・About Advanced Measurement 

Surface pressure, frictional stress, and temperature are essential quantities of state for the design of aircraft, railways, automobiles, and other vehicles. By measuring these state quantities, we can understand fluid phenomena such as boundary layer transitions, vortex emissions, and shock waves. Our group is working on molecular imaging to visualize and measure the state quantity using functional molecular sensors. Also, we are working on super-resolution measurements to improve the temporal and spatial resolution of the measurements using data-driven science.

・ Super-Resolution Measurement 

Experimental measurements have become more sophisticated, faster and with higher resolution every year. However, there are still some problems in the flow field, such as supersonic turbulence, which requires high speed and high-resolution measurements. Point sensors, such as unsteady pressure sensors, are capable of very fast sampling and high time resolution measurements, but even with multi-point measurements, fine structures, such as turbulence, are not fully resolved. On the other hand, measurements using high-speed cameras, such as PSP and PIV measurements, can measure in high spatial resolution, but the sampling speed is slower and the temporal resolution is lower than that of point sensors. Therefore, we are applying the techniques of data-driven science to realize high-spatiotemporal resolution measurements by combining measurements with low spatial resolution but high temporal resolution and measurements with high resolution but low temporal resolution.

1. Three-Dimensional Density Field Reconstruction of Supersonic Jet using BOS Method

The supersonic exhaust jets of rocket engines or jet engines of supersonic airplane generates strong acoustic wave. The generation of the acoustic wave is related to unsteady fluid phenomena such as turbulent-shock wave interaction and generation of large scale turbulent structure in the shear layer. In this research, in order to clarify the fluid phenomena of supersonic jets in detail, we visualize the three-dimensional fluid phenomena using the three-Dimensional Background Oriented Schlieren (3D-BOS) method. Currently, we are applying data-driven science techniques to the 3D-BOS method to reconstruct the 3D density field with high temporal and spatial resolutions.

Schematic diagram of the experimental setup of BOS measurement	Reconstructed density field

・Molecular Imaging 

We are developing technologies to measure state quantities by applying functional molecular sensors that emit fluorescence and phosphorescence, such as pressure-sensitive and temperature-sensitive paint (PSP / TSP) measurement and luminescent oil film (GLOF) method. In recent years, we have been conducting research to realize highly accurate and high spatiotemporal resolution measurements in various velocity regions, targeting "unsteady fluid phenomena". In the following, the research contents of PSP and GLOF are introduced.

1. Development of Fast-Responding PSP with Low Surface Roughness (joint research with JAXA)

Since a large surface roughness of conventional fast-response PSP, it could not be used for industrial wind tunnel tests and comparison with numerical simulation. Our group has developed the fast-responding PSP (AIAA Paper) with low surface roughness by developing and using the equipment to support PSP development such as an automatic calibration device and laser microscope. In addition to academic research, further improvements are being made to the low surface roughness fast-responding PSP for application to industrial product development.

2. Unsteady PSP Measurement in Low-Speed Wind Tunnel Tests

In the low-speed wind tunnel test, the pressure fluctuation is several Pa to several hundred Pa, and the signal-to-noise ratio is not sufficient for the conventional unsteady PSP measurement. We have developed a PSP with sufficient emission intensity change for small-amplitude pressure fluctuations and improved the signal-to-noise ratio by post-processing methods, and succeeded in quantitative measurements of pressure fluctuations caused by Kármán vortex vortices released from a square cylinder in a low-speed flow. Currently, we are developing techniques to measure even higher frequencies and smaller amplitudes of phenomena such as aerodynamic acoustics. We are also applying and developing data-driven noise reduction methods such as singular value decomposition and Kalman filters to expand the application of PSP measurement in terms of both measurement and post-processing.

Time-resolved visualization of Kármán's vortex street behind square cylinder using PSP

3. Time-Resolved Pressure Distribution Measurement on Wing Surface of Commerical Airplane using Fast-Responding PSP with Low Surface Roughness (joint research with JAXA)

When an aircraft flies at transonic speed (near the speed of sound), a shock wave is generated on the wings. This shock wave not only increases the drag on the aircraft, but also causes a phenomenon called "transonic buffet", which causes harmful vibrations to the aircraft depending on the flight conditions. In this laboratory, we measured the pressure distribution on the wing surface of a commercial aircraft model using the fast-responding PSP with low surface roughness, which was newly developed in our group in collaboration with JAXA. Our group succeeded in the quantitative measurement of the unsteady pressure distribution in transonic buffeting cases. Based on this data, we are now investigating the buffeting phenomenon of three-dimensional transonic velocity on the wing.

Visualization of transonic buffet using developed PSP (left: time-averaged pressure field; right: distribution of fluctuation component of pressure)

4. Visualization and Measurement of Skin Friction Field using GLOF Method

In order to measure and understand the frictional drag force that acts on an aircraft during cruising and the phenomena such as boundary layer transition and separation/reattachment, it is important to know the distribution of frictional stress on the surface of an object and its structure (topology). However, the conventional measurement method is point measurement, and it was difficult to measure the distribution. Recently, a global shear stress measurement method using the global luminescent oil film visualization (GLOF) has been developed. This method calculates the shear stress vector field by analyzing the time-resolved heat flux field obtained by the TSP measurement or by analyzing the visualization of the flow near the wall by the GLOF. It is expected to be applied to surface measurement of wall friction stress field. Our laboratory aims to establish a highly reliable method for measuring frictional stress fields by developing a new GLOF analysis method. Furthermore, we are applying the developed image analysis method to various flow fields to clarify the phenomenon.

Source code: Github (Link)
Demo datasets: GLOF_example.zip
Journal paper: ・Lee et al. (2018)

Experimental setup of TSP measurement (left) and estimated skin friction distribution based on the time variation of heat flux (right)