Two-color glowing molecule could make wind tunnel tests more accurate
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A two colour glowing molecule which can sense both pressure and temperature
view moreCredit: The University of Manchester
Key findings
- Researchers have developed a single molecule capable of measuring both air pressure and temperature at the same time, overcoming a longstanding challenge in sensor technology.
- By emitting two different light signals, one affected by both pressure and temperature and another affected only by temperature, the molecule can automatically correct for temperature effects and provide more accurate pressure readings.
- Designed for use in specialist sensor paints, the breakthrough could help engineers capture more reliable pressure measurements from aircraft models in wind tunnels, even when temperatures fluctuate.
Most sensors can only measure one thing at a time: like a thermometer for temperature, or a pressure gauge for pressure. Measuring two properties accurately from the same surface, at the same moment, usually means using two separate instruments – adding cost and complexity to the process.
Now, chemists and aerospace engineers at The University of Manchester, working with colleagues at the University of Eastern Finland, have designed a molecule that can help us to do both simultaneously. It emits two distinct colours of light at once: one that responds to both air pressure and temperature, and one that only responds to temperature. By comparing these two signals it’s possible to take a corrected pressure reading from the molecule itself, rather than having to use a separate temperature sensor alongside a pressure sensor.
The molecule has been designed to be an active ingredient in the specialist paints that measure pressure on aircraft models in wind tunnels, solving a major challenge in the aerospace industry. When engineers test a new aircraft design in a wind tunnel, they need to know exactly how air pressure is distributed across every surface. One of the best ways of doing this is by using a pressure-sensitive paint that glows in proportion to the air pressure pushing against it. The problem is that the light these paints currently emit is sensitive not only to pressure, but also to temperature. As a scale model heats and cools during a wind tunnel test, the glowing shifts in ways that have nothing to do with pressure, introducing errors that engineers then have to correct.
Dr Alexander Romanov, Senior Research Fellow in the Department of Chemistry, The University of Manchester, said: “Our new Manchester material emits red light, what we call phosphorescence, which is sensitive to changes in both pressure and temperature. At the very same time, this material emits blue light as a fluorescence – responding only to temperature. By measuring blue and red light at once, we’ve got everything we need to separate the pressure signal from the temperature interference. That kind of built-in self-correction is simply not possible with a standard light-emitting molecule.”
The team’s work, published in the journal Advanced Optical Materials, explains how the molecule is built around a gold atom bonded to a ring-shaped compound called acridine. The position of that bond within the structure determines which type of light the molecule produces. Gold was chosen because even subtle changes to where it sits within the molecule produce reliably different light-emitting behaviours, giving chemists precise control over the sensing properties. When the gold atom bonds to one position on the molecule, it triggers the pressure-sensitive red emission, whereas a bond at a different position triggers the temperature-only blue emission.
Calculations carried out by their colleagues in Finland, helped the team to understand and predict these differences before the molecule was built in the laboratory. Then in use, the molecule is embedded in a paint applied to test models in the standard way. A camera captures both emission colours simultaneously, and the ratio between the two signals automatically corrects for temperature, without any separate temperature sensor.
Dr Mark Quinn, Reader in the Department of Mechanical and Aerospace Engineering at The University of Manchester, said: “In wind tunnel testing, temperature correction is currently one of the main challenges to making pressure-sensitive paints reliable. Having both pressure and temperature measurements come from the same molecule simultaneously is a more elegant solution, and potentially a more practical one for real test conditions, where adding extra instruments creates its own complications.”
The team’s work sits within a broader programme of research at Manchester into improving pressure-sensitive paint for aerospace applications. A related study, recently published in ACS Applied Engineering Materials, addresses the temperature problem from a different direction, by designing a platinum-based paint with inherently low temperature sensitivity. Together, the two studies represent complementary approaches to one of the industry’s most persistent challenges.
This research has been published in the journal Advanced Optical Materials.
