脊状表面减阻特性的风洞试验研究

利用热线风速仪,对光滑表面和多个脊状表面在低速风洞中进行了表面流场测试。基于测得的边界层速度分布数据,利用对数律区速度分布公式,编程分别计算出光滑表面和脊状表面的壁面摩擦速度和虚拟原点。研究发现,脊状表面最大减阻量达13．5％;有减阻效果的脊状表面使边界层速度曲线上移、湍流强度下降;与光滑表面相比,脊状表面的位移厚度和动量损失厚度明显减小,也表明脊状表面具有减阻效果;位移厚度和动量损失厚度减少量随槽间距s^＋的增加呈现先变大后变小的趋势,在S^＋=12时达到最大。     

脊状表面减阻机理研究

针对脊状表面流场的特点,通过实验测量和数值模拟的方法对脊状表面微观流场进行了深入研究,获得了脊状表面湍流边界层的时均速度分布曲线、湍流度分布曲线和微观流场结构.为了得到脊状结构对壁面物性的影响,对脊状表面进行了疏水性测试,获得了液滴在脊状表面上的表观接触角,并通过水洞试验验证了脊状表面的减阻效果.研究表明,与光滑表面相比,脊状表面微观流场结构中存在"二次涡",近壁区的黏性底层厚度比平板的要厚得多,湍流度显著降低,且脊状表面表现出明显的疏水性.由此提出了基于壁面隔离效应、增大湍流阻尼效应和改变壁面物性效应的减阻机理.     

脊状表面航行器模型减阻特性的风洞实验研究

针对航行器提高航程和航速的需要,开展脊状表面湍流边界层减阻的实验和数值仿真研究。在航行器模型的外表面加工具有特定形状、尺寸的脊状结构,导致湍流边界层的流动稳定性增强,壁面摩擦阻力降低。在风洞中对具有光滑表面和脊状表面的航行器模型在不同风速和攻角下进行阻力测试,得到其减阻特性曲线。实验结果表明,具有横向脊状表面的航行器模型在一定来流速度范围内具有很好的减阻效果,实验获得的最大减阻量为23.5%。数值仿真结果则发现,在脊状结构内形成了稳定的"二次涡",边界层内湍动能和湍流猝发强度降低,很好地揭示了减阻机理。     

新型等离子体激励器对翼型的减阻效果

论述了在西北工业大学低湍流度风洞中采用新型等离子激励器对NACA0015翼型的减阻实验.实验风速为35m/s,攻角范围取0°~20°.并参照压力分布和总压分布实验结果对减阻效果进行了对比分析.本文还进行了有关等离子体激励抑制翼型流动分离的数值模拟研究,基于等离子体激励器的简化模型将体积力以源项方式引入到N-S方程中求解,得到激励器工作时的流场分布.结果表明在新型等离子体激励器开启后:在小攻角范围内,尾耙的总压分布曲线与坐标轴的纵轴(尾耙高度轴)所围面积变化不大;当攻角≥12°时,尾耙的总压分布曲线与坐标轴的纵轴(尾耙高度轴)所围面积明显减小.从而说明该新型等离子体激励器能够有效地减少翼型的阻力.     

水下减阻技术研究综述

在简要回顾早期减阻研究的基础上,对现有典型水下湍流减阻技术进行了较深入地分析.重点介绍了脊状表面减阻、微气泡减阻和疏水/超疏水表面减阻的研究现状.分别从实验研究和理论研究两方面对其进行了阐述,并着重强调了各自的减阻机理. 此外,还简要介绍了柔顺壁面减阻、壁面振动减阻等其它减阻技术.展望了水下减阻技术今后的研究重点及其应用前景.      

