Liquid-Crystal-Particle Thermometry and Velocimetry System

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

A three-dimensional (3D) defocusing liquid-crystal-particle thermometry and velocimetry (3DDLCPTV) system has been designed and constructed (but not yet operated) for use in measuring the 3D velocity and temperature fields, respectively, in a volume of interest in a possibly turbulent flow. As its name suggests, the system utilizes defocusing digital particle-image velocimetry (DDPIV) for obtaining velocity data and liquid-crystal thermometry for obtaining temperature data.

The system includes a total of three monochrome charge-coupled- device (CCD) electronic cameras and one color CCD camera, all aimed at the flow volume of interest (see figure). During operation of the system, the flow would be seeded with thermotropic liquid crystal (TLC) particles, which reflect light predominantly at a wavelength that varies with temperature. The flow would be illuminated by use of a high-intensity xenon flashlamp, and the cameras would acquire sequences of images of the illuminated TLC particles.

The 3DDLCPTV System Includes Four Cameras — one at the center and three at the vertices of an equilateral triangle. In operation, the system would acquire images of flash-illuminated TLC particles.

The three monochrome cameras are located at the vertices of an equilateral triangle that lies in a plane perpendicular to a common optical axis, which passes through the centers of both the triangle and the volume of interest. The digitized outputs of these three cameras would be processed to extract data on the positions and velocities of particles according to the established principles of DDPIV. The color camera lies at the center of the triangle; the digitized output of this camera would be processed to extract data on the temperatures of the illuminated particles.

The geometric arrangement of the cameras is such that in superposing the four camera images as part of the processing of image data, one would obtain, for each illuminated particle, a composite image that could be readily and uniquely identified as a combination of four blurred spots located at the vertices and center of an equilateral triangle. The 3D velocity of each particle could then be calculated by correlation analysis of the blurred spots at the vertices of the triangle in two consecutive images, while the temperature of the particle would be calculated through analysis of the hue of the central spot. Thus, the system should be capable of resolving the temperatures and velocities of all particles within the volume of interest.

This work was done by David Robert Schmitt of the University of Washington for the Air Force Research Laboratory.