NOVAlasers - Exp Flow Visualization with Laser Induced Fluorescence



		Experiment #5 Flow Visualization with Laser Induced Fluorescence Introduction It doesn’t take much to be a fluid mechanics engineer. Just grab an ordinary household fluorescent highlighter, a glass of water, and your X-Series laser with line generating lens attached and you’re set to go! Well, ok, being a fluid mechanics engineer is slightly more complicated than that . . . but you don’t have to be an engineer to replicate this simple experiment. In the fluid mechanics field the most advanced fluid visualization is done in sophisticated engineering laboratories using lasers and fluorescent dyes. This experiment will show you a simple flow visualization procedure you can do anywhere! The behavior of solids is easily visualized. If you roll a marble down an incline, you can take a video of its motion. The motion of a marble dropped into a liquid is also easily seen. But what about droplets of one liquid introduced into another? It is not as easy to observe the motion of liquids as it is of solids. To aid flow visualization a common technique employed by fluid mechanics engineers is that of planar laser-induced fluorescence (or PLIF for short). This method allows for the visualization of phenomenon such as flow turbulence, waves, wakes and the dispersion of liquids or particulates. A fluorescent compound is introduced into the liquid being visualized. A planar laser light source then illuminates the fluorescent compound. Because of the density of the fluorescent compound and its interaction with laser light to produce highly visible illumination, the flow of the fluid can be easily visualized. In this experiment an easily accessible fluorescent dye will be used to visualize the dispersion of one liquid within another using a planar laser light source. Background • To aid flow visualization a common technique used is that of planar laser-induced fluorescence (PLIF) in which a fluorescent compound is excited by a laser light source in order to visualize such phenomenon as waves, wakes or flow turbulence. • In this experiment an easily accessible fluorescent dye will be used to visualize the dispersion of one liquid within another using a planar laser light source. Purpose: to visualize fluid flow using fluorescence and laser light. Materials Laser light source: any output power and any wavelength of laser light can be used for this experiment. The wavelength chosen in combination with the color of fluorescent dye used will however determine the results obtained. Line generating optic with a mounting apparatus ex. NOVAlasers line generating optic with X-Series lens accessory. Fluorescent dye highlighters of various colors, including fluorescent yellow and fluorescent green. NOTE: ensure that the highlighters used have fluorescent dye as not all highlighters are made with dye that is fluorescent. A glass container (ex. glass cup) filled with water. Safety: if using a ClassIIIb (>5mW) laser the use of proper laser safety eyewear is recommended. Procedure Attach the line generating optic with the mounting apparatus to the laser you will be using. Ensure that you are getting a clear projection of the laser line. Fill the glass container with water. Dip the tip of the fluorescent highlighter into the water just so that some of the fluorescent dye leaks into the water. Aim the plane of laser light produced by the line generating optic connected to the laser at the glass of water. The plane of light should be orientated horizontally. Start by aiming the plane of light at the top of the glass where the fluorescent dye was introduced. Then proceed by slowly moving the plane of light down as the fluorescent dye travels down through the water. Observations • Observe how the fluorescent dye flows down through the water. Using a laser light source and a fluorescent compound allows for the clear visualization of fluid flow. You can clearly see the trajectory of the fluorescent dye. • Experiment by introducing currents and other flow characteristics into the cup of water prior to dropping the fluorescent dye into it. How does the trajectory of the fluorescent dye change when you stir the cup of water with a spoon prior to introducing the dye? • Observe the flow characteristics of layering the dyes introduced into the water. First dip one color of highlighter (ex. yellow) into the water and observe how it starts to flow down. After a few seconds dip another color of highlighter (ex. pink) into the water and observe its flow. The following are three photographs taken of this experiment. Due to the subtle color differences between the laser light and the fluorescent dyes as well as the sensitivity of the camera to the much brighter laser light, these photographs are not as clear as what you will observe when replicating this experiment. In the above image the laser light source used was an X-Series laser with lens accessory attached. The color of the highlighter was yellow. The water in the glass container was stirred with a spoon prior to the introduction of the fluorescent dye. In the above image the laser light source used was a blue laser pointer with a line generating lens attached. The color of the highlighter was green. The water in the glass container was not stirred prior to the introduction of the fluorescent dye. The above image is a full view of the glass container. The fluorescent dye has now travelled down to the base of the container and its trajectory can easily be visualized. Explanation A laser is a concentrated and coherent light source. These properties allow laser light to be projected in a plane and directed at a specific point. As laser light is concentrated, its interaction with a fluorescent dye causes the dye to fluoresce strongly. This allows for the clear visualization of the flow of the fluorescent dye. Conclusion Fluid flow can be visualized with the aid of a fluorescent compound and a planar laser light source. Copyright 2007 NOVAlasers INC