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Published November 14, 2014 | Submitted
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Laser Doppler Velocity & Vorticity Measurements in Turbulent Shear Layers


A Laser Doppler Velocimeter (LDV) system was developed to measure the instantaneous spanwise vorticity, - ω_z, in a turbulent shear layer. It was necessary to design and fabricate the LDV optics and processing electronics, as no commercially available LDV systems met the specifications of measuring the velocity at four closely spaced points to the requisite accuracy. Measurements were also made of the instantaneous u, v, u', v' and - u'v'. The instantaneous vorticity was processed to obtain an estimate of its probability density function, from which the mean and rms values were estimated. It was also possible to separate the irrotational fraction of the flow (-ω_z = 0) from the rotational (intermittent) fraction of the flow (-ω_z ≠ O). The development of the intermittency profiles, based on vorticity, as a function of the downstream distance from the splitter plate was studied. A notable feature is that the vorticity is found to have values opposite the mean sense of rotation, i.e., - ω_z(t) < 0, a significant fraction of the time. Additionally, a detailed study was performed to evaluate the approximation of -∂v/∂x, in terms of various local temporal derivatives ∂v/u(y)∂t. The optimum choice for u(y) can be found and is influenced by the relative local convection velocities of the small and large scale structures.

Additional Information

© 1986 California Institute of Technology. Final report NSF Award number MEA-8022945. Last, but not least, special thanks goes to my advisor, Dr. Paul Dimotakis, who helped define the goals of the project and provided me with much assistance and advice over the course of the project. The financial support of the Fannie and John Hertz Foundation and the California Institute of Technology is gratefully acknowledged. This project was supported by the NSF and AFOSR.

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