抄録
Rotating flows in a cylindrical open vessel show dynamic surface deformations between an axisymmetric and a non-axisymmetric shape accompanied by laminar-turbulent transitions of the global flow structure. Appearance of the dynamic surface deformation, termed surface switching in series of our papers, depends on the apparatus and a more precise system has a tendency to reduce frequency of appearance of the surface switching. The aim of this study is to clarify influences of imperfections of the flow system on the flow transition. We prepared three disks having different gap size against the cylinder wall and an elliptic disk for the rotor with expecting difference on noise generation in the flow system. As our expectation the disk with wider gap increases the appearance of surface switching. And a statistical analysis on the corresponding surface height variation suggests that the surface switching is a random event due to gap flow noise. Using the elliptic disk enhances the flow instability for axi-symmetry breaking on the free surface and leads toward the surface switching. But after the switching occurs, the disk affects to stabilize the surface motion.
