An axisymmetric streamline body supported by
the Magnetic Suspension and Balance System

The instability of axisymmetric wake behind a body of revolution with thick NACA airfoil section is studied experimentally.  A magnetic suspension and balance system is used to support the axisymmetric body to avoid undesirable influences of mechanical supports on the disturbance development.  Multi-hot-wire probe with six sensors is used to identify helical instability modes.  The Reynolds number based on the chord length of the airfoil section ranges from 5×10⁴ to 2×10⁵.  For the NACA0015 airfoil-section model, the wake is convectively unstable in the whole region at Reynolds numbers examined and helical modes with azimuthal wavenumber of unity are amplified downstream as predicted by the linear stability theory for axisymmetric wakes.  In case of NACA0018 airfoil-section model, the flow near the trailing edge become absolutely unstable as the Reynolds number is increased beyond R_L=1×10⁵, and distinguished disturbance growth occurs at and around the absolute instability frequency, leading to a self-sustained wake oscillation.  Occurrence of such a global instability mode is more clearly observed in the wake behind a thicker NACA0024 model at lower Reynolds numbers. [Asai, M. Inasawa, A., Konishi, Y. Hoshino, S. and Takagi, S.: J. Fluid Mech. 675 (2011) 574-595]

Disturbance growth in the wake of a circular cylinder moving at a constant acceleration is examined experimentally.  The cylinder is installed on a carriage moving in the still air.  The results show that the critical Reynolds number for the onset of the global instability leading to a self-sustained wake oscillation increases with the magnitude of acceleration, while the Strouhal number of the growing disturbance at the critical Reynolds number is not strongly dependent on the magnitude of acceleration.  It is also found that with increasing the acceleration, the Kármán vortex street remains two-dimensional even at the Reynolds numbers around 200 where the three-dimensional instability occurs to lead to the vortex dislocation in the case of cylinder moving at constant velocity or in the case of cylinder wake in the steady oncoming flow. [Inasawa, Toda & Asai, ASME/JSME Joint Conf. Fluid Eng., 2008]

Stability of compressible three-dimensional boundary layers is examined on the basis of the linearized stability theory. The stability analyses are made for subsonic and supersonic boundary-layers mainly at Mach numbers of external flow M = 0.2 and 2.0 by using a family of the Falkner- Skan- Cooke profiles as the base flows. The results clearly show that the cross flow instability becomes completely dominant when the magnitude of the cross flow velocity exceeds about 4 % of the external flow velocity both at the subsonic and supersonic Mach numbers over a wide range of Reynolds numbers. It is also shown that the influence of compressibility on the stability characteristics such as the oblique angle c and growth rate of the most amplified first mode is weak for the cross flow instability. [Asai, M. Saitoh, N., Seino, H. and Itoh, N., Trans. Jpn. Soc. Aeron. Space Sci. 42 (1999) 76-84.]
[Asai, M., Saitoh, N. and Itoh, N., Trans. Jpn. Soc. Aeron. Space Sci. 43 (2001) 190-195]

Development of localized disturbances generated by an oscillating point source in compressible boundary layers with zero pressure gradient at Mach numbers from 0.2 to 2.0 is studied theoretically on the basis of the linear stability theory. The method of complex characteristics recently proposed by Itoh as an extension of Whitham’s kinematic wave theory, is applied to describe the wave propagation from the oscillating source. The analysis demonstrates distinct differences in the development of localized disturbances between the subsonic and supersonic boundary layers. Importantly, the maximum growth occurs away from midspan in supersonic boundary layers, while it occurs at midspan in subsonic boundary layers. [Saitoh, N., Asai, M. and Itoh, N, Trans. Japan Society for Aeronautical and Space Sciences 45 (2003) 211-216]

Receptivity of the laminar shear layer separating from a 90-degree rear-edge of boundary-layer plate to acoustic disturbances is examined experimentally.  The experiment is focused on the dependency of the receptivity coefficient on the rear-edge curvature as well as the frequency of acoustic disturbance.  The receptivity coefficient decreases with increasing the disturbance frequency for the rounded rear-edge, while it is almost independent of the frequency for the sharp rear-edge over the frequency range examined. [Imai, T and Asai, M., Fluid Dyn. Res. 41 (2009) 035506]