Naval Underwater Systems Center Research Articles

Optimal shading of the general line array

Optimal synthesis for the general line array is defined here to mean the determination of element currents to minimize the sidelobe level for a specified beamwidth. Presently, available methods for the solution of this synthesis problem rely on either (1) a special set of positions (equispaced or symmetrically spaced) for which globally optimum currents can be computed or (2) some mathematical programming method (e.g., gradient descent) which can give only locally optimum results. In this paper, the author gives an algorithm which will yield globally optimum element currents for any given set of element positions. The first step of this algorithm maximizes the ratio of the power in the mainlobe region to the power in the sidelobe region and may be computed explicitly, since it can be expressed as an eigenproblem. Each additional step in the algorithm requires an iterative solution which may not be worth the effort, since the first step alone yields excellent results. The algorithm has the advantage of being applicable to broadband synthesis and the consequent suppression of grating lobes whenever the specified element positions make that possible. The primary drawback is that some sets of element positions result in numerically ill-conditioned eigenproblems. An example is given to illustrate the algorithm. [This work was sponsored by the Naval Underwater Systems Center.]

Analytical Prediction of the Influence of Polymer Additives on the Shear Drag of Bodies of Revolution

THIS Synoptic discusses an analytical method for the determination of shear drag on a submerged body of revolution when a polymer additive is injected into the boundary layer. In the case of external flow, as for bodies of revolution, the polymer is normally injected into the boundary layer from storage facilities within the body. Since the polymer will diffuse through the boundary layer, the polymer concentration will vary both normal to the surface of the body and along the surface of the body. This analysis must account for the variation in drag reduction effectiveness of the polymer as a result of the change in polymer concentration. In order to predict the drag reduction, it will first be necessary to predict polymer concentration. This calculation was performed by Fabula and Burns for flow over a flat plate. The case of drag on a flat plate with a pressure gradient was presented in a paper by White, but he assumed the polymer concentration to be constant. The analysis that will be presented in this report is a continuation of work initiated at the Naval Underwater Systems Center, Newport, R.I.

Stress-Induced Noise in Magnetic-Cored H-Field Antennas

Work is being conducted at the Naval Underwater Systems Center in the design and development of low-noise, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</tex> -field antennas for incorporation in submarine antenna systems consisting of towed, horizontal, electric dipole <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> -field antennas. Utilization of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> -field and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</tex> -field antennas in a common cable will permit omnidirectional extremely low frequency (ELF) reception by a submarine. This paper discusses various experiments that have been conducted to determine the effects of magnetic core geometry on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</tex> -field antenna SNR for the case where stress-induced core noise is the dominant antenna noise. A substantial improvement in the SNR is realized by cold-working the antenna core into a helically wrapped structure. The mechanical strain relief, or insensitivity to tensional stresses achieved, gives the helical structure a definite advantage over the axial design.

New low sound velocity magnetostrictive transducer

The magnetic and magnetostrictive properties of a magnetostrictive scroll wound ring transducer made from “cube textured” nickel developed by Inco were measured and compared to the corresponding properties of a conventional nickel ring transducer. The conventional ring was fabricated by the Naval Underwater Systems Center and the “cube textured” ring; with nearly identical dimensions, by Inco. Both transducers were separately excited in air under identical conditions and their electrical input impedances measured with a pulse vector immittance meter. Using Butterworth and Smith and NDRC analyses of a magnetostrictive ring transducer, the following parameters were obtained as a function of the bias induction (1 ⩽ B ⩽ 5kG): sound velocity, Young's modulus, reversible permeability, effective coupling coefficient, magnetostrictive and stress sensitivity constants. The Young's modulus of the “cube textured” ring was 40% lower than that of the conventional nickel ring, resulting in a 20% lower sound velocity and radial resonance frequency. This reduction was obtained without any appreciable sacrifice of electrochemical coupling coefficient or magnetostrictive constant. The “cube textured” ring also exhibited a much lower loss angle.

Wisconsin Test Facility Transmitting Antenna Pattern and Steering Measurements

During August and September 1971, the New London Laboratory, Naval Underwater Systems Center, performed pattern and steering measurements on the Wisconsin Test Facility (WTF) antennas. The pattern measurements were made at 13 locations in Minnesota and Wisconsin (covering approximately 120° of arc), while the far-field steering tests were taken in Maine and North Carolina. To be certain that the receiving sites were acceptable, several components of the received magnetic field were measured at each site and plotted on a normalized cosine curve. The 45- and 75-Hz pattern measurements were made at a range of approximately 300 km, while the steering measurements were performed at 1.7 Mm. The principal results from the pattern and steering measurements were 1) the east-west (EW) antenna pattern is skewed clockwise; 2) the north-south (NS) antenna pattern is skewed counterclockwise; and 3) the effective conductivity under the EW antenna is greater than the effective conductivity under the NS antenna.

Results of the August 1972 Wisconsin Test Facility effective earth conductivity measurements

During August 1972, the Naval Underwater Systems Center measured the effective earth conductivity at 45 and 75 Hz beneath both antennas of the Wisconsin Test Facility (WTF). The H/I induction method of conductivity measurements was utilized, with each WTF antenna alternatively employed as the source. These measurements were made in line with and broadside to each antenna mainly at distances 45–75 km from the transmitter. To ensure that the receiving sites were acceptable, 14 components of the received magnetic field were measured at each of the 50 sites and then plotted on a normalized cosine curve. The principal result obtained from these measurements is that the effective conductivity under the east-west antenna is greater than that under the north-south antenna.

