Randomization Process in Crossflow Instability Dominant Three-Dimensional Boundary-Layer Transitions

Abstract

Present investigation is concerned with the clarification of the randomization process in crossflow instability dominant boundary-layer transition. A swept wing model and a spinning disk, both of which produce crossflow dominant three-dimensional (3-D) boundary layer transition, are investigated experimentally in detail. Using accurate 3-D traversing system, hot wire is scanned in the transition region of a swept wing boundary layer, and also disk wall fixed hot wire system is newly developed in order to measure unsteady disturbances in a spinning disk boundary layer. Results showed that two different unsteady disturbances, frequencies of which are different in about one order, are detected in the transition regions of two different crossflow dominant experimental models. Such unsteady disturbances are also detected in a swept cylinder boundary layer transition region by Poll[1]. In the present paper, low frequency disturbance is called as f 1 , while high frequency one is as f 2 . Using rotatable parallel hot wire system, nature of those unsteady disturbances are considerably clarified. Results showed that origin of the low frequency disturbance f 1 is sitting very close to the wall in the boundary layer, where as that for the secondary instability is at the edge of the boundary layer. Numerical results obtained by Malik[2] showed the similar tendency with present results. One of the possible explanation for f 1 will be the crossflow primary instability unsteady mode. However, traveling direction is almost 180° different from that of measurement and calculation in different crossflow experimental models [3, 4]. Nature of the instabilities such as phase velocities and traveling directions are successfully detected, and discussion concerning the detailed randomization process are included in the present paper.