Rankine Source Method for Seakeeping Predictions

2012 ◽  
Author(s):  
Heinrich Söding ◽  
Alexander von Graefe ◽  
Ould el Moctar ◽  
Vladimir Shigunov
Keyword(s):  
Steady Flow ◽  
Potential Flow ◽  
Transfer Functions ◽  
Stokes Equations ◽  
Computational Time ◽  
Ship Motions ◽  
Periodic Flow ◽  
Source Method ◽  
Rankine Source ◽  
Rankine Source Method

Model tests are usually used for the traditional seakeeping predictions (transfer functions of ship motions and loads in regular waves). Experience shows that numerical solution of Reynolds-averaged Navier-Stokes equations (RANSE) can provide accurate results for this task, however, such computations require too much computational time for the required large number of the loading conditions, ship speeds and wave directions and periods. Traditionally, potential flow methods are used for such computations at early design stages. Although potential flow methods can produce results very quickly for large number of conditions, viscosity effects (most important for the roll motion) have to be taken into account using measurements or RANSE computations. Rankine source method, applied to seakeeping problems perhaps for the first time by Yeung [1] to oscillating ship sections, is increasingly used in practical seakeeping analysis. This paper presents a three-dimensional Rankine source code GL Rankine. Patch method is used instead of the usual collocation method to satisfy boundary conditions on the solid body surface. Periodic flow due to waves is linearized with respect to wave and motion amplitude, taking into account interactions between the nonlinear steady flow and periodic flow due to waves and ship motions. The steady flow solution accounts for the nonlinear free-surface conditions, ship wave and dynamic squat. The paper shows results of the method for ship motions in waves in comparison with model measurements and RANSE simulations.

Rankine Source Method for Ship-Ship Interaction Problems

2013 ◽  
Author(s):  
Alexander von Graefe ◽  
Vladimir Shigunov ◽  
Ould el Moctar
Keyword(s):  
Free Surface ◽  
Steady Flow ◽  
Test Data ◽  
Model Test ◽  
The Body ◽  
Ship Motions ◽  
Source Method ◽  
Rankine Source ◽  
Rankine Source Method ◽  
New Formulation

A Rankine source method is extended and applied to ship-ship interaction problems. The method covers the nonlinear steady flow and linear seakeeping in the frequency domain. The nonlinear steady flow solution accounts for the nonlinear free-surface conditions, ship wave and dynamic trim and sinkage. Periodic flow due to waves is linearized with respect to the wave amplitude, taking into account interactions with the nonlinear steady flow following Hachmann approach, which considers the steady perturbation potential as constant in the body-fixed reference frame. This is advantageous for the prediction of ship motions at moderate to high Froude numbers. In this context, a new formulation of the boundary condition for the multi-body case is derived. Two examples are considered, overtaking in calm water and replenishment at sea. For a feeder vessel overtaken by a container ship, computed forces and yaw moment are compared with model test data. As an example of replenishment operation, interaction between a frigate and a supply vessel is studied. Ship motions are computed for two relative positions and three forward speeds and compared with model test data for the largest forward speed. The Rankine source method proves as more accurate compared with a zero-speed free-surface Green function method.

Seakeeping Analysis of a SWATH-type Trimaran Using Potential Flow

2015 ◽  
Author(s):  
Adrian S. Onas ◽  
Jaye Falls ◽  
Ivan Stojanovic
Keyword(s):  
Potential Flow ◽  
Transfer Functions ◽  
Numerical Models ◽  
The United States ◽  
Ship Motions ◽  
Naval Academy ◽  
Viscous Effect ◽  
Viscous Effects ◽  
United States Naval Academy ◽  
Institute Of Technology

Potential flow methods tend to over-predict ship motions, especially the lightly damped, viscous effect dominated modes, such as roll. For SWATH ships, it is not uncommon to also over-predict heaving and pitching motions because of the strong viscous effects dominating the energy dissipation mechanism due to significant vortex shedding generated by the submerged hull(s). Care should be taken whenever such viscous effect become a significant part the total damping. Such fact appears to render the potential flow approach less useful when analyzing the restored response of SWATH ships compared to conventional ships, such as monohulls. The Atlantic Center for Innovative Design and Control of Small Ships (ACCeSS), under ONR sponsorship, has been studying a hybrid trimaran hull form with a small waterplane area center hull, so-called the Tri-SWACH. Extensive towing tank testing has been undertaken with the intent to characterize the behavior of the Tri-SWACH and to provide data for the verification and validation of predictive tools. For previous work, the scale models of the Tri-SWACH were identical below the waterline, but lacked any realistic cross-deck structure above the waterline. This setup is ideal to capture the nonlinear ship motion behavior in future studies since the scale and numerical models were made identical above and below the waterline, and include a representative cross-deck. For the current study, the linear transfer functions and short-term statistics are obtained using the potential flow code WASIM, and are partially validated using experimental results obtained from the United States Naval Academy and Stevens Institute of Technology.

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