Tunnel Vision


Far away and almost lost in the contrail of the aviation industry, a speck on the horizon suddenly became a little more distinct this year. A faster, cheaper and cleaner alternative to some forms of air travel appears to be gaining ground. It could be an illusion, of course, but in technical and cost estimations carried out by NASA, Elon Musk’s Hyperloop concept, a system of magnetically levitated pods accelerated along a tube in a low-pressure atmosphere, could represent a viable competitor to short-haul flights at least. There are many differences of opinion on the viability of such a system, but perhaps the most important impulse behind technologies such as the Hyperloop is the desire to reduce the environmental impact of high-speed transportation – to shift away from noisy, fossil fuel-burning aircraft, to renewable energy options.

NASA is of course engaged in its own era-defining program (the New Aviation Horizon initiative) to develop the next generation of aircraft to fly faster, cleaner and quieter. But while the Hyperloop, and other ‘vactrain’ concepts as they are known, would achieve their target in part by removing or at least greatly reducing the density of the atmosphere around the vehicle, that option is not available to researchers working for NASA’s Aeronautics Research Mission Directorate (ARMD).

To be faster, cleaner and in particular to reduce the boom of supersonic aircraft to enable overland flight, engineers must continue to work to unlock the secrets of how the aircraft interacts with its element, how it supports and impedes, in ever finer detail. Is testing a scale model in a wind tunnel the best way to expose the subtleties of that relationship, or will the wind tunnel ultimately be made redundant by advances in computational fluid dynamics?

The question is relevant to engineers working on NASA’s Commercial Supersonic Technology (CST) project, which is leading research on a number of key technical challenges: integrated low-boom aircraft design; sonic boom community response metrics and methodologies; and low-noise propulsion for low-boom aircraft. The CST project is also researching improvements in supersonic cruise efficiency, emission reduction, aeroservoelasticity and flight systems.

Low boom flight demonstrator

To accomplish its goals, NASA decided to commission a low boom flight demonstrator (LBFD) aircraft to validate the design tools and technologies applicable to low sonic-boom aircraft and to create a database of community response to support the development of a noise-based standard for supersonic overland flight. The objective is a research aircraft that creates a shaped sonic boom signature with a calculated perceived loudness level of 75dB or less during supersonic cruise flight at Mach 1.4. Although the aircraft will be smaller in size than future supersonic airliners, its sonic boom ground signature will be traceable to a larger aircraft.

The LBFD aircraft will be capable of performing multiple supersonic overflights of a single community with passes nominally 50 miles (80km) in length and up to 20 minutes apart on

a single flight.

In March 2016 Lockheed Martin was selected to carry out the preliminary design of its LBFD concept, called the Quiet Supersonic Technology (QueSST) aircraft. It completed a preliminary design review in June 2017.

David Richwine, the QueSST preliminary design project manager at NASA Langley, says that prior to the initiation of the QueSST preliminary design, the team conducted several concept feasibility studies with Lockheed Martin and Boeing, a major component of which was the maturation and validation of low-boom design tools and technologies, including both computation and experimental assessments of the design concepts in high-speed wind tunnel facilities. This was followed by a series of wind tunnel tests of increasingly larger scale models.

“Early in the QueSST preliminary design, Lockheed Martin used a 4.5%-scale low-speed wind tunnel at its Palmdale, California facility to collect initial aerodynamic performance and handling quality assessment data for the configuration,” explains Richwine.

Two further wind tunnel tests were planned as the core of the QueSST testing program.

“In spring 2017 a 9.5%-scale model of Lockheed Martin’s QueSST configuration was tested at the NASA Glenn 8 x 6ft supersonic wind tunnel to obtain a high-fidelity set of high- and low-speed aerodynamic performance and propulsion/airframe integration data,” continues Richwine. “The second test will use a 15%-scale model of Lockheed Martin’s QueSST configuration to obtain high-fidelity low-speed aerodynamic performance data. Model fabrication is nearly complete, with testing planned in the NASA Langley 14 x 22ft wind tunnel in fall 2017.”

Ray Castner, a NASA aerospace engineer at the Glenn Research Center, says that the wind tunnel tests focused on aerodynamic stability and control (S&C) and the performance of the propulsion inlet. The S&C aerodynamic lift and drag test completed a total of 86 test configurations and over 208 hours of wind-on testing, while the propulsion test completed a total of 12 configurations and underwent over 73 hours of wind-on testing. In both cases the results were compared with CFD simulations.

“CFD simulations were performed by both Lockheed Martin and NASA for the stability and control portion,” says Castner. “The wind tunnel performance database was compared with the computational modeling database, with good agreement of modeling capabilities with respect to the test data.

