超声波作用下气泡的非线性振动

Flynn方程在引入泡内气体的压缩性修正后能够更好地描述超声波作用下液体内气泡的非线性振动.以此为基础通过数值分析探讨了声场内气泡的非线性振动特征以及气泡初始半径、驱动声波频率和声压幅值对气泡非线性振动状态的影响.结果表明,声场内的气泡可分为3种类型:吸收声能量作准本征振动的大气泡、空化气泡和以驱动声波频率受迫振动的微气泡.空化气泡的尺度范围受到驱动声波频率和声压幅值的共同影响.利用高速摄影系统对换能器作用于未除气的自来水所引起的变化进行了实验研究,结果表明:超声波作用下液体内的气泡场是一个混合场,不仅有空化气泡,还有毫米级的大气泡;场内除了有气泡的非线性振动外,还有气泡间的相互吸引、碰撞和结合.

THE NONLINEAR OSCILLATION OF BUBBLES IN THE ULTRASONIC FIELD

The nonlinear oscillation can be described by Flynn Equation when considering the compressibility of inner gas of bubbles. Based on the model the displacement/the time relation and phase-trajectory of vibrating bubbles of different initial radii are simulated numerically, which shows that the movements of bubbles depend strongly on the initial conditions in the same sound field. When the driving frequency of 26.5kHz and intensity of 1.35atm is adopted, the bubbles oscillate forcedly with initial radii are smaller than 1\mu m, while oscillate quasi-intrinsically with initial radii larger than 200\mu m. The two kinds of bubbles are not cavitation ones. The changes of smaller bubbles are more intense to the sound intensity. If the sound intensity increases, a restricted range of bubbles smaller than 1\mu m can become cavitation ones. Furthermore, the smaller the initial radius is, the larger the increase of maximum displacement is. In addition, the driving frequency also affects the movement of bubbles. With the increase of driving frequency, the initial radii range of cavitation bubbles decrease, and the oscillation intensity reduces. The results present that there are three types of bubbles in the ultrasonic field: big bubbles oscillated quasi-intrinsically, cavitation bubbles and micro-bubbles oscillated forcedly at the driving frequency. The size range of cavitation bubbles is influenced jointly by driving frequency and pressure amplitude. At the same time, we researched the bubbles field in tap water by high speed photography when the ultrasonic transducer is working. The results show that there is a mixed field, where many bubbles with different radius are vibrating and moving. Not only cavitation bubbles but also bubbles with the millimeter scales coexist in the radiation field of ultrasonic transducer. The movements of bubbles can affect the intensity of cavitation directly. There is a complex physical field when cavitation comes into being in the liquid, where bubbles are vibrating, colliding and combining.