局部行波对流的时空结构

通过流体力学方程的数值模拟,研究了矩形腔体长高比Γ(28)20和混合流体分离比ψ(28)-0.4的无缺陷的局部行波对流和有缺陷的局部行波对流的特性。结果表明:局部行波可以存在于腔体左端壁附近,也可以存在于右端壁附近,但传播方向始终在背离端壁面的方向,对流存在区间长度随着相对瑞利数r的增加而增加;沿着对流分叉曲线增加r,当r超过临界值后,系统从无缺陷的局部行波过渡到有缺陷的局部行波状态;对于流体参数,无缺陷的局部行波存在于r∈[1.519,1.600],有缺陷的局部行波存在于r∈(1.600,1.670];有缺陷的局部行波中对流区间长度随着r增加基本保持为常数;缺陷出现在腔体中的相对位置基本不变,距背离传播方向的端壁5.8左右,缺陷出现的周期随着r增加呈增加趋势,腔体的长高比足够大是出现有缺陷的局部行波的一个必要条件。     

混合流体局部行波对流斑图选择的初值依赖性

利用Simple算法对流体力学基本方程组进行了数值模拟,初步研究了局部行波对流斑图选择的初值依赖性问题。分离比ψ-(28)6.0、相对瑞利数r(28)2.1时依赖于初值的有间歇性缺陷的行波,位于腔体右端的局部行波和位于腔体左端的局部行波的多重稳定性;分离比ψ(28)-0.6、相对瑞利数r在1.855~2.118范围内依赖于初值的位于腔体右端的局部行波和位于腔体左端的局部行波的多重稳定性等。虽然在不同初值下,局部行波存在的区间有所不同,局部行波的空间位置有所不同,但局部行波的特性参数变化规律基本一致。结果说明混合流体局部行波对流斑图选择的初值依赖性是存在的。     

二维矩形腔体中混合流体行进波对流

Rayleigh-Benard模型是研究对流稳定性,时空结构和非线性特性的典型模型之一。本文的兴趣集中在二维矩形腔体中混合流体对流场的结构方面。利用SIMPLE算法数值求解流体力学方程组,模拟了充分发展的二维矩形腔体中混合流体对流。结果说明偏离传导失去稳定的系统经过亚临界分叉产生了振动对流。进一步,我们给出了分叉曲线及其沿分叉曲线的上部分支三个Rayleigh数对应的对流图案的垂直速度场,流线,温度场,浓度场和Shadowgraph强度的等值线图。所有场的结构分析表明浓度场及Shadowgraph强度的等值线图可以很好的特征行进波的运动特性。     

侧壁面正弦加热条件下自然对流研究

通过采用SIMPLE算法求解二维流体力学基本方程组,研究了侧壁面正弦加热条件下普朗特数Pr=6.94的流体自然对流的动力学特性。结果表明,当格拉晓夫数Gr=4.4×10~3时系统由稳态阶段向周期阶段转变。在周期性阶段存在两种特殊的对流结构,它们是单局部对流卷和双局部对流卷;给出了特征物理量的分布及变化规律;说明垂直最大流速、水平最大流速、努塞尔数随着格拉晓夫数的增加而增加。最后,分析了腔体宽度对热壁面传热能力的影响,发现:当高宽比一定条件下,腔体宽度较大时,热壁面的传热能力较弱;对于宽度d分别为1cm和2cm的腔体,随着格拉晓夫数增大,最大努塞尔数的差值增大。     

小长高比腔体内的摆动行波

通过流体力学基本方程组的数值模拟,探讨了具有Soret效应(分离比ψ=?0.47)和小长高比(Γ=8)腔体中混合流体摆动行波对流的动力学特性。研究表明:在相对瑞利数r3.467时系统出现了行波状态;在r=3.647～6.227的范围内,发现了摆动行波对流;且对流振幅随着时间的变化存在两种不同特性,其摆动周期随瑞利数r增大而减小,对流振幅和努塞尔数随瑞利数r增大而增加;当r增大到r=6.228时,摆动行波过渡到定常对流状态。因此,在行波对流向定常对流过渡的过程中存在摆动行波对流.     

