The use of wind tunnels and scaled models to help solve the aerodynamics problems of aircraft as they are developed is a well-established method in the aerospace sector.
In the early days of aviation, wind tunnels were used to see if and how new aircraft designs would fly. Nowadays, with the tube and wing design of aircraft dominant in the market, they are more likely to be used for verification and certification.
But could wind tunnel’s role in aircraft development be about to change again?
According to the aviation experts who gathered at DNW’s Large Low-speed Facility (LLF) in the Netherlands this week, the answer is yes. Changing research requirements and closer integration with computational fluid dynamics (CFD) software will ensure wind tunnels remain an essential part of aircraft development for the next 20 years.
Can aerodynamics be solved
To some, aerodynamics is an engineering problem that has been solved by the development of high performance computers, able to perform complex CFD simulations. This software enables engineers to show how “flows” of wind will dynamically interact with 3D digital models.
As CFD algorithms and computing power improve in line with Moore’s Law, the simulations become more detailed and closer to reality. It becomes easy to predict a world where wind tunnels, especially complex large ones, are no longer required.
Despite the rapid and continual advance of computer simulation technology, the consensus at the “Future needs for experimental verification in aerodynamical discplines” symposium at the LLF was that that the need to verify computer simulations using wind tunnels at different stages of R&D will continue far into future. The simulations also need data to be based on and wind tunnels can provide this.
The LLF just happens to be the largest wind tunnel in Europe and one of the most complex. It’s unlikely many invited to the event would discuss the wind tunnel’s demise in its very shadow.
However, most agreed that instead of being replaced by computer simulation, wind tunnels and CFD will be used in a more complementary way in the future, while the development of hybrid wind tunnels, and the need to validate simulations for low-noise aircraft will grow.
Professor Chris Atkin, from the department of Mechanical Engineering and Aeronautics at City University of London, runs the UK low turbulence wind tunnel and gave examples of areas of recent research in support of wind tunnels’ continued usefulness. This included research into hybrid laminar flow systems and suction. “I believe it may limit the control of laminar flow,” he said.
Other examples included experiments looking at cross flow velocity and its ability to create instability, and research by engineers from the University of Nottingham into how the early stages of turbulence can be predictable.
“Let’s not write the history of wind tunnel testing too early. We don’t know what is around the corner, and once we get rid of them, we are well and truly in the world of numbers,” said Atkin.
“In the future we don’t know what the problems will be. We need to preserve the facilities and the capabilities.”
Dr Holger Mai, head of aeroelastic experiments for German research organisation DLR discussed how wind tunnels continued to be used in assessing aeroeleastics – the interaction of aerodynamic forces with structures.
Although flutter has been investigated since World War One using scaled models, the development of flexible wings in the Aerostabil project and laminar airfoils in the Alf-7 project still required the use of wind tunnels to verify numerical simulations, he said.
DLR also has a project, called Vicas, which aims to develop a digital-analogue hybrid wing tunnel. Mai said, “The aim is so that on one screen you have real time measurement from the tunnel, on the next screen real-time numerical results. If something happens in the wind tunnel, the simulation is automatically updated.
“They are complementary technologies and it should stay like that,” he added.
Reducing aircraft noise
Noise is a growing area of aviation research and one in which the LLF has invested in for equipment upgrades.
Professor Jan Delfs, head of DLR’s Institute of Aerodynamics and Flow Technology, gave an overview of the experimental techniques available to research aircraft noise within the context of the European Union’s Flightpath 2050 goals. Flightpath is aiming for a 65% reduction in aircraft noise by 2050.
“To test silent or nearly-silent aircraft you can either use high performance computers to simulate or experimental tests. If you want to test complete aircraft noise in a wind tunnel, you need an acoustic wind tunnel of excellent quality, bigger than three meters.
“You also need aeroacoustically similar turbofan simulators and pressurized air for them.
He also pointed to a need for hybrid wind tunnels for noise testing, which are neither open or closed at the sides, such as the one at Virginia Tech in the USA, where the side walls are made of Kevlar for acoustically transparent walls. Large wind tunnels are also needed to test large components and how noise interacts with different integrated components, he said.
“In the future we have to have combined wind tunnels with numeric data. It makes sense,” he said.
Phillipe Beaumier from French aerospace lab Onera, described how wind tunnel testing was essential in the development of rotor blades for the new Airbus H160 helicopter. He went on to outline the areas of helicopter development that wind tunnels would be needed for.
“They will be needed to support the development of high speed helicopters,” said Beaumier.
“For the forward flight of helicopters, we lack experimental data. It’s also needed for active rotors and multi-rotors. Perhaps the most critical thing is to understand the dynamics of helicopters better in the next five to ten years, better data is also needed for engine integration.”
He added that wind tunnel testing was essential to understand the physics of helicopters, such as unsteadiness, separation, vortical structures and turbulence.
“Digitalization will not kill wind tunnel testing, but data assimilation must occur,” he said.
Propellers and fans
Other speakers included Gustavo Trapp, senior propulsion systems engineer from Embraer, who discussed the importance of wind tunnels in verifying simulations of propellers. He said that they are needed to measure forces and moments, pressure distribution, transition and deformation, efficiently and cost-effectively.
“People are using data from the second world war to validate their models for propellers. It’s overdue that we have a new open dataset for this,” he said.
Ken Young, chief engineer at Rolls-Royce, outlined progress on the company’s next generation geared turbofan engine, Ultrafan, and discussed how it used wind tunnels when more of engineering and development was becoming digital.
“We have design tools for flutter and can design it out of the system. We use high fidelity CFD at every stage of design,” said Young.
“That impacts how we work with experimental organisations like DNW. We wouldn’t have fully understood the physics when we came to them with an engine 15 years ago.
“Now, they are helping us to understand the physics using high quality data, which we also use to validate our CFD against boundary conditions. That gives us the confidence to go out and use it [CFD] as a design tool.”
Finally, Floriane Rey, acoustics engineer at Dassault Aviation, described a research project that involved a two week test campaign at the LLF to develop a new tail for aircraft that reduces engine noise.
The Clean Sky research project’s U-tail shielding has been developed to TRL 4-5. It was fitted onto the airframe of Dassault’s Low Sweep Business Jet (LSBJ) and tested at the LLF, including with the moving belt to simulate the ground’s effect.
“It is a combination of testing and simulation that will give you the results. When we found different results from the wind tunnel, we developed a new turbulence model for CFD computations. That led to an accurate comparison with the test results,” said Rey.
The wind tunnel also enabled the comparison with classic tail shapes.
“Thanks to the large size of this wind tunnel, we have been able to measure the noise sources of landing gear and the nose. It’s really important to have that going forward,” said Rey.