Approximate methods in high-frequency scattering

1957 ◽  
Vol 239 (1218) ◽  
pp. 338-348 ◽  
Keyword(s):  
Boundary Condition ◽  
Circular Cylinder ◽  
High Frequency ◽  
Fourier Transforms ◽  
Scattering Coefficient ◽  
Complete Agreement ◽  
Approximate Methods ◽  
Exact Theory ◽  
Geometrical Acoustics

Two approximate methods for deriving the high-frequency scattering coefficient of twodimensional obstacles without edges are described. In the first method a simple field which satisfies approximately the boundary condition near the points of glancing incidence is found. Elsewhere the geometrical acoustics field is used. The scattering coefficient is about 7 % in error. In the second method Fourier transforms are employed to find a field which satisfies the boundary condition over a wider region. This leads to results which, for the circular cylinder, are in complete agreement with those of the exact theory.

An approximate method in high-frequency scattering

1962 ◽  
Vol 58 (4) ◽  
pp. 662-670
Author(s):  
A. Sharples
Keyword(s):  
Boundary Condition ◽  
Circular Cylinder ◽  
Integral Representation ◽  
Sound Wave ◽  
High Frequency ◽  
Correction Term ◽  
Scattering Coefficient ◽  
Extended Form ◽  
Geometrical Acoustics ◽  
Plane Sound

ABSTRACTThe diffraction of a high-frequency plane sound wave by a circular cylinder is investigated when the boundary condition on the cylinder is expressed by means of an equation of the form The special feature of this investigation is that an extended form of the Kirchhoff-Fresnel theory of diffraction is used to find an integral representation for the scattering coefficient. In order to avoid the complicated analysis which would be necessary to evaluate the integrals concerned, the more natural geometrical acoustics approach is used to find the first correction term in the scattering coefficient. Numerical results are given for large and small values of the impedance Z.

Conduction of heat in a solid with periodic boundary conditions, with an application to the surface temperature of the moon

1953 ◽  
Vol 49 (2) ◽  
pp. 355-359 ◽  
Author(s):  
J. C. Jaeger
Keyword(s):  
Boundary Condition ◽  
Thermal Properties ◽  
Circular Cylinder ◽  
Boundary Conditions ◽  
Surface Temperature ◽  
Numerical Method ◽  
Periodic Boundary ◽  
Periodic Boundary Conditions ◽  
The Moon ◽  
Non Linear

The object of this note is to indicate a numerical method for finding periodic solutions of a number of important problems in conduction of heat in which the boundary conditions are periodic in the time and may be linear or non-linear. One example is that of a circular cylinder which is heated by friction along the generators through a rotating arc of its circumference, the remainder of the surface being kept at constant temperature; here the boundary conditions are linear but mixed. Another example, which will be discussed in detail below, is that of the variation of the surface temperature of the moon during a lunation; in this case the boundary condition is non-linear. In all cases the thermal properties of the solid will be assumed to be independent of temperature. Only the semi-infinite solid will be considered here, though the method applies equally well to other cases.

scholarly journals The Scattering Coefficient for Shore-to-Air Bistatic High Frequency (HF) Radar Configurations as Applied to Ocean Observations

2019 ◽  
Vol 11 (24) ◽  
pp. 2978 ◽  
Author(s):  
Zezong Chen ◽  
Jian Li ◽  
Chen Zhao ◽  
Fan Ding ◽  
Xi Chen
Keyword(s):  
Electromagnetic Scattering ◽  
High Frequency ◽  
Doppler Frequency ◽  
Second Order ◽  
Hf Radar ◽  
Scattering Coefficient ◽  
Sea State ◽  
Scattering Coefficients ◽  
Wind Speeds ◽  
Ocean Observations

To extend the scope of high frequency (HF) radio oceanography, a new HF radar model, named shore-to-air bistatic HF radar, has been proposed for ocean observations. To explore this model, the first-order scattering coefficient and the second-order electromagnetic scattering coefficient for shore-to-air bistatic HF radar are derived using the perturbation method. In conjunction with the contribution of the hydrodynamic component, the second-order scattering coefficient is derived. Based on the derived scattering coefficients, we analyzed the simulated echo Doppler spectra for various scattering angles and azimuthal angles, operation frequencies, wind speeds, and directions of wind, which may provide the guideline on the extraction of sea state information for shore-to-air bistatic HF radar. The singularities in the simulated echo Doppler spectra are discussed using the normalized constant Doppler frequency contours. In addition, the scattering coefficients of shore-to-air bistatic HF radar are compared with that of monostatic HF radar and land-based bistatic HF radar. The results verify the correctness of the proposed scattering coefficients. The model of shore-to-air bistatic HF radar is effective for ocean observations.

Active Control Methods for Drag Reduction in Flow Over Bluff Bodies (Keynote)

2002 ◽  
Author(s):  
Haecheon Choi
Keyword(s):  
Circular Cylinder ◽  
Drag Reduction ◽  
Active Control ◽  
High Frequency ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Control Methods ◽  
Periodic Forcing ◽  
Time Periodic

In this paper, we present two successful results from active controls of flows over a circular cylinder and a sphere for drag reduction. The Reynolds number range considered for the flow over a circular cylinder is 40∼3900 based on the free-stream velocity and cylinder diameter, whereas for the flow over a sphere it is 105 based on the free-stream velocity and sphere diameter. The successful active control methods are a distributed (spatially periodic) forcing and a high-frequency (time periodic) forcing. With these control methods, the mean drag and lift fluctuations decrease and vortical structures are significantly modified. For example, the time-periodic forcing with a high frequency (larger than 20 times the vortex shedding frequency) produces 50% drag reduction for the flow over a sphere at Re = 105. The distributed forcing applied to the flow over a circular cylinder results in a significant drag reduction at all the Reynolds numbers investigated.

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