External flow around an acoustically levitated droplet
- Paper number
IAC-08.A2.4.3
- Author
Mr. Koji Hasegawa, University of Tsukuba, Japan
- Year
2008
- Abstract
Containerless technology has been well recognized as a method for obtaining liquid undercooling?Particularly in microgravity, a liquid can remain undisturbed and experience deep undercooling? One of the major recent advances for experiments in containerless processing is acoustic levitation?In terms of external flow around an acoustically levitated droplet by ultrasonic wave, Trinh et al.(1994) have qualitatively investigated external flow using flow visualization techniques?Acoustic streaming flow is generated and the flow field is characterized both in the absence and in the presence of levitated sample in a levitator. A standing wave is generated by an ultrasonic driver and coaxial reflector?Rednikov et al. (2006) provided an explanation of the flow phenomena observed by Trinh et al. (1994) by employing both computational and analytical techniques?Although their model was an idealization of the experimental situation, in the sense that we assume the primary flow to be a uniform stream, experimental result obtained by Trinh et al.(1994) show that is not in the case? With regard to internal circulation, configuration of it is influenced by viscosity of levitated droplet observed by Yutaka Abe et al.(2007)?This result is measured by comparing levitated glycerin droplet with water droplet? The purpose of the present study is to experimentally examine the characteristic of external flow of a droplet levitated by ultrasonic wave?In this study, influences of different viscosity of levitated of droplet and decreasing input voltage are mainly investigated. The purpose of the present study is to experimentally examine the characteristic of external flow around an acoustically levitated droplet? An ultrasonic single-axis levitator consisting of a transducer and a reflector generates a standing wave between the transducer-reflector gap?A sinusoidal wave signal is generated from a function generator?The signal produce acoustic standing wave provided the radiation pressure arising from a vertically oriented in the gap?A levitating droplet inserted by syringe is suspended at a velocity antinode?In a gaseous host medium, typical sound pressure level are in the 155-165 [dB] range, and the frequency is roughly 19.4 [kHz]?The transducer-reflector gap is 47.5 [mm]? Particle Image Velocimetry (PIV) is implemented and quantitative flow is visualized by using a sheet laser?a high speed video camera and a computer?Detailed observation of the fluid particle motion and of the local flow environment is carried out by PIV?The volume of the test section is enclosed by a transparent square soft chamber?The tracer particles have been generated by nebulizer, and the size is about 5 [µm] of water particle?A streaming flow field induced by acoustic standing wave can be visualized by illuminating entrained tracer particles?The flow pattern is recorded through a high speed video camera?In this study, two types of fluid, water and glycerin are used as test fluid?The viscosity of water and glycerin are 1 [mPa]?s and 1510 [mPa?s], respectively? In the case of any levitated droplet situated at a velocity antinode of standing wave, toroidal vortex are generated around levitated droplet?It is confirmed that flow structure around levitated droplet is strongly affected by viscosity of levitated fluid?Toroidal vortex generated below the water droplet?While, glycerin droplet has toroidal vortex below and above the droplet. In the cases where the inner to outer fluid viscosity ratio is low, one would also expected the appearance of internal circulation. This was experimentally confirmed Yutaka Abe et al.(2007). These experimental result regarding effect of the viscosity suggest that there is the relation between internal circulation and external toroidal vortex in the vicinity of a droplet? In this study, length of toroidal vortex is used as index for estimating external flow around a levitated droplet. It is also confirmed that length of toroidal vortex is affected by input voltage. In terms of water droplet, as input voltage is decreased?location of toroidal vortex are moved from bottom to top of levitated droplets. In conclusion?characteristic of external flow around an acoustically levitated droplet is experimentally examined?It is confirmed the flow structure of external flow is affected by the viscosity of levitated droplet?Location of toroidal vortex of water droplet is influenced by input voltage generated from a function generator? Experimentation of micro gravity to be conducted in the near future would allow the elimination of the natural convection contribution, and would therefore be expected drastically to simplify the analysis. We are expecting to obtain further information about origin of external toroidal vortex in the vicinity of droplet and the relationship between internal circulation and external toroidal vortex that is a fundamental scientific issue in fluid dynamics.
- Abstract document
- Manuscript document
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