First images of controlled convection in liquid helium

The state of the art in hydrodynamic turbulence: Past successes and future challenges

“…Where density gradients exist in the flow, visualiza-tion can occur in the absence of tracer particles, using shadowgraphs (which depends on the density gradient) or schlieren technique (which depends on the second derivative of the density). It has been demonstrated [96] that shadowgraphy can be used in helium I to visualize even weak flows near the convective onset. A light beam reflected from the cell displays intensity variations resulting from convergence or divergence as a result of gradients in the refractive index.…”

Turbulent flows at cryogenic temperatures: a new frontier

“…It has been known for many years that there are some advantages in conducting thermal convection experiments at cryogenic temperatures. There are also disadvantages, one of which is that visualizing the helium flow under cryogenic conditions is both conceptually and technically more difficult, although some progress has been reported by Woodcraft et al (1998).…”