Flow Visualization with Laser-Induced Thermal Tufts

Abstract

The laser-induced thermal tuft is a new flow visualization technique for simulating traditional tufts with a thermal plume. A laser is used to heat a point of interest on a windtunnel model, causing downstream convection of thermal energy from the heated spot. A temperature-sensitive coating is used to visualize the thermal plume. This technique can be used to indicate flow direction, locate regions of separated flow, and detect laminar/turbulent transition. One primary advantage of thermal tufts is that the measurement technique is less intrusive than traditional tufts. In addition, thermal tufts may be generated at any optically-accessible point during a test, whereas string tufts must be applied to specified locations before a test. This enables greater experimental efficiency, which is particularly important in large-scale ground-testing facilities. This work extends and develops the thermal tuft concept by employing temperature-sensitive paint, as well as the previously used thermochromic liquid crystals. The effect of various substrate materials on tuft quality is evaluated. Calibrations of tuft length dependency on Reynolds number and laser power are made. Furthermore, a computational model is developed to simulate the tuft shape and structure. Finally, a new variation of the technique is presented, based on thermal ablation of the substrate material.