Dynamics & Vibration Research Group
Mechanics, Materials, and Design
The Structural Dynamics of Complex Systems
Engineering structures are subjected to a wide range of dynamic excitation forces; for
example, an aerospace structure may be subject to jet noise, propeller (or rotor) noise, boundary layer
turbulence, and mechanical excitation arising from the engines and the transmission system. Similar examples
can be cited from the automotive, civil, and marine industries, and in all cases the designer must avoid
fatigue failure, damage to sensitive equipment and payloads, and unacceptable noise and vibration
levels.
The physical nature of the vibrational behaviour of a built-up structure is highly dependent upon the frequency of excitation. At low frequencies only the first few modes of vibration are excited, and the response can normally be predicted to a good degree of accuracy by using the finite element method of analysis. At medium to high frequencies, many hundreds of modes can be excited and it becomes extremely difficult to predict the detailed response of the structure. Moreover, the response can become extremely sensitive to small changes in the structural properties, so that successive items from a production line can have very different levels of performance: for example a 15dB variation in helicopter interior noise has been reported over nominally identical vehicles. Ongoing research is directed at the analysis of this type of complex system.
The physical nature of the vibrational behaviour of a built-up structure is highly dependent upon the frequency of excitation. At low frequencies only the first few modes of vibration are excited, and the response can normally be predicted to a good degree of accuracy by using the finite element method of analysis. At medium to high frequencies, many hundreds of modes can be excited and it becomes extremely difficult to predict the detailed response of the structure. Moreover, the response can become extremely sensitive to small changes in the structural properties, so that successive items from a production line can have very different levels of performance: for example a 15dB variation in helicopter interior noise has been reported over nominally identical vehicles. Ongoing research is directed at the analysis of this type of complex system.
Project Details
Various topics relating to the dynamics of complex systems are being addressed under two main themes, which are summarised below.- Statistical Energy Analysis (SEA). This method of predicting high frequency vibration levels is based on
the analysis of energy flow. Recent work in collaboration with Vibro-Acoustic Sciences Inc. has considered a
hybrid analysis procedure whereby SEA is coupled to the finite element method to capture both the
deterministic and statistical components of the response of a built-up system [1]. This work is currently
being extended to allow general coupling between the finite element package NASTRAN and the SEA package
Auto-SEA.
- The study of random systems. The natural frequencies of a system with uncertain properties can be considered to be random quantities. A non-Poisson point process model of the natural frequencies has recently been investigated [2], and this has led to a number of insights into the statistics of the frequency response function. Further work is considering elements of random matrix theory and the applicability or otherwise of certain results in statistical physics to engineering systems.
Relevant/Recent Publications
- [1] R. S. Langley, P. G. Bremner. 'A hybrid method for the vibrational analysis of complex structural-acoustic systems'. J. Acoustical Society of America, 105, 1657-1671, 1999.
- [2] R. S. Langley. 'A non-Poisson model for the vibration analysis of uncertain dynamic systems'. Proc. Roy. Soc, Series A, 455, 3325-3349, 1999.
Principal Investigator & Researchers
Funding Body
- Ongoing research
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