A laser-induced surface flow visualization technique using liquid crystal thermography

Observation of flow field characteristics such as flow separation and reattachment are important in many industries. Current methods for flow visualization can be difficult to implement, expensive, and highly intrusive. The objective of this project is to develop an inexpensive, user-friendly, non-intrusive measurement technique useful to engineers interested in surface flow visualization. This is accomplished using liquid crystals in conjunction with a laser heat source to generate a thermal tuft. The shape and size of the thermal tuft is used to characterize the flow field. Wind tunnel experiments are conducted to validate this concept and examine flow behavior over a flat plate in a low Reynolds number environment. The plate is coated with liquid crystals of one-degree and fivedegree bandwidths. A 150-mW infrared, diode laser provides a constant heat source and generates a high temperature thermal spot on the model. The results obtained during the wind tunnel experimentation show that an irradiated spot on a liquid crystal coated surface will produce a tuft. The shape and size of the thermal tuft is indicative of the direction and magnitude of the flow conditions. As the wind speed increases from 2 to 10 m/s, it was shown that the length of the thermal tuft increases linearly. The tail of the tuft was also found to follow the direction of flow. Turbulent and laminar flow conditions can be distinguished; however, the angle of attack could not be realized with this technique. Developing a technique for generating a matrix of heated spots on the model indicates that the results of using this method can be viewed over a large area. Overall, it was shown that this is an easy, inexpensive, and non-intrusive technique for visualizing flow on the surface of an object.