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.

Underwater Acoustic Navigation Using a Beacon With a Spiral Wave Front

Abstract: In this paper, a method for performing underwater acoustic navigation using a spiral wave-front beacon is examined. A transducer designed to emit a signal whose phase changes by 360 $^{\circ}$ in one revolution can be used in conjunction with a reference signal to determine the aspect of a remote receiver relative to the beacon. Experiments are conducted comparing spiral wave-front beacon navigation to Global Positioning System (GPS) onboard an unmanned surface vehicle. The advantages and disadvantages of several outgoing signals and processing techniques are compared. The most successful technique involves the use of a phased array projector utilizing a broadband signal. Aspect is determined by using a weighted mean over frequencies. Sources of error for each of the techniques are also examined.

Multi-antenna near field cancellation duplexing for concurrent transmit and receive

Abstract: Concurrent transmit and receive with multiple antennas is demonstrated through near field cancellation. Full-duplex operation is possible through cancellation of the transmit signal at the receiver as much as 50dB. By exploiting symmetry in an array of antennas, a centrally located receiver is able to operate without saturation in the presence of high power transmit signals. The transmitters are able to provide spatial diversity and 360-degree azimuthal coverage while maintaining the requisite cancellation. We demonstrate the basic concept and then explore the criteria within the RF components needed to maintain suitable cancellation.

Hilbert-Huang Transform Stability Spectral Analysis Applied to Flutter Flight Test Data

Abstract: A new method, the Hilbert-Huang transform (HHT), has been applied to analyze the aerostructures test wing (ATW) flight flutter data from NASA Dryden Flight Research Center. The analysis shows the yielding of the wing after the onset of flutter, but just before breaking off of the wingtip. Based on HHT, a new stability spectral analysis is also proposed that shows both positive (stable) and negative (unstable) damping. The stability spectral analysis further shows that the flutter of ATW bending occurs at 2-5 Hz in addition to 18 Hz as determined by modal analysis and identification. Both HHT and the Teager energy operator based nonlinearity indicator show that the vibrations of the ATW are nonlinear throughout the flight-test flutter maneuver.

Finite-time Lyapunov exponent-based analysis for compressible flows

Abstract: The finite-time Lyapunov exponent (FTLE) technique has shown substantial success in analyzing incompressible flows by capturing the dynamics of coherent structures. Recent applications include river and ocean flow patterns, respiratory tract dynamics, and bio-inspired propulsors. In the present work, we extend FTLE to the compressible flow regime so that coherent structures, which travel at convective speeds, can be associated with waves traveling at acoustic speeds. This is particularly helpful in the study of jet acoustics. We first show that with a suitable choice of integration time interval, FTLE can extract wave dynamics from the velocity field. The integration time thus acts as a pseudo-filter separating coherent structures from waves. Results are confirmed by examining forward and backward FTLE coefficients for several simple, well-known acoustic fields. Next, we use this analysis to identify events associated with intermittency in jet noise pressure probe data. Although intermittent events are known to be dominant causes of jet noise, their direct source in the turbulent jet flow has remained unexplained. To this end, a Large-Eddy Simulation of a Mach 0.9 jet is subjected to FTLE to simultaneously examine, and thus expose, the causal relationship between coherent structures and the corresponding acoustic waves. Results show that intermittent events are associated with entrainment in the initial roll up region and emissive events downstream of the potential-core collapse. Instantaneous acoustic disturbances are observed to be primarily induced near the collapse of the potential core and continue propagating towards the far-field at the experimentally observed, approximately 30° angle relative to the jet axis.