壁乱流におけるストリークの不安定と流れ方向過度の生成メカニズム

概要

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The linear stability analysis has been performed at Re_τ=180 for a turbulent-channel-type base flow with a periodic undulation in the spanwise direction in order to elucidate the generation mechanism of streamwise vorticity through the instability of streaks in near-wall turbulence. It is found that there appear three different instability modes depending on the spanwise wavenumber of streaks. In the case of the streak with around 100 wall-unit spanwise wavelength its critical velocity amplitude lies at 〓U^+_c〓3, above which streaky flow is unstable to an infinitesimal sinuous disturbance, i.e.a bending mode along the streamwise direction. The instability is identified to originate from inflection points, i.e.wake-like instability, in the spanwise variation of the streaky flow. In this case, unstable eignmodes take a form that is inclined towards the streamwise direction from the wall-normal direction, and they directly induce the streamwise vorticity on low-and high-speed streaks. In addition, the streamwise vorticity is secondarily produced pricipally through tilting of the wall-normal disturbance vorticity by the base flos whear across the wall-normal direction.
2000-02-25