Propagation of the difference frequency wave generated by a truncated parametric array through a water‐sediment interface

1988 ◽  
Vol 84 (S1) ◽  
pp. S8-S9
Author(s):  
Liu Wensen ◽  
Xu Zhenxia
Keyword(s):  
Difference Frequency ◽  
Frequency Wave ◽  
Parametric Array ◽  
The Difference ◽  
Sediment Interface

Practical method for parametric array source evaluation at 120 kHz frequency range in the near-field region

2020 ◽  
Vol 68 (5) ◽  
pp. 389-398
Author(s):  
Hongmin Ahn ◽  
Yonghwan Hwang ◽  
Yub Je ◽  
Wonkyu Moon
Keyword(s):  
Near Field ◽  
Sound Field ◽  
Simulated Data ◽  
Practical Method ◽  
Difference Frequency ◽  
Water Tank ◽  
Small Aperture ◽  
Frequency Wave ◽  
Parametric Array ◽  
The Difference

A parametric array is a non-linear conversion process that can generate a narrow beam of low-frequency sound with a small aperture. One of the challenging issues with a parametric array is precise measurement of the sound field generated. In particular, near the transducer, it is not easy to measure the sound field generated by a parametric array precisely, because the amplitude of the difference frequency wave is much lower than the amplitude of the primary wave. In this study, the practical issues that should be considered in the design of near-distance experiments with a parametric array are examined. Limiting effects were examined, and their associated characteristics were identified by numerical simulations. Experiments were performed in a water tank (18 x 12 x 10 m) to assess these characteristics, using a custom-designed acoustic filter; the beam pattern and propagation curve of the difference frequency wave generated by the parametric array were measured and compared with simulated data.

Wave Drift Forces and Low Frequency Damping on the Exwave Semi-Submersible

2017 ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Potential Flow ◽  
Low Frequency ◽  
Second Order ◽  
Difference Frequency ◽  
Frequency Wave ◽  
Wave Drift ◽  
The Difference ◽  
Wave Drift Forces ◽  
The Relationship ◽  
Drift Forces

The paper presents realistic horizontal wave drift force coefficients and low frequency damping coefficients for the Exwave semi-submersible under severe seastates. The analysis includes conditions with collinear waves and current. Model test data is used to identify the difference frequency wave exciting force coefficients based on a second order signal analysis technique. First, the slowly varying excitation is estimated from the relationship between the incoming wave and the low frequency motion using a linear oscillator. Then, the full quadratic transfer function (QTF) of the difference frequency wave exciting forces is defined from the relationship between the incoming waves and the second order force response. The process identifies also the linear low frequency damping. The paper presents results from cases selected from the EXWAVE JIP test matrix. The empirical wave drift coefficients are compared to potential flow predictions and to coefficients from a semi-empirical formula. The results show that the potential flow predictions largely underestimate the wave drift forces, especially at the low frequency range where severe seastates have most of the energy.

Wave Drift Forces and Low Frequency Damping on the Exwave FPSO

2017 ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Low Frequency ◽  
Second Order ◽  
Difference Frequency ◽  
Frequency Wave ◽  
Force Coefficients ◽  
Wave Drift ◽  
The Difference ◽  
Wave Drift Forces ◽  
The Relationship ◽  
Drift Forces

A method is followed in the present analysis to estimate realistic surge and sway wave drift force coefficients for the Exwave FPSO. Model test data is used to identify the difference frequency wave exciting force coefficients based on a second order signal analysis technique. First, the slowly varying excitation is estimated from the relationship between the incoming wave and the low frequency motion using a linear oscillator. Then, the full QTF of the difference frequency wave exciting forces is defined from the relationship between the incoming waves and the second order force response. The process identifies also the linearized low frequency damping. The paper presents results from a few cases selected from the Exwave JIP test matrix. Empirical mean wave drift coefficients are compared to potential flow predictions. It is shown that the latter underestimate the wave drift forces, especially at the lower frequency range where severe seastates have most of the energy. The sources for the discrepancies are discussed.

Sum and Difference Frequencies in Vibration of High Speed Rotating Machinery

1972 ◽  
Vol 94 (1) ◽  
pp. 181-184 ◽  
Author(s):  
F. F. Ehrich
Keyword(s):  
Spectral Analysis ◽  
Gas Turbine ◽  
High Speed ◽  
Beat Frequency ◽  
Difference Frequency ◽  
Wave Form ◽  
Low Frequencies ◽  
Frequency Wave ◽  
Excitation Frequencies ◽  
The Difference

A vibration incident on a gas turbine engine was noted where two major excitation frequencies were involved—an excitation synchronous with rotor rotation, associated with rotor unbalance, and an asynchronous excitation associated with fluid inadvertently trapped in the rotor. Spectral analysis of the vibration wave form revealed not only the two base excitation frequencies, but also a component at the difference frequency. A mechanism for generating such a difference frequency is hypothesized—the truncation of the basic “beat frequency” wave form by virtue of clearance in the rotor bearing system. Fourier analysis of the hypothesized excitation wave form indicates that components at difference frequency are indeed generated, and also at the sum frequency and a spectrum of higher harmonics and side band frequencies. The hypothesized wave form’s spectral analysis bears a remarkable resemblance to the measured spectrum, except that low frequencies appear to have been greatly amplified in the experimental case, and high frequencies attenuated. This latter fact is attributed to the transmission characteristics of the gas turbine stator system, and is probably responsible for the lack of precise correspondence between the measured and hypothesized wave forms.

Tunable Far-Infrared Radiation Generated from the Difference Frequency between Two Ruby Lasers

1969 ◽  
Vol 180 (2) ◽  
pp. 363-365 ◽  
Author(s):  
D. W. Faries ◽  
K. A. Gehring ◽  
P. L. Richards ◽  
Y. R. Shen
Keyword(s):  
Infrared Radiation ◽  
Far Infrared ◽  
Difference Frequency ◽  
The Difference ◽  
Far Infrared Radiation

DIFFERENCE FREQUENCY HARMONIC ION HEATING

1967 ◽  
Vol 45 (5) ◽  
pp. 1771-1781 ◽  
Author(s):  
C. R. James ◽  
W. B. Thompson
Keyword(s):  
High Frequency ◽  
Collision Frequency ◽  
Difference Frequency ◽  
Fourth Power ◽  
Heating Process ◽  
Ion Heating ◽  
Frequency Signal ◽  
Transverse Waves ◽  
The Difference ◽  
Frequency Harmonic

The heating of a magnetized hot diffuse plasma using the difference frequency signal generated from two high-frequency (35 GHz) transverse waves is examined. The plasma is described by the cold plasma model and a series expansion of harmonics is used to obtain a solution to the equations. It is shown that the energy absorbed by the ions can be made inversely proportional to the collision frequency and the fourth power of the driven frequency and proportional to the fourth power of the driven electric field intensity. An investigation of the sensitivity of the heating process to fluctuations in frequency, density, and d-c. magnetic field is carried out.

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