Dynamics of noise

reducing unwanted noise is one of the main problems for mechanical engineers. The automobiles which are not perfectly tuned cause a lot of stress to the passengers and also shorten the life of the component. Engineers mostly rely on computer models to see the response of a new design to disturbance of various kinds.

Although the design software is highly advanced and the complex geometries involved are conventionally modelled using finite element analysis, the physical test results often show that there is need for a more accurate computer model to be developed. A lot of research is going on all over Europe for developing a new range of software to create 'virtual reality' mathematical models of future generations of designs.

Researchers in the department of mechanical engineering at the University of Wales, Swansea, uk, are collaborating in this area with car manufacturers such as bmw and Peugeot Citroen and with railway rolling stock manufacturers. The Welsh team is concerned with the finite element modelling aspect of the task. Finite element analysis is a method of analysing the static and dynamic behaviour of a continuous structure such as a car body, by breaking it down into simple geometrical shapes. Steel rods or pistons become straight lines, and plates become quadrilaterals. All these simplified shapes are combined to form complex geometries.

Micheal Friswell, the project leader at Swansea says, "The elimination of noise, vibration and harshness is vital to produce successful vehicle designs. However, the vibration patterns involved are extremely complicated. They combine the interaction between the engine, power train, road or track surface, aerodynamics, structure-borne transmission and the acoustics of the passenger accommodation itself."

These interactions are too complex and the current analytical models are seen by engineers as the chief limitation to improving the efficiency of the design process in this area. The full model, after feeding all the simplified elements, may be composed of hundreds of thousands of simplified elements, demanding powerful computers to solve these equations. The difficulty of the models obtained through computers is the accuracy in predicting the dynamic behaviour of joints. Even if the errors in modelling are very small, the combined effect of such errors in many joints considerably reduces the accuracy of the model as a whole. One serious limitation of the finite element analysis is its ineffectiveness if used for bodies with vibration frequencies above the order of 80 to 100 Hz. The inaccuracies start popping up above this range due to a number of factors which may range from inaccurate modelling of joints or pressings in the body structure, or variations in the thickness of sheet metal used in its construction. If the model can be modified to take these into account, it can be used to predict the effect of design changes.

The recent finite element models impose an upper limit of 2,000 Hz in analysing power unit vibrations. The Swansea researchers, by including more precise measurements aim to enhance the models to a capability for covering frequencies upto 200 Hz in the car and up to 3,000 Hz in the power unit.

Another priority is to reduce the time taken for vibration analysis. Currently, bringing out a body design to the presently attainable limits takes several months, but the new models being developed will be able to handle the work in about two or three weeks. If the team succeeds, the resulting computer models will give users significant advantages in performance and cost.

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