Naval Surface Warfare Center

About: Naval Surface Warfare Center is a facility organization based out in Washington D.C., District of Columbia, United States. It is known for research contribution in the topics: Radar & Sonar. The organization has 2855 authors who have published 3697 publications receiving 83518 citations. The organization is also known as: NSWC.

Micro-magnetostrictive vibrator using iron–gallium alloy

Abstract: A micro-magnetostrictive vibrator using an iron–gallium alloy (Galfenol) drive element was investigated. Galfenol is an iron-based magnetostrictive material with magnetostrictions greater than 200 ppm, a high relative permeability μ r > 70 and a Young’s modulus of ∼70 GPa. This material is machinable by conventional cutting techniques, and can operate under tensile, bending, and impact loads without degradation in performance. A micro-actuator using Galfenol, therefore, has advantages over a PZT type actuator in design simplicity, low drive voltage requirements, high robustness, and a wide temperature operating range. This paper describes the design, fabrication process, and performance of a micro-vibrator which utilizes either an un-annealed or stress-annealed Galfenol pin as the drive element. A displacement of 1.2 μm was observed with a high bandwidth of 30 kHz and a high tensile robustness withstanding a suspended weight of 500 g (6 MPa). The vibrator was also verified to be useful as a speaker which can generate clear sound from the power of a portable music player.

Survey of Detection, Mitigation, and Repair Technologies to Address Problems Caused by Sensitization of Al-Mg Alloys on Navy Ships

Abstract: Structural aluminum alloys are used in marine applications where light weight and increased speed are paramount. The 5xxx series alloys are the ideal choice, providing an optimum combination of as-welded strength and general corrosion resistance. A particular concern is sensitization in service. Aluminum is considered sensitized when a nearly continuous network of β phase forms along the grain boundaries. The β phase is anodic to the Al matrix, and, when exposed to sea water and sufficient loading, provides a clear pathway to stress corrosion cracking (SCC). The degree of β-phase precipitation is driven by a combination of time and elevated temperature. For over a decade, SCC of Al superstructures has plagued Navy ships. To address this issue, relevant technologies have been adopted, adapted, and developed to combat this issue. More specifically, the technology areas involve the detection of sensitization, the mitigation of SCC, and the replacement of corrupt materials. The technologies discussed herein a...

Motion planning for an autonomous Underwater Vehicle via Sampling Based Model Predictive Control

Abstract: Unmanned Underwater Vehicles (UUVs) can be utilized to perform difficult tasks in cluttered environments such as harbor and port protection. However, since UUVs have nonlinear and highly coupled dynamics, motion planning and control can be difficult when completing complex tasks. Introducing models into the motion planning process can produce paths the vehicle can feasibly traverse. As a result, Sampling-Based Model Predictive Control (SBMPC) is proposed to simultaneously generate control inputs and system trajectories for an autonomous underwater vehicle (AUV). The algorithm combines the benefits of sampling-based motion planning with model predictive control (MPC) while avoiding some of the major pitfalls facing both traditional sampling-based planning algorithms and traditional MPC. The method is based on sampling (i.e., discretizing) the input space at each sample period and implementing a goal-directed optimization (e.g., A★) in place of standard numerical optimization. This formulation of MPC readily applies to nonlinear systems and avoids the local minima which can cause a vehicle to become immobilized behind obstacles. The SBMPC algorithm is applied to an AUV in a cluttered environment and an AUV in a common local minima problem.

Turbulent kinetic energy transport in a corner formed by a solid wall and a free surface

Abstract: High-resolution DPIV and LDV measurements were made in a turbulent mixed- boundary corner, i.e. a turbulent boundary layer generated by horizontal flow of water along a vertical wall in the vicinity of a horizontal free surface. This work is an extension of an earlier numerical/experimental study which established the existence of inner and outer secondary flow regions in the corner. The inner secondary motion is characterized by a weak, slowly evolving vortex with negative streamwise vorticity. The outer secondary motion is characterized by an upflow along the wall and outflow away from the wall at the free surface. The objective of the current investigation, then, was to understand the combined effects of a horizontal, shear-free, free surface and a vertical, rigid, no-slip boundary on turbulent kinetic energy transport. The context of this work is providing physical insights and quantitative data for advancing the state of the art in free-surface turbulence modelling. Experiments were conducted in a large free-surface water tunnel at momentum-thickness Reynolds numbers, Reθ, of 670 for the DPIV studies, and 1150 for the LDV measurements. A high-resolution, two-correlation DPIV program was used to generate ensembles of vector fields in planes parallel to the free surface. These data were further processed to obtain profiles of turbulent kinetic energy transport terms, such as production and dissipation. In addition, profiles of streamwise and surface-normal velocity were made (as functions of distance from the wall) using two-component LDV. Key findings of this study include the fact that both turbulent kinetic energy production and dissipation are dramatically reduced close to the free surface. Far from the wall, this results in an increase in surface-parallel uctuations very close to the free surface. The degree of this anisotropy and the spatial scales over which it exists are critical data for improved free-surface turbulence models.

Microstructural banding and failure of a stainless steel

Abstract: The presence of microstructural bands in AL-6XN stainless steel plate has been examined. The bands, which consist of a high density of second-phase particles, are located near the midthickness of the plate, range in thickness up to 300 µm, and are continuous over lengths up to 50 mm. Chemical analyses of the microstructural bands indicate elevated levels of chromium and molybdenum, while orientation imaging microscopy identified primarily sigma-phase particles within the bands; a small volume fraction of chi phase was also found. Tensile specimens oriented in the short transverse direction of the plate show low ductility and exhibit a large variation in failure strains, depending on the continuity of the bands as well as the presence of large precipitate particles within the bands. When oriented in either the longitudinal or the long transverse direction of the plate, circumferentially notched tensile specimens exhibit comparatively high ductility, although at high stress triaxialities, the material was susceptible to specimen splitting parallel to the tensile axis due to cracking along microstructural bands.