缺陷源摆动的对传波

基于流体力学方程组，对长高比 腔体内混合流体对流中缺陷源摆动的对传波的特性进行了数值模拟．结果发现，当分离比 时，缺陷源摆动的对传波的缺陷源由两个对流圈同时被拉长同时被分裂后形成．对传波的存在区间为 ，对传波的摆动周期较小并且基本都稳定在 ．对传波的摆动幅度较小并且几乎不随着相对瑞利数 变化．当 时，对传波的缺陷源下方対流圈轴线与缺陷源移动方向保持一致，没有对传波分支产生．对传波的缺陷源上方存在多个与缺陷源移动方向几乎垂直的対流圈的轴线，存在对传波分支，在对传波分支上出现间歇性的缺陷．从而形成具有单侧缺陷的缺陷源摆动的对传波．对传波的存在区间为 ．对传波的摆动周期为 并且随着相对瑞利数的增加而增加．对传波的摆动幅度较大，并且随着相对瑞利数的增加而减小．     

具有大负分离比的混合流体局部行进波对流

从流体层底部加热引起的对流运动是研究非平衡对流的时空结构或斑图(Pattern)及非线性动力学特性的典型模型之一.本文通过流体力学基本方程组的数值模拟,探讨了具有强Soret效应的混合流体局部行进波的形成过程,发现当分离比ψ=-0.6时,在局部行进波的存在范围内,向局部行进波过渡的不同过程依赖于相对瑞利数r.进一步,讨论了具有强Soret效应的混合流体局部行进波流速场,温度场, 浓度场的结构和特性,分析了局部行进波的存在区间对分离比ψ的依赖性.发现随着Soret效应的增强或负分离比ψ的绝对值的增加,局部行进波稳定存在的区间Δr也在增加.     

瑞利数对行波对流缺陷特性的影响

通过流体力学方程的数值模拟,研究了瑞利数对分离比ψ=-0.6的混合流体行波对流缺陷结构的影响。结果表明:对于给定的相对瑞利数r,缺陷的出现是间歇性的,缺陷出现的位置固定,缺陷出现的周期保持为常数;对于不同的r,缺陷形成时滚动的分裂是不对称的,可以在原滚动的上方也可以在原滚动的下方形成一个新的滚动,从而形成缺陷,缺陷出现的位置基本稳定在腔体的中部,缺陷出现周期随着r的增加而增加;在具有缺陷的行波存在的下限附近,缺陷出现周期减小得较快;在具有缺陷的行波存在的上限附近,缺陷出现周期增加得较快。有缺陷和无缺陷的行波的对流垂直流速最大值δwmax都随着时间在周期变化,但规律是不同的。具有缺陷的行波中,垂直流速最大值δwmax的周期代表缺陷出现的周期;在无缺陷的行波中,垂直流速最大值δwmax的周期代表行波的周期。     

具有强SORET效应的充分发展的混合流体行进波对流

混合流体Rayleigh-Benard对流是研究对流稳定性,时空结构和非线性特性的典型模型之一。本文利用流体力学扰动方程组的数值模拟,讨论了偏离传导状态具有强SORET效应的混合流体行进波对流的温度场和浓度场的成长过程,分析了充分发展对流情况下的对流振幅,Nusselt数及混合参数与相对瑞利数的关系。并给出了行进波相速度对相对瑞利数的依赖关系。结果说明混合参数的曲线与行进波相速度的分布曲线是类似的。文末,给出了垂直速度,温度和浓度场的分布并讨论了相对瑞利数对场的分布及不同场之间的相位差的影响。     

周期加热的Rayleigh-Benard对流时空结构

通过二维流体力学基本方程组的数值模拟,研究了普朗特数Pr=6.99时矩形渠槽周期加热对Rayleigh-Benard对流时空结构的影响．当水平流动强度Re=0时,发现稳定的由周期加热引起的局部定常对流．当Re比较小时,对流滚动抑制水平流动,获得了由周期加热引起的局部行波对流．当水平流动强度比较大时,由于周期加热与水平流动相互作用,水平流动抑制部分对流滚动,导致对流区域上游附近出现传导区域,对流区域减小,从而形成一种新的局部行波对流结构．并进一步讨论了Rayleigh-Benard对流时空结构的动力学特性．     

