Development of New Ultrasonic Measurement System for Turbulence and Elucidation of the Limit of Transport in Turbulence

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Study on turbulence is important for every area of science and technology. Among them thermal turbulence has been popular research subjects, since the prediction of heat transport, diffusion and mixing in thermal turbulence is indispensable fur both fundamental and application researches. In this research project we aimed to develop a new ultrasonic measurement method far thermal turbulence and explore stricture in turbulence to elucidate the scaling law of beat transport due to turbulence. We used mercury as a test liquid. Because, the fact that its kinematic viscosity is low and kinetic boundary layer is thinner than the thermal boundary layer enables us to explore high Rayleigh, high Reynolds number in laboratory Although ultrasonic velocimeter (USV) is known for long time for flow rate measurements, it has never been applied to measurement of turbulence due to limitation in temporal resolution and accessible maximum velocity We succeeded to use USV for thermal turbulence, because t … More he maximum velocity and the highest frequency in thermal turbulence is relatively low even at high Rayleigh number compared with other turbulence, such as grid turbulence and wake turbulence. We developed 3 measurement systems far thermal turbulence 1. Three ultrasonic transducers monitoring along X, Y, and Z axes simultaneously. 2. One moving transducer without destructing the wall of the convection cell. 3. Multi-thermosensors (32 thermistors) embedded in the cell bottom and the side wall which enable us to monitor the structure of the mean flow in thermal turbulence. By using these measurement systems, we successfully measured the energy spectrum directly from instantaneous measurement of the velocity filed without utilizing Taylor hypothesis, and found that Bolgiano-Obkhov scaling law holds in thermal turbulence. We further elucidated structure of the mean flow existing in thermal turbulence. Their structures, dynamics, and stabilities strongly depend on the aspect ratio of the convection cell. Less