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Keyword : Reynolds-Averaged Navier-Stokes (RANS)
Results 1 - 5 of 50
Towards Unsteady Approach for Future Flutter Calculations
A new free surface flow RANSE solver has been developed based on the OpenFOAM framework. The solver addresses some of the main deficiencies of OpenFOAM’s standard free surface solver. It uses advanced higher order discretization schemes for the volume of fluid variable, a reconstruction of the pressure at the free surface for proper treatment of the jump of the pressure gradient at the free surface and a special method for the generation and damping of sea waves and ship generated waves at inlet and outlet of the flow domain. This new solver is used for the simulation of advanced flow problems for sailing yachts and small boats: resistance investigations at very high Froude number, investigation of the behaviour of sailing yachts in head waves and the surfing behaviour of a sailing yacht in following waves. The paper outlines the new solver and presents some case studies demonstrating its abilities.
Aerodynamic Analysis Around a C-Class Catamaran in Gust Conditions Using LES and Unsteady RANS Approaches
Wingsail is the propulsion mean adopted by the America’s Cup and C-class catamarans. This rig has improved aerodynamic performance with respect to conventional soft sails enhancing the yacht performance. However due to the higher forces acting wingsails, the yacht stability can easily be compromised especially in heavy gust wind conditions. The wingsail response to a gust has been then investigated performing numerical analysis in a C-class catamaran in downwind navigation conditions. Both a LES and an unsteady RANS approach were used for the simulations. The solutions given by the two approaches have been compared analyzing both the aerodynamic coefficients and the flow characteristics. The effect of the wind gust on the wingsail has been further investigated at different gust frequencies. Stall cells appear on the flap surface when the gust is taking into account affecting the wingsail aerodynamic performance.
This work presents an investigation into the aerodynamics of downwind sailing using different meth- ods for modelling turbulence, comparing Large Eddy Simulation (LES) and Reynolds-Averaged Navier- Stokes (RANS) methods. Pubblished experiments on a downwind sail plan [2] [8] showed areas of flow separation and vortex structures at the leading and trailing edge of the gennaker. The ability of LES to model the transition to turbulence within the shear layer leads to an accurate prediction of the leading edge separation bubble, which significantly influences the flow field around the top half of the sail. The tran- sient nature of the LES solution allows the computation of the creation and shedding of unsteady vortices at the leading edge and downstream of the sail draft. The effect of the vortex rolls being convected towards the trailing edge is to generate a boundary layer which is more resistent to separation. Comparison with the experimental pressure distribution shows the correct prediction of the separation by LES, while the RANS result shows a large area of stalled flow which limits the suction on the sail. As a result, the overall drive and side forces computed by the LES are in good agreement with the experiments with less than 3.5% error, while RANS underestimates their magnitude by more than 14%.
Hydrodynamics of Wind-Assisted Ship Propulsion Validation of RANS-CFD Methodology
A Reynolds-Averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for Wind- Assisted commercial ships. The modelling challenge presented by large separated flow structures in the wake of the sailing ship points to a conscientious validation study. A validation data set, consisting of hydrodynamic forces acting on the ship sailing with a leeway angle, was collected at the Delft University of Technology towing tank facility, for bare-hull and appended cases. Four hull geometries were selected to represent of the Delft Wind-Assist Systematic Series. Appended cases were designed to represent a broad range of appendage topologies: Rudder, Bilge-keels, Skeg, and Barkeel. The direct validation exercise for the bare-hull case was successful, with the validation level for the sideforce equal to 9.5% (fine mesh: 9M cells). An extended validation statement is made for simulations for the entire series. This exercise was successful for leeway angles equal to = [3 , 6 ]. The validation level (base mesh, 3M cells) for each force component is: ′=12%, ′=17%, ′=10%. The validation for appended geometries was not regarded as successful, with the exception of the Rudder case. The numerical uncertainty is the dominant contribution for the validation level, motivating a proportionate refinement of the grid. Here, it is sufficient to achieve parity with other contributions to the uncertainty within the larger context of the project.