vAEROsimT

MIRA's vehicle aerodynamics simulation process

MIRA’s vAEROsim™ aerodynamics simulation process allows engineers to address a number of different areas of vehicle aerodynamics at a very early stage in the development process.  These cover lift and drag reduction, aerodynamic stability, aero-acoustics, brake cooling and vehicle soiling. The process involves an integrated approach using Computational Fluid Dynamics (CFD), MIRA’s Full-Scale Wind Tunnel (FSWT), MIRA’s extensive vehicle proving ground and specific simulation software for vehicle stability, acoustics and thermal analysis.

The image above is meant to convey the unique ability MIRA has to use simulation and experiment to provide a complete aerodynamic development service.  The simulation tools, as well as providing answers prior to physical prototypes being available, also give MIRA’s aerodynamic consultants a much greater understanding of the flow around the vehicle, which in turn allows them to be more focussed in their development approach.

Computational Fluid Dynamics is used extensively at MIRA for aerodynamics development on a range of vehicles from single-seater racecars up to heavy goods vehicles. MIRA have experience of most of the major CFD codes, including Fluent, Star and PowerFLOW, and as well as using them for development purposes have also educated major OEMs in their correct implementation.  This process can either take place at MIRA as part of MIRA’s Aerodynamic Training Course, as part of a technology transfer project or, indeed, with resident engineers educating OEMs ‘from within’.

MIRA’s Full-Scale Wind Tunnel was built in 1960 and has been the cornerstone of automotive aerodynamic testing in the UK for the last 45 years. Membership of the European Aerodynamic Data Exchange (EADE) and the European Car Aerodynamic Research Association (ECARA) has kept MIRA at the forefront of aerodynamic testing techniques. MIRA also run the only dedicated Vehicle Aerodynamics Conference in the world (www.aerodynamics-conference.co.uk).

Probably the most common application of vAEROsim™ is for drag or lift reduction on passenger cars or commercial vehicles. Drag predictions using CFD are now usually accurate to within 2% of experimental values, giving MIRA the confidence to direct the early development process using simulation alone. A combination of CFD, wind tunnel testing and validation using coastdown techniques on the track has delivered significant (up to 25% in some cases) reductions in vehicle drag from first concept to final prototype. Indeed on a recent project a reduction of 10% was achieved after the style was signed off, by using add-on parts alone. It is this ‘real world’ experience allied with the cutting edge simulation techniques that set MIRA apart.

With increasing vehicle speeds, lower weight to meet emissions targets and an increasingly litigious society aerodynamic stability is at the front of many motor manufacturers’ minds. MIRA have pioneered an integrated approach using CFD, and/or wind tunnel techniques, allied with vehicle dynamics simulation using MSC.ADAMS/Car to enable early, and accurate, prediction of vehicle aerodynamic stability.

MIRA have also spent a lot of time on developing new techniques for aero-acoustics, or wind noise. An integrated approach is used that takes pressure information from CFD, or the wind tunnel, and uses a technique called Statistical Energy Analysis (SEA) to predict what the sound pressure levels in the vehicle cabin will be. This differs from a number of other simulation techniques that only predict what the noise sources will be. The strength of this technique is that where CFD and the wind tunnel can be used to reduce the level of the noise sources, SEA can then be used to evaluate sound deadening, material properties in the cabin and give a prediction of what the new ‘in-car’ sound levels will be.

Brake cooling simulation involves CFD for prediction of convective heat transfer and a simulation code called Radtherm to predict the radiative heat transfer. This combined approach allows MIRA to identify problems early and make changes before physical prototypes exist. Once hardware is available MIRA’s Full-Scale Wind Tunnel has a dynamometer that, when used in conjunction with MIRA’s Thermal Imaging Camera, can be used to aid any brake development programme.

Finally CFD and MIRA’s wind tunnels are also used to evaluate water management / vehicle soiling. MIRA have worked closely with windscreen wiper manufacturers, for instance, to improve both wiper performance and wind noise characteristics. Results were then validated on MIRA’s vehicle proving ground.

MIRA’s vAEROsim TM process covers the integration of a number of different simulation packages, experimental facilities and a significant knowledge base to offer a proven development tool to automotive companies worldwide, helping them to reduce timescales, minimise the number of physical prototypes and solve problems quickly and efficiently.

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