表面张力对疏水微结构表面减阻的影响

通过构造具有棋盘状微结构的疏水表面,考虑表面张力的影响,利用定常与非定常结合的数值模拟方法,研究了疏水表面在湍流状态下的减阻特性以及微结构内气体封存的效果,其中Re=3000—30000.在低雷诺数下,疏水表面微结构内气体封存状态良好,减阻率最高约为30%;随着雷诺数的增大,压差阻力增大,减阻率有下降趋势.当来流速度过大时,水会大量进入微结构,疏水表面的减阻率变化剧烈,且已经不再减阻.结果表明,表面张力削弱了壁面切应力的影响,使得低雷诺数下微结构内气体能够有效封存,进而减小壁面阻力.     

不同流变性质的表面活性剂湍流减阻的直接数值模拟研究

采用直接数值模拟方法研究了不同流变性质的表面活性剂湍流减阻流动的规律.数值计算方法为有限差分法,时间项采用Adams-Bashforth格式,空间项采用二阶有界格式.讨论了粘弹性能与减阻率的关系.     

疏水表面减阻的格子Boltzmann方法数值模拟

采用格子Boltzmann方法的多松弛模型和Shan-Chen多相流模型对雷诺数为100的疏水表面方柱绕流进行了数值模拟, 分析了疏水表面接触角和来流含气率对方柱绕流流场的影响. 研究结果表明: 疏水表面接触角一定时, 来流含气率在一定范围内, 疏水表面具有减阻的能力, 超出这一范围时会出现阻力系数、升力系数升高的现象, 同时在方柱近壁面处伴随涡的形成产生了气团脱落; 当来流含气率处于适当水平时, 接触角越大, 绕流物体近壁面处含气率越稳定, 减阻效果越明显. 分析发现疏水表面减阻的关键在于保证近壁面处气层的稳定性, 此时接触角越大, 减阻效果越明显. 本文从含气率角度出发分析疏水表面的减阻现象, 为进一步探索疏水表面减阻机理提出了新的思路.     

聚合物湍流减阻流动的直接数值模拟研究

采用离散弹性元素.珠簧哑铃模型方程模拟了聚合物减阻流动.模拟出了高分子聚合物浓度的空间分布.得到了减阻率与粘弹性应力,雷诺应力等的定量关系.分析了聚合物湍流结构和离散弹性元素模型中弹性系数的关系.     

基于蚯蚓背孔射流的仿生射流表面减阻性能研究

为了减小流体对固体壁面的阻力, 基于蚯蚓生物学特征, 对蚯蚓背孔射流特性进行分析, 建立仿蚯蚓背孔射流的仿生射流表面计算模型, 采用SST k-ω 湍流模型对仿生射流表面的减阻特性进行数值模拟, 同时对数值模拟结果进行实验验证, 并以此研究了仿蚯蚓背孔射流表面的减阻机理.结果表明, 在一定条件下, 仿蚯蚓背孔射流的仿生射流表面具有较好的减阻效果; 在同一射流方向角下, 随着射流速度的增加, 减阻率逐渐增大; 在同一射流速度下, 随着射流方向角的增加, 减阻率呈先减小后增大的变化趋势; 数值模拟与实验均在射流速度为1 m·s^-1、射流方向角为-30°时达到最大, 分别为8.69%, 7.86%; 射流表面改变了原有光滑壁面的边界层结构, 对壁面边界层进行了有效的控制, 减小了壁面的剪应力, 降低了壁面边界层的速度.     

仿生射流孔形状减阻性能数值模拟及实验研究

针对横流中的侧向射流能够减小仿生射流表面摩擦阻力问题, 建立仿生射流表面模型, 利用SST k-ω湍模型对不同射流孔形状的仿生射流表面模型进行数值模拟, 并对数值模拟结果进行了实验验证. 结果表明: 当射流孔的流向长度和展向长度不变时, 3号模型的折线形射流孔减阻效果最好; 将折线形射流孔简化为圆弧形, 当r=3–5 mm时, 减阻率随着射流速度的增大而增大, 当r=4 mm时减阻效果最好, 最大减阻率为9.51%. 减阻原因: 通过射流孔向横向主流场中注入射流流体, 改变了射流表面附近边界层的流场结构, 使得边界层黏性底层厚度增加, 垂直于射流表面的法向速度梯度减小, 从而减小了壁面剪应力; 低速的射流流体被封锁在边界层内, 降低了高速流体对壁面的扫掠, 达到了减阻目的.     

