Key structure in laminar-turbulent transition of boundary layer with streaky structures

A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place.However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that,vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.

THE NEW SOLUTION FOR THE AXIALLY SYMMETRICAL LAMINAR BOUNDARY LAYER EQUATIONS BETWEEN TWO PARALLEL SPHERICAL SURFACES

The transformations, which are similar to Mangler's transformations, are given in this paper. They change the entrance region flow of axially symmetrical laminar boundary layer between two parallel spherical surfaces into the flow of two-dimensional boundary layer, and simplify the problems. The simplified equations can be solved by the two-dimensional boundary layer theory and numerical methods. Therefore, a new way is opened up to solve the diffusive laminar flow in the entrance region between two parallel spherical surfaces.

AN APPROXIMATE ANALYTICAL SOLUTION OF THE LAMINAR BOUNDARY LAYER EQUATIONS

Using the pressure gradient as the new variable instead of. the ordinary longitudinal coordinate x, Liu transformed the ordinary laminar boundary equations into a new form. On this base Liu obtained the frictional stress factor by using the graphical method.In this paper the same variable replacement as in [1] is used and an approximate analytical solution of the laminar boundary layer equations by the series method is obtained. The author also obtains a formula of frictional stress factor. For the case of the main function without the term of constant, the author makes a further simplification. The error of the frictional stress factor obtained by the author is still less than 10%, compared with that of [1].

Entropy Generation through the Interaction of Laminar Boundary-Layer Flows: Sensitivity to Initial Conditions

A modified form of the Townsend equations for the fluctuating velocity wave vectors is applied to the interaction of a longitudinal vortex with a laminar boundary-layer flow. These three-dimensional equations are cast into a Lorenz-format system of equations for the spectral velocity component solutions. Tsallis-form empirical entropic indices are obtained from the solutions of the modified Lorenz equations. These solutions are sensitive to the initial conditions applied to the time-dependent coupled, non-linear differential equations for the spectral velocity components. Eighteen sets of initial conditions for these solutions are examined. The empirical entropic indices yield corresponding intermittency exponents which then yield the entropy generation rates for each set of initial conditions. The flow environment consists of the flow of hydrogen gas with impurities at a given temperature and pressure in the interaction of a longitudinal vortex with a laminar boundary layer flow. Results are presented that indicate a strong correlation of predicted entropy generation rates and the corresponding applied initial conditions. These initial conditions may be ascribed to the turbulence levels in the boundary layer, thus indicating a source for the subsequent entropy generation rates by the interactive instabilities.

Similarity solutions of Prandtl mixing length modelled two dimensional turbulent boundary layer equations

The exact similarity solutions of two dimensional laminar boundary layer were obtained by Blasius in 1908,however,for two dimensional turbulent boundary layers,no Blasius type similarity solutions(special exact solutions)have ever been found.In the light of Blasius’pioneer works,we extend Blasius similarity transformation to the two dimensional turbulent boundary layers,and for a special case of flow modelled by Prandtl mixing-length,we successfully transform the two dimensional turbulent boundary layers partial differential equations into a single ordinary differential equation.The ordinary differential equation is numerically solved and some useful quantities are produced.For numerical calculations,a complete Maple code is provided.

Mathematical Formulation of Bubble Formation after Compressible Boundary Layer Separation: Preliminary Numerical Results

Laminar boundary layer (BL), under adverse pressure gradient, can separate. The separated shear layer reattaches to form a laminar separation bubble. Such bubbles are usually observed on gas turbine blades, on low Reynolds number wings and close to the leading edges of airfoils. Presence of bubbles has a weakening effect on the performance of a fluid device. The understanding of the prevailing mechanism of the separation bubble and ways to control it are essential for the efficient design of these devices. This is due to the significance of drag reduction in these various aerodynamic devices, such as gas turbines, re-entry space vehicles and airfoils. This study introduces a two-dimensional mathematical formulation of bubble formation after flow separation. The laminar BL equations with appropriate boundary conditions are dimensionalized using the Falkner-Skan transformation. Additionally, using the Keller-box method, the nonlinear system of partial differential equations (PDEs) is numerically solved. This study presents preliminary numerical results of bubble formation in low Mach numbers. These results reveal that after separation, a laminar bubble is formed in all studied cases, for Mach numbers, M = 0.2, 0.33 and 1.0. The flow after separation reverses close to the wall and finally reattaches downstream, in a new location. As the Mach number increases, this effect is more intense. After reattachment, the BL is again established in a lower energy level and the velocity field is substantially reduced, for all cases.

Linear Instability of the Supersonic Boundary Layer on a Compliant Surface

In the paper the influence of flexible covering properties on the linear development of disturbances in a supersonic boundary layer is investigated for Mach numbers M = 1.0, 2.0, 5.3, 6.0. As a model of a covering the porous plate closed by a flexible film is used. In the absence of gas in pores it is established that the flexible covering stabilizes boundary layer in the area of large Reynolds numbers and destabilizes it at small Reynolds numbers. Joint influence of the thickness and tension of a film leads to an appearance of additional unstable waves. For filled with gas pores the researches are conducted as taking into account losses of energy of disturbances in pores and in their absence. Calculations without power losses indicate possibility of existence of an absolute instability of the boundary layer on the flexible surface. The damping properties of a flexible covering connected with power losses in pores reduce their stabilizing role.

