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Laser Velocimetry
While there are many methods of determining flow rates and fluid velocity, this lab explores a very accurate and noninvasive method called laser velocimetry and aims to give the students experience in measurements and calculations involved with the process.
The laser velocimetry experiment makes use of a setup that includes a laser source, mirrors, beam splitter, convex lens, glass pipe with water flowing through it, and a photon detector. The photon detector gets hooked up to an amplifier and oscilloscope. Differing flow velocities are used for the water for different trials. The laser gets reflected into the beam splitter, and then has two separate beams travel through the convex lens. The beams go through the water in the glass pipe, and then strike the photon detector and are measured are on the oscilloscope for variations.
The experiment works by utilizing the concept that as a particle travels through a beam of light it creates diffraction in much the same way that a small opening does, as pictured above. The variations in photon detection caused by the fluid flow in the glass pipe aremeasured by the photon detector and plotted on the oscilloscope versus time. The frequency can be derived from this and, when combined with the known wavelength of light of the laser, is used to detemine the velocity of the fluid flow. A sample scattering diagram and oscillation plot are pictured below. The two separate laser beams originate from the same source, so they are known to have equal wavelengths.
Laser velocimetry is a method of measuring particle velocities and flow rates that has become standard in fluid research labs across the world, thanks to its accuracy and noninvasive properties. The Laser Doppler Velocimetry used in this lab is especially useful, working in air or water, and has been used not only for fluid flows but even measuring the wake left by rotor blades on a helicopter or the concussion waves of a shock tube. |
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NEEP Lab Course Instructor:
Yaron Danon, vist Dr.
Danon's web site |
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