疏水表面滑移流动及减阻特性的格子Boltzmann方法模拟

采用格子Boltzmann方法研究了固体壁面对流体的作用强度与其润湿性的关系,在此基础上进一步模拟了疏水表面微通道内的流体流动,获得了润湿性对疏水表面滑移流动及减阻特性的影响规律,证实了疏水表面表观滑移的存在性并揭示了其产生机理.结果表明,疏水性作用在疏水表面的近壁区诱导了一个低密度层,而表观滑移则发生在低密度层上.表观滑移是疏水表面具有减阻作用的直接原因,减阻效果随滑移长度的增大而增大.对于特定的流体系统,滑移长度是疏水表面的固有属性,仅是壁面润湿性的单一函数,而与流动本身的性质无关.     

三角肋条大涡模拟与减阻机理研究

已有实验表明三角肋条具有较好的表面减阻效果,但雷诺时均湍流模型难以模拟与揭示肋条减阻机理。本文采用大涡模拟(LES)对下壁面安装肋条的槽道流进行模拟,通过减阻率与实验值的对比以及平均速度剖面和脉动速度均方根等与直接数值模拟(DNS)结果的对比确认本文计算的准确性。根据计算的流场细节分析表明:肋条通过增加近壁区的边界层厚度,抑制流向涡的发展,减少近壁活动,使得近肋条壁面的剪切力比平板的剪切力小而获得减阻。     

煤油超燃冲压发动机三维大规模并行数值模拟

在我国巨型计算机上,采用1024个CPU对煤油燃料超燃冲压发动机燃烧流场进行三维大规模并行数值模拟.计算软件采用自主研发的并行软件AHL3D,控制方程采用雷诺平均的N-S方程,无粘项计算采用Steger-Warning矢通量分裂格式,湍流模型采用k-ω双方程模型,煤油分子式采用正癸烷代替.计算给出的发动机壁面压强分布与试验测量结果有较好的一致性.结果表明,凹槽是发动机主要的着火区和火焰稳定区,同时,由于燃料的喷注形成的回流区也起到一定的稳焰作用.计算结果验证了AHL3D程序和采用的理论模型可以用于模拟煤油燃料超燃冲压发动机内部的复杂流动.     

确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.     

八重准周期排列的短沟槽结构减阻机理分析

设计构建了一排和三排阵列的八重准周期短条纹沟槽减阻结构,以及作为对比研究的无序和周期结构,并采用雷诺Navier-Stokes方程和RANGκ-ε湍流模型,系统计算了这些结构表面的湍流边界层状态和总应力,模拟结果显示:八重准周期沟槽结构相对于周期和无序结构具有更优的减阻效应,且为三排阵列时的减阻效果明显优于单排阵列,这一结果得到了减阻实验的验证,通过分析比较不同结构的流体边界层特性发现,八重准周期结构可有效抑制附面层的涡强度,减小湍流耗散速率,保持流体条纹相的稳定性,结合二维光栅的夫琅禾费衍射波模型分析表明,八重准周期结构可减弱衍射谱在大角度方向上的谱强度,揭示出该结构抑制流体相干扰动波扩展的物理机制,并与流场分析结果相符合。     

一维周期与准周期排列沟槽结构的流体减阻特性研究

本文设计了具有相同平均沟槽密度的三种排列类型的一维沟槽结构: 密排列、周期间隔排列和两种准周期间隔排列, 并采用数值模拟和实验验证相结合的方法研究了一维沟槽结构在不同排列下的流体减阻特性. 模拟计算分析流场特征和总阻力, 发现相对于密排列和周期间隔排列的沟槽结构, 准周期间隔排列具有更好的减阻特性, 并且这一结果得到减阻实验的验证. 通过流场分布特性进一步分析沟槽结构的减阻机理. 机理分析发现高速流在经一维准周期结构的扰动波调制后形成了准周期间隔排列的速度条纹相, 这有效地抑制了大涡在流向和展向上的形成, 从而实现较大幅度的减阻. 同时对比分析沟槽排列结构调制展向涡和流向涡各自对流动减阻的贡献, 结果表明, 调制流向涡对减阻的作用更大. 关键词： 流体减阻 沟槽结构 准周期     