Session 5A (CONFIDENTIAL) 2:00 P. M. Thursday, December 6 UNDERWATER PROBLEMS II Chairman: Dr. William W. Murray, Naval Ship Research &amp; Development Center, Bethesda, Md. Cochairman: Mr. William Dale, Naval Underwater Systems Center, New port, R. I

Session 5A (CONFIDENTIAL) 2:00 P. M. Thursday, December 6 UNDERWATER PROBLEMS II Chairman: Dr. William W. Murray, Naval Ship Research &amp; Development Center, Bethesda, Md. Cochairman: Mr. William Dale, Naval Underwater Systems Center, New port, R. I

Engineering Acoustics Workshop on the Accuracy of Calibrating Underwater Sound Transducers

Measurement accuracy in the calibration, test, or evaluation of underwater sound transducers is a somewhat neglected subject. It is the purpose of this workshop to shed some light on the subject and the reasons for its neglect. A panel of four, representing three naval laboratories and an observer of European laboratories will address several questions. What do the laboratories claim for accuracy? What is meant by “accuracy”? What is the claim based on? Can more accurate measurements be made? What would it cost? Is there a need or demand for more accuracy? After a ten-minute informal presentation by each panel member, the floor will be open for comment and contributions by others who perform measurements, use the data, or are otherwise interested in the subject. Representatives of several industrial and university, as well as naval, organizations have been invited to comment. The panel members are: Joseph Blue, Underwater Sound Reference Division, Naval Research Laboratory, Orlando, Florida; Charles Green, Naval Undersea Center, San Diego, California; Joseph Navin, Naval Underwater Systems Center, New London, Connecticut; W. James Trott, Acoustics Division, Naval Research Laboratory, Washington, D. C., and Chairman, Working Group on Hydrophones, International Electrotechnical Commission Technical Committee on Ultrasonics.

Parametric Acoustic Waveform Generation from Spherical Primary Fields

Parametric acoustic secondary signals which are generated largely within the nearfield of the primary sound beam exhibit a waveform proportional to ∂2F2/∂t2, where F is the envelope of the primary waveform and t is the time. However, the secondary waveform can be expected to tend toward ∂F2/∂t as more of the parametric generation occurs in the farfield of the primary beam. In an experiment conducted at the Millstone Quarry facility of the Naval Underwater Systems Center, the latter waveform has been produced by the modulation of a 330-kHz primary pulse from a 1.5-in.-diam underwater sound projector. [This work was supported in part by the National Science Foundation.]

Far‐field, extremely‐low‐frequency propagation measurements, 14 March to 9 April 1971

From 14 March through 9 April 1971, the New London Laboratory of the Naval Underwater Systems Center conducted a far‐field, extremely‐low‐frequency (ELF) attenuation‐constant measurement test. Two sites located along the same great‐circle path were utilized. One site (3.9 Mm) was in St. John Island, Virgin Islands; the other (1.7 Mm) was near Swansboro, North Carolina. The horizontal magnetic‐field strengths were measured at a band of frequencies centered at 45 Hz and at 75 Hz to determine the attenuation rates for daytime and nighttime propagation conditions. The EW antenna of the US Navy ELF Wisconsin Test Facility, Clam Lake, Wisconsin, was the transmission source. The principal results obtained from these measurements were that (a) the daytime attenuation rate is higher than the nighttime rate at both 45 and 75 Hz and (b) the ionospheric excitation factors are quite different for daytime and nighttime propagation conditions. A comparison of these results with the data taken previously and concurrently shows excellent agreement.

Sound Propagation along a Sloping Bottom in Shallow Water

One of a series of sound propagation experiments recently conducted by the Naval Underwater Systems Center in the Gulf of Aaen took place across a basin with a sloping bottom. Three receiving hydrophones were located near one edge in 60 m of water. Results of this experiment in which bomb shots were used were compared with both a propagation model based on ray theory by J. P. Jones and P. W. Smith, Jr. (Bolt Beranek and Newman TM 84 of 7 July 1971) and a model based on normal mode theory. Since the received signal appeared as an impulse followed by a reverberant portion, each with different spatial properties, the two segments were treated separately.

Shallow Water Investigations at Naval Underwater Systems Center

The objective of these efforts is to formulate a model suitable for sonar performance prediction in shallow water. For this purpose, the BIF1 range, which is 18.6 miles long and 110 ft deep, was established with bottom-mounted acoustic sources and receivers and moored oceanographic telemetering buoys. Measurements of propagation loss, time smear, frequency smear, and, in addition, temperature profiles and salinity provided a data bank for average values and fluctuations of a number of shallow water acoustic and oceanographic parameters. These oceanographic data were supplemented by others obtained from hydrographic and aerial radiometer surveys and from geological studies. Evidence from tests using explosives as sound sources indicated that the lowest of the normal modes was predominant in the received signal. This led to the design and implantment of a normal mode source consisting of 25 hydrophones capable of transmitting any desired “low” mode in the frequency range 1–4.5 kHz. Normal mode theory and experiment helped resolve many previously noted anomalies. Scheduled additional studies should provide the data required for formulation of the shallow water model.