British team uses DSSP, extreme visualization to streamline skeleton bobsled aerodynamics

Strip bobsledding down to its essence – one human being and the slightest of sleds – and you have skeleton, a sport whose name comes from the bare-bone metal frames first used in 1892.

In many ways, skeleton is one of the simplest sports.  After the initial push-off, the only factors determining success are the driver’s skill and the external forces acting on driver and sled: gravity, airflow and friction.  Yet, these forces can take on so many characteristics and have such a profound effect on performance that researchers are using advanced approaches such as digital shape sampling and processing (DSSP), computational fluid dynamics (CFD), and extreme visualization to shave off the precious tenths of seconds that can make the difference between an Olympics medal and disappointment.

From bikes to sleds

In the forefront of research on applying advanced technologies to sports that rely on the closest of man-machine interaction is Sports Engineering @ CSES, operating out of Sheffield Hallam University in Sheffield, UK.  Led by Dr. John Hart, Sports Engineering @ CSES combined DSSP, CFD and extreme visualization to help the British Cycling team win four medals in the 2004 Summer Olympics in Athens.

The work of Sports Engineering @ CSES did not escape the attention of Dr. Kristan Bromley, a former engineer with British Aerospace.  After leaving the aerospace industry in the mid-1990s, Bromley began applying aerospace technology such as finite element analysis (FEA) and physical and structural simulation to skeleton bobsledding.  Along the way, he became a world-class athlete, competing for Great Britain in the Olympics and winning the world skeleton championship in the 2003/2004 season.

Bromley decided that the Sports Engineering group’s work could be a key factor in preparing for the 2006 Winter Olympics in Turin.  There was plenty of motivation for Bromley: Despite the fact that he would be going into the 2006 Olympics as the reigning skeleton champion, he had finished 13th in the 2002 Winter Olympics in Salt Lake City.  While that might be an expected result for an athlete from a country devoid of mountains and snow, it was not even close to being satisfactory for Bromley.

Bromley’s team, called Pro RACE, develops, manages and delivers R&D as part of a focused sled-development program.  As part of that program, Pro RACE turned over the CFD simulation research to Sports Engineering @ CSES.

Capturing sled and athlete

Although they might seem vastly different on the surface, there is a lot of commonality between Sports Engineering’s work with cyclists and the skeleton research: Both involve men riding on very lightweight vehicles, where the interaction of the human with the surrounding environment is just as important or more than the structural dynamics of the bike or sled.  And, in both cases aerodynamics is a major performance factor.

Simulating a real-world skeleton environment required much of the same type of work that Sports Engineering @ CSES did for the British Cycling team.  Sports Engineering researchers needed to capture precise geometry for the driver (also called a “slider”) and the sled, create a highly accurate digital model of the two, then simulate and visualize the complex airflow factors that affect performance.  Hart’s digital toolbox for making that happen included a ModelMaker X70 laser scanner from 3D Scanners, Geomagic Studio digital reconstruction software, Fluent GAMBIT software for preprocessing, FLUENT software for CFD simulation, and CEI’s EnSight for visualizing the myriad factors that come into play among driver, sled and environment over time.

Sports Engineering @ CSES initially scanned a skeleton sled with a mannequin to capture data and test out a few theories.  But the real work was done based on a scan of Bromley in racing position on a competition sled.

“It is essential to have the true geometry of the actual athlete for whom the equipment is being designed in an event like the skeleton, where aerodynamics can be so important and so athlete-specific,” says Hart.  Those specifics can come down to such physical characteristics as the size of the athlete’s posterior, which can contribute significantly to the overall drag according to Hart.

Hart used the ModelMaker X70 with a FARO Gold Arm to capture the sled alone and Bromley in position on the sled.  Besides the standard problems associated with scanning, such as shiny surfaces and areas difficult to access, capturing a live athlete entails other challenges, most notably trying to keep the athlete still during a process that can take around an hour.

“The athletes begin to twitch and ache, so you have to try to make them as comfortable as possible, and then make certain they get back in the correct positions if they need to get up and move about,” says Hart.  “It’s not so much of an issue with a skeleton athlete, as their sport-specific posture is lying down.”

