共查询到19条相似文献,搜索用时 62 毫秒
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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. 相似文献
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针对疏水功能材料在流动减阻方面的应用, 选取典型不同粗糙度、不同疏水性的功能涂层表面, 通过新型环带实验研究了其阻力特性, 并获得了相应的扭矩和减阻率曲线. 实验采用测量圆盘带动环带旋转时的扭矩的方法间接计算环带表面所受的摩阻, 突破了传统微管道实验在尺度上的限制, 避免了水洞实验中影响因素过多的弊端, 对疏水材料的宏观应用有着重要意义. 实验证实了在宏观尺度下疏水涂层在低雷诺数时的减阻作用; 但在高雷诺数时, 减阻作用减弱, 甚至部分涂层有增阻作用, 而压差阻力的迅速增大是造成增阻的主要原因. 通过对比分析认为: 低雷诺数时, 疏水特性对于减阻效果影响更大; 而高雷诺数时, 粗糙度起更大作用, 甚至可能起到增阻的反效果.
关键词:
疏水表面
环带实验
粗糙度
减阻 相似文献
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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. 相似文献
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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. 相似文献
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了解弹跳液滴在固体表面的动力学和传热特性对于促进自清洁、防冰和热管理等领域的发展是至关重要的。本文通过数值模拟的手段研究了冷液滴撞击热超疏水表面的过程。本文中提出了一种混合相场格子玻尔兹曼模型来描述液滴动力学和液固传热过程,并通过单个液滴在固体表面的铺展过程和定量冷却效率验证了所提出模型的正确性。随后对单个液滴碰撞超疏水表面的动力学行为和换热过程进行了模拟,结果表明热流密度不仅与液固接触面积有关,还与液滴与基体之间的温差有关。此外增大液滴的韦伯数对换热过程有促进作用,而增大液滴的雷诺数反而会抑制换热。 相似文献
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超疏水表面在水下的减阻效果随着来流冲刷时间的增加会逐渐减小甚至会出现粗糙增阻的现象,而这种现象的本质在于超疏水表面裂隙中驻留的气相结构在来流的作用下会不断地从表面脱离.针对超疏水表面的裂隙中驻留的气相结构在水下不稳定的情况.本文通过对表面微结构的设计,利用疏水性展向微沟槽结构使驻留在沟槽内部的气相结构被相邻沟槽间的脊状结构挡住,从而不能轻易的被水流冲刷掉.实验结果表明该表面不仅能使气相结构在表面微结构内稳定驻留,而且基于稳定驻留在表面结构内的气相结构,在来流作用下会有新的气相结构生成.虽然表面上不稳定的气相结构会随流速的增加而加剧地脱离表面,但是可再生的气相结构能够补充由于冲刷从表面脱离的气体.最终在固/液界面间构建相对稳定的气模.通过粒子图像测速系统(PIV)对近壁面流场进行分析,可得到大于15%的速度滑移量. 相似文献
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液滴的快速脱落和移除对蒸汽滴状冷凝传热具有重要的影响,超疏水表面由丁二具有接触角大,接触角滞后小的优点而用于驱动冷凝液滴的自发运动,但是,常压蒸汽在超疏水表面冷凝时,液滴的润湿形态还没有定论。本文设计了超疏水疏水条纹间隔排列的超疏水一疏水组合表面,研究了常压蒸汽在组合表面上的冷凝过程,观测了液滴的运动特性,测量了超疏水一疏水组合表面上常压蒸汽冷凝传热性能。实验结果显示疏水区液滴在表面张力差的作用下从疏水区向超疏水区自发迁移,说明超疏水区液滴处于Wenzel润湿形态,超疏水一疏水组合表面蒸汽冷凝传热性能比完全超疏水和完全疏水表面传热性能的面积加权平均值大。说明液滴的自发迁移运动强化了疏水区的传热性能。 相似文献
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通过化学气相沉积方法获得了具有良好超疏水特性的微纳跨尺度结构ZnO表面, 其表面接触角为150.7°.扫描电镜(SEM)的测试结果表明, 样品结构为ZnO微米柱阵列和在上面交织生长的高密度ZnO针状纳米线的复合结构.通过流变仪, 采用分步流动模式对样品表面在不同的剪切速率和不同间距的情况下进行测量, 得到了扭矩与剪切速率之间的关系.进一步选择覆盖硅烷的光滑Si表面作为对比样品, 选用40%的甘油作为试验液体, 当剪切速率接近20 s-1时, 测试的表面滑移长度为46.8 μm.这表明微纳跨尺度结构的ZnO表面可有效增加流体减阻特性, 有利于制备具有减阻效应的微器件. 相似文献
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An opposition control scheme with strengthened control input is proposed and tested in turbulent channel flows at friction Reynolds number Reτ = 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 Reτ = 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. 相似文献
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Marcel Sperling Vivian J. Spiering Orlin D. Velev Michael Gradzielski 《Particle & Particle Systems Characterization》2017,34(1)
This article presents a study on the formation of anisometric, ellipsoidal supraparticles by evaporation‐induced self‐assembly from multicomponent colloidal dispersion droplets deposited on a superhydrophobic surface. Performing the formation process on bent surface substrates grants precise control on the shape and spatial orientation of the final dried supraparticles. Due to the V‐shaped surfaces providing interfacial blockage, anisotropic evaporation rates occur with respect to the direction of the bending channel. This proportionally leads to inhomogeneous accumulation of fumed silica (FS), used as structure guiding component. Thus, upon the increase of FS‐particle interaction via ionic strength (NaCl), this so‐formed shell provides enough anisotropic stiffness resulting in predictable droplet deformation with the elongation orientation being perpendicular to the bending axis. The anisotropic evaporation rates were monitored and quantified using an established, empiric kinetic model and taking into account surface geometry. Employing this reliable control of elongation direction and using additional Fe3O4@SiO2 core–shell nanoparticles, anisometric magnetic Janus supraparticles with defined patch position were prepared, which are not accessible on flat surfaces. The results can find application in the controlled, easy to scale up, nanofabrication process of patchy anisometric supraparticles. 相似文献
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Turbulent control and drag reduction in a channel flow via a bidirectional traveling wave induced by spanwise oscillating Lorentz force have been investigated in the paper. The results based on the direct numerical simulation (DNS) indicate that the bidirectional wavy Lorentz force with appropriate control parameters can result in a regular decline of near-wall streaks and vortex structures with respect to the flow direction, leading to the effective suppression of turbulence generation and significant reduction in skin-friction drag. In addition, experiments are carried out in a water tunnel via electro-magnetic (EM) actuators designed to produce the bidirectional traveling wave excitation as described in calculations. As a result, the actual substantial drag reduction is realized successfully in these experiments. 相似文献