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Keyword : free surface
Results 1 - 5 of 15
A Case Study on the Effect of Sweep and Variations in Free-Surface Cross Section Geometry on the Lift and Drag of Transom-Hung Sailboat Rudders
Conventional transom-hung rudders are often used on small sailboats because of their simplicity compared to rudders mounted under the hull; however, they present substantial performance penalties, including (1) the rudder is more likely to ventilate by drawing air down from the free surface, (2) the effective aspect ratio, and therefore the lift-to-drag ratio, is not increased by the mirror-plane of the hull bottom and (3) there is additional spray and wavemaking resistance that arises as a result of the rudder passing through the free surface. This case study focuses on a means to mitigate the last of these penalties, the increased spray and wavemaking resistance. While many transom-hung rudders are essentially parallel, or tapered with the maximum chord at the top where it meets the tiller handle, the reader will recognize that having the largest cross section of rudder at the free surface will generate significant spray and wavemaking resistance, especially when the rudder is turned. This study investigated the use of minimizing the rudder chord length where it passes through the free surface, demonstrating the findings by full-scale towing tests of a series of rudders designed for a Fireball-class dinghy. Running the tests at full-scale, therefore matching Reynolds number and Froude number, eliminated questions on scaling. Experimentation on the effects of sweep angle, section shape and chord length at varying angles of attack and velocities showed a noticeable increase in lift-to-drag ratio of foils with reduced chord length at the free surface and by sweeping the rudder forward. To complete the case study, a velocity prediction program was used to estimate the change in speed around a notional race course.
Investigation of Modern Sailing Yachts Using a New Free-Surface RANSE Code
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.
Numerical Simulations of a Surface Piercing A-Class Catamaran Hydrofoil and Comparison against Model Tests
Hydrofoil supported sailing vessels gained more and
more importance within the last years. Due to new
processes of manufacturing it is possible to build slender
section foils with low drag coefficients and heave stable
hydrofoil geometries are becoming possible to construct.
These surface piercing foils often tend to ventilate and
undergo cavitation at high speeds. The aim of this work is
to define a setup to calculate the hydrodynamic forces on
such foils with a RANS based CFD method and to
investigate whether the onset of ventilation and cavitation
can be predicted with sufficient accuracy...
Advanced CFD-Simulations of Free-Surface Flows Around Modern Sailing Yachts Using a Newly Developed OpenFOAM Solver
While plain vanilla OpenFOAM(OF) has strong capabilities with regards to quite a few typical CFD-tasks, some problems actually require additional solvers and numerical methods for efficient computation of high-quality results. One of the fields requiring these additions is the computation of large-scale free-surface flows as found e.g. in naval architecture. This holds especially for the flow around typical modern yacht hulls, often planing, sometimes with surfacepiercing appendages. Particular challenges include, but are not limited to, breaking waves, sharpness of interface, numerical ventilation (aka streaking) and a wide range of flow phenomenon scales. A new OF-based application including newly implemented discretisation schemes, gradient computation and rigid body motion computation is described. The new code is validated against published experimental data; the effect on accuracy, computational time and solver stability is shown by comparison to standard OF-solvers (interFoam / interDyMFoam) and Star-CCM+. The code’s capabilities to simulate complex ”real-world” flows are shown on a well-known racing yacht design.
Advancements in Free Surface RANSE simulations for Sailing Yacht Applications
The analysis of yacht hulls performance using RANSE based free surface simulations has become an accepted approach over the last decade. Access to this technology has been eased by the development of user-friendly software and by the increase of computational power. Results are widely accepted as superior to previous non-viscous approaches and have to compete with towing tank results in terms of accuracy. However, many practical applications suffer from a numerical smearing of the free surface interface between air and water which can be described as numerical ventilation. This problem occurs when the intersection between bow and calm water surface form an acute angle and is further pronounced if the stem is rounded or blunt. It is therefore especially linked to sailing yacht applications. The problem manifests itself as a non-physical suction of the air-water mixture under the yacht hull, causing a significant underprediction of viscous resistance. While this is the easily observable appearance of the problem, a second issue is its effect on wave resistance. It can be shown that wave damping is significantly increased, causing a prediction of wave resistance which is also too low. The paper provides a review of the Volume-of-Fluid method. It discusses the resultant implications for practical applications. A remedy to circumvent the problem is described and its impact on the accuracy of the result is shown. Simulations on an identical appended hull with and without interface smearing are compared. Effects on free surface visualization and numerical accuracy are shown. The paper finishes with a thorough verification and validation of a fully appended yacht in accordance with ITTC standards.