This paper is concerned with the laminar-turbulent transition of three-dimensional boundary layers on cones rotating in external axial flow. The experiment was carried out for three cone angles of 15 degree , 30 degree and 60 degree in a range of free-stream turbulence (0004% to 3. 5%) using a hot-wire anemometer and a flow-visualization technique. The transition region was determined with the critical Reynolds number and the transition Reynolds number in relation rotational speed ratio. The spiral vortices were found to fix relative to the rotating surface. The relation of the direction of the spiral vortices to the rotational speed ratio remains unchanged as the cone angle becomes larger, while the number of vortices increases with increased cone angle. These experimental results were compared with the present numerical results based on the linear stability theory. The free-stream turbulence has no effect on the transition Reynolds number, while the effect becomes more sensitive to the critical Renolds number as the cone angle is increased.
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