Testing : Aerodynamics

Opening the world's first dedicated automotive wind tunnel in 1960, MIRA has been at the forefront of research into ground vehicle aerodynamics.  Since then our aerodynamicists have used our model scale and full scale wind tunnels to further understanding of vehicle aerodynamics for MIRA and for the industry as a whole.

As an aerodynamic development tool the wind tunnels are invaluable providing detailed and accurate data in a very short amount of time.  However since the 1980's MIRA's aerodynamicists have been investigating the use of computers to predict the aerodynamic performance of different vehicle shapes initially with in-house developed empirically based codes.  As the power of computers and software increased MIRA's engineers have been able to use Computational Fluid Dynamics (CFD) to evaluate vehicle design and styling before physical models or prototypes are available.  Having the resources to combine computational methods with wind tunnel testing gives MIRA a considerable advantage over many other aerodynamic consultancies.

Wind Tunnel Testing

The first wind tunnel was designed for aeronautical purposes in the 1870's, however as mentioned above it wasn't until 1960 that automotive engineers had access to a purpose built automotive wind tunnel.

Wind tunnels provide an idealised environment for the evaluation of a vehicle's aerodynamic performance.  A wind tunnel typically comprises a test section where a model or vehicle can be mounted and viewed whilst air is either blown or more usually sucked over it by a fan or number of fans.  Data can usually be gathered from a balance on which the model or vehicle is mounted and visualization techniques such as adding smoke trails to the airflow can be used to gain an understanding of how certain geometric features affect its aerodynamic performance.  Wind tunnels enable the following data to be acquired:

• aerodynamic forces; drag, lift, side force and moments; pitch, yaw, roll
• variation of aerodynamic forces and moments with yaw
• surface pressure distribution
• the influence of different vehicle details on the above
• vehicle cooling drag
• assessment of brake cooling flows
• aero-acoustic data
• affect of aerodynamic features and aids

Model Scale Wind Tunnel Testing (MWT)

To some extent model scale wind tunnel testing has been superseded by Computational Fluid Dynamics (CFD) although it still plays a significant role in the aerodynamic development of motorsport vehicles, training and research.

Depending on the size of the vehicle and the limitations of the test facility models typically lie between 30% - 60%.  Model scale testing is ideal for rapid evaluation of the influence of different body styles and features on the vehicle's aerodynamics.  Models can be made by several techniques including:

• stylist's clay models
• rapid prototype models (direct from CAD data)
• milled foam (usually direct from CAD data)
• fibre glass
• or a combination of the above

By its very nature model scale testing uses smaller facilities with lower running costs than their full scale counterparts.  Sophisticated test methods such as employing a moving ground plane (typically a moving belt beneath the model) can be done without great expense.  Particularly if styling models (used for styling reviews) can be used then model testing can be a cost effective method of developing a vehicle's aerodynamics.

It is important to bear in mind a couple of limitations with model scale wind tunnel testing i.e.:

• ensuring geometric similarity - at smaller model scales certain features can be difficult to model (interestingly there is tendency for stylists and modellers to over emphasize details such as window recesses and radii)
• Reynolds number effects - the reduction in scale decreases the Reynolds number (Re) of the flow around the vehicle requiring a commensurate increase in the airflow.

Model Scale Wind Tunnel (MWT)

Full Scale Wind Tunnel Testing (FSWT)

It is increasingly common to eliminate the model scale stage of an aerodynamic development programme and proceed directly to full scale testing.  This can be on programmes where a large proportion of the styling evaluations have been carried out virtually with CFD being used to predict the vehicles aerodynamic performance or on programmes where full scale styling models have been produced.

Testing at full scale has several advantages:

• Reynolds number effects are eliminated
• If full scale styling models or prototypes are available then the cost of model production is avoided
• The influence of small geometric changes can be determined
• Benchmark data for competitor vehicles can be obtained

Full Scale Wind Tunnel (FSWT)

Coast Down Testing

Some aerodynamic characteristics can be determined on the road.  Coast down testing is one method used to determine a vehicle's aerodynamic drag.  Being straight, flat, relatively smooth and protected from gusts of wind MIRA's mile straights provide excellent conditions for coast down testing. 

Proving Ground

Non-Automotive Applications

MIRA’s aerodynamics expertise is frequently called upon for a diverse selection of non-automotive products. From other forms of transport such as trains and bicycles through to totally unrelated products... Lifeboats, aerials, wheely bins, tents, sports clothing and even world class skiers! All have benefited from our aerodynamics expertise and development facilities.

Complementary Services and Facilities

Aerodynamic testing is part of MIRA's comprehensive capabilities for aerodynamics, fluid dynamics and thermal management.  Further details of which can be found using the following links:

Aerodynamic Consultancy

Thermal Management Consultancy

Thermal Management Testing

Climatic Wind Tunnel – CWTone
Climatic Wind Tunnel – CWTtwo
Fluid Systems Engineering Centre
Full Scale Aerodynamic Wind Tunnel
Model Scale Aerodynamic Wind Tunnel
Computational Fluid Dynamics (CFD) suite
Extensive Proving Ground test circuits