TY - JOUR

T1 - Waves and radiation conditions in a cuspidal sharpening of elastic bodies

AU - Kozlov, V.A.

AU - Nazarov, S.A.

N1 - Funding Information: Acknowledgements This work was written within the project 17-11-01003 of Russian Science Foundation. The authors are grateful to the anonymous referee whose comments helped to improve the presentation.

PY - 2018/6/1

Y1 - 2018/6/1

N2 - Elastic bodies with cuspidal singularities at the surface are known to support wave processes in a finite volume which lead to absorption of elastic and acoustic oscillations (this effect is recognized in the engineering literature as Vibration Black Holes). With a simple argument we will give the complete description of the phenomenon of wave propagation towards the tip of a three-dimensional cusp and provide a formulation of the Mandelstam (energy) radiation conditions based on the calculation of the Umov-Poynting vector of energy transfer. Outside thresholds, in particular, above the lower bound of the continuous spectrum, these conditions coincide with ones due to the traditional Sommerfeld principle but also work at the threshold frequencies where the latter principle cannot indicate the direction of wave propagation. The energy radiation conditions supply the problem with a Fredholm operator of index zero so that a solution is determined up to a trapped mode with a finite elastic energy and exists provided the external loading is orthogonal to these modes. An example of a symmetric cuspidal body is presented which supports an infinite sequence of eigenfrequencies embedded into the continuous spectrum and the corresponding trapped modes with the exponential decay at the cusp tip. We also determine (unitary and symmetric) scattering matrices of two types and derive a criterion for the existence of a trapped mode with the power-low behavior near the tip.

AB - Elastic bodies with cuspidal singularities at the surface are known to support wave processes in a finite volume which lead to absorption of elastic and acoustic oscillations (this effect is recognized in the engineering literature as Vibration Black Holes). With a simple argument we will give the complete description of the phenomenon of wave propagation towards the tip of a three-dimensional cusp and provide a formulation of the Mandelstam (energy) radiation conditions based on the calculation of the Umov-Poynting vector of energy transfer. Outside thresholds, in particular, above the lower bound of the continuous spectrum, these conditions coincide with ones due to the traditional Sommerfeld principle but also work at the threshold frequencies where the latter principle cannot indicate the direction of wave propagation. The energy radiation conditions supply the problem with a Fredholm operator of index zero so that a solution is determined up to a trapped mode with a finite elastic energy and exists provided the external loading is orthogonal to these modes. An example of a symmetric cuspidal body is presented which supports an infinite sequence of eigenfrequencies embedded into the continuous spectrum and the corresponding trapped modes with the exponential decay at the cusp tip. We also determine (unitary and symmetric) scattering matrices of two types and derive a criterion for the existence of a trapped mode with the power-low behavior near the tip.

KW - Cuspidal singularity

KW - Elastic waves

KW - Energy radiation conditions

KW - Fredholm operator

KW - Trapped modes

KW - Vibration Black Holes

KW - Weighted spaces with detached asymptotics

UR - http://www.scopus.com/inward/record.url?scp=85032822176&partnerID=8YFLogxK

U2 - 10.1007/s10659-017-9658-x

DO - 10.1007/s10659-017-9658-x

M3 - Article

VL - 132

SP - 103

EP - 140

JO - Journal of Elasticity

JF - Journal of Elasticity

SN - 0374-3535

IS - 1

ER -