INFLUENCE OF CAVITATION ON TURBULENT SEPARATED FLOW
INFLUENCE OF CAVITATION ON TURBULENT SEPARATED FLOW
复制标题
空化对湍流分离流的影响
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
K. Okabayashi
中科院分区：
文献类型：
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作者：
T. Kajishima;T. Ohta;Hiroki Sakai;K. Okabayashi
涡与空化的相互作用是研究空化非定常湍流流动的基础。首先，直接模拟了含空化的分离剪切层。采用一个简单的模型来描述高剪切层中典型的展向涡和流向涡所引起的空化现象。实验观察到的趋势的位置的旋涡形成，旋涡脱落的频率和强度的雷诺应力合理地再现。以一个典型的流向涡为例，观察了空化对流向涡的影响。结果表明，Burgers型涡核的涡度明显减小。为了建立对它的描述，我们使用了人工维持的Burgers涡旋。因此，假设由于空腔的突然发展，沿沿着闭合圆的恒定环量可以代表旋涡的修改。水力机械中的流动受到各种类型的空化的影响。自20世纪90年代以来，针对包括非定常空化在内的流场的数值模拟提出了多种方法。合理地再现了附着（片）空化和云（泡）空化。然而，尽管空化流动大多为湍流，但在以往的数值模拟中并未完全考虑湍流的影响。这导致预测空化起始的不准确性，因为局部压力最小值被认为对应于湍流涡的核心。另一方面，空化对湍流的影响通常被忽略。因此，要建立湍流空化流动的计算方法，就必须正确地模拟空化与湍流涡的相互作用。我们的研究的目的是解决建模策略考虑两个问题。(1)湍流中的细尺度涡与空化初生有何关系？(2)空化是如何改变湍流旋涡和湍流统计的？由于两者是相互作用的现象，因此必须用双向的方法来分析。对于我们的目标的一个典型的和适当的例子是在一个二维通道中的薄栅栏的尾迹分离流。Iyer和Cessio（2002）可视化了各种类型的涡流空化，并报告了尾流区域的湍流统计数据。在本报告的前半部分，我们展示了由Okita和Kajishima（2002）开发的空化模型的直接数值模拟结果。我们的计算结果与Iyer和Cessio（2002）的实验结果进行了定性的比较，因为我们的计算简化了流动结构。然后，我们研究空化和涡结构之间的关系，即，初级（展向）涡和次级（流向）涡。在后半部分中，在理想情况下直接模拟了Burgers涡与空化的相互作用。原因之一是在前一部分中模拟的剪切层中的流向涡具有Burgers涡轮廓。此外，已发现在充分发展的湍流中的最小尺度涡是Burgers型的。为此，在计算区域的入口截面处给出了均匀来流中的Burgers涡。在减少空化数的情况下，分析了空化初生后旋涡的衰减结构。然后我们给出了一个描述这一过程中涡旋结构的唯象模型。空化模型和数值方法的研究过程应符合湍流剪切层中旋涡非定常运动的时空尺度。在
Attention was focused on the interaction between vortex and cavitation as a basic study on modeling unsteady turbulent flows with cavitation. First, separated shear layer with cavitation were directly simulated. Cavitation caused by spanwise and streamwise vortices, which are typical features in high shear layer, was represented by a simple model. Experimentally observed tendencies in the location of vortex formation, the frequency of vortex shedding and the intensity of Reynolds stresses were reasonably reproduced. Influence of cavitation on a typical example of streamwise vortex was observed. It was found that the vorticity in the core of Burgers type vortex was significantly reduced. To establish a description of it, we used an artificially maintained Burgers vortex. As a result, an assumption of constant circulation along a closed circle expanding due to sudden development of cavity could represent the modification of the vortex. INTRODUCTION Flows in hydro-machineries are affected by various types of cavitation. Since 1990’s, several methods have been proposed for the numerical simulation of flow fields including unsteady cavitation. Attached (sheet) cavitation and cloud (bubble) cavitation have been reasonably reproduced. However, the influence of turbulence has not been taken into account completely in previous simulations although most of cavitating flows are turbulent. This causes inaccuracy in predicting the cavitation inception because the local minimum of pressure is thought to be corresponding to the core of turbulence vortices. On the other hand, the effect of cavitation on turbulence has usually been omitted. Therefore, to establish the computational method for turbulent cavitating flows, the interaction between cavitation and turbulence vortices should be correctly modeled. The aim of our study is to address the modeling strategies considering two questions. (1) How are fine-scale vortices in turbulence related to cavitation inception? (2) How does the cavitation modify turbulent vortices and turbulence statistics? Since both are interactive phenomena, they must be analyzed by two-way methodology. One of typical and appropriate examples for our objective is the separated flow in the wake of a thin fence in a two-dimensional channel. Iyer and Cessio (2002) visualized various types of vortex cavitation and they reported the turbulence statistics in the wake region. In the first half of this report, we show the results of the direct numerical simulation with cavitation model, which was developed by Okita and Kajishima (2002). Our result is compared with experimental observation by Iyer and Cessio (2002) qualitatively, because the flow configuration was simplified in our simulation. Then, we investigate the relationship between cavitation and vortical structure; namely, primary (spanwise) vortices and the secondary (streamwise) vortices. In the second half, the interaction between a Burgers vortex and cavitation is directly simulated in an idealized situation. One reason is that streamwise vortices in a shear layer, simulated in the former part, has a profile of Burgers vortex. Moreover, the finest scale eddies in fully developed turbulence has been found to be Burgers type. A Burgers vortex in a uniform stream is given at the inlet cross section of the computational domain for this purpose. Reducing the cavitation number, the structure of vortex decaying by cavitation inception is analyzed. Then we give a phenomenological model that represents the vortical structure during this process. OUTLINE OF COMPUTATION The procedure including cavitation model and numerical method should fit in with the spatio-temporal scale of unsteady motion of vortices in the turbulent shear layer. In