Behaviour of small-scale turbulence in the turbulent/non-turbulent interface region of developing turbulent jets
Behaviour of small-scale turbulence in the turbulent/non-turbulent interface region of developing turbulent jets
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DOI：
10.1017/jfm.2019.676
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发表时间：
2019-09
影响因子：
3.7
通讯作者：
M. Breda;O. Buxton
中科院分区：
文献类型：
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作者：
M. Breda;O. Buxton
在两种不同类型的射流的近场和中场进行了层析粒子图像测速实验，一个装有圆形孔，另一个装有重复分形图案的孔。布雷达和巴克斯顿（J.维斯，第21（4）卷，2018年，第21页。525-532; Phys. Fluids，第30卷，2018年，035109）表明，相对于圆形射流，这种分形几何结构抑制了近场中存在的大尺度相干结构，并影响了背景流体夹带到分形射流中的速率以及分形射流的后续发展。根据这些发现，我们现在检查修改的湍流/非湍流界面（TNTI）和空间演变的小尺度行为的这些不同的射流，这两个重要因素背后确定的夹带率。这种演变是检查在两个流向方向和内的TNTI本身的流体适应从一个非湍流状态，最初通过粘度的直接作用，然后通过非线性惯性过程，在短距离内的大部分流量的湍流状态。我们表明，抑制分形射流中的相干结构导致一个较少扭曲的接口，与大规模的内部TNTI（射流的方位角剪切层和潜在的核心之间）的偏移被抑制。进一步下游，TNTI的行为被证明是可比的两个射流。速度梯度发展成一个规范的状态与流向的距离，表现为经典的泪滴形轮廓的速度梯度张量不变量$\mathit{Q}$和$\mathit{R}$的统计分布的发展。速度梯度也通过TNTI从无旋边界的体积流量空间发展，特别是，有一个强大的小尺度各向异性在这个区域。TNTI区域中速度梯度的这种强烈不均匀性对这些空间发展流中TNTI厚度的缩放具有强烈的影响，因为泰勒和科尔莫戈罗夫长度尺度都是直接根据速度梯度计算的。
Tomographic particle image velocimetry experiments were conducted in the near and intermediate fields of two different types of jet, one fitted with a circular orifice and another fitted with a repeating-fractal-pattern orifice. Breda & Buxton (J. Vis., vol. 21 (4), 2018, pp. 525–532; Phys. Fluids, vol. 30, 2018, 035109) showed that this fractal geometry suppressed the large-scale coherent structures present in the near field and affected the rate of entrainment of background fluid into, and subsequent development of, the fractal jet, relative to the round jet. In light of these findings we now examine the modification of the turbulent/non-turbulent interface (TNTI) and spatial evolution of the small-scale behaviour of these different jets, which are both important factors behind determining the entrainment rate. This evolution is examined in both the streamwise direction and within the TNTI itself where the fluid adapts from a non-turbulent state, initially through the direct action of viscosity and then through nonlinear inertial processes, to the state of the turbulence within the bulk of the flow over a short distance. We show that the suppression of the coherent structures in the fractal jet leads to a less contorted interface, with large-scale excursions of the inner TNTI (that between the jet’s azimuthal shear layer and the potential core) being suppressed. Further downstream, the behaviour of the TNTI is shown to be comparable for both jets. The velocity gradients develop into a canonical state with streamwise distance, manifested as the development of the classical tear-drop shaped contours of the statistical distribution of the velocity-gradient-tensor invariants $\mathit{Q}$ and $\mathit{R}$ . The velocity gradients also develop spatially through the TNTI from the irrotational boundary to the bulk flow; in particular, there is a strong small-scale anisotropy in this region. This strong inhomogeneity of the velocity gradients in the TNTI region has strong consequences for the scaling of the thickness of the TNTI in these spatially developing flows since both the Taylor and Kolmogorov length scales are directly computed from the velocity gradients.