An examination of turbulent flow with an ultramicroscope

Abstract

1. Hitherto, the majority of researches into the character of turbulent fluid flow have been concerned with the motions of relatively large molar masses of fluid, and the methods used to obtain visual impressions of the flow pattern have usually involved the introduction into the fluid of particles of extraneous matter, such as aluminium particles, oil drops, etc. It is questionable whether such methods are permissible for the examination of microturbulence, especially near the boundary of the fluid where the scale of the turbulence is small, since if the particles introduced are comparable in size with the molar masses, their internal motions may not be faithfully represented. In a study of this kind of motion it is very desirable therefore to avoid any such interference with the flow, and the ultramicroscope offered a possible means of doing this provided the difficulties in applying the instrument could be surmounted. 2. The principle of the ultramicroscope depends on the fact that minute particles usually present in most fluids, but invisible in ordinary light even under the most powerful microscope, become visible when intensely illuminated provided they are seen against a dark background. Particles whose shapes are not discernible, because they are smaller than the wave-length of light, then become visible as bright points of light.