New Method of Flow Visualization for Low‐Density Wind Tunnels

A new method of visualizing low‐density flows is described which employs the absorption by oxygen of radiation in the wavelength region 1400 A to 1500 A. It was developed for use in the low‐density supersonic continuous flow wind tunnels at Berkeley because the conventional methods (shadowgraph, schlieren, and interferometer) were predicted to have inadequate sensitivity at the unusually low densities in the test section. A xenon discharge is used for the source of radiation, its 1470 A resonance line being isolated by a calcium fluoride vacuum monochromator. A 26 mm diameter beam of parallel radiation is passed through the test section where it is absorbed more or less depending on the oxygen density along the radiation path. The center of the test section is then focused by a calcium fluoride vacuum camera on an ultraviolet‐sensitive photographic plate. Pictures of shock waves from cylinders ⅛ and ¼ inch in diameter and from a 60° ⅜ inch wedge were obtained with exposure times of about 4 minutes. Development of a stronger source would reduce the exposure time appreciably. This oxygen absorption method is shown to be suitable, in principal, for general wind tunnel work where the absolute pressures are below about 1 mm Hg.