Random Vibration & Vibroacoustics

Certain types of dynamic excitation can be best described by a statistical description. These include the acoustic environments due to rocket exhaust and pressure fluctuations due to turbulent boundary layer. The structural system response to these random excitations requires the methods and techniques of random vibrations. The resulting component structural environments due to these excitations is typically referred to as the “structure-borne” random environments. Structure-borne random environments, whether derived through analysis, test, or flight data, play a critical part in structural design and flight certification. Realistic utilization of structure-borne random environments is contingent on a formalized, robust, and accurate method of random vibration “mass attenuation”. Because these environments are most often defined without the presence of component mass, and because a viable, robust “mass attenuation” method has not been available, over-conservative component response predictions have all too often been imposed on hardware developers.

In fact, the need for an “improved method” of random vibration “mass attenuation” has recently been identified by the NESC Vibroacoustics Best Practices Team (ASD is a member) as the #1 risk to new launch vehicle/payloads programs.

ASD has developed a new generalized method (see ASD Advanced Method) for the prediction of random vibration “mass attenuation”. This method has yielded the capability of simultaneously accounting for all component interface degrees of freedom, with specified correlations, and producing the predicted attenuated environments at each component’s interface degrees of freedom, including accelerations, interface forces and the resulting cross-correlations. This new ASD method reduces to Barrett’s method for the single rigid component case and Norton-Thevenin method for single drive degree of freedom case. The new method has exhibited highly rational attenuated environments, even for the complex analytical examples and has the potential applications to mitigate issues relative to random vibration over-testing.

Relative to the random vibration/vibroacoustic system level environment predictions, ASD utilizes the powerful method of component-mode synthesis, with increased component frequency contents, to build-up the coupled system subject to the random pressure field with careful consideration to spatial cross correlation modeling of the excitation. Acoustic volumes are accounted for via enforcing constraints between the finite element nodal displacements and the acoustic volume nodal pressures. The coupled vibroacoustic analysis is done in physical or modal coordinates depending on the problem. Typically, this approach is more than sufficient if the analysis goals are relative to primary structure responses, where the very high frequencies are attenuated. If the analysis goal involves environment recoveries on low mass items, e.g., thin skin structure, where the higher frequencies may be a contributing factor, higher fidelity finite element models in the areas of interest may be required.

Alternatively, for such cases, ASD may utilize the method of Statistical Energy Analysis (SEA) only when warranted and when the SEA parameters can be clearly calculated/defended and underlying SEA assumptions not violated.

ASD is a team member in the NASA Engineering and Safety Council (NESC) Vibroacoustics Best Practices Working Group.