Influence of viscous effects on numerical prediction of motions of SWATH vessels in waves

Abstract

The accurate prediction of motion in waves of a marine vehicle is essential to assess the maximum sea state vs. operational requirements. This is particularly true for small crafts, such as Autonomous Surface Vessels (ASV). Two different numerical methods to predict motions of a SWATH-ASV are considered: an inviscid strip theory initially developed at MIT for catamarans and then adapted for SWATHs and new a hybrid strip theory, based on the numerical solution of the radiation forces by an unsteady viscous, non-linear free surface flow solver. Motion predictions obtained by the viscous flow method are critically discussed against those obtained by potential flow strip theory. Effects of viscosity are analyzed by comparison of sectional added mass and damping calculated at different frequencies and for different sections, RAOs and motions response in irregular waves at zero speed. Some relevant conclusions can be drawn from this study: influence of viscosity is definitely non negligible for SWATH vessels like the one presented: amplitude of the pitch and heave motions predicted at the resonance frequency differ of 20% respectively and 50%; in this respect, the hybrid method with fully non-linear, viscous free surface calculation of the radiation forces turns out to be a very valuable tool to improve the accuracy of traditional strip theories, without the burden of long computational times requested by fully viscous time domain three dimensional simulations.

Comparative Performance of Optimum High Speed SWATH and Semi-SWATH in Calm Water and in Waves

“…We agree with him that the viscous effects are very relevant for the prediction of SWATH vertical motions and should be always included if the final scope of the study is the exact prediction of motions in waves. In fact, using a unsteady viscous flow solver, for a different type of SWATH (Brizzolara et al, 2013), we recently demonstrated that it is not possible to get a reliable prediction of heave and pitch responses if viscous forces are not included in the estimation of the radiation forces. Additional references on this theme and on the fully non-linear viscous flow solver that we have adapted to the scope are added here (Bonfiglio & Brizzolara, 2013;Bonfiglio & Brizzolara, 2014).…”

“…Hydrodynamic forces are predicted by the inviscid BEM without correction for viscous effects of any kind. If this is not a drastic approximation in case of conventional monohulls and catamarans (Bonfiglio & Brizzolara, 2013), in case of SWATHs viscous radiation forces were proven to affect the amplitude of the response, particularly around the resonance frequency (see for instance Brizzolara et al, 2013 andCenteno et al, 2000).…”

The second generation of autonomous surface vessels: Optimized performance for AUVs assistance at Sea

“…Close to resonance, viscous and non-linear free surface effects are heavily affecting added mass and damping forces. The exact estimation of these forces is essential to obtain reliable motion predictions, as demonstrated in [18] by means of the viscous strip theory, previously mentioned.…”

“…The relevance of viscous effects for the accurate prediction of SWATH motions in regular and irregular waves was addressed in [18], where the added mass and damping of the 3D ASV-SWATH hull were calculated and compared with a potential flow based strip theory developed years ago at MIT [19]. Motions evaluated with the viscous method result considerably different from the pure potential ones: around the first resonance mode (piston model frequency) the calculated differences are 20% on the heave amplitude and 50% on the pitch amplitude.…”

“…The new model created for the two different ASVs uses a mesh morphing technique to allow for the motion of the body boundary. First results obtained with the new model were presented in [18] for the SWATH alone and more recently they have been completely re-executed in a new study aimed to simulate the multi-body motions of the ASV-SWATH with a submerged AUV in its proximity at the same time [19]. We resume herebelow the main results of the comparative investigation of the vertical motions (pitch and heave) predicted for the ASV-SWATH and the same equivalent catamaran design used for resistance comparison.…”