Use of an Infrared Laser to Determine Separated Flow

Abstract

This experiment used an infrared laser technique to determine where flow separation and reattachment occurred on a backward facing step. The step size was 6.35 cm, and the overall length of the model was 60.96 cm, with 30.48 cm for the bottom step. Two types of tufts were used in this experiment to define the flow field: string tufts and thermochromic liquid crystal (TLC) tufts. In this experiment, the string tuft was attached to the end of a wand that was placed in the flow path. String tufts are intrusive to the flow and give a less accurate surface visualization because the weight of the string affects the results. Consequently, string tufts do not respond well to low flow velocities. In contrast, TLC tufts do not have these limitations. Thermochromic liquid crystals do not impede the flow and they give accurate surface visualizations. They respond to changing temperature by reflecting light at different wavelengths, resulting in a changing observable color as the temperature changes. An infrared laser heated the surface of the model in a circular spot resulting in crystal color change. Airflow over the heated surface convected energy in the direction of the flow. This resulted in a visible TLC "tail" pointing in the direction of the flow. With this technique, both forward and reverse flow can be detected. Images of these tufts were recorded with a digital video camera. Eccentricity of the tufts indicated the direction of flow. Two velocities of upstream flow were tested: 100% of the blower's voltage, a velocity of 1.13 m/s, and 80% of the blower's voltage, a velocity of 1.07 m/s. For both velocities, reattachment occurred using the TLC tuft method at approximately 17.145 cm from the step. Using the string tuft attached to a wand, these results were verified as the reattachment region was between 16.51 cm and 17.78 cm.