Hydrodynamic Modeling
Tuesday 9th November 2010, 1600hrs–1730hrs
Chaired by Jim van Antwerp Propulsor Performance Scaling Utilising a Combined Experimental/Numerical Methodology Ms. Amanda Dropkin, Naval Undersea Warfare Center (Division Newport), United States Dr. Stephen A. Huyer/Dr. Charles Henoch Naval Undersea Warfare Center, USA Propulsor design methods utilise Computational Fluid Dynamics (CFD) to develop initial propulsor configurations and predict the full-scale in-water performance of these optimal designs. Like all numerical models, these CFD models need experimental validation to provide a sufficient level of confidence in the design. The actual data needed to validate CFD models, which include propulsor inflow and outflow velocities and thrust, are impractical to collect for full-scale vehicles in water. As a result, the in-water propulsor performance is often significantly different than CFD predictions. Another approach in the propulsor design process is to experimentally test a subscale version of the vehicle and appropriately scale the results. This scaling is often unreliable due to fundamental differences between open water conditions and the flow in the laboratory facility. This paper will present a method to combine CFD modeling with subscale experiments to improve full-scale propulsor performance prediction. Laboratory experiments were conducted on a subscale generic torpedo model in a 12’’ by 12’’ square water tunnel. This model included an operational ducted post-swirl propulsor. Laser Doppler Velocimetry was used to measure several velocity profiles along the torpedo hull. A force dynamometer was used to measure the vehicle forces to estimate the propulsor thrust and torque. The CFD models were constructed using the commercial CFD code, Fluent®. Initially, two-dimensional simulations investigated a shrouded hull case without a propulsor to understand the axisymmetric flow development and investigate methods to best match the propulsor inflow. Full 3-D flow simulations were then conducted with an operating propulsor and compared with the corresponding subscale experimental data. Finally, simulations were conducted for full-scale tests and compared with actual in-water data. It was expected that the numerically found advance ratio will be higher than the experimental value as it is not computationally feasible to model every nuance of the torpedo geometry, underestimating the body drag. Therefore, the numerically estimated advance ratio of 1.81 was satisfactorily close to the experimentally found value of 1.74. Comparison of Computational Meshing Approaches for a Generic Propeller Mr David Clarke, Defence Science and Technology Organisation (DSTO), Australia Greg Seil, Sinclair Knight and Mertz, Australia; Daniel Norrison/Ronny Widjaja/Brendon Anderson, Defence Science Technology Organisation, Australia; Paul Brandner, Australian Maritime College, Australia Computational fluid dynamics (CFD) packages that employ Reynolds Averaged Navier Stokes (RANS) approaches are increasing in their application. Comparison with model test data is still necessary to complete validation of the CFD simulations. However, whether due to expense or other reasons, it’s not always possible to undertake validation activities. So, in order to be confident with the use of a particular set of CFD tools for a problem type, it’s useful to benchmark the various approaches to the problem. As in many applications, modeling of a propeller, particularly a highly skewed one, can be a complex task. The choice of approach to the meshing is often a compromise between accuracy and complexity of the task. The structure and resolution of a mesh is crucial to the accuracy of CFD predictions. An understanding of the y+ requirements for particular turbulence models is also necessary. A proper grid sensitivty study is essential to the prediction of the hydrodynamic performance of a propeller. Earlier work by the authors compared the use of the ANSYS solvers FLUENT and CFX for a highly skewed 7-bladed propeller using a hybrid mesh. This paper extends that work and provides comparison of CFD simulation results for structured, unstructured and hybrid meshes of the same propeller, with simulations using the ANSYS CFX and FLUENT solvers and variouvarious turbulence models. These will be compared to previously published experimental data. Small Scale Model Testing of the SUBOFF Submarine Shape Mr David Clarke, Defence Science and Technology Organisation (DSTO), Australia Paul Brandner, Australian Maritime College, Australia; Brendon Anderson/Ronny Widjaja, Defence Science Technology Organisation, Australia While expensive, model scale experiments continue to play an important role in determining the performance characteristics of submarine hullforms. Infrastructure costs to maintain facilities, instrumentation and trained staff are high, not to mention the costs associated with model production. It goes without saying that these costs increase with the size of the facility (and model) and depending on the critical nature of the data required, a compromise takes place between the use of computational tools and the need to have real physical data for validation. Many universities maintain small to medium experimental facilities, compared to larger research organisations, to enable them to undertake research and activities complementary to the teaching program. While the use of smaller facilities is more affordable, applied research, particularly high Reynolds number applications, may be affected by the use of smaller facilities. Much like the work that’s been undertaken to benchmark computational tools, it is useful to benchmark a facilities’ experimental capability. The DARPA SUBOFF Submarine shape has been extensively tested and the data published in the literature. This has provided good benchmark data for computational methods and can be used for benchmarking experimental capability. The Australian Maritime College has recently completed the commissioning of its new cavitation tunnel. The variable pressure tunnel has a 0.6m x 0.6m square x 2.6m long test section which can reach flow speeds in excess of 12m/s. The size of models that can be tested in the facility is limited. This paper will describe a series of measurements made on a 1.8m model of the SUBOFF Submarine shape in the AMC cavitation tunnel with Reynolds numbers approaching 20x106. Measurements of surface pressure, drag, and wake velocity are compared to published data on SUBOFF for comparable Reynolds numbers which were achieved with a larger model scale.
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