基于POD-Galerkin降维方法的热毛细对流分岔分析

作为流动与传热相互耦合的非线性过程,热毛细对流有着复杂的转捩过程,探究流场和温度场随参数变化而发生的分岔现象,是热毛细对流研究的一个重要课题.基于本征正交分解的POD-Galerkin降维方法可以通过提取特征模态,构建低维模型,实现流场的快速计算.数值分岔方法可以通过求解含参数动力系统的分岔方程,直接计算稳定解和分岔点.探究了将直接数值模拟方法、POD-Galerkin降维方法、数值分岔方法的优势结合,以提高热毛细对流转捩过程分析效率的可行性.利用直接数值模拟得到的流场和温度场数据,构建了不同体积比下,二维有限长液层热毛细对流的POD-Galerkin低维模型,在低维模型上采用数值积分及数值分岔方法计算了分岔点,得到了低维方程的分岔图.在一定参数范围内,在低维模型上模拟热毛细对流,对雷诺数和体积比进行参数外推,通过与直接数值模拟的结果对比,验证了低维模型的准确性与鲁棒性.说明了低维方程可以定性反映原高维系统的流动特性,而定量方面,由低维模型和直接数值模拟计算得到的周期解频率的相对误差大约为5%.验证了利用POD-Galerkin降维方法研究热毛细对流的可行性

界面张力梯度驱动对流向湍流转捩的研究

作为空间自然对流热质输运的基本形式,界面张力梯度驱动对流是流动和传热强耦合的复杂非线性过程,也是一个多控制参数耦合作用的过程,表现出丰富的流动时空特征.界面张力梯度驱动对流是微重力流体物理的重要研究内容和学科前沿,同时在空间燃料输运过程和空间能源或热管利用等空间流体管理问题中均有重要应用.本文综述了界面张力梯度驱动对流向湍流转捩研究的背景意义、地面实验、空间实验及数值模拟的研究现状,重点介绍了从非线性动力学角度来研究转捩规律的具体方法,目前最常见的手段是对观测量的时间序列进行分析,通过频谱分析及相空间重构等方法计算时间序列的特征量,从而判断流动模式,这类方法理论成熟,计算简单,但需要对大量数据进行繁琐的处理;另一种方法是通过数值计算分岔来研究对流在时空中的转捩模式,这类方法可以直接计算出分岔点,但是复杂之处在于需要求解大规模的线性或非线性方程组,本文详细阐述了两种方法的理论背景,应用状况及局限性,探讨了将两种方法相互结合,在研究中互为补充的可能,并对今后的研究方向提出了建议

Dependency of transition in thermocapillary convection on volume ratio in annular pools of large Pr fluid in microgravity

Thermocapillary convection driven by the horizontal temperature gradient is a typical nonlinear dynam ical system. As the applied temperature difference increases, the flow undergoes a series of transitions into turbulence. Volume ratio of the liquid layer Vr (liquid volume/volume of the container) is a critical parameter that affects transition behaviors of the convection. Direct numerical simulations on thermo capillary convection in annular liquid pools with various volume ratios are carried out. Characteristics of the oscillatory convection are detected by time series and frequency spectrum analyses. The dynamic mode decomposition (DMD) method is also adopted to investigate inherent structures of the flow field. Effect of Vr on oscillation characteristics under a wide range of temperature differences from 10K to 90K is discussed. Results show that in annular pools with lager volume ratios, the thermocapillary convection is chaotic under very large AT and its transition behaviors are more complicated.(c) 2023 Elsevier Ltd. All rights reserved

Effect of volume ratio on thermocapillary convection in annular liquid pools in space

In systems with liquid/liquid or liquid/gas interface under microgravity, and even in shallow liquid layers in the terrestrial conditions, thermocapillary force takes the principal role to drive natural convections. A series of numerical simulations are conducted to investigate the stability limit of axisymmetric steady thermocapillary flow in annular liquid pools with curved and adiabatic liquid surface for eight volume ratios 0.809 <= Vr <= 1.173, where Vr is defined as (liquid vol/vol of the annular gap). Simulations provide the critical temperature difference Delta T-c, frequency f(c), and azimuthal wave number mc for each liquid pool. At the critical condition, oscillations start in form of standing wave. At the slightly supercritical condition (Delta T*), the standing waves turn to traveling oscillations. The calculated Delta T-c values decrease with the increase of Vr. A simulation code with a convective thermal boundary condition in the liquid surface suggests that heat transfer through the liquid surface significantly increases the Delta T-c value

Thermal stratification and self-pressurization in a cryogenic propellant storage tank considering capillary effect in low-gravity

Thermal stratification and self-pressurization in a propellant storage tank due to heat leakage from the wall are key issues of space fluid management. Under low-gravity conditions, the gas/liquid two-phase flow in a tank is complicated owing to the irregular interface morphology caused by the capillary effect. To clarify the heat and mass transfer process, the gas/liquid two-phase flow with the capillary effect accounted at the interface is systematically investigated by taking into account the volume of fluid (VOF) method for two-phase capturing and the Lee model for phase change. Spatial-temporal evolutions of thermal and pressure distributions and mass transfer rates at the interface in an axisymmetric scaling capsule tank of ethanol are studied depending on various gravity levels, liquid filling ratios, and boundary heat fluxes. The results show that the overall temperature, pressure, and thermal distributions inside the tank are significantly affected by the gravity level, liquid filling ratio, and boundary heat flux, while the pressure distributions are quite similar under different conditions. The influence of gravity levels mainly originated from various interface configurations due to the capillary effect. Therefore, the capillary effect plays an important role in the heat and mass transfer process in low-gravity environments