Determining the locations of flow transition on an aircraft is important for validating designs that may be useful in reducing drag and improving fuel economy.
A paper listed in NASA’s PubSpace open-access database presents a temperature-sensitive paint (TSP) applied over a carbon nanotube (CNT) layer to map airflow over aircraft in cryogenic wind tunnels. The CNT introduces a temperature step to amplify the adiabatic temperature change on an aircraft when airflow becomes more turbulent due to aerodynamic drag. The paper compares the CNT/TSP system to other methods for airflow mapping as well.
Cryogenic temperatures may be introduced in wind tunnels for reasons like generating higher Reynolds numbers in gas flow. While methods like infrared thermography are useful for airflow mapping at ambient temperatures, they are not practical for cryogenic wind tunnels, as IR radiation decreases in colder conditions. And while CNT layers have been used to produce a temperature step to amplify surface temperature changes, they tend to see degradation at only 110 K when based on an acrylic binder.
By basing the CNT on a polyurethane binder, the paint could remain intact at temperatures as low as 77 K. A DC electrical current was run through the CNT layer to produce the temperature step. Thermal mapping of flow transitions was successful at temperatures ranging from 110 to 200 K. The team showed that running a DC current through the CNT layer was more effective than injecting cryogenic liquid helium into the chamber to induce a temperature step because it preserved wind tunnel conditions.