Effects of different boundary condition formulations on internal acoustic pressure and vibration loading of an aircraft compartment

During the assessment of rocket components vibroacoustic loading using the finite element modeling, it is not feasible to analyze the entire rocket due to the design's complexity and its large spatial scales compared to high-frequency wavelengths. Therefore, in practice, individual structural groups (compartments, rocket fuel tanks, and the warhead) must be considered. This limited model for calculations under rigid constraint can produce distorted vibroacoustic parameter characteristics; these errors will be greater the shorter the considered compartment. To address this issue, it is proposed to use beam finite elements for modeling the missing components of the rocket. This should allow to consider the missing vibration modes propagating along its entire body and affecting the compartment under consideration, as well as the dynamics of its end frames. In this regard, in the proposed article, a finite element model of the stiffened shell of a launch vehicle's dry compartment was developed. The model is constructed from flat and solid finite elements, with brackets containing instruments positioned within it. These brackets represent concentrated masses with specified centering and inertial characteristics. Such structural elements, described by a system of oscillator equations, interact with acoustic (fluid) elements in the model, where the acoustic wave equations are solved. The missing parts of the launch vehicle were modeled using beam elements, considering the existing design of tunnel pipelines, fuel tanks, propulsion systems, etc. The rocket's beam structure is connected to the compartment frames by elastic contact elements. Solving the resulting systems of equations allowed us to determine the displacements of the structural finite elements and pressure pulsations in the acoustic domain. The calculations are presented under various boundary conditions, which take into account low-frequency vibrations that occur during engine operation and their influence on the internal acoustic pressure and the dynamics of the structure, with the beam part of the model and without it.