Reconstructing the physical world

The combined scan data from the sled and Bromley on the sled was about five million points when it was brought into Geomagic Studio software for refinement and surfacing.  Geomagic Studio is the central tool for realizing DSSP, a term that describes the ability to capture shape data from the physical world and duplicate it accurately in a computer so it can be used for downstream design, engineering and custom manufacturing.

Once the raw point-cloud data was imported into Geomagic Studio, it was refined to remove any noise picked up from the scanner.  The data was automatically reduced to a workable size, while still maintaining dense point cloud data where needed for detail.

“Geomagic allowed us to easily clean up any imperfections in the scanned data, such as areas where Bromley might have moved inadvertently,” says Hart.  “The software smoothed the data, and enabled us to produce the high-quality NURBS model that we required for the simulation.  No other tool could have produced these types of accurate surfaces so quickly and easily from such complex scan data.”  

The Geomagic model was output as an IGES file and imported into GAMBIT, Fluent’s geometry and mesh-generation software.  The completed mesh was imported into FLUENT CFD software, which was used by Sports Engineering to simulate the aerodynamics of the driver and sled, and to determine where improvements can be made.

Seeing the data in new ways

While FLUENT results gave Sports Engineering good data on aerodynamics, researchers needed a higher level of visualization to illuminate the CFD results.  The FLUENT results were brought into EnSight for what is called “extreme visualization,” not because it is used only in extreme cases, but because the software enables interactions to be seen in new and revealing ways.

“There is no substitute for this type of visualization,” says Hart.  “It enables us to produce high-quality graphical output easily and quickly for detailed analysis, communication of results, and even marketing presentations.”

Hart’s group exported case file data directly from FLUENT into EnSight and then manipulated the model on screen to get the best views on what was taking place.  To display factors such as surface pressures and surface oil flows, researchers swept clip planes through the model.  This enabled them to analyze the entire model quickly.  Streamlines within EnSight were used to obtain detailed information on how swirling airflows are formed and where they migrate over time.

When researchers found something interesting, they generated a reference image within EnSight or produced an animation to convey results to the client.

Although Sports Engineering often uses CEI’s free EnLiten geometry viewer to distribute results to colleagues and customers, for this project results were communicated with still images for short technical reports and with animations generated directly from EnSight for face-to-face meetings.

From digital back to physical

Sports Engineering @ CSES cannot divulge details from its research for obvious competitive reasons, but Hart says results revealed some surprising regions of flow separation, and showed that certain factors have a larger influence than was previously believed.

As might be expected in a sport that entails a driver on top of a basic sled, body make-up plays a prominent role.

“Heavier sliders have an energy advantage over lighter ones, so the lighter ones will attempt to put on weight, usually through muscle bulk, which can actually have an adverse effect on drag depending on how the weight is distributed,” says Hart.

The CFD visualization results, along with structural analysis, enabled Bromley to refine the sled design and implement equipment changes that increase aerodynamic efficiency.  Design modifications from the digital environment were built into new sleds and equipment that were tested under training and race conditions.

“The research shows that drag can have a big impact over a typical skeleton run,” says Hart.  “As with all theoretical studies, however, this relies on the slider having a near-perfect run.  You can provide a slider or any athlete with the most aerodynamic piece of equipment available, but if they have an off day, any advantage quickly disappears.”

In Bromley’s case, it was an off day in Turin.  After placing third following the first run, he fell to fifth place, out of medal contention, after the second and final run.  Still, from 13th in 2002 to fifth four years later is remarkable progress, and only three-tenths of a second separated Bromley from a bronze medal.

No doubt there are other flow dynamics that can be tweaked based on DSSP and extreme CFD visualization.  If there are new efficiencies to be found for skeleton bobsledding, Bromley’s Pro RACE team and Sports Engineering @ CSES certainly have the tools and the know-how to find them.

Additional Resources

Engineers in many disciplines are increasingly combining DSSP, including reverse engineering, with CFD and extreme visualization to simulate real-world aerodynamics. Click here for more examples. For information on Sports Engineering @ CSES, visit: www.shu.ac.uk/cses

Images courtesy of Sports Engineering @ CSES, Sheffield Hallam University.