Detection of Transition by Infrared Image Techniques

Abstract

Transition lines where laminar flow changes to turbulent flow are usually detected by the well-known oil-flow method or sublimation techniques (acenaphthene). Both these methods have the disadvantage that the flow may be affected by the visualization material. Besides this, these methods are rather time consuming and, therefore, depending on tunnel size, also costly. Other methods like microphone, hot wire and hot film can give only a point by point information whether the boundary layer is laminar or turbulent. Therefore, the use of these methods is also time consuming or very large and costly equipment is required. The infrared technique is a powerful tool for transition line detection. The whole model area can be under surveillance with respect to turbulent wedges, or even smaller sub-critical disturbances in the laminar boundary layer. This technique has been successfully applied in different wind tunnels and in free flight experiments. In most cases "natural" temperature differences between the model and the fluid gave reasonable or good infrared images. In one case, switching on and off the wind tunnel cooling systems gave good results. Nevertheless, the most comfortable means is the internally heated model. Infrared technique is also suitable for free flight experiments.