Reflection of incident pressure waves by ribbed panels

Abstract

The definitions of the reflection coefficients, in the absence of ribs and due to ribs, of an incident pressure wave on a plane at the surface of a ribbed fluid-loaded panel are defined, examined, and computed. The incident pressure wave is described by a plane wave and by a collimated beam. The results of representative computations are displayed. Through these displays, some aspects of the influence on the reflection coefficients caused by changes in the parameters that describe the ribbed panel, the fluid loading, and the incidence are investigated. These aspects include wave-number aliasings and symmetries in some of the terms and factors that compose the expressions for the reflection coefficients; the wave number of concern lies across the ribs. Aliasings are present, however, only when the separations between adjacent ribs are conditioned to be equal. Moreover, in some of these aliased terms and factors, symmetry is present only when the incidence obeys specific conditions. Disturbing these conditions tends to spoil the aliasing and/or the symmetries in these terms and factors. It is shown, however, that the aliasing and symmetry properties are invariant to changes in the parameters that describe the ribbed panel and/or the fluid loading to which it is subjected. The role that the phenomenon of pass and stop bands (bands in reference to the frequency domain) plays in the reflection properties of ribbed fluid-loaded panels is of particular interest here. It is illustrated that in the frequency ranges of pass bands, diffraction orders and aliasings tend to fade, and in the frequency ranges of stop bands they are enhanced. Fluid loading subdues the pass and stop bands; however, even substantial fluid loading does not eliminate these bands. Finally, the relationship between the reflection coefficients defined on the surface of the panel and on a control surface placed in the farfield is formulated and discussed.