Hot-Wire Investigations of Smoke Patterns Caused by a Spherical Roughness Element

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The transition process from laminar to turbulent flow caused by a sphere placed on a flat plate has been investigated using smoke technique in detail by the author. These results are compared with those obtained in the present paper using hot-wire method. From the measurements of mean wind velocity, it is found that the smoke filaments appearing behind the sphere exhibit the central part of the longitudinal vortices in the case of low wind velocity and non-turbulent flow. As the turbulence wedge appears, there exists a longitudinal vortex close to the plate along the boundary of the wedge. In the case when clear periodic vortices appear, the wave forms of the velocity fluctuations are observed at various three dimensional positions of hot-wire relative to the vortices, taking simultaneously photographs of the smoke patterns and hot-wire position using movie camera. From the results of these observations, it is found that the longitudinal vortices act to transfer the fluid up and down and the equalization of wind velocity takes place in the wake of the sphere by the vortices which exist there.
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