Visualization of the quiet test region in a supersonic wind tunnel using luminescent paint

A temperature-sensitive luminescent paint technique has been used to visualize boundary-layer transition on a 10-degree cone model in the 0.2-m Supersonic Wind Tunnel at National Aerospace Laboratory. A luminescent paint based on EuTTA was applied on the model surface using airbrush. The paint coating was excited by a Xenon light and the luminescence image was acquired using a high resolution cooled-CCD camera. To enhance thermal signature across transition, tunnel flow was heated by a sudden shutdown of cooling water supply to heat exchanger. Two images were taken for each condition, one is transient image taken in heating process and the other is reference image taken in steady condition. By ratioing these two images, global transition patterns on the cone have been visualized as the edges of brightness. From transition images at various model locations in the test section, it was found that transition remains at a fixed spatial location with respect to the tunnel. This indicates that the cone transition is induced by radiated noise propagating from turbulent boundary layers on the tunnel walls. Effects of the stagnation pressure were also studied over the range from 55 to 150 kPa. The extent of quiet test region was found to be strongly sensitive to the pressure or unit Reynolds number. At the NAL 0.2-m SWT, natural transition did not occur on the cone at lower values of unit Reynolds number