Abstract:

Second sound entropy waves in helium II are considered as a scalar temperature field imposed on an underlying counterflow velocity vector field. This unusual viewpoint offers a conceptual explanation of the form of nonplanar pulses of second sound, as well as a tool for understanding, and perhaps solving, the evolution of nonlinear pulses of second sound. A simple model of two point sources will be used to show how the large and negative finite amplitude nonlinearity near the phase transition of helium II is responsible for some of the observed features of the recently observed self-focusing of second sound.