The Journal of the Acoustical Society of Korea (한국음향학회지)

The Acoustical Society of Korea (한국음향학회)
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Investigation of standing wave acoustic levitation with Bernoulli principle and bolt-clamped Langevin type ultrasonic transducer

베르누이 원리와 bolt-clamped Langevin type 초음파 진동자를 이용한 정상파 음파 공중부양의 탐구

  • 박민철 (KAIST 부설 한국과학영재학교 응용음향학연구실) ;
  • 박두재 (한림대학교 나노융합스쿨) ;
  • 김영환 (KAIST 부설 한국과학영재학교 응용음향학연구실)
  • Received : 2018.09.17
  • Accepted : 2018.11.22
  • Published : 2018.11.30
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Abstract

The purpose of this study is to investigate the effect of acoustic radiation force on the standing wave acoustic levitation phenomenon, which is the levitation of small objects near the pressure node of the standing wave, using the Bernoulli principle. The source and scheme of the acoustic radiation force, which is the cause of the levitation, are conceptually explained through comparison with the graph of the acoustic radiation force versus the distance from the transducer. A series of experiments supporting this explanation was performed with a BLT(Bolt-clamped Langevin Type) ultrasonic transducer to confirm that the objects are floating near the pressure nodes and that it satisfies the condition for the standing wave formation when the object is levitating. Furthermore, the vertical alignment of floating objects, which is a characteristic of standing wave acoustic levitation phenomenon, could be explained.

본 연구에서는 소형 물체가 음파 정상파의 압력 마디 부근에 소형 물체가 부양하는 정상파 음파 공중부양(standing wave acoustic levitation) 현상에 대해 베르누이 원리를 이용하여 부양의 원인이 되는 음향 방사힘(acoustic radiation force)의 근원과 개형을 기존에 알려진 진동자에서 떨어진 거리에 따른 음향 방사힘의 그래프와 비교함을 통해 개념적으로 설명했다. 이러한 설명을 뒷받침하는 일련의 실험들을 BLT(Bolt-clamped Langevin Type) 초음파 진동자를 이용해서 수행하여, 물체들이 공기의 압력 마디 부근에 부양하고 있음을 확인했고, 물체가 부양하고 있는 상태에서 정상파가 형성되는 조건임을 확인했다. 더불어, 정상파 음파 공중부양 현상에서 부양하는 물체들이 수직하게 일렬로 정렬하는 현상 역시 설명할 수 있었다.

Keywords

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Fig. 1. Pressure, velocity graph of air element as height varies, and resulting acoustic radiation force[1,2,10]. Objects are located slightly below the pressure node.

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Fig. 2. Conceptual Model which relates the speed of air and the force exerting of the particle. Pbelow and Ptop illustrate the pressure of air exerting at the upper side and the lower side of the object respectively. Vtop and Vbelow illustrate the velocity of air at the top of the object and at the below of the object respectively.

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Fig. 3. Distribution of squared speed of air between the reflector and transducer as height. A ~ D illustrate each case when the object is located at each position.

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Fig. 4. Phenomenon observed which shows the vertical line-up of the levitating objects even though the planar wave was generated by planar transducer.

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Fig. 5. Height of the sphere from the transducer surface vs sphere’s order (0th sphere means the lowest position).

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Fig. 6. Result of experiment in changing the height of reflector and check the height where the object gets levitated. The zero height point is not on the surface of the transducer in this graph.

Table 1. Force acting on the levitating object at each case in A~D.

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