focus to the non-cavitating part.