低Pr数流体自然对流边界层流动的直接数值模拟

在太阳能采暖通风系统中,空气在太阳辐射作用下于吸热壁上所形成的非稳态自然对流边界 层流动是决定系统热性能的关键所在. 对于线性分层的低普郎特数($Pr<1$)流体而言,标度分 析表明在起始阶段边界层的发展是与高度无关而只依赖于时间的,而处于稳态时,各项标度 关系与处于起始阶段终结时刻的大不相同,且都与高度和时间无关,而只与$Pr$数和线性分 层度相关. 直接数值模拟计算结果表明由标度分析所得的各项标度关系揭示了流动的特性参 数与控制参数之间的决定性内在联系. 但同时也表明标度关系并没有充分体现出 对$Pr$数的所有依赖关系,这一不足可利用直接数值模拟计算结果得到弥补.     

非稳态Couette流动阶段的边界位移与黏性能量耗散

本文研究了牛顿流体非稳态Couette流动阶段的边界位移与流体黏性摩擦能耗.在上边界始终保持静止,下边界以恒定速度或恒定内壁剪切应力突然运动的情况下,建立了Couette非稳态流动的数学模型.求解定解问题,获得流体速度分布函数;通过渐近分析,获得不同边界条件下流动充分发展的临界时间,进而获得非稳态过程中的边界位移量.利用边界剪切应力和位移量的结果,计算非稳态过程中移动平板做功;再结合流体动能增加量,计算得出不同边界条件下非稳态流动阶段流体黏性摩擦能耗.     

壁湍流相干结构尺度及边界层内的SL标度律

利用高分辨率、高帧率PIV系统对湍流边界层中相干结构的多种空间尺度和边界层内SL 标度律在不同尺度下的具体表达形式进行了实验研究. 实验在两个动量损失厚度雷诺数 (Re_{\theta}=628.5和Re_{\theta}=1032.9)下测量平板湍流边界层中缓冲 层、对数区和外区的二维瞬时速度场. 应用 小波分析以及传统的统计学方法，在垂直于平板和平行于平板的平面内考察平板湍流边界层 中存在的相干结构的流向和展向尺度，并与已知的相干结构尺度实验结果进行了对比分析. 利用在动量损失厚度雷诺数628.5下测得的数据，对多种脉动结构(脉动速度结构等) 的空间关系及其标度律进行了研究. 第2项工作直接利用湍流边界层空间速度分布，对多种 流场尺度结构内部的She-Leveque(简称SL)标度律及自相似律进行了验证. 结果表明，各 单一流场尺度结构内部，流向脉动速度{\pmb u}'、法向脉动速度{\pmb v}'及 脉动涡分量\d {\pmb v}'/\d {\pmb x}的统计结构量均存在明显的标度律，标度 指数的形式与自相似律和SL标度律均非常吻合，只是常数随流场尺度的不同而不同， 且呈现一定的规律性. 但对于结构量的五阶矩随距离l的研究表明，自相似律和SL 标度律成立的范围并不完全一致，同时标度律成立的范围大小与流场尺度有明显关系.     

壁湍流边界层奇异标度律的实验研究

采用热线风速仪对平板湍流边界层的流向速度进行测量,用速度结构函数研究不同尺度结构标度律的变化规律,结果显示小尺度区的概率密度曲线尾部明显偏离高斯型,说明高幅值间歇性事件占的份额较多;惯性子区的曲线向高斯型靠近,间歇性事件所占份额减少;大尺度结构的曲线趋于高斯型,间歇性事件所占份额最小。在耗散区、惯性子区和较大的尺度结构区存在大小不同的绝对标度指数,越靠近壁面这些区域的标度指数均越偏离p/3而逐渐变小。绝对标度指数与边界层位置有关,在缓冲层各阶标度指数与线性标度律偏差很大,显示较强的奇异性,当过渡到对数层及外区,标度指数逐渐增大,接近均匀各向同性湍流的状态。缓冲层、对数层及外区具有各异的绝对标度指数增长率,与各层的不同湍流结构特征和运动形式有关。     

超声速边界层/混合层组合流动的稳定性分析

利用可压缩线性稳定性理论研究了超声速混合层考虑壁面影响流动时的失稳特性. 基本流场选取了具有不同速度特征的2 股均匀来流,进入存在上下壁面的流道中. 混合层与边界层的距离为1~3 个边界层厚度,其中壁面取为绝热壁. 分析了该流动在超声速情况下的稳定性特征,同时还讨论了不同波角下的三维扰动波的演化特点,并与二维扰动波进行了比较和分析. 研究结果表明,在此流动情况下,边界层流动和混合层流动的稳定性特征同时存在,并互有影响,其流动稳定性特征既有别于单纯的平板边界层,也有别于单纯的平面混合层,呈现出了新的稳定性特征.      

