Temperature elevation, streaming, and self-action in sound beams

当超声波在热粘性流体中向下传播时，声能被耗散，并且波束中的流体温度升高。除了这样的超声波加热之外，流体的整体运动被诱导为声流。该流动带走下游的流体的局部加热的质量，并且作为冷却效果降低了梁中的温度。本课题研究了粘性流体中声束的流动冷却效应和热自作用两种物理现象。由于流体中的粘性和热传导引起的声能损失在梁中引起局部温度升高和全局流动。流动强制带走介质的加热质量，并且温度升高降低。然而，在高粘度流体中，由于粘度，流动不会大大增加。由于在大多数流体中，声速等物理参数依赖于温度，声束本身的振幅随照射时间而变化。本文用有限差分法建立了一种复杂但有效的数值计算格式，可以同时求解基本的流体力学方程、热输运方程和波动方程，这些方程都具有独立的温度依赖性参数。

When ultrasound is traveling down in a thermoviscous fluid, the sound energy is dissipated, and the fluid temperature in the beam is elevated. Inaddition of thus ultrasonic heating, bulk movement of the fluid is induced as acoustic streaming. The streaming carries away locally heated mass of the fluid downstream and reduces the temperature in the beam as cooling effect. Two physical phenomena of cooling effect by streaming and thermal self-action of sound beam in a viscous fluid have been investigated for the term of the present project. Sound energy losses due to viscosity and heat conduction in the fluid induce temperture elevation locally and streaming globally in the beams. The streaming carries forcibly away the heated mass of the medium and the temperature elevation is reduced. In a high viscous fluid, however, the streaming does not greatly build up because of the viscosity. As physical parameters such as sound speed depend on temperature in most fluids, sound beam changes its amplitude itself with irradiation time. A complicated but profitable numerical calculation scheme to solve simultaneously the basic hydrodyamic equations, heat transport equation, and wave equation, all the equations have individually temperature-dependent parameters, has been established using a finite difference method.