Wind tunnels - why do they matter

This is what engineers do when they build and test an aircraft model in a wind tunnel. In the world of aerodynamics, the wind tunnel has been a critical facility for measuring aircraft performance for decades. Wind tunnels form part of a strong sovereign defence capability in the UK - a capabilty that is strategically important today and into the future.

In this feature we spoke to engineers and business leads from across the Air sector to understand the value of the wind tunnel in the design of an aircraft and beyond that, as a critical and national asset that supports the UK Combat Air Sector.  

Wind Tunnels – In a digital world, why are these physical test facilities still needed?

Let’s start first with understanding what it is that wind tunnels are designed to do.  By their broadest definition, they are tunnels through which air is blown to allow the impact of the airflow on an object to be measured, and in this case, this is most often a model of a military aircraft.

Designing an aircraft 

To address these complex design challenges, aerodynamicists use a combination of digital techniques and physical tests, including wind tunnel testing.  Before arriving at an initial aircraft design, aerodynamicists use computational fluid dynamics (CFD), a method of simulating airflow using computing power, to test the feasibility of a range of initial concepts. By using some simplifications of the exact flow solution, the CFD methods in use today allow engineers to quickly filter down a very large number of configurations to eliminate those that aren’t feasible/going to fly.

As the design matures, aerodynamicists turn increasingly to wind tunnel testing, which typically employs a reduced scale aircraft model mounted inside the tunnel.  Whilst it remains in a fixed configuration, it will be tested at different airspeeds and repositioned at different angles compared to the airflow inside the tunnel to replicate different flight conditions.

Large amounts of aerodynamic test and qualification data are generated from the tests so that engineers can understand the effect of forces like lift, drag and turbulence and can use this data to further refine and perfect the shape of the aircraft to deliver maximum performance.

Once the fundamental shape for the aircraft is determined (concept selection), the purpose of the wind tunnel then focuses increasingly on generating the data that will be used by other systems on the aircraft. Most notably the next phase of wind tunnel testing will create the data set needed by the flight control system (FCS), responsible for flying the aircraft.

Move further through the aircraft’s development and the wind tunnel will be used (alongside CFD) to generate aerodynamic data to design for the safe separation of payloads.  This is another complex area of military aircraft development because you have different sets of aerodynamic behaviours for the aircraft, the payload and then again for the two combined.

A combined approach – wind tunnel testing and digital modelling

Aerodynamicists have been using this combination of wind tunnel testing and digital techniques to design aircraft for decades. While in the past CFD was mainly used early on in the design process before switching to wind tunnel testing as the design matures, CFD and wind tunnel use is now much more complimentary with CFD used throughout the design process supported by wind tunnel testing.

Looking forward, CFD and wind tunnel testing will continue to be required, although the way we use them will evolve further. As CFD/computing capability improves, it will be used more and more to explore and understand product performance, with targeted wind tunnel testing used to validate the computational models and populate the aerodynamic dataset in regions where there is lower confidence in CFD, such as the extremes of the operating envelope.

Rather than signalling the end of the need for wind tunnels, this transition to more widespread use of digital techniques relies on continued development of wind tunnel testing capability; physical testing will be fundamental to establishing the required confidence in the use of the computational methods in future.

This will require new technologies for wind tunnel testing to be developed in industrial facilities, such as Pressure Sensitive Paint and other advanced measurement techniques that allow the full air flow field to be measured on and off the surfaces of the model, enabling better comparison and understanding of any differences with the CFD generated flow field.

The most effective tool for the job

As powerful as modern computational fluid dynamics software has become when used with the latest supercomputers, it cannot affordably generate the volume and accuracy of data required across the entire flight envelope.  A good example here is the measurement of intake engine aerodynamic compatibility, which generates and analyses large quantities of unsteady data.

Understanding aircraft performance requires data to be obtained across the operating envelope, including for challenging regions where unsteady separated flows are common. 

Current best practice CFD is unable to reliably predict these aerodynamic phenomena and methods that offer higher accuracy (such as scale-resolving simulations) are often prohibitively expensive to run. Consequently, current confidence in CFD is restricted to a narrower portion of the operating envelope with wind tunnel testing being relied on towards the extremes of the envelope.

CFD offers greater flexibility to quickly generate a lower fidelity solution, making it invaluable for rapid turnaround in concept assessment and detailed design, and as a tool to investigate unexpected test results or flight phenomena. However, despite advances in computing power, the efficiency of CFD simulations can be a barrier to its use as a tool to gather large amounts of data, such as when generating the data set required by the aircraft flight control system. In these cases, when many thousands of data points are required, wind tunnel testing can be more efficient as the up-front cost and lead-time may be offset by the speed of data acquisition during testing.

Leading engineering expertise

This complex and critical area of engineering means that BAE Systems has a deep and broad level of expertise in aerodynamics.  BAE Systems engineers and technologists participate in wider research and development programmes at an international level.  That includes for example having a seat at the NATO Science and Technology Organization (STO).