超声速边界层/混合层组合流动的稳定性分析简

利用可压缩线性稳定性理论研究了超声速混合层考虑壁面影响流动时的失稳特性. 基本流场选取了具有不同速度特征的2 股均匀来流,进入存在上下壁面的流道中. 混合层与边界层的距离为1~3 个边界层厚度,其中壁面取为绝热壁. 分析了该流动在超声速情况下的稳定性特征,同时还讨论了不同波角下的三维扰动波的演化特点,并与二维扰动波进行了比较和分析. 研究结果表明,在此流动情况下,边界层流动和混合层流动的稳定性特征同时存在,并互有影响,其流动稳定性特征既有别于单纯的平板边界层,也有别于单纯的平面混合层,呈现出了新的稳定性特征.     

液桥中不同Prandtl数流体的热毛细对流

本文研究从上面加热的液桥中不同 pr 数流体的热表面张力驱动对流。在 Ma 数相同的条件下,不同 pr 数流体液桥中的温度分布和流场结构定性相似,但定量结果不同。小pr数(pr<<1)流体液桥中的粘性边界层远小于热边界层,最大流函数所在位置向冷端偏移,有较大的流动速度。结果表明,Ma 数是描述这种流动的敏感参数。     

环形浅液池内热流体波的本质特征

为了了解环形浅液池内热毛细对流的特征,利用有限容积法对外壁受热、内壁冷却、厚度为1 mm的环形浅液池内硅油的热毛细对流进行了三维数值模拟.结果表明,当Marangoni(Ma)数小于临界值时,随着Ma数的增加,内、外壁附近的温度梯庹上升,稳定的二维轴对称流动的径向速率增加;超过临界Ma数后,漉动转化为三维振荡对流,形成热流体波.沿径向运动的同时,伴随着热流体波的传播流体质点成对地绕顺时针和逆时针方向旋转.热流体波的周向传播速度较快,而流体质点的周向速度很小.分析发现,热流体波为对数螺线形波纹,其传播角为常数;随着Ma数的增大,传播角增大.     

湍流能量耗散率标度律实验研究

应用粒子图像测速(PIV)系统对平板湍流边界层内流向和法向的瞬时速度进行了测量。湍流的能量耗散率由轴对称假设得到,同时在研究湍流动能耗散率标度律的过程中采用传统的统计学方法。实验结果显示,对于不同尺度上和不同法向位置湍流耗散率标度律来说,湍流耗散主要发生在小尺度上,也就是说湍动能耗散率标度律在小尺度上具有普适性。另外,根据层次结构理论假设,通过PIV实验数据对最高激发态的标度指数进行了研究,结果发现,最高激发态存在绝对标度指数,并且绝对标度律是由信号中最强耗散涨落的局部结构产生的。     

竖直平板间自然对流的湍流频谱特性

根据直接数值模拟的计算结果，对竖直平板间湍流自然对流的脉动动能、速度及温度等物理量的时间序列进行频谱分析．结果表明，流动达到充分发展状态后，小尺度到大尺度的能谱很宽，计算的分辨率足够．从能谱分布可以观察到含能区、惯性子区和耗散区的存在，文中对各区的特性进行分析．由于该流动的强各向异性，惯性子区很窄．并讨论了法向位置对脉动动能的影响以及大尺度结构的特性．     

金属丝壁面加热湍流标度律的实验研究

通过在固壁表面的平板湍流边界层沿流向平行放置若干通电加热的金属细丝,在平板表面形成沿展向周期性分布的温度场,利用该温度场引起的空气热对流,在湍流边界层近壁区域产生一组沿湍流边界层展向周期分布的流向涡结构。对壁湍流小尺度结构标度律统计特性的研究表明,金属丝加热后形成的规则流向涡结构将壁湍流各种尺度湍涡结构不规则的脉动有序地组织起来,增强了湍流小尺度结构的层次结构相似性,减小了壁湍流中小尺度结构的间歇性和奇异性,抑制了壁湍流中奇异的湍涡结构。     

Thermal Capacity Effect on Transient Free Convection Adjacent to a Vertical Surface in a Porous Medium

In this paper we analyse how the presence of the thermal capacity of a vertical flat plate of finite thickness, which is embedded in a porous medium affects the transient free convection boundary-layer flow. At the time t = 0, the plate is suddenly loaded internally with a constant heat flux rate q, so that a transient boundary-layer flow is initiated adjacent to the plate. Initially, the transient effects due to the imposition of the uniform heat flux rate at the plate are confined to a thin fluid region near to the surface and are described by a small time solution. These effects continue to penetrate outwards and eventually evolve into a new steady state flow. Analytical solutions have been derived for these transient (small time) and steady state (large time) flow regimes, which are then matched by a numerical solution of the full boundary-layer equations. It has been found that the non-dimensional fluid temperature (or fluid velocity) profiles are reduced when the thermal capacity effects, described by a parameter Q ^*, are reduced. For small values of Q ^*, the approach of these profiles to their steady state values is monotonic. However, for large values of Q ^*, the temperature profiles are observed to locally exceed (pass through a maximum value) the final steady state values at certain distances from the plate. In general, the maxima in the temperature profiles increase in size as Q ^* increases and the time taken to approach the steady state solutions increases significantly.     

