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The local flowfield about large roughness elements placed in a flat plate
laminar boundary was studied. Flowfield measurements were taken upstream,
over, and downstream of an isolated hemispherical roughness element and
distributed hemispherical roughness elements. The height of the roughness
elements was large compared to the local undisturbed boundary layer
thickness. The roughness flowfields were scaled such that the roughness
Reynolds number, Rek, and the ratio of
roughness height to boundary-layer
thickness, k/ d, were typical of those found in
initial glaze-ice
accretions. For the isolated roughness case, k/d
= 2.5 and at the
leading edge of the distributed roughness, k/d
=
3.4. The nominal value
of Rek for both cases was 3700. The large physical size of the elements
improved the spatial resolution of the measurements and helped facilitate
the flow visualization. Velocity measurements were obtained with a 2-D
laser Doppler velocimeter (LDV). Two-dimensional velocities,
time-averaged three-dimensional velocities, turbulence intensities, and
vorticity were all calculated from the measured velocities.
Fluorescent-oil flow and laser-sheet/smoke-wire visualization data were
obtained in order to help qualitatively explain the field. Results showed
that the horseshoe vortex system in front of the isolated roughness
element contained three primary vortices and demonstrated the
characteristics of an unsteady amalgamation phenomena. Distinct
spiral-type vortices and a hairpin-type vortex structure formed behind the
isolated element. In comparison, the horseshoe vortex system that formed
in from of each element on the from row of the distributed roughness
contained only two primary vortices, but it also appeared to be unsteady.
Although a vortical structure was observed immediately behind the last row
of the distributed roughness, it was weaker than, and did not appear to
have the same structure as, the hairpin-type vortex behind the isolated
roughness element. The distributed roughness was thicker and more
persistent than the isolated roughness wake. This was attributed
partially to the stronger vortex systems behind the isolated roughness
element enhancing the mixing of the wake with the outer inviscid flow,
thereby re-energizing and thinning the isolated roughness wake.
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