Zhang Likun, Marston Philip L
Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164-2814, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2011 Dec;84(6 Pt 2):065601. doi: 10.1103/PhysRevE.84.065601. Epub 2011 Dec 22.
An acoustic vortex in an inviscid fluid and its radiation torque on an axisymmetric absorbing object are analyzed beyond the paraxial approximation to clarify an analogy with an optical vortex. The angular momentum flux density tensor from the conservation of angular momentum is used as an efficient description of the transport of angular momentum. Analysis of a monochromatic nonparaxial acoustic vortex beam indicates that the local ratio of the axial (or radial) flux density of axial angular momentum to the axial (or radial) flux density of energy is exactly equal to the ratio of the beam's topological charge l to the acoustic frequency ω. The axial radiation torque exerted by the beam on an axisymmetric object centered on the beam's axis due to the transfer of angular momentum is proportional to the power absorbed by the object with a factor l/ω, which can be understood as a result of phonon absorption from the beam. Depending on the vortex's helicity, the torque is parallel or antiparallel to the beam's axis.
对无粘流体中的声涡旋及其对轴对称吸收物体的辐射扭矩进行了傍轴近似之外的分析，以阐明与光学涡旋的类比。利用由角动量守恒得到的角动量通量密度张量来有效描述角动量的传输。对单色非傍轴声涡旋光束的分析表明，轴向角动量的轴向（或径向）通量密度与能量的轴向（或径向）通量密度的局部比值恰好等于光束的拓扑电荷l与声频ω的比值。由于角动量的传递，光束对以光束轴为中心的轴对称物体施加的轴向辐射扭矩与物体吸收的功率成正比，比例因子为l/ω，这可以理解为物体从光束中吸收声子的结果。根据涡旋的螺旋度，扭矩与光束轴平行或反平行。
