Serge Toxopeus

Collaborative CFD Exercise for a Submarine in a Steady Turn

The application of viscous-flow solvers to calculate the forces on ship hulls in oblique motion has been studied for a long time. However, only a few researchers have published work in which the flow around ships in steady turns was studied in detail. To predict ship manoeuvres, an accurate prediction of the loads due to rotational motion is also required. In a collaborative CFD exercise, the Submarine Hydrodynamics Working Group (SHWG) performed calculations on the bare hull DARPA SUBOFF submarine to investigate the capability of RANS viscous-flow solvers to predict the flow field around the hull and the forces and moments for several steady turns. In the study, different commercial as well as bespoke flow solvers were used, combined with different turbulence models and grid topologies. The work is part of a larger study aiming to improve the knowledge and understanding of underwater vehicle hydrodynamics. In this paper, the results of the exercise will be presented. For several cases, verification studies are done to estimate the uncertainties in the results. Flow fields predicted by the different members of the SHWG are compared and the influence of the turbulence model will be discussed. Additionally, the computed forces and moments as a function of the drift angle during the steady turns will be validated. It will be demonstrated that using sufficiently fine grids and advanced turbulence models without the use of wall functions will lead to accurate prediction of both the flow field and loads on the hull.© 2012 ASME

Viscous-flow calculations for KVLCC2 in deep and shallow water

In the SIMMAN 2008 workshop, the capability of CFD tools to predict the flow around manoeuvring ships has been investigated. It was decided to continue this effort but to extend the work to the flow around ships in shallow water. In this paper, CFD calculations for the KLVCC2 are presented. The aim of the study is to verify and validate the prediction of the influence of the water depth on the flow field and the forces and moments on the ship for a full-block hull form. An extensive numerical investigation has been conducted. For each water depth, several grid densities were used to investigate the discretisation error in the results. In general, the uncertainties were found to increase with increased flow complexity, i.e. for larger drift angles or yaw rates. A dependency of the uncertainty on the water depth was not found. The predicted resistance values were used to derive water-depth dependent form factors. Comparisons with resistance measurements and with an empirical formula given by Millward show good agreement for deep as well as for shallow water depths. The CFD results give insight into the forces and moments acting on the ship as a function of the drift angle, yaw rate and water depth. A clear dependence of the forces and moments on the water depth is found for steady drift conditions. For pure rotation, this dependence is much more complex and only develops fully for larger non-dimensional rotation rates. The paper shows that CFD is a useful tool when studying the flow around ships in restricted water depths.

Incompressible flow calculations of blunt leading edge separation for a 53 degree swept diamond wing (Invited)

One of the challenges in simulating aerodynamic flows is the accurate prediction of onset and progression of separation. In earlier STO research, it was found that various predictions for three-dimensional airfoils at angle of attack showed different pitch moments due to differences in the predicted location of separation areas. Therefore, a diamond shaped wing was developed within the NATO STO AVT-183 research group, to isolate the blunt leading edge separation and understand the underlying physical mechanisms. In the present paper, two viscous-flow solvers dedicated to hydrodynamic applications are used to predict the flow around the diamond wing. Both codes solve the incompressible flow using unstructured grids and finite volume discretisation. The results comprise different grid set-ups, with/without peniche and different turbulence models ranging from isotropic or anisotropic statistical turbulence closures to hybrid LES turbulence models. Moreover, the role played by the discretisation error is assessed by using anisotropic automatic grid refinement procedures based on flow-related criteria. A detailed discussion is made, highlighting the similarities and differences of the results, the respective influence of modeling and discretisation errors and showing the potential of CFD tools to predict the type of flow under consideration.

Improvement of Resistance and Wake Field of an Underwater Vehicle by Optimising the Fin-Body Junction Flow With CFD

To control underwater vehicles appendages such as rudders or fins are generally used. These appendages induce added resistance and deteriorate the quality of the inflow to aft control surfaces or propeller, due to the formation of amongst others horseshoe vortices.In this paper, CFD is used to study the flow around a typical wing-body junction and to obtain insight in how to suppress the horseshoe vortex. For a generic submarine the impact of a range of modifications of the sail on resistance, propulsion and wake field is investigated. Design guidelines regarding the most promising modifications will be given. It will be shown that quite significant improvements of the resistance as well as the wake quality can be obtained by properly designing the junction between the appendage and the hull.Copyright

Optimisation of Resistance and Towed Stability of an Offshore Going Barge With CFD

For the transport of large structures over sea, offshore going barges are one of the preferred means of transportation. The resistance and towed stability of these vessels are of prime importance with respect to the economic feasibility of transport. To ensure acceptable towed stability behaviour, side skegs are generally placed under the aft ship of the barge. However, these skegs give additional resistance and thus undesired fuel consumption.SMIT and MARIN joined forces with the intention of designing a new ocean going barge with sufficient towed stability and superior resistance characteristics compared to the already low drag Giant 4 barge. The Giant 4, for which favourable tow characteristics were observed during its service, was used as a reference vessel. The first step was to modify the hull form in order to fulfil SMIT’s design requirements. Subsequently, a detailed CFD study into the flow around the barge and the size, shape and positioning of the side skegs was performed leading to a significant resistance reduction. However, this reduction in resistance led to a degradation of the towed stability of the vessel, which was compensated for by adding a centreline skeg. The final design has a 14% percent reduction in resistance compared to the existing barge, with comparable towed stability.A model testing program was performed in order to validate the CFD computations. The agreement between the CFD and experimental results was found to be very good, demonstrating that CFD tools can be used to optimise the resistance and towed stability of barges.Copyright