Tip Geometry Effects on Performance and Erosion for Tip Rake Propellers
The search for increasing propulsive efficiency of ship propellers leads to creation of propeller blades with unconventional geometries. The prediction of the performance of such propellers is studied experimentally in model scale. However, the methods to extrapolate propeller performance data of innovative propeller designs to full scale is not fully developed. Therefore, there are uncertainties related to the propeller performance in full scale. But the availability of comprehensive computational fluid dynamic (CFD) tools, like Simcenter STAR-CCM+, can play a major role in exploring the propeller performance under different operational conditions for different propeller configurations i.e. different tip configurations. With ever-increasing computing power, it is becoming possible to develop robust and accurate computational tools that can resolve the behavior of multiphase complex fluids, involving the effects of cavitation and flow-unsteadiness, in addition to complex propeller-ship interaction simulations.
Scientific Challenge
Ship propellers need to offer good performance in an operational profile, meaning that good performance characteristics are required at design and off-design conditions. In order to single out propeller designs with overall good characteristics for further studies, it is important to have a reliable calculation to assess propeller behavior at off-design conditions. The complex unsteady flow characteristics for unconventional propellers that needs to be simulated to evaluate these operational conditions is a ubiquitous problem in predicting potential problems the propeller might face under operation, e.g., high pressure pulses, cavitation, vibrations and in the worst cases erosion.
In addition to the complexities coming from the flow around unconventional propellers, the study of the flow around the propeller should consider the interaction between ship and propeller. In order to account for this interaction to the model, the wake field at the propeller plane is set as input boundary to the steady and unsteady CFD simulations. The process for generating, modifying and evaluating design candidates should be performed automatically within a lead time of maximum 2 weeks.
Solution
To assess the performance of propellers with different tip geometries, the propeller is modeled in the computer aided design and optimization software CAESES based on a method presented by Praefke (2017)[1]. The fully parametric model allows to generate a Design of Experiments (DOE) with several variants that is otherwise difficult to generate with conventional propeller design software. The first stage of the study aims to find the design candidates that offer enhanced open-water efficiency at design point and it is evaluated with steady state RANS simulations. Then, the second stage is outlined to study and evaluate the propeller performance at off-design condition in a two-staged design exploration with a RANS simulations and surrogate method with subsequent DES assessment of selected candidates to account for complex flow and limit computational effort. Thereafter, surrogate models are constructed to further search for promising designs in an enhanced exploitation. Finally, two designs with negative and positive rakes are selected from the secondary exploitation for further analysis at design and off-design condition, where cavitation patterns and Erosion Potential Power (EPP) (Eskilsson & Bensow 2015) are evaluated with URANS-DES method.
Cavitation, vapor pressure and risk of erosion evaluation on propeller blade for positive and negative rake propellers
Results and Benefits
The access and use to Gompute computational facilities allows us to perform the large number of fully parametric numerical studies mandatory to devise accurate propeller performance, covering a wide range of operational conditions within a timeframe that outperformed the expected computational time compared to previous experience with other HPCs used before. Moreover, with such computational power, the unsteady DES assessment of selected candidates could be included in the design loop to assess other relevant variables needed to measure and evaluate the performance of the design candidates. These possibilities are expected to allow a better understanding of the variable and parameters that influence propeller performance as well as ship-propeller interaction.
- Praefke, E., Stoye, T. & Abt, C. (2017). ‘A generalized description of hydrodynamic parts based on aerodynamic profile sections’, Fifth International Symposium on Marine Propulsors, Espoo, Finland.