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Japan Aerospace Exploration Agency

Optical measurement technologies

Optical measurement technologies using cameras and lasers are non-intrusive and can acquire various and large information of the flow fields around the wind tunnel test models. JAXA has developed three optical measurement technologies in its wind tunnel testing to expand the lineup of the measurement techniques in JAXA wind tunnels: pressure measurement technology using pressure-sensitive paint (PSP), which changes color in response to the pressure acting on the model; particle image velocimetry (PIV), which mixes microscopic oil particles into the flow to measure flow velocity; and model deformation measurement (MDM), which measures deformation and displacement of the wind tunnel model, including wing bending caused by lift.

Pressure-sensitive paint (PSP)

Conventional pressure measurement using pressure taps and pressure transducers have to manufacture taps on the test models in order to measure pressure on the model surface. Pressure-sensitive paint (PSP) is a luminous paint whose luminescence intensity changes depending on the surrounding pressure. By measuring the luminescence from PSP with a CCD camera, PSP measurement can give users global pressure distribution on the entire surface of the PSP painted model. The Figure below - Cp distribution measurement using PSP measurement on an aircraft configuration model - provides quantitative information and visual representation of the global pressure distribution on the model. Aiming to make PSP an efficient and user-friendly standard measurement technique, JAXA is developing and using the JAXA backbone PSP measurement system that enables quasi-real-time measurement by introducing automation of the measurement procedure and improvements to minimize required time for PSP data reduction.

Cp distribution measured using a PSP measurement for an aircraft configuration model

Particle image velocimetry (PIV)

Particle image velocimetry (PIV) measures the velocity distribution and direction of flow around a model, identifies vortices, and performs other assessments. Under the PIV method, the user mixes "tracers" - fine particles of oil around 1 micrometer in diameter - into the air and photographs the tracers as they pass through a laser light sheet. By then calculating the distance particles travel based on a comparison of two images taken at a given interval, the user can measure the velocity and direction of the flow containing the particles. Each image shows tens of thousands of tracers, whose exact travel distances users can determine by performing computer-based statistical processing on the images. JAXA’s setup uses two cameras to photograph tracer images, enabling measurements of three-dimensional velocity vectors. Researchers are conducting a wide variety of tests to establish PIV as an increasingly versatile technology capable of measuring the velocity and directional attributes of the invisible flows around models in wind tunnels of all speeds - from low-speed tunnels to transonic tunnels.

The PIV measurement-derived space velocity distribution
for the slipstream of a wing on an aircraft configuration model
The image shows the wingtip vortex (y=-500 mm) and the slipstream of the engine nacelle (y=-200 mm)

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