USA: NSWC’s Engineers Complete Surface Ship Model Testing
Aerospace engineers at Naval Surface Warfare Center (NSWC) Carderock completed surface ship testing in the subsonic wind tunnel (SWT), Nov. 29, collecting data to improve the aerodynamics of current and future ship designs.
When a ship is underway, the above-waterline structure experiences aerodynamic effects due to the wind which impacts the ship’s performance, particularly in severe conditions. Since ships are typically non-aerodynamic bodies and are operating on the sea surface, aerodynamic test techniques need to be appropriate for these configurations and operational conditions.
The data collected from the testing will enhance NSWC Carderock’s support to the Navy’s current and future shipbuilding programs by determining the proper sizing of ship models for testing, creating a more realistic test environment in the wind tunnel, as well as developing guidance for the future aerodynamic testing of ships.
“For over 70 years, Carderock engineers have served as the Navy’s technical experts in ship design,” said Kevin Kimmel, NSWC Carderock aerospace engineer and test director. “This testing is an important next step in understanding and simulating the aerodynamic environment around a ship at sea.”
NSWC Carderock partnered with the Office of Naval Research and the U.S. Naval Academy to complete the testing. Testing included the evaluation of the impact of wind tunnel wall boundary conditions on the aerodynamic loads experienced by the ship, evaluation of methods of simulating an atmospheric boundary layer (ABL), and study of the impact of model scale effects and wall boundary effects on ship airwake measurements.
The ABL is the region of the atmosphere adjacent to the sea surface and experiences significantly reduced wind speeds as the wind is slowed by friction with the ocean surface. Accurate determination of wind loads on a ship and their effect on stability and performance within the ABL is important for Navy ship design.
NSWC Carderock’s 8 foot by 10 foot SWT is used to evaluate models of ships, submarines, aircraft, buoys, marine structures such as oil rigs, unmanned air vehicles (UAV), unmanned underwater vehicles, and even Olympic bobsled designs.
For this testing, a set of four generic frigate models, at various scales, were fabricated from high-density foam material. The models were mounted above a ground board to simulate the ocean’s surface and to raise the model above the wind tunnel boundary layer that is built up along the tunnel floor.
NSWC Carderock engineers measured forces and moments on the models at various ship headings and took high-frequency flow measurements in the turbulent airwake near the ship’s stern, a region that is critical for safe helicopter and UAV launch and recovery operations.
Phase I of the testing measured the wall and blockage effects on the ship models inside the wind tunnel. Wind accelerates as it flows around models within the closed walls of the wind tunnel test section. That acceleration produces higher aerodynamic forces than would be experienced without the constraints of the tunnel walls. Proper test procedures and accurate evaluation of results requires a thorough understanding of these effects for non-aerodynamic ship geometries.
Phase II evaluated the ABL and its effects on ship forces and moments. Phase III evaluated model scale and wind tunnel wall effects on the ship airwake. Results from this test program will be used to plan more effective and meaningful wind tunnel tests for future Navy ships as well as current ship programs such as DDG 1000 and LCS.
“The techniques that we are developing will enable more effective ship performance simulation and ship design capabilities,” said Kimmel. “This can lead to cost savings for the Navy fleet by avoiding unexpected problems requiring fixes or redesign.”
Press Release, December 17, 2012