Improved methods for thin, surface boundary layer investigations |
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Authors: | H J Lin M Perlin |
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Institution: | (1) Department of Mechanical Engineering and Applied Mechanics University of Michigan Ann Arbor, MI 48109-2125, USA, US;(2) Department of Naval Architecture and Marine Engineering University of Michigan Ann Arbor, MI 48109-2145, USA, US |
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Abstract: | New techniques are developed to improve the velocity flow-field measurement capability within a free-surface boundary layer
region on which progressive capillary-gravity waves are present. Due to the extremely thin but rather vortical characteristics
of the aforementioned boundary layer, conventional particle image velocimetry (PIV) methods fail to estimate velocity (and
vorticity) vectors at an acceptable detection rate. This failure is a direct consequence of optimal PIV parameters that are
difficult to achieve in practice for such flow situations. A new technique, Sub-pattern PIV, is developed. This method has
features similar to both the super-resolution PIV (Keane et al. 1995) and the particle image distortion (PID) technique (Huang
et al. 1993), but is predicated upon a very differential philosophy. Another difficulty that arises in experiments to investigate
surface boundary layer flows is that the oscillating and deforming air–water interface has a mirror-like behavior that affects
the images, and generates very noisy data. An alternative experimental setup that utilizes the Brewster angle phenomenon is
adopted and the specular effects of the free-surface are removed successfully. This Brewster angle imaging, along with the
Sub-pattern PIV technique, is used for the target application – a free-surface boundary layer investigation. It proved to
be very effective. The methodology of both techniques is discussed, and the modified PIV procedure is validated by numerical
probabilistic simulations. Application to the capillary-gravity wave boundary layer is presented in a subsequent paper.
Received: 31 July 1997/Accepted: 4 February 1998 |
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