Spalding公式在脊状表面湍壁摩擦力测量中的应用

在低速风洞中来流速度一定的情况下使用IFA300恒温热线风速仪测量了光滑表面和两种不同尺寸的脊状表面湍流边界层平均速度分布剖面,并验证了试验段湍流发展的充分性;通过应用Spalding壁面公式使用最小二乘法精准拟合了实验测量的边界层内层速度分布曲线,得到了湍流边界层壁面摩擦速度并进一步求得湍流壁面摩擦应力,较准确地计算出脊状表面的虚拟原点位置,并通过与对数律公式拟合结果比较分析,证实了该方法更加准确有效. 最后分别计算了3种实验模型的湍流边界层动量损失厚度. 通过对比脊状表面与光滑表面动量损失厚度和壁面摩擦应力,反映了动量损失厚度的大小与壁面摩擦应力的大小具有一致性,充分证实了脊状表面在湍流中具有一定的减阻效果.      

微纳结构超疏水表面的湍流减阻机理研究

超疏水表面的优异性质使其在现代生活和工业生产中具有十分广泛的潜在应用价值. 本文采用了碳纳米管缠绕技术和聚氟硅氧烷疏水化处理方法制备了具有二级微纳米结构的超疏水表面. 测量了由该超疏水表面构建的槽道中的流动压降,将其与普通表面构建的槽道内的流动压降进行比较,发现在层流情况下,流动阻力减小最多达到了22.8%. 在湍流的情况下,超疏水表面的减阻比例约为53.3%,减阻效果比层流更加明显.利用PIV (particle image velocimetry) 技术测量了具有超疏水表面的槽道内的速度场,通过超疏水表面速度滑移和湍动脉动场信息,分析了湍流减阻效果比层流更加明显的物理机制.     

表面活性剂减阻溶液湍流流动研究进展

与聚合物添加剂相比,表面活性剂具有寿命长,不受机械力和高温影响发生降解的特点,目前被认为是最具有适用价值的减阻添加剂．针对表面活性剂湍流减阻机理,尽管已经开展了大量的实验研究、理论分析和数值模拟,但仍处于探索阶段,尚未定论．文中对目前国内外有关表面活性剂减阻溶液湍流流动的研究如平均速度、湍流强度、雷诺应力和相关系数等湍流统计量以及取得的成果进行了归纳和总结．目前普遍认为阻力减小是由流动垂直方向的湍流强度受抑和脉动速度分量解耦导致雷诺应力极大降低引起的．分析了目前研究存在的问题,对今后继续开展研究提出了自己的观点．      

壁湍流相干结构和减阻控制机理

剪切湍流中相干结构的发现是上世纪湍流研究的重大进展之一,这些大尺度的相干运动在湍流的动力学过程中起重要作用,也为湍流的控制指出了新的方向.壁湍流高摩擦阻力的产生与近壁区流动结构密切相关,基于近壁区湍流动力学过程的减阻控制方案可以有效降低湍流的摩擦阻力,但是随着雷诺数的升高, 这些控制方案的有效性逐渐降低.近年来研究发现, 在高雷诺数情况下外区存在大尺度的相干运动,这种大尺度运动对近壁区湍流和壁面摩擦阻力的产生有重要影响,为高雷诺数湍流减阻控制策略的设计提出了新的挑战.该文将对壁湍流相干结构的研究历史加以简单的回顾,重点介绍近壁区相干结构及其控制机理、近年来高雷诺数外区大尺度运动的研究进展,在此基础上提出高雷诺数减阻控制研究的关键科学问题.      

多孔翼型流场的数值模拟研究

空化和空蚀是影响水力机械能否安全、经济运行的主要因素之一.为了研究新的翼型结构——多孔翼型对于水力机械的空化及空蚀现象的影响,利用PROE建立具有不同微孔数的翼型模型,再借助Fluent软件对多孔翼型的三维绕流流场进行了定常数值模拟计算.基于伯努利定律,对具有不同孔数的翼型绕流流场进行流态分析.根据模拟结果显示的不同翼型结构的速度场和压力场的分布特征,从能量转换的角度出发,揭示了多孔翼型绕流流场的内在特性,即绕流翼型的流场(速度场、压力场)是随着孔数而变化的.结合空化发生的条件,说明采用多孔翼型结构对改善水力机械内部空化现象是有一定的作用的,通过数值模拟方法来分析研究水力机械内部的空化空蚀现象,可以作为实验与理论分析的参考.从一个新的角度,为改善水力机械的空化和空蚀现象提供了新思路.     

