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中国物理学会期刊

旋转肥皂泡热对流能量耗散与边界层特性的数值模拟

CSTR: 32037.14.aps.71.20220693

Boundary layers and energy dissipation rates on a half soap bubble heated at the equator

CSTR: 32037.14.aps.71.20220693
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  • 将底部加热的半个肥皂泡作为一个新的热对流模型, 结合了肥皂泡固有的球面与准二维特征, 由此有助于理解行星大气流动中的复杂物理机制与热对流特性. 本文使用直接数值模拟方法计算了旋转肥皂泡上的湍流热对流, 研究了肥皂泡上的温度与黏性边界层以及拟热能和动能耗散规律. 结合肥皂泡上温度场与速度场特征, 分别根据温度脉动均方根最大值以及速度脉动边界处斜率延长线与最大值交点提出了肥皂泡上温度与黏性边界层的识别方法. 研究发现, 当肥皂泡从边界吸收能量时, 拟热能耗散与动能耗散均集中在边界层中, 肥皂泡上的温度边界层与黏性边界层厚度与瑞利数Ra存在明确的标度关系. 相比经典Rayleigh-Bénrad对流(RB对流)模型, 温度标度指数具有较为接近的结果, 但速度标度指数存在一定的差异. 此外, 在混合区, 均方根温度(T^*)随纬度(\theta)具有近似T^*\sim\theta^0.5的标度关系, 这与RB对流模型及其相应的理论预测一致. 最后通过能量平衡方程发现, 肥皂泡上拟热能内耗散率\varepsilon_T^0和动能内耗散率\varepsilon_u^0比拟热能外耗散率\varepsilon_T^1和动能外耗散率\varepsilon_u^1大1个量级, 拟热能与动能的内部耗散率在边界层中具有支配地位, 随着肥皂泡旋转速率的增加, 热羽流难以输运到高纬度地区, 进一步降低了拟热能与动能外耗散率的影响.

     

    The soap bubble heated at the bottom is a novel thermal convection cell, which has the inherent spherical surface and quasi two-dimensional features, so that it can provide an insight into the complex physical mechanism of the planetary or atomspherical flows. This paper analyses the turbulent thermal convection on the soap bubble and addresses the properties including the thermal layer and the viscous boundary layer, the thermal dissipation and the kinetic dissipation by direct numerical simulation (DNS). The thermal dissipation and the kinetic dissipation are mostly occur in the boundary layers. They reveal the great significance of the boundary layers in the process of the energy absorption. By considering the complex characteristics of the heated bubble, this study proposes a new definition to identify the thermal boundary layer and viscous boundary layer. The thermal boundary layer thickness of \delta_T is defined as the geodetic distance between the equator of the bubble and the latitude at which the the mean square root temperature (T^*) reaches a maximum value. On the other hand, the viscous boundary layer thickness \delta_u is the geodetic distance from the equator at the latitude where the extrapolation for the linear part of the mean square root turbulent latitude velocity (u^*_\theta) meets its maximum value. It is found that \delta_T and \delta_u both have a power-law dependence on the Rayleigh number. For the bubble, the scaling coefficent of \delta_T is -0.32 which is consistent with that from the Rayleigh-Bénard convection model. The rotation does not affect the scaling coefficent of \delta_T. On the other hand, the scaling coefficent of \delta_u equals -0.20 and is different from that given by the Rayleigh-Bénard convection model. The weak rotation does not change the coefficent while the strong rotation makes it increase to -0.14. The profile of T^* satisfies the scaling law of T^*\sim\theta^0.5 with the latitude of (\theta) on the bubble. The scaling law of the mean square root temperature profile coincides with the theoretical prediction and the results obtained from the Rayleigh-Bénard convection model. However, the strong rotation is capable of shifting the scaling coefficent of the power law away from 0.5 and shorterning the interval of satisfying the power law. Finally, it is found that the internal thermal dissipation rate and kinetic dissipation rate \varepsilon^0_T and \varepsilon^0_u are one order larger than their peers: the external thermal dissipation and kinetic dissipation rates \varepsilon^1_T and \varepsilon^1_u based on a thorough analysis of the energy budget. The major thermal dissipation and kinetic dissipation are accumulated in the boundary layers. With the rotation rate increasing, less energy is transfered from the bottom to the top of the bubble and the influence of the external energy dissipations is less pronounced.

     

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