Abstract: An experimental investigation on the buoyancy instability in the natural convection boundary layer is presented, including its basic characteristics such as power spectrum, wavelength, phase velocity and related critical layers, and its developing process of growth and dissipation with the variation of Grash of number. Results indicate that the natural convection boundary layer becomes unstable due to buoyancy at a critical modified Grashof number G around 40; the neutral curve obtained by the growth rates of the temperature fluctuation has a ring-like curve separated from the main branch of Nachtsheim's calculation by velocity fluctuation at the lower Grashof number side; and its phase velocity is close to the maximum mean velocity where the critical layer of the eigenmode locates. The thermal fluctuation decays rapidly as the non-dimensional frequency above 0.4, so that it means that the coupling effect between instability wave generated by buoyancy fluctuation and velocity waves by invicid instability seems weak enough for their critical layers in the range of the non-dimensional frequency B＜0.4.

Key words: buoyancy instability, neutral curve, vortex wave, eigenmode

摘要： 详细研究了自然对流边界层中最早失稳的浮力振型的功率谱、波长、波速与临界层位置等基本特征和它为主导其他振型尚未失稳时按线性理论得到的3种相互独立振型的振幅分布。系统地测量了它所对应的温度和速度涨落的振幅增长规律和中性曲线，实验结果表明，温度涨落的中性曲线在低格拉斯霍夫(Grashof)数G_r时有环状封闭曲线，随后是和数值计算结果相似的鼻部。至修正格拉斯霍夫数G＞100时，浮力振型迅速衰减，无粘性振型成为主导振型，特征曲线中由3种相互独立的特征解组成的3个峰形曲线的特征不复存在。

关键词: 浮力不稳定性, 中性曲线, 涡量波, 特征振型