Effects of riblet bends on pipe flows

Four riblet bends were tested to investigate the effects of riblets on pipe flows including the secondary flow on the Reynolds numbers; Re _D =6×10³–4×10⁴. The pressure gradients on the smooth pipe downstream from the riblet bends were measured, and also the pressure losses of the bends only were measured. All riblet bends reduced the pressure gradient on the smooth pipe downstream from them, which means a drag reduction. Two of the riblet bends showed the maximum drag reduction of about 4 percent at Re _D = 6500; this reduction rate was significant considering the uncertainty of the present experiments. Since the pressure losses of these two riblet bends were almost identical to that of the smooth bend at Re _D = 6500, they could cause a net drag reduction of about 4 percent on the piping system including these bends at that Reynolds number. Furthermore, the velocity profiles measured by LDV indicated that the secondary flow becomes weaker downstream from the riblet bends when a drag reduction is recognized there.Nomenclature D pipe diameter - D ₀ the distance from the valley to the valley passing through the pipe center - H height of groove - P nondimensional static pressure (p/it/(U ₀ ² ):p is gauge pressure) - dP/dX nondimensional pressure gradient - Rc curvature of bend - Re _D Reynolds number based on bulk velocity and pipe diameter - s spacing of groove - U mean streamwise velocity along the horizontal diameter - U ₀ bulk velocity - V mean vertical velocity along the horizontal diameter - x streamwise direction along the pipe axis (see Fig. 1) - X nondimensionalx (=x/D) - y radial direction in the horizontal plane which is perpendicular to the plane including the bend (see Fig. 1) - yUV swirl intensity (nondimensional swirl intensity:yUV/(DU ₀ ² ))     

On near-wall turbulence-generating events in a turbulent boundary layer on a riblet surface

The boundary layer over a drag reducing riblet surface is investigated using hot-wire anemometry and flow visualisation. The concept of a riblet sublayer is introduced, and a definition is proposed in terms of a region of reduced turbulence energy production formed near the wall by the addition of riblets. The hot wire records are examined using a modified form of quadrant analysis, and results obtained over plain and riblet surfaces are compared. Close to the wall, the addition of riblets produces a marked reduction in the occurrence of ejection (2nd quadrant) events. A corresponding increase in the incidence of sweep (4th quadrant) events is accompanied by the development of a strong tendency toward a preferred event duration, and a preferred interval between events. These changes diminish rapidly with distance from the surface, becoming almost undetectable beyondy ⁺=40. They are discussed in the light of flow visualisation results, and interpreted in terms of mechanisms associated with the interaction between the riblets and the inner boundary layer flow structures. A conceptual model of the flow mechanisms in the riblet sublayer is proposed.     

Suppression of force fluctuations in flow past an aerofoil

Force fluctuations on a solid body are associated with unsteadiness in the wake, e.g. vortex shedding. Therefore, the control of force fluctuations can be realised by suppressing the flow unsteadiness. A NACA0024 aerofoil closed with a round trailing edge is chosen to represent the solid body throughout this investigation, with the Reynolds number fixed at Re = 1000 and angle of attack α ≤ 15^o, at which the uncontrolled flow is two-dimensional. A linear optimal control is calculated by analysing the distribution of sensitivity of unsteadiness to control around the entire surface of the body. The nonlinear effects of the calculated control, which can be actuated through surface-normal suction and blowing across the surface of the aerofoil, are tested through two-dimensional direct numerical simulations. It is observed that a surface-normal velocity control with a maximum magnitude less than 8% of the free stream velocity completely suppresses unsteadiness at α = 10° with an overall drag reduction of 14% and a 138% increase of lift.     

