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Type of Document Dissertation Author Othman Bekhit, Hesham Abdel Ghafar Author's Email Address hothman@nd.edu URN etd-07222005-120133 Title Experimental Study of Absolute Instability over a Rotating Disk Degree Doctor of Philosophy Department Aerospace and Mechanical Engineering Advisory Committee
Advisor Name Title Prof. Thomas Mueller Committee Member Keywords
- rotating disk
- absolute instability
Date of Defense 2005-07-14 Availability unrestricted Abstract A series of experiments were performed to study the absoluteinstability of Type I traveling cross-flow modes in the boundary layer
on a smooth disk rotating at constant speed. The basic flow agreed with
analytic theory, and the growth of natural disturbances matched linear theory
predictions. Controlled temporal disturbances were introduced by a short-duration air pulse from
a hypodermic tube located above the disk and outside the boundary layer. The
air pulse was positioned just outboard of the critical radius for Type I cross-flow modes.
A hot-wire sensor primarily sensitive to the azimuthal velocity component, was positioned
at different spatial locations on the disk to document the growth of disturbances
produced by the air pulses. Ensemble averages conditioned on the air pulses revealed
wave packets that evolved in time and space. Two amplitudes of air pulses were
used. The lower amplitude produced wave packets with linear amplitude characteristics
that agreed with linear-theory wall-normal eigenfunction distributions and spatial growth rates. The
higher amplitude pulse produced wave packets that had nonlinear amplitude characteristics. The
space-time evolution of the leading and trailing edges of the wave packets were followed
well past the critical radius for the absolute instability based on Lingwood (1995).
With the linear amplitudes, the absolute instability was dominated by the convective
modes, agreeing with the linear DNS simulations of Davies and Carpenter (2003). With the
nonlinear amplitudes, larger temporal growth of the wave packets existed which
supports the finite amplitude analysis of Pier (2003), and more closely resembles
the wave packet evolution in the experimental study of Lingwood (1996). This suggests that
the disturbance levels in the experiment that was intended to demonstrate the linear analysis,
were likely finite.
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