Experiments on drag-reducing surfaces and their optimization with an adjustable geometry
Experiments on drag-reducing surfaces and their optimization with an adjustable geometry
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DOI：
10.1017/s0022112096004673
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发表时间：
1997-05-10
影响因子：
3.7
通讯作者：
Hoppe, G
中科院分区：
文献类型：
--
作者：
Bechert, DW;Bruse, M;Hoppe, G
先前的研究已经证实，具有沿流方向排列的小肋条（纹条）的表面可以将湍流壁面剪切应力降低到光滑表面之下。典型的皮肤摩擦减少约为5%。然而，目前的调查结果表明，这一价值有相当大的改善。这种改进是通过以理论概念为指导的系统实验优化来实现的。我们实验的一个关键特点是利用了一个油通道。以前在风洞中的实验必须与非常小的肋尺寸相抗衡，通常肋侧间距约为0.5毫米或更小。相比之下，在我们的油通道中，肋的横向间距可以在2到10毫米之间。这种增加的表面结构尺寸使测试表面能够用传统的机械方法制造，它也使我们能够构建具有可调节几何形状的测试表面。此外，柏林石油通道具有新颖的剪应力平衡，精度达到前所未有的+/- 0.3%。后一个特征是系统实验优化的先决条件。在本研究中，研究了带有纵肋和附加狭缝的表面。实验涵盖了相当大的参数范围，从而最终计算出带有肋和/或狭缝的表面的减阻潜力。由于表面的可调节性以及油通道的自动操作，使大的参数范围成为可能。特别是，运行了以下测试。(i)用常规的波纹结构，即三角形和半圆形凹槽进行了剪切应力测量。这些测量是必要的，以便在我们的石油通道数据和以前的风洞数据之间建立联系。正如之前所确定的那样，我们发现阻力减少了约5%。（ii）建立并测试了具有纵向叶片肋和狭缝的可调表面。实验过程中，槽深和狭缝宽度均可单独连续变化。结果表明，表面上的裂缝并没有减少阻力。然而，这些研究表明如何设计穿孔表面（例如用于边界层控制）以实现最小的寄生阻力。另一方面，封闭的狭缝可以确定肋面的最佳凹槽深度，即侧肋间距的一半。对于这种配置，我们发现表面摩擦减少了8.7%。通过仔细消除有害影响（由小间隙等引起），可以将表面摩擦减少率提高到9.9%的记录值。（iii）理论与实验进行了定量比较。该理论是基于这样的假设，即波纹阻碍了壁面附近的波动湍流横流。这样，减小了动量传递和剪应力。Luchini（1992）提出的简化理论模型得到了本实验的支持。（iv）对于肋条的技术应用，例如远程商用飞机，上述薄叶片肋条不实用。因此，我们设计了一种表面，它结合了显着提高的性能（8.2%）和具有更好耐用性的几何形状，并使以前开发的塑料波纹膜生产制造方法得以使用。我们的肋条几何形状呈现出楔形肋条的梯形凹槽。梯形沟槽的平坦地板允许通过透明的条纹膜，这是至关重要的裂纹检查飞机不失真的可见性。
Previous research has established that surfaces with tiny ribs (riblets) aligned in the streamwise direction can reduce the turbulent wall-shear stress below that of a smooth surface. Typical skin-friction reductions have been found to be about 5%. The results of the present investigation, however, demonstrate a considerable improvement over this value. This improvement is achieved by a systematic experimental optimization which has been guided by theoretical concepts.A key feature of our experiments is the utilization of an oil channel. Previous experiments in wind tunnels had to contend with very small riblet dimensions which typically had a lateral rib spacing of about 0.5 mm or less. By contrast, in our oil channel, the ribs can have a lateral spacing of between about 2 and 10 mm. This increased size of the surface structures enables test surfaces to be manufactured with conventional mechanical methods, and it also enables us to build test surfaces with adjustable geometry. In addition, the Berlin oil channel has a novel shear stress balance with an unprecedented accuracy of +/-0.3 %. This latter feature is a prerequisite for a systematic experimental optimization.In the present investigation, surfaces with longitudinal ribs and additional slits are studied. The experiments cover a fairly large range of parameters so that the drag reduction potential of a surface with ribs and/or slits is worked out conclusively. A large parameter range is made possible because of the adjustability of the surfaces as well as the automatic operation of the oil channel. In particular, the following tests were run.(i) Shear stress measurements with conventional riblet con figurations, i.e. with triangular and semi-circular grooves, have been carried out. These measurements were necessary in order to establish the connection between our oil channel data and previous data from wind tunnels. As was previously established, we found a drag reduction of about 5%.(ii) An adjustable surface with longitudinal blade ribs and with slits was built and tested. Both groove depth and slit width could be varied separately and continuously during the experiment. It turned out, that slits in the surface did not contribute to the drag reduction. Nevertheless, these investigations show how perforated surfaces (e.g. for boundary-layer control) can be designed for minimal parasitic drag. On the other hand, with closed slits, an optimal groove depth for the rib surface could be determined, i.e. half of the lateral rib spacing. For this configuration, we found an 8.7% skin-friction reduction. By carefully eliminating deleterious effects (caused by little gaps, etc.), the skin-friction reduction could be improved to a record value of 9.9%.(iii) A quantitative comparison between theory and experiment was carried out. The theory is based on the assumption that riblets impede the fluctuating turbulent crossflow near the wall. In this way, momentum transfer and shear stress are reduced. The simplified theoretical model proposed by Luchini (1992) is supported by the present experiments.(iv) For technological applications of riblets, e.g. on long-range commercial aircraft, the above thin-blade ribs are not practical. Therefore, we have devised a surface that combines a significantly improved performance (8.2 %) with a geometry which exhibits better durability and enables previously developed manufacturing methods for plastic riblet film production to be used. Our riblet geometry exhibits trapezoidal grooves with wedge-like ribs. The flat floor of the trapezoidal grooves permits an undistorted visibility through the transparent riblet film which is essential for crack inspection on aircraft.