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1、Computational Fluid Dynamics or How to Make a Good Boat FastDavid VacantiThe term CFD is showing up more often these days in articles describing the design efforts used tomake Volvo 60 round the world racers and Americas Cup yachts faster. Computational Fluid Dynamics or CFD actually covers a great
2、many engineering specialties and is not the sole domain of boat and ship design. In this article we will review what types of CFD products exist and hopefully provide some understanding of when and how CFD products are best suited to a project.Computational Fluid Dynamics is the application of compu
3、ters to the modeling of fluid characteristics when either the fluid is in motion or when an object disturbs a fluid. A few examples of a fluid in motion are water or chemical flow in pipes, heating and ventilation systems conducting cooling, heating or fresh air supplies to a building. Fluids in mot
4、ion also include flame and fire effects in combustion or jet engines. Surprised by these fields of interest?What about examples of an object disturbing a fluid? Examples include stirring paddles submerged in a tank of water and effluent in a waste treatment plant, aircraft of all kinds, cars and tru
5、cks at highway or racing speeds and even monohull sailboats, ship, multihull sailboats to name but a few.Obviously, an open mind is important when considering what constitutes a fluid. Fluids can exist in gaseous and liquid states and science has recently found that even some solids can exhibit flui
6、d like characteristics under right conditions. Scientists have found that some of the most spectacular and deadly landslides or rock falls behave as a fluid while the mass of stone and soil or sand is in motion, only to return to a most decidedly solid form when the motion subsides.The general field
7、 of fluid dynamics differs from the field of boat design in one critical way. Only boat design deals with a vehicle passing through the two fluids of air and water simultaneously. Our atmosphere is a compressible fluid, though not at yachting or even high-powered boat racing speeds. Air can change i
8、n density according to altitude, temperature and humidity. Water is anincompressible fluid that can vary in viscosity according to its salinity and temperature.For most of us, small effects such as variable salinity and temperature are not of concern, but can make the difference between winning and
9、loosing a major international yacht race.How do CFD programs Work?CFD programs are based on the laws of physics, such as the law of conservation of momentum, and special “boundary conditions”. The law of conservation of momentum states that the total momentum of a system remains constant regardless
10、of how the system may change. A boundary condition limits how and where a fluid can travel. A simple example is that motion of the fluid must emain tangent or parallel to the surface of an object passing through it. Another example is that pressure applied by the fluid against the object must be per
11、pendicular to the surface at all points.These laws and conditions are critical to the development of a CFD program because they allow an aerodynamicist to write equations that describe the system that is being studied. Without the physical laws and boundary conditions there would be no way to write
12、equations that describe fluid motion. The complex equations that result take into account the viscosity, mass and other characteristics of the fluid. The equations are written using integral and differential calculus and require specialized computer techniques to solve them. Typically the programmer
13、 writes an algorithm that makes a series of estimates using algorithms that iteratively solve the sets of equations by looking for “balance” in the system of equations. A final answer is obtained when the algorithm converges on a solution with an error that is sufficiently small for the desired accu
14、racy.Once an algorithm has been developed to implement the laws of momentum and boundary conditions, it cannot be applied to the entire surface of the hull and appendages at once. The surface area of the hull, keel and rudder are broken into thousands of small patches (collectively called a mesh) an
15、d the algorithm applied to each patch. Each patch in turn influences the fluid flow on the patch area of its neighbors and therefore the solution must account for the conditions surrounding the patch currently being solved. As a result the program must solve and resolve the equations for all of the
16、patches until the solution obeys the physical laws and boundary conditions. Sometimes the complexities of the laws of physics are too difficult to implement all at the same time. As a result the aerodynamicists choose to write programs that make certain limiting assumptions that permit the programming to become more practical and still result in reasonable results. A specific example arises in the case of what actually happens to fluids very near the surface of an object
