22nd Congress of International Council of the Aeronautical Sciences, Harrogate, UK, 28 August - 1st September, 2000
Paper ICAS 2000-3.4.3

DOWNSTREAM EVOLUTION OF A LAMINAR SPOT

A. Matsumoto
Nihon University, Japan

Keywords: boundary layer transition, hot wire anemometer, turbulent spot

It was well known that the disturbance, introduced artificially into a supercritical twodimensional laminar boundary layer along a flat plate by ejecting an intermittent jet from a small hole on the flat plate, developed downstream and finally evolved into a turbulent spot. As a jet and the Reynolds number match well, however, this disturbance was still laminar during the initial stage of its downstream evolution and resembled a turbulent spot. Therefore, we called it a ‘laminar spot’. We focused our attention on the transition of a laminar spot to a turbulent spot. Measurements were made of streamwise mean velocity and turbulence intensity profiles of a laminar boundary layer in detail using the multi-channel data processing connected with the rake type 16-channel hot-wire anemometer system. Moreover, we visualized the flow field with a laminar spot by drawing several contours of difference between streamwise mean velocity and Blasius velocity. As a result, we found that a laminar spot consists of high-speed and lowspeed regions lined up alternately in spanwise direction. At the first stage of its downstream evolution, a laminar spot grows slowly to spanwise direction. The half of lateral growth can be estimated as 5! , which was almost half of a turbulent spot growth. At the next stage, lateral growth of a laminar spot is repressed. After then, it is traveling downstream with the same growth angle, about 10! , as that of a turbulent spot. Thus, we deduce that this stage might be the beginning of transition from a laminar to a turbulent spot.

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