A Review of the Attachment Line Instability for Hybrid Laminar Flow Control

This paper focuses on the investigation of the attachment line instability for Hybrid Laminar Flow Control(HLFC),one of the most promising drag reduction technologies for modern transport aircraft respect to high Reynolds numbers and large sweep angles.The attachment line instability also plays an important role during laminar-turbulent transition control and HLFC design on a swept wing.The overview of historical research is presented and knowledge gaps are pointed out as the conclusion.

EXPERIMENTAL INVESTIGATION ON THE TURBULENT COHERENT STRUCTURES OF LAMINAR SEPARATION FLOW OVER A BACKWARD FACING STEP

The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.

Visualization of a Laminar Necklace Vortex System in Front of a Vertical Rectangular Plate on a Ground Wall

Various properties of a necklace vortex system formed around a rectangular plate standing vertically on a flat ground wall were investigated by visualizations produced by injecting fluorescent dye into a water channel flow. As a result, it was found that the necklace vortex pattern had three steady systems depending on the Reynolds number Reh, the relative height h/δ of the rectangular plate compared with the laminar boundary layer thickness and the aspect ratio w/h. As it is expected that the aspect ratio of the rectangular plate will typify the projected area configuration of various three-dimensional bodies, the aspect ratio is varied widely from 0.5 to 7.0. The transitional boundaries of Reh and h/δ in the 2-vortex, 4-vortex and 6-vortex systems for each aspect ratio decreased when w/h < 3.0, and increased when w/h ≥ 3.0 as w/h increased. The x-direction length of the main vortex position XV1/h was almost constant when w/h < 3.0, and decreased when w/h ≥ 3.0 as Reh increased. Then, the separation length r1/h increased when w/h < 3.0 and Reh < 1000, and became almost constant when w/h ≥ 3.0 and Reh ≥ 1000 as w/h and Reh increased. Moreover, the increase of r1/h is notably rapid when w/h < 3.0.

Simulation of Random Waves and Associated Laminar Bottom Shear Stresses

This work presents a new approach for simulating the random waves in viscous fluids and the associated bottom shear stresses. By generating the incident random waves in a numerical wave flume and solving the unsteady two-dimensional Navier-Stokes equations and the fully nonlinear free surface boundaiy conditions for the fluid flows in the flume, the viscous flows and laminar bottom shear stresses induced by random waves axe determined. The deterministic spectral amplitude method implemented by use of the fast Fourier transform algorithm was adopted to generate the incident random waves. The accuracy of the numerical scheme is confirmed by comparing the predicted wave spectrum with the target spectrum and by comparing the nanlerical transfer function between the shear stress and the surface elevation with the theoretical transfer function. The maximum bottom shear stress caused by random waves, computed by this wave model, is compared with that obtained by Myrhaug＇ s model （1995）. The transfer function method is also employed to determine the maximum shear stress, and is proved accurate.

管内充分发展流动与传热数值模拟的教学方法探讨

由于缺乏数值计算的基础理论知识,工科本科生在学习流体流动与传热过程的数值模拟方法并进行程序设计时,往往觉得难度较大。本文围绕圆管内不可压缩流体充分发展流动与传热,采用边界层积分法,推导出无量纲控制方程组;针对层流工况和湍流工况,开发出相应的数值方法。基于该方法的程序代码易于理解,且计算结果表明该方法具有预测精度高、收敛速度快的优点。本文的工作可以为计算流体力学、数值传热学及热工计算等系列课程的本科教学提供参考。

成层地基振动台试验双向剪切箱设计与边界效应测评

为满足散粒体材料(如土体)在多向振动台试验中截断边界模拟的需求,设计并制作了一种刚度可调节的层状双向剪切箱。剪切箱由矩形钢管框架堆叠而成,层间设置万向轴承与滚珠,可实现更加真实的双向协调运动;刚度调节装置与复位装置保证箱体复位、运动一致性和整体性,能够满足层间剪切变形。利用所设计的双向层状剪切箱进行上软下硬成层地基的振动台试验,根据边界效应指数与2-范数对边界效应进行定量分析。结果表明,研制的双向剪切箱性能良好,能够正确模拟成层地基场地的地震动特性,边界效应得到有效控制。

