Structural Analysis with Vibro-Acoustic Loads in LS-DYNA

Many structures are designed to operate in hot temperature and stringent aero-acoustic fatigue environment, e.g. the engine inlet and the heat shield of aircraft are subject to high temperature and sonic pressure level. It is important to evaluate the dynamic response of the structures exposed to both vibration and acoustic sources of excitations. A new feature of structural analysis with vibro-acoustic loads has been implemented in LS-DYNA®. This feature is based on N-FEARA® finite element analysis tool developed by The Boeing Company. This new capability in LS- DYNA treats the structural response by finite element method coupled with acoustic field based on a known acoustic source behavior by spatial correlation function. The added capabilities enable the users to evaluate the response of structure to both base-excitation, or vibration and acoustic source in the frequency domain. Various acoustic environments and sources of excitations can be considered, including base excitation defining random vibration, in addition to plane wave, progressive wave, reverberant wave, turbulent boundary layer, shock wave, representing various fields for acoustic sources of excitation. Modal acceleration method as well as modal superposition method is used to evaluate the dynamic behavior of structures in the frequency domain. The acceleration power spectral density (PSD) is defined in term of g2/Hz used for vibration analysis. The input spectrum for the acoustic excitations can be either pressure PSD in psi2/Hz or sound pressure level (SPL) in dB. For the latter, sound pressure level is first converted into pressure PSD. The coupling between the structures (represented by the modal shapes) and the acoustic excitations is expressed through the concept of joint acceptance. The results are presented in terms of PSD of the nodal displacements, velocities, accelerations, and element stresses, and the RMS (Root Mean Square) of those variables for a frequency range of interest. Several efficient numerical techniques have been implemented to accelerate the solution phase, including the partition of the panel and subdivision of the range of frequencies. A restart option is provided in case users need to change the input acoustic spectrum or change the range of frequency in the output. Furthermore, this novel feature of LS-DYNA provides users a method to replace an acoustic test environment by a shaker table test as a virtual qualification for testing method. This method is based on a conversion factor between the maximum of root mean square of displacement response due to acoustic pressure load and due to base acceleration load. Several keywords have been introduced in LS-DYNA to facilitate this new feature. Numerical examples are given to demonstrate the new vibro-acoustic analysis capability which will be available in the next release of LS-DYNA.