Natural Convection Induced by a Heated Vertical Plate Embedded in a Porous Medium with Transpiration: Local Thermal Non-equilibrium Similarity Solutions

This paper is concerned with the thermal non-equilibrium free convection boundary layer, which is induced by a vertical heated plate embedded in a saturated porous medium. The effect of suction or injection on the free convection boundary layer is also studied. The plate is assumed to have a linear temperature distribution, which yields a boundary layer of constant thickness. On assuming Darcy flow, similarity solutions are obtained for governing the steady laminar boundary layer equations. The reduced Nusselt numbers for both the solid and fluid phases are calculated for a wide range of parameters, and compared with asymptotic analyses.     

Turbulent thermal convection in wide horizontal fluid layers

Turbulence in thermal convection is investigated for flows in which the production of turbulence energy is due solely to buoyancy, and the statistics of the flow are homogeneous in horizontal planes. New experimental results for high Rayleigh number unsteady turbulent convection in a horizontal layer heated from below and insulated from above are presented and compared to turbulent Rayleigh convection, convection in the planetary boundary layer, and laboratory penetrative convection. Mean temperature fields are correlated in terms of wall layer scales and convection scales. Joint statistics of turbulent temperature and horizontal velocity and vertical velocity through fourth order are presented for the core region of the convection layer.This paper was presented at the Ninth Symposium on Turbulence, University of Missouri-Rolla, October 1–3, 1984     

Thermal radiation effect on non-Darcy natural convection with lateral mass transfer

 A boundary layer analysis has been presented to study the influence of thermal radiation and lateral mass flux on non-Darcy natural convection over a vertical flat plate in a fluid saturated porous medium. Forchheimer extension is considered in the flow equations, and the Rosseland approximation is used to describe the radiative heat flux in the energy equation. Similarity solution for the transformed governing equations is obtained and the combined effect of thermal radiation and fluid suction/injection on the heat transfer rate is discussed. Numerical results for the details of the velocity and temperature profiles as well as Nusselt number have been presented. Received on 7 July 1999     

EXPERIMENTAL STUDY OF MEASUREMENT FOR DISSIPATION RATE SCALING EXPONENT IN HEATED WALL TURBULENCE

Experimental investigations have been devoted to the study of scaling law of coarse-grained dissipation rate structure function for velocity and temperature fluctuation of non-isotropic and inhomogeneous turbulent flows at moderate Reynolds number. Much attention has been paid to the case of turbulent boundary layer, which is typically the non-istropic and inhomogeneous trubulence because of the dynamically important existence of organized coherent structure burst process in the near wall region . Longitudinal velocity and temperature have been measured at different vertical positions in turbulent boundary layer over a heated and unheated flat plate in a wind tunnel using hot wire anemometer. The influence of non-isotropy and inhomogeneity and heating the wall on the scaling law of the dissipation rate structure function is studied because of the existence of organized coherent structure burst process in the near wall region . The scaling law of coarse-grained dissipation rate structure function is foun     

Heat transfer enhancement for laminar natural convection along a vertical plate due to sub-millimeter-bubble injection

Sub-millimeter-bubble injection is one of the most promising techniques for enhancing heat transfer for the laminar natural convection of liquids. However, flow and heat transfer characteristics for laminar natural convection of water with sub-millimeter bubbles have not yet been fully understood. The purpose of this study is to experimentally clarify the effects of sub-millimeter-bubble injection on the laminar natural convection of water along a heated vertical plate. The use of thermocouples and a particle tracking velocimetry (PTV) technique are applied to temperature and velocity measurements, respectively. The temperature measurement shows that the ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection increases with an increase in the bubble flow rate or a decrease in the wall heat flux and that the ratio ranges from 1.35 to 1.85. Moreover, it is concluded from simultaneous measurement of temperature and velocity that the heat transfer enhancement is directly affected by flow modification due to bubbles rising near the heated vertical plate.     

The Opposing Effect of Viscous Dissipation Allows for a Parallel Free Convection Boundary-Layer Flow Along a Cold Vertical Flat Plate

External free convection boundary-layer flows are usually treated by neglecting the effect of viscous dissipation. This assumption always results in a non-parallel flow, besides a strong parallel component also a weak transversal component of the (steady) velocity field occurs. The present paper shows, however, that the weak opposing effect of the buoyancy forces due to heat release by viscous dissipation, can give rise along a cold vertical plate adjacent to a fluid saturated porous medium to a strictly parallel steady free convection flow. This boundary-layer flow is described by an algebraically decaying exact analytical solution of the basic balance equations.