火星超低雷诺数下螺旋桨气动性能研究

火星超低雷诺数环境导致螺旋桨系统气动特性相比于地球环境显著恶化,翼型表面层流分离现象影响了火星螺旋桨的气动特性.为设计出适应火星低雷诺数环境的螺旋桨,深入了解超低雷诺数对翼型气动特性的作用机理,采用Transition SST转捩模型求解非定常可压缩N-S方程的数值模拟技术,对几种适用于低雷诺数环境的翼型进行火星超低雷诺数环境下气动特性模拟计算,从而选取最适应火星大气环境的翼型.结果表明CLF5605翼型具有更好的气动性能.对选定的翼型进行不同超低雷诺数下气动特性模拟计算,揭示了超低雷诺数对翼型气动特性的影响,即在火星大气雷诺数范围内,更高的雷诺数对应更好的气动性能;对雷诺数从100~20000翼型表面边界层状态进行数值模拟,发现翼型层流分离结构发生显著变化,从低雷诺数下的层流边界层状态,随着雷诺数的增加开始发生层流分离,在翼型尾缘产生层流分离泡,并最终变成湍流边界层.采用最小能量损失的方法设计火星螺旋桨,对其气动性能进行了数值模拟仿真,并对轻量化制造的螺旋桨进行了地面台架实验验证,通过将地面实验结果与CFD数值模拟仿真结果对比,验证了螺旋桨轻量化设计合理性以及数值计算的准确性.

Prediction of Boundary Layer Transition Based on Modeling of Laminar Fluctuations Using RANS Approach

This article presents a linear eddy-viscosity turbulence model for predicting bypass and natural transition in boundary layers by using Reynolds-averaged Navier-Stokes (RANS) equations. The model includes three transport equations, separately, to compute laminar kinetic energy, turbulent kinetic energy, and dissipation rate in a flow field. It needs neither correlations of intermittency factors nor knowledge of the transition onset. Two transition tests are carried out: flat plate boundary layer under zero ...

γ-Re_θ转捩模型的标定研究

介绍了γ-Reθ转捩模型,补充了相应的关联函数。基于平板的试验数据在中国空气动力研究与发展中心(CARDC)的trip软件平台上对该关联函数进行了标定,并研究了关联函数的变化对转捩结果的影响规律。通过修正与SST湍流模型的结合,改进了转捩位置后壁面摩擦阻力系数偏低的状况。

不同截面形状微肋片内流动阻力特性

以去离子水为工质,实验研究了横截面形状为圆形、椭圆及菱形的叉排微肋片组成的流道内的流动阻力特性。结果表明,3种形状肋片流道内压降随流量增大而增大。受微/小尺度下层流边界层的影响,流量较低时微肋形状对流动影响较弱,椭圆形肋片与菱形肋片内压降几乎相同,而圆形截面肋片内由于流动距离较长导致流动阻力最大;当流量较大时,椭圆形肋片内压降最小。Re较小时,长短轴一致的菱形肋片内的流动阻力系数f比椭圆形肋片略低,但在Re较高时,椭圆形肋片内的f值仍然最低。研究还表明,在现有各种关联式中,只有菱形微肋片关联计算值与本实验值较吻合。

温室作物叶-气系统水流阻力研究初探

温室作物叶-气系统水流阻力的确定对计算作物蒸腾量、制定温室作物灌溉制度以及调控温室小气候具有重要意义,但测量难度较大。该文通过对温室作物周围环境微气象条件的连续观测,计算分析了温室作物叶-气系统水流阻力各分项即叶片周围层流副层边界层阻力(rb)、冠层上方湍流边界层阻力(rg)、空气动力学阻力(re)和叶片气孔阻力(ri)的变化规律。结果表明:温室内rb比较稳定,平均约235s/m,且与环境因素关系不甚密切;温室内黄瓜、西红柿类植物生殖生长期rg<<rb,在计算re时,rg的影响可忽略,取re≈rb;利用基于能量平衡方程和空气动力学方程得出的叶-气温差计算公式计算得到ri,符合其变化的一般规律;在此基础上计算了温室黄瓜的蒸腾速率,与实测值的一致性较好。

多孔表面抑制第二模态失稳的最优开孔率和孔半径分析

多孔介质、微槽道和超声波吸声材料等可用于抑制高超声速边界层第二模态扰动波。通过声波在无穷长小管道中的传播模型给定多孔介质表面的扰动边界条件,采用时间模式的线性稳定性理论分析了多孔介质表面对边界层稳定性的影响。对马赫数6热壁平板边界层的考察表明,多孔介质表面不仅可以大大推迟第二模态扰动波的中性点,还可以大大抑制最不稳定第二模态扰动波的幅值增长率。为了找到最优控制参数,在较广的参数范围内考察了多孔介质表面的开孔率和孔半径对第二模态扰动波幅值增长率的影响,得到了最优开孔率和孔半径沿流向的分布。最后,还考察了基本流的当地非平行性(法向速度)对控制效果和最优控制参数的影响。

M6高超声速静风洞的气动设计和结构研究

论述了发展高超声速静风洞设备与技术的重要性，对静风洞的基本特点进行了简要讨论。根据国内外发展静风洞的经验和成果，针对开展静风洞的实验技术及边界层稳定性问题研究的背景，提出一座M6高超声速静风洞（M6HQWT）的气动和结构设计，重点讨论了阀门管路、稳定段和喷管抽吸装置的设计特点。M6HQWT的设计马赫数为6．0，喷管出口直径120mm，总压l一5MPa，总温约500K，运行时间20～30s。M6HQWT是国内第一座高超声速静风洞，具备实验教学和基础科学研究的能力。