Designed with STAR-CCM+: The Becker Mewis Duct
By   |  December 21, 2016

Designed with STAR-CCM+

Steve Leonard is the Head of Research & Development at IBMV who are a wholly owned subsidiary of Becker Marine Systems, tasked with developing, engineering and launching innovative technological solutions into the maritime market. Leonard and his team performed the CFD calculations for the first Becker Mewis Duct in 2008, and have subsequently developed a process which employs 13 engineers and naval architects, delivering 100s of ducts per year.

“The success of the Becker Mewis Duct depends almost entirely on the CFD process that we use to define it,” says Leonard. “Without accurate CFD simulations, we wouldn’t be able to tune each duct to the specific flow conditions generated around each hull. Although there are similarities, the duct that we design for each vessel is absolutely unique and a result of the careful tuning of over 40 design parameters. No two are ever alike.”

Not only does Leonard’s team have to deliver guaranteed energy savings they also have to deliver them within a strict timescale. “From the moment we receive a new order, we have typically six weeks to find the required energy savings,” says Leonard. “This timescale is strictly fixed, by the fact that the towing tank slot is reserved well in advance and cannot be moved. If we can’t improve the energy efficiency of a given vessel within that time, then we’ve basically failed. There are no second chances.”

Dynamics pressure distribution on the duct and rudder

The marine industry tends to be conservative, and self-propulsion tests remain the benchmark for proving the powering performance of vessels for most shipbuilding contracts. Few customers are even aware of the intensive CFD effort that goes into designing and tuning their Becker Mewis Duct, concentrating only on the final fuel savings demonstrated during model testing. Any variation between CFD and towing tank predictions is investigated thoroughly using further CFD calculations.

The vast majority of CFD calculations are performed at model scale. To verify that scaling effects do not have a significant influence and also ensure good cavitation performance, the IBMV team runs a series of final full scale calculations

Although this problem seems well suited for an automated “optimization” process, in which a computer algorithm chooses the next design configuration (rather than an actual human), based on the parametric exploration of previous iterations, the Becker Mewis Duct does not lend itself easily to automated design exploration.

Vorticity magnitude on a cylinder section inside the duct showing the effect of the rotating propeller

The reason for this, Leonard explains, is that it is almost impossible to reduce the flow around the duct to a handful of numerical parameters that could be used to fully define the next design iteration. Instead, Leonard relies on a team of experienced Naval Architects and Hydrodynamicists who are tasked with visually inspecting all data that are automatically generated at the end of each STAR-CCM+ simulation, and identifying adverse flow features through the duct, fins and propellers, and suggesting a corrective action for the next iteration.

In most cases, the team is able to obtain optimal energy savings within about 10 design iterations, although some credit here must also go to the experience of Leonard’s team, who through the experience of fine-tuning many hundreds of these ducts are able to use their engineering judgement to define an initial design that offers a solid foundation for further improvement. The better designed the hull of the vessel is, the less energy is wasted in the wake, and the harder it is for Leonard’s team to obtain big savings.

With some excitement, Leonard fondly recalls the team’s solitary “one and done” duct design, in which it was subsequently shown that the initial design iteration delivered the required energy saving without the need for any further optimization. In reality, this is also a victory for the IBMV process, as the initial design was configured by an engineer who used knowledge from the hundreds of previous duct design studies when choosing the design parameters for this particular duct.

Nominal wake behind the duct colored by axial (longitudinal) component of velocity

Conclusion

The success of IBMV in delivering over 1000 Becker Mewis Ducts offers a clear demonstration of the value of engineering simulation (and in particular CFD) as a tool in the marine design process, informing decisions, and providing a constant stream of data to improve the real-world performance of vessels.

Without intensive design exploration, driven by experienced engineers, it would be impossible for Becker Marine Systems to deliver finely tuned energy saving devices that offer guaranteed performance within a strictly controlled time scale. Not only has this delivered multiple millions of dollars of fuel savings to their customers, but it has also played a significant role in reducing harmful CO2 and NOX for the shipping industry as a whole.

STAR-CCM+ allowed IBMV to discover better designs, faster.

More Info:
The Becker Mewis Duct was first introduced to the market in September 2008. The first full-scale installation was completed on the 54,000 tdw multi-purpose carrier STAR ISTIND of the Grieg Shipping Group, Bergen, Norway in September 2009. The estimated power saving for that ship is about 6%.

The AS Valeria, a 57,000 tdw bulk carrier, achieved fuel savings of 5.0% (predicted by CFD and confirmed in sea trials) resulting in the reduction of 1,002 tons of CO2 per year.

A vessel of 55,000 tdw will use about 160 tonnes of fuel per day at normal cruising speed. Over the course of a year, a 5% improvement in fuel consumption would save over 2,000 tonnes of fuel over the course of a year, resulting in cost savings of around $500,000.

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