Surface Flow Visualization Using Thermal Tufts Produced by Evaporatively Cooled Spots

Abstract

Surface flow visualization is useful for understanding flow in many applications and can be used to complement and validate studies of complex flow using computational fluid dynamics (CFD). A recent development in non-intrusive surface flow visualization has been the development of “laser thermal tufts”. Baughn et al. 1 first described this technique in a study to determine where flow separation had occurred on the suction side of a turbine blade. This technique was later patented by the USAF (Rivir et al. 2). The principle behind this method was to use a laser to heat a spot on a surface coated with thermochromic liquid crystals (TLCs), showing up on an image of the surface as a round dot. This circular laser spot (∼3 mm) heated the surface above the color play temperature, creating a distinct thermal tuft downstream. The thermal tuft was created by advection in the direction of surface airflow. The result was a teardrop shaped color change in the TLCs pointing in the downstream direction. Using this method, they were able to determine the location of boundary layer separation on a turbine blade model in a cascade wind tunnel. They pointed out that multiple spots could be obtained by using a laser tuft matrix (LTM). In the location of boundary layer separation or reattachment, the thermal tuft is circular and centered around the laser spot with no hint of a teardrop shape. Further demonstrations of the laser thermal tuft were done by Townsend 3. Baughn et al. 4 presented the results for a laser thermal tuft created by an infrared (IR) laser on the flow separation and reattachment on a turbine blade at low Reynolds numbers. The IR laser produced a clear red spot at the center of the thermal tuft. Their results are shown in Fig. 1.