Strengthened opposition control for skin-friction reduction in wall-bounded turbulent flows

An opposition control scheme with strengthened control input is proposed and tested in turbulent channel flows at friction Reynolds number Re_τ = 180 by direct numerical simulations. When the detection plane is located at less than 20 wall units, the drag reduction rate can be greatly enhanced by increasing the control amplitude parameter. The maximum drag reduction rate achieved in the present study is around 33%, which is much higher than the best value of 25% reported in literature. The strengthened control can be more efficient to attain a given drag reduction rate. Based on the total shear stress at the virtual wall established between the real wall and the detection plane by the control, a new friction velocity is proposed and the corresponding coordinate transform is made. Scaled by the proposed friction velocity, the wall-normal velocity fluctuation and the Reynolds shear stress of the controlled flows are collapsed well with those of the uncontrolled flow in the new coordinate. Based on the similarity, a relation between drag reduction rate and the effectiveness of the virtual wall is deduced, which disclosed that the elevation and residual Reynolds shear stress at the virtual wall are the key parameters to determine the drag reduction rate. The conclusion are also validated at Re_τ = 395 and 590. The decrease of the drag reduction rate with the increase of the Reynolds number is attributed to the enhanced residual Reynolds shear stress at the virtual wall.     

Suboptimal control of wall turbulence with arrayed dimple actuators for drag reduction

Direct numerical simulation of a turbulent channel flow with moving dimples at the bottom wall is carried out using the pseudo-spectral method and the curvilinear coordinate system. Suboptimal control based on the spanwise wall shear stress is applied for skin-friction drag reduction, and is implemented by the finite-size sensor-actuator system. The control law is realised in physical space by using a cross-shaped truncation of the wall shear stress information, which can be measured by the sensor. Only the information of wall shear stress inside the sensor area is utilised and that outside the sensor area is obtained by a linear reconstruction from the averaged value over the sensor. To effectively intervene the near-wall coherent structures, low-pass filtering of the spanwise wall shear stress is additionally implemented to eliminate the extra disturbances induced by the presence of dimple actuators, and the filtered stress is used as the control input. Numerical tests on the present control strategy show that the pressure form drag caused by the presence of dimples is reduced significantly as compared with the original suboptimal control, and the reduction of total drag is comparable with that of the opposition control. The underlying mechanism is further analysed by looking into the interaction between the moving dimples and the near-wall coherent structures.     

Drag reduction on a patterned superhydrophobic surface

We present an experimental study of a low-Reynolds number shear flow between two surfaces, one of which has a regular grooved texture augmented with a superhydrophobic coating. The combination reduces the effective fluid-surface contact area, thereby appreciably decreasing the drag on the surface and effectively changing the macroscopic boundary condition on the surface from no slip to limited slip. We measure the force on the surface and the velocity field in the immediate vicinity on the surface (and thus the wall shear) simultaneously. The latter facilitates a direct assessment of the effective slip length associated with the drag reduction.     

具有微型沟槽结构旋转圆盘减阻的实验研究

建立了开式旋转圆盘系统减阻实验平台,对圆盘螺线沟槽减阻进行实验研究,并可以结合数值研究手段对微型沟槽减阻的机理进行研究。实验结果表明圆盘开槽面积为7.4%时,螺线微型沟槽使得旋转圆盘的最大减阻率达6.1%,V型沟槽两侧的压力差所产生的正扭矩是其减阻的主要原因。沟槽还能起到提高圆盘内径与外径处的静压差的效果。研究结果还表明,采用和平板减阻相同的无量纲尺寸的沟槽会引起圆盘表面的剪应力较大增加。     

Effect of Microbubble Gas Saturation on Near-Wall Turbulence and Drag Reduction

The article presents results of an experimental study of the effect of gravitational orientation of the flow along its lower/upper solid boundaries on reduction of turbulent drag and void fraction profiles with injection of gas through a porous channel wall. The shear stress on the wall was measured in the Reynolds number range Re_x = (0.23–1.1) × 10⁷ by floating element transducers; the void fraction profile was determined using a fiber-optic sensor. The void fraction in the inner (near-wall) region of the boundary layer was shown to be a key parameter for turbulent drag reduction. The size of the inner region depends on the gas flow rate, the fluid velocity, the distance downstream of the gas generator, and the gravitational orientation of the wall.