Full title: Temperature Correction in a Single Luminophore Pressure-Sensitive Paint Using a Dual-Emitting Gold Acridine Complex
DOI: 10.1002/adom.71493
URL: https://advanced.onlinelibrary.wiley.com/doi/10.1002/adom.71493?af=R
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Journal
Advanced Optical Materials
Article Title
Temperature Correction in a Single Luminophore Pressure-Sensitive Paint Using a Dual-Emitting Gold Acridine Complex
Glowing paint that maps air pressure could improve the design of future aircraft
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Aerodynamic performance data on a truncated cone model at supersonic flow collected using low temperature sensitivity PSP.
view moreCredit: The University of Manchester
Key findings
- Scientists have developed a new pressure-sensitive paint that delivers more accurate pressure measurements by minimising the effects of temperature fluctuations.
- The material combines a platinum-based light-emitting compound with a specially engineered polymer, allowing it to respond more reliably during wind tunnel testing.
- Trials showed a 25% reduction in temperature sensitivity compared with the current industry standard.
- The improvement comes from chemically locking the active molecules into the polymer structure, preventing molecular clustering that can distort measurements.
- The advance could help aerospace engineers make better-informed design decisions using cleaner aerodynamic data.
When engineers design a new aircraft, they need to know exactly how air flows over every surface. The pressure distribution must be controlled or the fuel efficiency, handing, or even structural safety can be affected.
A promising new technique for measuring this is by applying a special paint to scale models tested in wind tunnels, which glows in proportion to the air pressure pushing against it. However, these paints have one main flaw – they’re sensitive to temperature as well as pressure. As a model heats up during testing, the paint output can shift and introduce errors that engineers then have to unpick.
Now, a team from The University of Manchester’s Departments of Mechanical and Aerospace Engineering, and Chemistry, have created a paint that substantially reduces the problem. This new material uses a light-emitting, platinum-based compound, locked into a specially engineered plastic. The results of trials using the paint, published in ACS Applied Engineering Materials, show a drop in temperature sensitivity to just 0.3% per degree Celsius – 25% less than the current industry benchmark.
Dr Elliott Nunn, first author based in the Department of Chemistry, The University of Manchester, said: “When you’re testing a vehicle at high speed it can heat and cool dramatically based on its aerodynamic design. By creating a pressure-sensitive paint which doesn’t respond as strongly to this heat, we’ve got something that’s much closer to measuring exactly what we want to measure. Our hope is that this will really help the engineers designing the next generation of high-performance and more sustainable aircraft and spacecraft, to make better-informed decisions through cleaner data.”
Their breakthrough comes down to how the active ingredient sits within the material. In many existing paints, the molecules responsible for glowing can cluster together, and this clustering makes the paint more sensitive to heat. The Manchester team fixed this by anchoring this ingredient, from the same family of molecules that give blood its red colour, or make leaves green, directly into a tough, Teflon-like plastic. When held in place at the chemical level, the molecules are far less likely to cluster and the paint’s temperature sensitivity drops.
Dr Louise Natrajan, Reader in the Inorganic Chemistry Group, The University of Manchester, said: “Getting this chemistry right was thanks to a creative collaboration between our chemistry group and the aerospace engineering team – basically, they knew what the paint needed to do in a wind tunnel, and we knew how to create something that could do it.”
To test their paint under realistic conditions, the team applied it to a cone-shaped model designed to produce complex airflows, then ran this model through a supersonic wind tunnel where airflows can exceed Mach 5 – 5x the speed of sound. At these conditions the model’s temperature varies drastically across its surface. However, the new paint measured pressure accurately throughout the test, with results aligning closely with the values predicted by computer simulations.
This paint was also able to help the researchers visualise the corkscrew-shaped swirls of air that develop along concave curved surfaces – known as Görtler vortices – which are important for understanding how the thin layer of air next to a surface behaves at speed.
More accurate pressure measurements at high speeds and temperatures, could translate directly into helping the aerospace industry to develop safer, more efficient transport. The team are now planning to test their paint across a wider range of conditions, to build confidence in how reliably it can perform.
This research has been published in the journal ACS Applied Engineering Materials
Full title: Low-Temperature Dependency Pressure-Sensitive Paints for Wind Tunnel Testing Based on Luminescent Polymer-Bound Porphyrins
DOI: 10.1021/acsaenm.6c00751
URL: https://doi.org/10.1021/acsaenm.6c00751
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Journal
ACS Applied Engineering Materials
Article Title
Low-Temperature Dependency Pressure-Sensitive Paints for Wind Tunnel Testing Based on Luminescent Polymer-Bound Porphyrins
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