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Feature 3 | CFD & HYDRODYNAMICS


Accurate model and full-scale viscous flow computations on propulsors


Marin’s Douwe Rijpkema & Guilherme Vaz explain the importance of accurate model and full scale viscous flow computations for propulsors.


A


substantial part of hydrodynamic research is related to the design of high-efficiency propulsors. Any


increase in propulsor efficiency leads to a reduction of fuel costs. In addition to the optimisation of conventional propulsors, such as (ducted) propellers or water-jets, new innovative concepts are realised, e.g. biomechanical propulsion, large area propulsion and pump-jets. In all cases an accurate prediction of propulsor performance for design and off-design conditions is essential. Viscous Computational Fluid Dynamics (CFD) methods are able to provide invaluable knowledge about the propeller performance and corresponding flow field. The open-water characteristics (thrust,


torque and open-water efficiency) of propulsors are traditionally determined experimentally. Next to experiments, the use of numerical methods in the propulsor design process is increasing. Lifting line methods or boundary element methods (BEM) are still the industry standard and applied in daily practice for propeller design. Recent developments in numerical methods and computational capacities show a shiſt towards viscous CFD solvers, RANS (Reynolds-Averaged Navier-Stokes) or LES (Large Eddy Simulations) [1]. Te advantages of viscous CFD include the improved modelling accuracy, the amount of detailed information extracted from the simulations and the possibility of full-scale analysis. At the recent The Royal Institution of Naval Architects


(RINA) symposium


Developments in Marine CFD 2011, a paper was presented by Rijpkema and Vaz [2] of Marin on numerical simulations for propellers in open-water conditions. The main points are summarised in this article. For the calculations the Marin in-house CFD code ReFRESCO was used [3]. ReFRESCO is an unstructured finite volume method, parallelised and targeted for HPC clusters, and solves the multiphase unsteady incompressible RANS equations. It designed, optimised and validated exclusively


is 52


Figure 1: Normalised open water diagrams for a skewed propeller. The lines represent a fit through the experimental values, the filled symbols the ReFRESCO (RANS) results and the open symbols the PROCAL (BEM) results.


for hydrodynamic applications. For the open-water analysis there were


presented, three propeller geometries which selected with varying design characteristics: 1) the INSEAN E779A propeller, a well-known and documented test case [4] with a relatively simple blade geometry and small variations in pitch angle, skew and rake in radial direction, see Figure 3 (top); 2) a contemporary propeller with moderate skew, see Figure 3 (bottom leſt) used in the European project Leading Edge and tested at SSPA [5]; 3) a ducted propeller as shown in Figure 3 (bottom right) tested at MARIN. Te presence of a duct adds additional complexity to the grid generation, the calculation, and the flow. For the latter an investigation of scale effects was made.


Open-water results The open-water characteristics for the three propellers have been determined both experimentally and numerically. A comparison of open-water characteristics


for the skewed propeller is presented in Figure 1. The RANS and experimental results are in good agreement, the trends in the open-water diagram are closely followed and the difference is in the order of 2 to 3% near the design condition and within 5% for off-design conditions. A similar accuracy was obtained for the E779A and ducted propeller in open water for model-scale. Additional to the CFD and experimental


results, a comparison with the potential flow method PROCAL [6] is included. PROCAL is a MARIN BEM potential-flow tool for propeller flow analysis. The potential-flow results presented in Figure 1 also show a good agreement with the experimental results. However, larger deviations are found with experiments than for the RANS results for complex geometries (high skew) and at off-design conditions, especially at higher loadings where viscous effects are not negligible. Figure 1: Normalised open water diagrams for a skewed propeller. Te lines represent a


The Naval Architect July/August 2011


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