Fluid flows get more dynamic with CFD

As well as industrial applications, CFD software is seeing rapid take-up in other areas such as renewable energy and swimwear, writes Dean Palmer

Computational Fluid Dynamics (CFD) is successfully being used to optimise renewable energy production, and its use in power generation looks set to grow rapidly if the number of wind farms and tidal power plants increases as expected by 2010.

By allowing users to ‘virtually’ model air and fluid flow, CFD can, for example, help to identify the best site for a potential wind farm and optimise the efficiency of a wind or tidal turbine rotor.

Take the Coal Clough wind farm based in Lancashire. It is run by Renewable Energy Systems who wanted to find out how closely CFD predictions would agree with experimental data taken at the site, with a view to then using the technology to analyse the suitability of other potential sites.

An analysis was conducted, orientated in the prevailing wind direction, with sufficient upstream and downstream distance from the existing location of the turbines. The local topography was taken into account, based on gridded, topographic data with a 50m horizontal resolution and a 1m vertical accuracy. A mesh of one million cells was generated in which the necessary equations were performed iteratively, to create an overall picture of the farm. The grid was progressively coarsened in the vertical direction with the first cell layer approximately 0.05m off the ground and gradually increasing to 25m at the top boundary of the domain. By calibrating the CFD predictions with wind speed measurements taken at the actual locations of the turbines, the margin of error in initial tests, caused by the variation in topography, was almost entirely eliminated, leading to the production of an accurate wind map, predicting prevailing wind speeds across all points of the site.

A series of further studies focusing on wind speed have been conducted by TUV Nord, one of Germany’s Technical Inspection Agencies. It’s using CFD analysis methods to study the effects of spacing between individual turbines. The spacing is important because a lesser distance gives rise to wake effects on other turbines downwind, which can lead to decreased and changeable wind loads, reduced energy yield and vibration-induced fatigue on the rotors and on other nearby structures such as power lines.

Thomas Hahm, researcher at TUV Nord, told Eureka: “In order to accurately model the wake effect, blade pitch, wind speed and direction, turbulence intensity and length scale, and rotor speed are input for each simulation. Downstream distances of between six and 10 times the rotor diameter have been modelled so far, making it possible to identify and examine variations in wind speed, both within and at the boundary of wake effects resulting from differing turbine spacing arrangements."

The type of information resulting from the two examples given above can be extremely important in developing new wind farms as well as optimising existing ones. Wind turbines start to generate electricity at speeds of 10mph, with maximum rated power output achieved at 33mph, so electricity is produced for about 80 to 85% of the time. As wind speed varies over the terrain, and at differing heights, as well as in the wake of individual turbines, such analyses is critical in identifying the optimum locations for the turbines to be situated. Also, the visual impact of a wind farm can be an important factor in a successful wind farm planning application. By locating the turbines at the most efficient point, the number required could be minimised, increasing the likelihood of an outcome in favour of the developer.

CFD technology is also finding its way into other areas. Speedo, a leading swimwear company, has brought Formula One technology to the swimming pool with the launch of the fastest swimsuit in the world, Fastskin FSII. Speedo's R&D team has developed a swimsuit that increases the speed of the wearer by reducing passive drag by up to 4% more than other suits. Passive drag affects a swimmer in the streamline position, which is achieved after the initial dive and after each turn. During a 50m race, a swimmer is likely to be in the streamline position for up to 15m.

Speedo's R&D team began developing Fastskin FSII after the Sydney Olympics in 2000 by evaluating emerging engineering technologies that would enable them to take swimsuit design to a new level. It identified CFD, never before used in swimwear development, to achieve its objective.

Speedo worked closely with Fluent, a leading CFD software provider, to develop a ‘virtual swimmer’. This allowed Speedo to monitor fluid flow and make the swimsuit as hydrodynamic as possible. Fluent's software was incorporated into Speedo's R&D engineering design process to evaluate the drag and fluid flow characteristics around swimmers.

During the development of Fastskin FSII, Fluent’s software eliminated the variability of the results and enabled Speedo to conduct a greater number of tests and achieve more accurate results, a technique used extensively in Formula One.

Dr Keith Hanna, director of marketing and communications at Fluent, commented: "What Speedo has achieved by transferring motor racing technical design methodologies to the hydrodynamics of swimming and swimsuits is probably a first for an Olympic discipline, and certainly a milestone of applied engineering. I believe that in its wake this integrated technical design approach will be used in many other Olympic sports."

Speedo worked with elite athletes from around the world to develop the new swimsuit. These athletes will be at the forefront of Speedo's challenge in the Athens Olympics.

Tom Shelley

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