GT AE 3051 - AE3051 Experimental Fluid Dynamics

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Copyright  1999, 2000, 2005, 2007-2009 by H. McMahon, J. Jagoda, 1 N. Komerath, and J. Seitzman. All rights reserved. AE3051 Experimental Fluid Dynamics UNSTEADY VELOCITY MEASUREMENTS IN A JET USING A LASER DOPPLER VELOCIMETER Objective In this lab, you will learn the basic principles of laser Doppler velocimetry. You will use a laser Doppler velocimeter to measure velocity across a jet in a coflow; this is the same flow used in the hot wire anemometer lab. You will obtain similar data to that measured with the hot wire and compare the results for the two systems. Please note the following safety precautions very carefully: 1. Your eyes, and your friends' and instructors' eyes, can get hurt by lasers. Take no chances or shortcuts: laser beams travel a lot faster than you can react! To ensure safety, no one who has not read the instructions thoroughly can be allowed near the experiment. Anyone disobeying safety rules will be ordered out of the lab immediately. 2. Remove all rings and watches when working in the vicinity of lasers. 3. Do not go to the back side of the test section unless directly supervised by a TA. 4. Do not open the curtains if they are closed. Only the person in charge may do this. 5. Do not lean over the laser beam, or stick your head between the laser transceiver and the test section when the laser is on. 6. Do not touch the laser or fiber optic coupler, whether the laser is on or off. If they go out of alignment, the TA will realign the system. Also, do not touch the transceiver unless instructed to. 7. Be very careful where you move the traverse: you must know clearly where all the reflections are (if any)! Background In many flows of interest, it is not desirable to use intrusive probes to make measurements. There may be several reasons for this. First, it may be physically difficult to design a probe that can survive in the flow to be measured. For example, consider the problem of measuring velocity inside the passages between the blades of a jet engine compressor while the engine is operating. In other situations, the presence of the probe itself can greatly change the flow fieldAE 3051 Unsteady Velocity Measurements in a Jet Using a Laser Doppler Velocimeter 2 that is being measured. As an example, consider a Pitot tube inside the test section of a small supersonic wind tunnel, or a probe inside a vortex that is shed from a wing. A third problem occurs when several properties are varying simultaneously, as in a turbulent flame (e.g., inside an engine combustion chamber). For example, the velocity, pressure, and temperature may all be changing from one instant to the next. Most probes, such as Pitot tubes and hot-wires, respond to changes in several of these properties, and so it may be quite difficult to separate out the part due to velocity alone. Still another problem arises in two-phase flows (not all gas): there may be liquid droplets, e.g., liquid oxygen in the Space Shuttle main engine turbopumps and jet fuel sprays in engine combustion chambers, or solid particles, as in the exhaust from a solid rocket motor. Droplets and particles can clog or even instantly destroy sensitive probes. The laser Doppler velocimeter (LDV, also known as a laser Doppler anemometer - LDA) is a non-intrusive measurement device that is sensitive only to velocity. Thus, it is one of just a few techniques that can be used in complex flowfields to measure velocity. This is why, despite the greater cost and difficulty usually involved in making measurements with this technique, laser velocimeters are highly regarded instruments. Principles of Operation of LDV When electromagnetic waves are scattered from moving objects, the scattered light has a frequency which is different from that of the incident waves. This is similar to the phenomenon that we have all observed: The horn of an oncoming car changes pitch suddenly as it passes us. The difference in frequency is known as the Doppler shift, and is proportional to the relative velocity between the moving object and the observer (or receiver). This principle is also used in radar and laser speed detection. A laser is a source of radiation that generally has most of the following properties; it is: 1) well-collimated, which means the beam will not diverge or change in size much; 2) highly coherent, meaning its electromagnetic wavefronts are all in phase; and 3) nearly monochromatic, which means that the radiation is composed of a very narrow range of wavelengths. Thus, it is ideal for making measurements of Doppler shift; you can precisely and easily calculate what the shift should be for a given velocity. Unfortunately, the frequency of light is very high, and the Doppler shift caused by the kinds of velocity that we encounter isAE 3051 Unsteady Velocity Measurements in a Jet Using a Laser Doppler Velocimeter 3 extremely small by comparison. So, if one were to shine a laser beam at a moving object, measure the incident light frequency and the scattered frequency, and try to find the difference, one would probably find that one's measuring instrument cannot make out the slight difference between these frequencies. The solution to this problem is to find the difference in Doppler shift caused by two beams, hitting the same object at slightly different angles. If one tries to measure only this difference, one can get good accuracy. This is the principle of the dual-beam interferometer-type LDV that is widely used today (see Figure 1). Dual-Beam Laser Velocimeter When two laser beams of the same frequency cross in a small region, stationary interference fringes are formed, as shown in Figure 2. This is because light behaves as a kind of wave, with an amplitude and a phase. At some points, the two beams are in phase and the amplitudes add together; there the light’s amplitude is large. At other points, the amplitudes cancel each other out (out-of-phase), and you get nothing at all. Thus, fringes are formed, with each bright fringe being a thin disc inside which the light intensity is large. The bright fringes are separated by dark fringes, where there is hardly any light intensity at all. As seen from Figures 2 and 3, the region where the fringes exist is an ellipsoid of revolution. Consider a small solid or liquid


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