Consultancy : Noise, Vibration & Harshness

MIRA uses Nastran for linear elastic frequency domain CAE modelling and for input into CAE/empirical hybrid models. Nastran is used extensively for modal studies right from the initial concept stage and into the validation phases of the prototype, estimating transfer functions, calculating forced responses and for interior acoustic problems. MIRA can use the MSC.Nastran parametric optimisation solver for modal alignment specially in powertrain mount positioning where frequency dependent stiffness are used to achieve the target isolation between suspension, powertrain and vehicle critical modes.

These primary ride models, simulating rigid modal behaviour are coupled with flexible body models to predict the structural transfer functions for higher frequency range.

Modal Analysis

A normal modes solver is used for lightly damped structures, such as untrimmed vehicle bodies, together with sensitivity based modal updating to assist in aligning predicted modes with targets. Modal results can also be output for correlation with equivalent empirical modal data. MIRA uses CADA-X and TestLab for Spectral Acquisition and Modal Analysis and Design for test based modal analysis.

Frequency Response Analysis

MIRA combines flexible bodies and lumped parameters in a frequency response analysis in similar fashion to a conventional time domain multi-body ( ADAMS ) simulation. The model typically incorporates frequency dependent representations of the stiffness and damping parameters of elastomeric components such as hydromounts and bushes. For a vehicle, a model may contain a flexible representation of the body and lumped parameter representations of the most significant masses such as the powertrain, suspension and occupants. A frequency domain forced response is used to, for example, calculate the response at an occupants contact points to a force input at road wheel.

Acoustic Modelling

In order to calculate acoustic transfer functions, a structural model is coupled to an acoustic cavity model. MIRA has experience of modelling both coupled/uncoupled trimmed BIW models of multi-million degrees of freedom including suspension and powertrain modal representation.

These highly complex trimmed models, coupled with structural and cavity modes are used for various transfer function calculations up to 300 Hz including acoustic energy absorber elements using various acoustic energy sources. The effective panel contribution/modal participation factors are calculated to identify the critical panels where vibrations can be reduced to minimise the noise. Interior noise is managed by identifying the measures for controlling the body structural contribution in combination with acoustically absorbent trim.

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