IWAMOTO Yukiharu Ehime University, Faculty of Engineering, Instructor, 工学部, 助手 (80325357)
KAWAHARA Genta Ehime University, Faculty of Engineering, Associate Professor, 工学部, 助教授 (50214672)
AYUKAWA Kyozo President of Ehime University, 学長 (30036230)
|Budget Amount *help
¥3,500,000 (Direct Cost : ¥3,500,000)
Fiscal Year 2000 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1999 : ¥2,300,000 (Direct Cost : ¥2,300,000)
Fiscal Year 1998 : ¥700,000 (Direct Cost : ¥700,000)
In this research, we make both experimental and theoretical approaches to the self-sustaining mechanisms of near-wall turbulence.
In the experiments, the generation mechanism of Reynolds stresses, which play a significant role in maintenance of turbulence activity, has been demonstrated based upon the results obtained from turbulence measurement of a square-duct flow using LDV.In the near-wall region of the duct, streamwise vortical motions induce the ejection and the sweep, both of which generate intense Reynolds stresses, so that turbulence is sustained. In the corner region of the duct, on the other hand, streamwise vortices transport fluid momentum from one wall to another adjacent wall, which leads to the reduction of the contribution of the sweep. In this region, therefore, turbulence is sustained principally through the ejection. The characteristics of the streamwise vortices, e.g. dimension and circulation on cross-flow plane, are shown in the corner region. In addition, the spa
tial distribution of local wall shear stress on the wall is presented.
In the theoretical investigation, the generation mechanism of streamwise vortices, which induce the Reynolds stress responsible for the maintenance of turbulence, has been demonstrated. The hydrodynamic instability of near-wall streaks is investigated to prove that streaks are unstable to sinuous (or bending) disturbances. Because streaks are affected by a shearing motion across the wall-normal direction, the unstable mode for streaks has the inclination from the wall-normal direction in the streamwise direction, which means that if streaks are bent in the spanwise direction, through the instability, the streamwise vorticity is directly generated to consequently involve the streamwise vortices. In addition to the above analysis, the direct, numerical simulations of a minimal plane Couette flow (i.e. minimal flow units of near-wall turbulence) have been performed to obtain time-periodic solutions to the Navier-Stokes equation, which well represent the whole of the self-sustaining cycle in near-wall turbulence (in preparation for publication). We have numerically shown the dominant role in turbulence of coherent structures consisting of streamwise streaks and vortices. Less