Application of a general CFD code to planning craft performance

Thornhill, Eric (2002) Application of a general CFD code to planning craft performance. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Performance prediction is an important part of vessel design. Common methods for predicting planing hull performance include the use of empirical equations and model testing. Empirical equations are often only applicable to similar hull types over a small range of parameters, while model testing is often prohibitively expensive, particularly for small craft. Ever increasing computer power is making the use of computational fluid dynamics (CFD) as a performance prediction tool a practical alternative. This work presents the results of a study involving CFD to evaluate the performance of a high-speed planing vessel moving at steady speed through calm water. -- After a review of the state-of-the-art in CFD methods, it was decided that an unstructured, multiphase, finite volume code employing the volume-of-fluid (VOF) method for free surface capturing would be best suited for the study. The use of a commercial CFD code was found to be the best alternative as they are publicly available, generally undergo extensive validation, have a wide user-base, and receive periodic upgrades and improvements. The code chosen was Fluent (v5.3). -- Fluent could not, however, be used directly to simulate the behaviour of a planing vessel. The performance of high-speed craft is intimately linked to the orientation of the hull at speed, which cannot be known a priori. Planing hulls rise and change trim angle in response to the pressure field generated by the flow. In order to solve for these changes in hull position, the simulation method had to ensure that dynamic equilibrium was achieved in terms of lift and trimming moment. This was accomplished with an iterative scheme wherein the flow field was solved for discrete hull orientations that were then adjusted based on force and moment results until the conditions of equilibrium were met. -- The work began with a set of physical model experiments used to provide the baseline from which the numerical results would be evaluated. Three sets of simulations were then performed to evaluate the prediction method. The first set fixed the hull orientation to match those measured in the physical experiments. This enabled a direct comparison of the numerical results to the physical results. A second set of simulations was then performed where only the equilibrium condition of lift was satisfied; trim angles remained fixed at the experimental values. The last set of simulations solved for equilibrium in both lift and trimming moment and represent the results of predictions that would be produced without the benefit of physical experiments. -- It was found that the CFD simulations produced high hull pressure forces compared with the experimental results. In the first set of simulations, this meant high drag forces. When the model was permitted to move vertically (with a fixed trim angle) in the second set of simulations, the numerical model lifted higher in the water. This reduced the pressure drag results, but decreased the wetted area and hence the frictional drag. During the last set of simulations, which permitted the model to trim and heave to achieve dynamic equilibrium, trim angle and sinkage were found to decrease relative to the previous simulation set. As a result, pressure drag was found to decrease, but frictional drag predictions improved. Values for total resistance were found to be low when compared with experimental results. The under predicted resistance results from the second and third set of simulations were a direct consequence of the over predicted hull pressures identified during the first set of simulations. Despite these high pressure values, the results of the predictions still followed experimental trends, and the procedure for solving dynamic equilibrium was successful.

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