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首页> 外文期刊>International Journal of Heat and Mass Transfer >Characterization of linear and oscillatory behaviours of radiation-induced natural convection boundary layer in response to constant and time-varying thermal forcing
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Characterization of linear and oscillatory behaviours of radiation-induced natural convection boundary layer in response to constant and time-varying thermal forcing

机译：辐射诱导自然对流边界层响应恒定和时变热强迫的线性和振荡行为的表征

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摘要

This paper considers linear and oscillatory behaviours of the natural convection boundary layer induced by the absorption of incoming radiation in response to constant and time-varying (ramp) thermal forcing. The considered configuration is relevant to the flow in littoral regions of lakes and reservoirs in response to the daytime thermal forcing due to solar radiation. The absorption of the incoming solar radiation is depth-dependent, following Beer's law, whilst the residual radiation at the bottom bathymetry is absorbed and released back to the water body as a boundary flux. Therefore, a potentially unstable thermal boundary layer forms adjacent to the bottom bathymetry and the associated thermoconvective instability induces vertical mixing in the form of rising plumes (i.e. intermittent convection). The linear theory is applied to investigate the incipient evolution of the instability, and direct comparisons of linear and direct stability analyses are performed. The fastest growing mode and the corresponding temporal growth rate obtained via a quasi-static linear stability analysis are shown to be in excellent agreement with the results of the direct stability analysis based on numerical simulations of the flow. Further, the time and frequency scales of the unstable bottom thermal boundary layer are derived via a local Rayleigh number analysis. It is then demonstrated that the boundary layer response to the time-varying thermal forcing is in equilibrium with the forcing intensity at each instant of time.
机译：本文考虑了自然对流边界层的线性和振荡行为，该自然对流边界层是由入射辐射的吸收对恒定和随时间变化的（斜坡）热强迫引起的。考虑到由于太阳辐射引起的白天热强迫，所考虑的构造与湖泊和水库沿岸区域的流量有关。遵循比尔定律，入射太阳辐射的吸收取决于深度，而底部测深仪中的剩余辐射则被吸收并作为边界通量释放回水体。因此，在底部测深仪附近会形成潜在的不稳定热边界层，并且相关的热对流不稳定性会以上升的羽状流形式（即间歇对流）引起垂直混合。应用线性理论研究不稳定性的初期演变，并对线性和直接稳定性分析进行直接比较。通过准静态线性稳定性分析获得的最快增长模式和相应的时间增长率显示出与基于流动数值模拟的直接稳定性分析的结果非常一致。此外，不稳定的底部热边界层的时间和频率尺度是通过局部瑞利数分析得出的。然后证明了边界层对时变热强迫的响应与每个时刻的强迫强度是平衡的。

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