Atmospheric acoustic gravity modes at frequencies near and below low frequency cutoff imposed by upper boundary conditions

The low-frequency vibration and radiation performance of a locally resonant (LR) plate with periodic multiple resonators is studied in this paper, with both infinite and finite structure properties examined. For the finite cases, taking the LR plate attached with two periodic arrays of resonators as an example, the forced vibration response and the radiation efficiency are theoretically derived by adopting a general model with elastic boundary conditions. Through a comparison with the band structures calculated by the plane-wave-expansion method, it shows that the band gaps in the infinite LR plate are in good agreement with the vibration-attenuation bands in the finite LR plate, no matter what boundary conditions are applied to the latter. In contrast to the vibration reduction in the band gaps, the radiation efficiency of the finite LR plate is sharply increased in the band-gap frequency ranges. Furthermore, the acoustic power radiated from the finite LR plate can be seriously affected by its boundary conditions. For the LR plate with greater constraints, the acoustic power is reduced in the band-gap frequency ranges, while that from the one with fully free boundary conditions is increased. When further considering the damping loss factors of the resonators, the attenuation performance can be improved for both the vibration and radiation of the LR plate.

Download Full-text

A note on the upper boundary conditions for turbulence models in the neutral atmosphere

Boundary-Layer Meteorology ◽

10.1007/bf02033596 ◽

1981 ◽

Vol 21 (4) ◽

pp. 489-493 ◽

Cited By ~ 5

Author(s):

D. M. Deaves

Keyword(s):

Boundary Conditions ◽

Turbulence Models ◽

Neutral Atmosphere ◽

Upper Boundary

Download Full-text

Use of experimental dynamic substructuring to predict the low frequency structural dynamics under different boundary conditions

Mathematics and Mechanics of Solids ◽

10.1177/1081286517727147 ◽

2017 ◽

Vol 23 (11) ◽

pp. 1444-1455

Author(s):

Walter D’Ambrogio ◽

Annalisa Fregolent

Keyword(s):

Boundary Conditions ◽

Rigid Body ◽

Frequency Response ◽

Response Function ◽

Frequency Response Function ◽

Low Frequency ◽

Response Functions ◽

Frequency Response Functions ◽

Different Boundary Conditions ◽

Rigid Body Modes

Flexible structural components can be attached to the rest of the structure using different types of joints. For instance, this is the case of solar panels or array antennas for space applications that are joined to the body of the satellite. To predict the dynamic behaviour of such structures under different boundary conditions, such as additional constraints or appended structures, it is possible to start from the frequency response functions in free-free conditions. In this situation, any structure exhibits rigid body modes at zero frequency. To experimentally simulate free-free boundary conditions, flexible supports such as soft springs are typically used: with such arrangement, rigid body modes occur at low non-zero frequencies. Since a flexible structure exhibits the first flexible modes at very low frequencies, rigid body modes and flexible modes become coupled: therefore, experimental frequency response function measurements provide incorrect information about the low frequency dynamics of the free-free structure. To overcome this problem, substructure decoupling can be used, that allows us to identify the dynamics of a substructure (i.e. the free-free structure) after measuring the frequency response functions on the complete structure (i.e. the structure plus the supports) and from a dynamic model of the residual substructure (i.e. the supporting structure). Subsequently, the effect of additional boundary conditions can be predicted using a frequency response function condensation technique. The procedure is tested on a reduced scale model of a space solar panel.

Download Full-text