有效射流结构模式的数值模拟

应用LS-DYNA程序及示踪点信息后处理方法,将典型小锥角药型罩沿母线方向分为4段,研究其微元在炸药爆炸驱动下的运动规律及有效射流结构模式。射流形成并稳定后,将罩质材料按速度大小分为6段,得到各段结构组成模式。结果表明,有效射流高速段由药型罩顶部材料组成,次高速段及中速段由罩中部及中下部材料组成,靠近罩底部约0.25倍罩高范围内的材料不形成射流,有效射流段为初始材料微元的管状分层分布形结构。

     

Numerical simulation of thin airfoil stall by using a modified DES approach

The detached‐Eddy simulation (DES) method was applied to calculate pre‐ and post‐stall aerodynamic characteristics of airfoil stall. A discrepancy between numerical and experimental data was observed near the stall regime for the airfoil NACA64_A‐ 006 which is a thin airfoil stall type. The reason of this discrepancy and one possible way for improvement of the numerical model are discussed here. It is shown that the use of the Baldwin–Lomax model in the RANS region improves the DES results in this case. If the relevant factors (grid density, time step, turbulence model, etc.) are appropriately taken into account, the DES approach could reliably predict stall aerodynamical characteristics. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of anisotropic resistance characteristic on boundary-layer transitional flow

It is observed that the feather surface exhibits anisotropic resistances for the streamwise and spanwise flows. To obtain a qualitative understanding about the effect of this anisotropic resistance feature of surface on the boundary-layer transitional flow over a flat plate, a simple phenomenological model for the anisotropic resistance is established in this paper. By means of the large eddy simulation (LES) with high-order accurate finite difference method, the numerical investigations are conducted. The numerical results show that with the spanwise resistance hindering the formation of vortexes, the transition from laminar flow to turbulent flow can be delayed, and turbulence is weakened when the flow becomes fully turbulent, which leads to significant drag reduction for the plate. On the contrary, the streamwise resistance renders the flow less stable, which leads to the earlier transition and enhances turbulence in the turbulent region, causing a drag increase for the plate. Thus, it is indicated that a surface with large resistance for spanwise flow and small resistance for streamwise flow can achieve significant drag reduction. The present results highlight the anisotropic resistance characteristic near the feather surface for drag reduction, and shed a light on the study of bird’s efficient flight.     

Numerical simulation on the NACA0018 airfoil self-noise generation

Airfoil self-noise is a common phenomenon for many engineering applications. Aiming to study the underlying mechanism of airfoil self-noise at low Mach number and moderate Reynolds number flow, a numerical investigation is presented on noise generation by flow past NACA0018 airfoil. Based on a high-order accurate numerical method, both the near-field hydrodynamics and the far-field acoustics are computed simultaneously by performing direct numerical simulation. The mean flow properties agree well with the experimental measurements. The characteristics of aerodynamic noise are investigated at various angles of attack. The obtained results show that inclining the airfoil could enlarge turbulent intensity and produce larger scale of vortices. The sound radiation is mainly towards the upper and lower directions of the airfoil surface. At higher angle of attack, the tonal noise tends to disappear and the noise spectrum displays broad-band features.     

Numerical simulation and hydrodynamic visualization of transient viscous flow around an oscillating aerofoil

Unsteady viscous flow around a large-amplitude and high-frequency oscillating aerofoil is examined in this paper by numerical simulation and experimental visualization. The numerical method is based on the combination of a fourth-order Hermitian finite difference scheme for the stream function equation and a classical second-order scheme to solve the vorticity transport equation. Experiments are carried out by a traditional visualization method using solid tracers suspended in water. The comparison between numerical and experimental results is found to be satisfactory. Time evolutions of the flow structure are presented for Reynolds numbers of 3 × 10³ and 10⁴. The influence of the amplitude and frequency of the oscillating motion on the dynamic stall is analysed.     

高耸筒形结构爆破拆除的数值模拟

采用共节点分离式钢筋混凝土模型,通过比较分析塔体缺口中间有切缝与无切缝设计2种方案,对典型高耸筒形结构冷却塔爆破拆除过程进行了三维数值模拟。对混凝土和钢筋单元的受力过程进行分析,共节点分离式模型可以体现混凝土和钢筋材料的力学性能差异;采取切缝设计可减小结构倒塌过程中的下坐与后坐倾向,促使塔体后部严重扭曲变形,使塔身解体比较完全,缩短整体倒塌时间,减小爆堆范围。