2000 Fiscal Year Final Research Report Summary
Study on the Prediction and Control of By-Pass Transition to Turbulence
| Project/Area Number |
10650897
|
|---|
| Research Category |
Grant-in-Aid for Scientific Research (C)
|
| Allocation Type | Single-year Grants |
|---|
| Section | 一般 |
|---|
| Research Field |
Aerospace engineering
|
|---|
| Research Institution | Tokyo Metropolitan Institute of Technology |
|---|
Principal Investigator |
MASAHITO Asai Tokyo Metropolitan Institute of Technology, Department of Aerospace Engineering, Professor, 工学部, 教授 (00117988)
|
|---|
| Project Period (FY) |
1998 – 2000
|
| Keywords | Boundary Layer / Laminar-Turbulent Transition / Flow Instability / Vortices / Streaky Structure |
|---|
| Research Abstract |
First the instability of a near-wall low-speed streak is examined experimentally. Symmetric varicose modes which develop into hairpin-shaped vortices are governed by the Kelvin-Helmholtz instability of inflectional velocity profiles in the normal-to-wall direction. Anti-symmetric sinuous modes due to the inflectional instability of the spanwise velocity distributions evolve into a train of quasi-streamwise vortices with vorticity of alternating sign. The streak meandering caused by the sinuous mode continues without disappearing over a few wavelengths even after the breakdown. It is also found that the anti-symmetric mode is more dangerous than the symmetric mode as the streak width becomes comparable with the shear layer thickness.
Next examined are disturbance developments in the relaminarization and retransition process of boundary layer. Low-frequency fluctuations, which are markedly suppressed near the wall during laminarization due to the boundary-layer suction, start to grow algebraically downstream of the trailing edge of the suction region, leading to the retransition to wall turbulence. The algebraic growth of near-wall low-frequency fluctuations in the retransition process is found to be due to appearance and meandering of near-wall low-speed streaks, which are responsible for the by-pass boundary-layer transition caused by high-intensity background turbulence.
In addition, the instability of compressible three-dimensional boundary layers at Mach numbers of 0.2 to 2.0 is examined theoretically. The boundary layer becomes unstable to stationary three-dimensional modes when the cross flow velocity exceeds a rather small threshold less than 1% of the external flow velocity. Importantly, the critical Reynolds number for stationary modes does not strongly depend on the Mach number.
|
