山谷 千秋, 井上 浩

Chiaki YAMAYA, Hiroshi INOUE

秋田大学工学資源学部電気電子工学科

Department of Electrical and Electronic Engineering, Faculty of Engineering and Resource Science,Akita University

キーワード : absorption of ultrasound, estimation of medium constant, heat analysis, finite-difference time-domain method, 3D simulation

目的：超音波照射による媒質の温度上昇を利用した媒質定数の推定のため，推定に応用出来る方法の開発を目的として検討を行った．対象と方法：水中の伝搬路中に設置した試料媒質に超音波を照射し，細い熱電対を用いて媒質内の温度を測定し，さらに超音波伝搬と熱伝導を連結したシミュレーション法により超音波照射された媒質内の温度を計算する方法を用いた．結果と考察：測定では，（1）媒質の温度上昇とともに（2）温度センサ自身の温度上昇と（3）温度センサが超音波を受信する瞬間に発生するアーティファクトが含まれるので，媒質の温度上昇のみを計算するシミュレーションとの差を正規化表現で検討した．（1）‐（3）はいずれも超音波強度の距離特性に依存し，吸収係数を定数とした指数関数曲線に比例する．超音波照射の一定時間経過後の温度空間分布を正規化することにより実験とシミュレーションが一致した．結論：正規化温度分布を媒質定数の推定に用いるとき，熱の拡散が無い数秒間の加熱であれば熱伝導率や比熱などの熱物性値の影響は現れず，吸収係数の特徴が温度分布の傾きに顕著に現れることを明らかにした．本方法は吸収係数の推定にとって実用上有用な方法と考えられる．

Purpose: To develop a method for estimating the medium constant from the temperature rise produced by ultrasound irradiation. Subjects and Methods: Ultrasound was irradiated to a sample medium placed in the path of propagation through water. We used a thin thermocouple to measure the temperature in the medium. The temperature in the medium was calculated using a simulation method that combined ultrasonic propagation and heat conduction. Results and Discussion: (1)Temperature rise of the medium, (2) temperature rise of the thermocouple, and (3) artifacts generated when ultrasound reaches the temperature sensor are included in the temperature measurement. The measured temperature differs from the simulation in which only the temperature rise of the medium is calculated. These three factors of the measured temperature depend on the distance characteristic of ultrasound intensity and the characteristic is proportional to the exponential curve on which the constant is the absorption coefficient. The normalized temperature distribution of measurement agrees with that derived from the simulation. Conclusion: Since the influence of the thermal conductivity and specific heat are not reflected in the normalized temperature distribution, this study shows that the absorption coefficient is significantly affected by the inclination of the temperature distribution. This practical estimation method is thus expected to prove useful in estimating the absorption coefficient.