“For inlet performance, computational tools are in the development phase, so the wind tunnel data was critical in the understanding of the top-mounted, aft-mounted inlet location. To NASA’s knowledge, this is the first experimental database of a supersonic inlet in this location as tested on a full vehicle configuration. Wind tunnel test data showed the inlet to have adequate performance to fly the mission.”

So while others might appear to be making progress in developing alternatives to high-speed air travel, those at the forefront of aviation research believe that the experimental data sets, combined with the computation output, will form the core of the performance data used for configuration assessment and control law design of the QueSST, and will also help to improve computational modeling for low-boom vehicle designs in the future.

As Castner says, this is really how it has always been: “Validation data sets are important to make sure that modeling is correct over the range of expected flight conditions, as computer models are sometimes not accurate when used outside the range of validation.

This modeling and wind tunnel test approach is typically used as risk reduction in aerospace vehicle development projects.”

Second opinion

Castner isn’t alone in his view. Advances in CFD will never becalm the wind tunnel business, according to Peter Curtis, chief technical officer at the Aircraft Research Association (ARA), a UK-based independent research and development organization specializing in high-speed wind tunnel testing, CFD and high-precision wind tunnel model design and manufacture. However, Curtis believes that better computation will have an impact on the way tunnels are used. “There is a resurgence in new wind tunnels around the world at the moment, but the nature of tunnel testing is changing with the improvements in CFD capability,” he says.

While CFD can reliably model flows “in the core of the envelope”, says Curtis, wind tunnels still hold many advantages, particularly in terms of testing time when exploring less stable parts of the airflow, further from the surface of the aircraft.

“CFD is very good at predicting flows in the on-design area, where there are no separated flows or significant instabilities. To predict those correctly takes orders of magnitude more computing time,” Curtis continues. “Wind tunnels have the advantage around the edges of the envelope in the off-design regime, where there are separations and instabilities. In such cases CFD can often tell you that something interesting is happening, but not confidently what that is. Once one has a model in the tunnel the number of cases that can be tested in a day is enormous.”

But there is an important two-way relationship to be developed between experimental data and that produced by CFD.

“We believe it is essential that CFD and the wind tunnel are seen as complementary tools for the aerodynamicist,” he says. “High-quality experimental data is needed to validate and calibrate computational tools, which can in turn be used to enhance data quality from the tunnel, for instance in improving blockage corrections or the scaling of Reynolds numbers. CFD data can also be used to help refine the test matrix for a tunnel campaign by highlighting areas where unusual phenomena are anticipated.”

But in order to gather data for CFD validation, high-fidelity measurements must be taken at the surface of the test model and above. Traditional surface pressure ports can produce very high quality data, but they are limited by the number that can be integrated into a model. ARA, among others, has been developing the use of pressure-sensitive paint, which effectively increases the number of ‘ports’.

“The current initiative with this technique,” says Curtis “is to extend it into the time domain, by taking dynamic data with high-speed cameras, and making measurements in hard-to-access areas using miniaturized cameras. It is also necessary to know exactly what shape the wind tunnel model is when the measurements are being taken, so high-fidelity dynamic model deformation measurement systems are required. While such systems are relatively commonplace, their use in wind tunnels is fairly recent.”

Curtis adds that until recently off-surface measurements systems, such as particle image velocimetry, have mostly been used in academic research. Now modern systems are being used in the industrial context, to help diagnose interesting phenomena and assist in CFD validation and calibration.

“Wind tunnels will always be necessary, says Curtis, “but there is also a need to make sure they can produce the best quality data and at reasonable cost.

“The constant drive from our customers is to maintain the highest quality while taking data faster and keeping a lid on costs. Therefore the upgrading of basic control systems, data acquisition systems and improvements in automation are all absolutely essential just to stay competitive in the commercial environment. To improve their commercial offering, modern tunnels need to add in enhanced test techniques, acoustic measurements and multichannel dynamic measurements. The growth of interest in dynamic measurements, as we do more testing toward the edges of the envelope, has been most striking in recent years.”

ARA has made a number of enhancements to its capabilities lately. Curtis concludes, “We have developed the use of whole aircraft coverage with pressure-sensitive paint and are still working to reduce the time taken for processing and analysis of the data. We have also installed two 144-microphone arrays in our acoustic liner for performing beam forming measurements and are looking to extend this from analyzing stationary, or time-average, noise sources to dynamic ones, such as propellers.”

George Coupe is a journalist and editor with many years of experience writing for science, engineering and technology publications, as well as the national press

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