半浮区液桥热毛细振荡流

采用非定常、三维直接数值模拟方法研究大Pr数半浮区液桥热毛细对流从定常流向振荡流的过渡过程．文中详细描述了热毛细振荡流的起振和振荡特征,给出了液桥横截面上振荡流的流场和温度分布．在地面引力场条件下计算的结果与地面实验的结果进行比较,得出液桥水平截面上的流场和温度分布图样以一定的速度旋转,自由表面固定点处流体的环向流速正、负交替变化的一致结论．     

Rayleigh-Bénard湍流热对流研究的进展、现状及展望

对流现象广泛存在于恒星和行星里.在地球上,对流现象在诸如大气、海洋、地核和地幔等众多动力学系统中起着重要作用.Rayleigh-Bénard(RB)湍流热对流系统是从这些复杂的自然现象中抽象出来的研究对流问题的经典流体力学模型.本文主要从湍流传热、羽流、大尺度流动结构、速度和温度脉动的小尺度统计和非传统RB对流等几个方面着重评述近年来RB对流的若干研究新进展,并对今后的研究做出展望.     

Steady and transient free convection of an electrically conducting fluid from a vertical plate in the presence of a magnetic field

Summary An analysis is made for the laminar free convection and heat transfer of a viscous electrically conducting fluid from a hot vertical plate in the case when the induced field is negligible compared to the imposed magnetic field. It is found that similar solutions for velocity and temperature exist when the imposed magnetic field (acting perpendicular to the plate) varies inversely as the fourth root of the distance from the lowest end of the plate. Explicit expressions for velocity, temperature, boundary layer thickness and Nusselt number are obtained and the effect of a magnetic field on them is studied. It is found that the effect of the magnetic field is to decrease the rate of heat transfer from the wall. In the second part, the method of characteristics is employed to obtain solutions of the time-dependent hydromagnetic free convection equations (hyperbolic) of momentum and energy put into integral form. The results yield the time required for the steady flow to be established, and the effect of the magnetic field on this time is studied.     

Space–time correlation measurements of high-speed axisymmetric jets using optical deflectometry

An optical deflectometry system is used to provide unique space–time correlation measurements at two positions separated by varying axial distances within a high-speed jet shear layer. The measurements were made for both pure air and for helium/air mixture jets at Mach numbers M=0.9 and M=1.5. The jets issue from round nozzles and the sensing volumes at the two measurement positions consist of small light filaments along spanwise lines that are tangential to the annular jet shear layer. Applying this technique to obtain measurements detailing the level of correlation, spectral content, and convection velocity for jet flows in these flow regimes near the end of the potential core is particularly important in the understanding and prediction of jet noise. Measurements near the end of the potential core along the jet lip line exhibit distinct cross-correlation curves for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structure in the measured spectra. It is believed that the thick visual density gradients dominated by smaller scales throughout the shear layer of the helium/air mixture jets effectively mask the large-scale structure, thus, reflecting a limitation of this optical deflectometer. Finally, a decrease in normalized convection velocity with helium addition is observed.     

Stability of Solutal Convection in a Rotating Mushy Layer Solidifying from a Vertical Surface

We consider the effects of rotation in a mushy layer being cast from a vertical surface where the effects of Coriolis acceleration, gravity and centrifugal effects are included. It is demonstrated that the Coriolis acceleration and gravity play a passive role in convection and are excluded from the stability criteria. The stability criteria is presented as the critical centrifugal Rayleigh numbers referenced for locations far away (start of solidification) and close to (nearing end of solidification) the axis or rotation.     

Single-phase convection and boiling heat transfer: Confined single and array-circular impinging jets

In the present study, the effects of diverse situations of confinement on heat transfer from single and array-circular jet impingements are carefully investigated over various heat transfer regimes of single-phase convection and fully developed nucleate boiling. For the single, circular, unconfined free-surface jet, the transition to turbulence was observed to start around x/d = 5.5 and end around x/d = 9. For the array-circular jet, however, the wall jet structure yielded no transition to turbulence for all the tested cases, instead monotonically decreasing the convection coefficient. Conversely, the single-circular jet experienced the transition for V ? 6.1 m/s. For the confined submerged jet, the transition length was very short due to the vigorous mixing driven by lateral velocity components, and the locus of the secondary peak moved downstream as velocity increased. The temperature distributions of the confined array-circular jet were fairly uniform over the whole heated surface. The averaged single-phase convection coefficients indicated that the confined jet provided the most uniform convection in the lateral direction.     

