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Laser Dopper Velocimetry
Laser Doppler Velocimetry (LDV) provides a single point measurement of one component of the gas velocity. A laser is separated into two beams that cross at point that is typically about 0.5 mm in size. Light that is scattered from this point is interpreted to determine velocity. Multiple lasers at different wavelengths can be used to measure more than one component of velocity at the same time.

Particle Image Velocimetry
Particle Image Velocimetry (PIV) provides a spatially resolved measurement of the velocity field at an instant in time. The technique is deceiving in its simplicity. Small particles are added to the flow at a sufficient density to fill the field, and two images, separated by a known time, are acquired. The local velocity is determined by interrogating a small portion of the image to determine the particle displacement between exposures. Using the known time separation and calculated particle displacements, the local velocity can be determined. The process is repeated throughout the image in order to provide the full flow field.

Basic Quantum Mechanics
Optical processes such as absorption, emission, and their combination in a process called laser-induced fluorescence have been proven useful for studying engines. Several examples of their utility are provided elsewhere on this webpage. In many cases, proper application of optical techniques requires some understanding of quantum mechanics.

Absorption Spectroscopy
As you learned in the quantum mechanics tutorial, light can interact with molecules. Here we will briefly examine this effect for the case of directing light (such as from a lamp) through a medium that can absorb the light (such as the gas in an engine).

Emission Spectroscopy
As you learned in the quantum mechanics tutorial, molecules can store energy, for example by rotating rapidly. Furthermore, they can release this energy in the form of photons (light). We are familiar with hot objects emitting light. Here we will see that, as in the case of absorption spectroscopy, hot gases will have information-rich spectra relative to dense matter such as molten metal. We will also see that, in addition to heating a gas, a gas can be made to give off light by exciting it in other ways (chemically or optically). The optical excitation case is treated in the laser-induced fluorescence tutorial.

Laser-induced Fluorescence
As discussed in the quantum mechanics tutorial, there are discrete energy levels available to molecules (atoms that are bonded to each other). This is also true for the electrons that are bound to the molecule as well. In the left figure two energy levels available to the electron are shown as two separate orbital paths X and A. For each electronic energy state there will be a number of vibrational and rotational states that are available to the molecule as well. It is more convenient to think in terms of energy level, so the figure at the right shows the manifold of possible states for each electronic state.