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Publication : Self Published
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TP52 Performance Data - Spookie3 -- Superseries Cascais 2015
One way of trying to understand the amount of downwash produced by a lifting surface is called the "momentum" theory of lift. In this theory, the lift produced by a wing (fin, rudder, sail) is equal to the downward "push" it gives to the air that it passes through. By deflecting the air downwards, the wing is lifted upwards. The following analysis is fairly standard and can be found, for example, in Marchaj's Aerohydrodynamics.
A Smoothed Particle Hydrodynamics Study of Ship Bow Slamming in Ocean Waves
Smoothed Particle Hydrodynamics (SPH) is a mesh-free Lagrangian computational method suited to modelling fluids with a freely deforming surface. This thesis describes the development, validation and application of a two-dimensional Smoothed Particle Hydrodynamics algorithm to the problem of ship bow slamming in regular ocean waves. Slam events often occur in rough seas and have the potential to cause significant structural and payload damage due to the loads and subsequent whipping experienced by the ship. SPH is well suited to modelling ship bow slamming because the interaction between the bow of the ship and the water surface is of a freely deforming transient nature. The developed SPH algorithm was subjected to an extensive validation using both analytical and experimental data as a basis for comparison. The influence of each numerical correction – necessary for SPH stability – was evaluated using two theoretical problems free from the influence of external forces: the evolution of initially circular and square patches of fluid. Solid boundaries treated by the ghost particle technique were introduced and evaluated by way of the hydrostatic tank and the two-dimensional dam break. Still water impacts of two-dimensional wedges and hull cross-sections were simulated using the SPH algorithm and the results were compared with the experimental data of Aarsnes (1996), Whelan (2004) and Breder (2005). The complexity of the slamming problem was then increased by imposing the relative vertical velocity profile (between the hull and the water surface) measured during the ocean wave basin experiments of Hermundstad and Moan (2005) on a hull cross-section. Reasonable agreement between the simulated and experimental slamming pressures confirmed that the two-dimensional SPH algorithm could be applied to a three-dimensional problem through the use of a relative vertical velocity profile. Finally, the commercial ship motion prediction software SEAWAY and the validated SPH algorithm were combined in a 2D + t method to simulate bow slamming of a slender hull. The relative motion between the bow and the free water surface was extracted from the ship motion data and then imposed on a cross-section of a given hull form. Satisfactory agreement with the peak pressures measured on a model V-form hull in regular waves (Ochi, 1958) demonstrated that the developed two-dimensional SPH code is capable of modelling three-dimensional ship bow slamming.
This primer is about why we have wings at all. As Jack Moran says, wings are a thrust amplifier. Their magic is in their ability to defy gravity without using raw thrust from a fuel-guzzling rocket. Instead, wings use the air flowing past to create a vertical force called lift which seems to defy gravity. This tutorial is about how lift is created, how to estimate it, and how to make it happen.
The origin of lift is very simple: it is the result of having higher air pressure below the wing than you have above it. Very unlike a hammer, air can only impart forces to solid objects via pressure and friction. Those are the only two methods. I will repeat: lift is the result of having higher pressure below the wing than you have above it. Pretty simple eh?
The Dirty Little Secrets of Hull Design by Computer
There is nothing more flexible than a piece of paper, a pencil, and the human brain. Once you begin using a computer for hull shape design, you are forced into the limitations and idiosyncrasies of the program and its underlying hull geometry technique. In exchange, however, you get the following and more; automatic matching of all lines in all views, automatic lines drawings and offsets tables, accurate full-size templates, plate developments, hydrostatic and stability calculations, 3D Rendering. Many feel that because of these benefits and capabilities, there are few or no complications or difficulties in designing by computer. This could be blamed on something I call "the rendering effect." One look at a 3D rendered boat hull with all of the colors, shading, and all of the details defined in 3D, and it is hard to imagine anything a computer can't do. When I look at a picture like that, I think of the following: Is the geometry accurate enough for construction use? How long did it take to model the geometry? (Rendering is easy, modeling is difficult.) Is the hull fair? Renderings may look good in a brochure, but the 3D model used to generate the rendering may not be accurate enough for construction purposes. Most rendering programs hide unfairness in curved surfaces because they process the surfaces into triangles and the shading routines smooth the triangles to hide their edges. This article discusses some of the flexibility you give up and some of the difficulties you encounter to take advantage of these benefits. These are the "dirty little secrets" that most designers learn about only after they buy and start to use a hull fairing program. Designers should also know more about the tools they use and understand what restrictions are placed on them by the program's user interface and what restrictions are placed on them by the underlying NURB hull geometry.