Stability of Thermal Convection in a Fluid-Porous System Saturated with an Oldroyd-B Fluid Heated from Below

A linear stability analysis is conducted for thermal convection in a two-layer system composed of a fluid layer overlying a porous medium saturated with an Oldroyd-B fluid heated from below. It is found that the convection pattern in the system is controlled by the porous medium when the ratio of the depth of the fluid layer to that of the porous medium is small. However, the fluid layer takes a predominant role if the depth ratio exceeds a critical value. Compared with stationary convection, the switching point from a porous-dominated mode to a fluid-dominated mode for oscillatory convection is located at a lower depth ratio. The effects of different parameters on stationary convection and oscillatory convection are also investigated in detail.     

Layered Thermohaline Convection in Hypersaline Geothermal Systems

Thermohaline convection occurs in hypersaline geothermal systems due to thermal and salinity effects on liquid density. Because of its importance in oceanography, thermohaline convection in viscous liquids has received more attention than thermohaline convection in porous media. The fingered and layered convection patterns observed in viscous liquid thermohaline convection have been hypothesized to occur also in porous media. However, the extension of convective dynamics from viscous liquid systems to porous media systems is complicated by the presence of the solid matrix in porous media. The solid grains cause thermal retardation, hydrodynamic dispersion, and permeability effects. We present simulations of thermohaline convection in model systems based on the Salton Sea Geothermal System, California, that serve to point out the general dynamics of porous media thermohaline convection in the diffusive regime, and the effects of porosity and permeability, in particular. We use the TOUGH2 simulator with residual formulation and fully coupled solution technique for solving the strongly coupled equations governing thermohaline convection in porous media. We incorporate a model for brine density that takes into account the effects of NaCl and CaCl2. Simulations show that in forced convection, the increased pore velocity and thermal retardation in low-porosity regions enhances brine transport relative to heat transport. In thermohaline convection, the heat and brine transport are strongly coupled and enhanced transport of brine over heat cannot occur because buoyancy caused by heat and brine together drive the flow. Random permeability heterogeneity has a limited effect if the scale of flow is much larger than the scale of permeability heterogeneity. For the system studied here, layered thermohaline convection persists for more than one million years for a variety of initial conditions. Our simulations suggest that layered thermohaline convection is possible in hypersaline geothermal systems provided the vertical permeability is smaller than the horizontal permeability, as is likely in sedimentary basins such as the Salton Trough. Layered thermohaline convection can explain many of the observations made at the Salton Sea Geothermal System over the years.     

Stability Analysis of a Porous Layer Heated from Below and Subjected to Low Frequency Vibration: Frozen Time Analysis

We investigate the convection amplitude in an infinite porous layer subjected to a vibration body force that is collinear with the gravitational acceleration and heated from below. The analysis focuses on the specific case of low frequency vibration where the frozen time approximation is used. The results reveal that for moderate Vadasz numbers, increasing the magnitude of the acceleration stabilizes the convection. The results of the large Vadasz number analysis reveals that the acceleration plays a passive role in the stability of convection and the classical stability criteria for Rayleigh–Benard convection applies.     

Linear stability analysis of convection in two-layer system with an evaporating vapor-liquid interface

Classical theories have successfully provided an explanation for convection in a liquid layer heated from below without evaporation. However, these theories are inadequate to account for the convective instabilities in an evaporating liquid layer, especially in the case when it is cooled from below. In the present paper, we study the onset of Marangoni convection in a liquid layer being overlain by a vapor layer. A new two-sided model is put forward instead of the one-sided model in previous studies. Marangoni-Bénard instabilities in evaporating liquid thin layers are investigated with a linear instability analysis. We define a new evaporation Biot number, which is different from that in previous studies and discuss the influences of reference evaporating velocity and evaporation Biot number on the vapor-liquid system. At the end, we explain why the instability occurs even when an evaporating liquid layer is cooled from below. The project supported by the National Natural Science Foundation of China (10372105) and the Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-SW-L05 and KGCX-SW-409) The English text was polished by Keren Wang.