An application of the thermal image velocimetry to detect the dust-devil like vortex

This study investigated 2-dimensional horizontal velocity distribution around a dust-devil like vortex observed by thermal image velocimetry (TIV, Inagaki et al. 2013). TIV is a method to measure the two-dimensional velocity distribution near the ground, which is accomplished by tracking footprints of the coherent structures of turbulence, which is moving with wind, within a time-sequential thermal image on the ground surface. This method has an advantage to observe the flow field among an area of few hundred meters by few-centimeter resolutions. Field observation was conducted in a sports ground of Tokyo Institute of Technology at a noon time in discontinuous days from March to September in 2013. A thermo-camera (FLIR, SC5200) was installed at 55 m from the ground at the top of a building in the university campus, and to take images of surface brightness temperature on a sports ground which is covered by artificial turf. The sampling frequency of the camera is at 100 Hz, and camera view covers in a horizontal range of about 100 m by 200 m. At the same time, 5 sonic anemometers were also installed within a target area to measure the velocity distribution at 5 levels from 10 cm to 1.2 m from the ground. During the observation period, two dust-devil like vortices were observed in different datys by chance. The diameter of the vortices were more than 30 m. We analyzed one of them. The core of the vortex was detected by checking the correlation between the flow field and the Rankine vortex model. This makes it possible to measure the advection speed of the vortex, and also to obtain the ensemble mean profile of the radial and tangential velocity of the vortex along the radial direction, and also the size of the vortex core. In addition, the anomaly of the velocity distribution around the vortex, unsteady deformation of the shape of vortex, and meteorological environment when the vortex appeared were also examined.

Reference: Inagaki A, Kanda M, Onomura S, Kumemura H (2013) Thermal Image Velocimetry, Boundary-Layer Meteorol 149(1), 1-18.