Slipping smoothly through the sea

Slipping smoothly through the sea
by Staff Writers
Washington DC (SPX) Sep 19, 2023

As ships, boats, and uncrewed underwater vehicles (UUVs) move through the water, they experience resistance, or drag, caused by waves and friction from water contact with the hull. Drag increases as the water traveling around the hull transitions from laminar flow - water that moves along smooth paths in neat layers - to chaotic, turbulent flow - water that moves in chaotic and unpredictable paths. Vessels require more power to accelerate in turbulent conditions.

With the goal of overcoming the effects of drag, DARPA announced its Drag Reducing Architected Geometries (DRAG) program. The program seeks to create optimized shapes and surface characteristics for ship and UUV hulls to reduce drag in transitional and fully turbulent flow conditions. This would enable increased speed and endurance, while reducing fuel use and emissions. The effort will focus on novel solutions that reduce drag on flat, curved, and complex surfaces.

"Previous research has enabled ship drag reduction in laminar flow conditions but not in transition and fully turbulent flow," said Dr. Susan Swithenbank, program manager in DARPA's Defense Sciences Office. "With the availability of new modeling tools and the ability to optimize highly complex shapes, we're now seeking novel approaches to achieve a breakthrough in more challenging turbulent flow conditions, where the drag coefficient is much higher.

"For naval vehicles you either need more power or less drag to go farther. The problem with adding power is that it means you need more batteries, fuel, or a bigger engine, which increases the size of your ship or UUV, adding more drag. We aim to lower the drag coefficient to allow increased speed without increasing power or to go the same speed using less power."

The program will explore various material structures, shapes, and coatings to decrease friction where water contacts the hull. DRAG is not focused on biofouling mitigation, development of new hull materials, manufacturing scale-up, or methods that require active air and/or polymer injection.

DRAG is part of DARPA's Disruptioneering effort designed to rapidly explore bold, high-risk ideas with the goal of accelerating scientific discovery. The 18-month effort comprises two phases: a nine-month first phase to develop a modeling and simulation tool to optimize surface solutions from flat plates to complex curvatures and then fabricate the optimized geometry for testing in a water tunnel. In the second phase, performers will use the design tool to develop the optimal solution for a three-dimensional surface and also fabricate and test it.

"This is a proof-of-concept research effort to see what's possible," Swithenbank said. "If we're successful in showing drag can be reduced in turbulent flow, it could lead to a larger follow-on program to address engineering and manufacturability challenges, as it applies to potential use cases for the technology."

For technical details and proposal instructions visit the DRAG Disruption Opportunity solicitation at SAM.gov