自然对流边界层中湍流的发生

自然对流边界层中从层流到湍流的转捩经历了浮力振型、无摩擦振型和黏性振型的三重流动不稳定性相继产生的前转捩过程,以及近壁迅速出现强湍流源,随之平缓地向自模拟的湍流边界层过渡的热转捩过程.浮力振型在修正Grashof数G>40时开始失稳并成为主要振型,在振幅分布中3种振型的临界层位置处出现3个峰值;在G>100时浮力振型消失,无摩擦振型失稳并成为主要振型,振幅分布中在近壁区还出现黏性振型的峰值;在G>170时无摩擦振型经非线性演化在外层形成较弱的湍流,但内层黏性应力仍远高于湍流应力,振幅分布中仅有与黏性振型相应的峰值,在频谱中黏性振型的基频、第一、第二、第三阶亚谐频随G的增加相继出现,此时黏性不稳定波的高频成分已转化为湍流,但低频成分仍按线性规律增长,直至湍流惯性子区开始形成;至G>800时黏性振型消失,并在G=850附近时近壁区出现强湍流源,湍流应力、湍能产生项和近壁湍流热流率剧增.在热转捩后期,湍流应力和湍能产生项明显下降,流动在内外层趋于平衡.

Origin of turbulence in natural convection boundary layer

Experiments indicated that the transition of the natural convection boundary layer from laminar to turbulent flow passed through a pre-transitional process with triple instabilities of buoyancy, frictionless, and viscous eigenmode dominating sequentially, and a thermal transition process where a strong turbulence-production source appeared rapidly at the near wall region before the beginning of the smooth evolution to self-similarity characterized turbulent boundary layer. Buoyancy eigenmode became unstable and dominated while modified Grashof number G > 40, and 3 peaks corresponding to the triple-eigenmode appeared at their respective critical layer in the normal amplitude distribution of velocity and temperature fluctuations. Buoyancy eigenmode disappeared at G > 100, and the frictionless eigenmode became dominant with the viscous eigenmode peak still existed at the near-wall region in the normal amplitude distributions. While G > 170, weak turbulence emerged at the outer layer with the non-linearization of the frictionless eigenmode, but the viscous stress in the inner layer was still much higher than the turbulence stress therein, and the viscous peak was the only one existing in the normal amplitude distributions, in the spectra of the viscous eigenmode, its fundamental frequency, 1st, 2nd, and 3rd subharmonics appeared sequentially with the increasing of G, while the higher frequency components already became turbulent, the lower frequency components still kept growing linearly until the turbulent inertial subrange came into being. Viscous eigenmode vanished at G > 800, then a strong turbulence-production source could be observed in the near-wall region at G = 850, and turbulent stress, turbulent energy production term, the near wall turbulent heat flux grew sharply. In the later stage of thermal transition, turbulent stress and turbulent energy production term dropped obviously and the flow equilibrated gradually at both inner and outer layer.