Simulations of rib-roughened rough-to-smooth turbulent channel flows
Simulations of rib-roughened rough-to-smooth turbulent channel flows
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DOI：
10.1017/jfm.2018.119
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发表时间：
2018-03
影响因子：
3.7
通讯作者：
U. Ismail;T. Zaki;P. Durbin
中科院分区：
文献类型：
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作者：
U. Ismail;T. Zaki;P. Durbin
高保真度的紊流模拟表明，在一个具有粗糙壁面和光滑壁面的通道中，恢复区内存在高度的不平衡。实际上，由于计算域的流向退出，所研究的所有流统计量的恢复是不完整的。在薄壁层以上，湍流强度明显高于完全发育的光滑壁水平，在发展中区域持续存在。在薄壁层内，湍流应力的剖面形状恢复得非常快，特征峰的壁法向位置得以建立。然而，即使在这种薄层中，完全恢复湍流应力的大小也是异常缓慢的。皮肤摩擦也表现出类似的最初迅速但最终不完全和缓慢的松弛行为。在湍流剪切应力和顺流应力之间，湍流剪切应力在薄壁层上方表现出较快的恢复速度。在发育中的光滑壁面上，平衡不仅仅是由于压力和剪应力引起的通量之间的平衡。受上游粗糙度大小直接影响的强动量通量对这种平衡起着重要作用。近似曲线拟合估计了在大约$20\unicode[STIX]{x1D6FF}{-}25\unicode[STIX]{x1D6FF}$处达到接近完全发育水平所需的流向距离，其中$\unicode[STIX]{x1D6FF}$为通道半高。平均速度可能需要更长的距离，超过$50\unicode[STIX]{x1D6FF}$，才能接近完全发育的星等。可视化和相关性显示，在粗糙度上方产生的大规模涡流在下游持续存在，偶尔会扰乱细长的条纹。这些高动量和低动量交替的条纹在去除粗糙度后几乎立即出现。在计算域内，平均流并没有重新建立一个平衡的对数层，并且由粗糙度造成的速度赤字在整个域内持续存在。在粗糙度阶跃变化的壁面附近，湍流动能耗散率曲线$\unicode[STIX]{x1D716}$和能谱表明在小尺度上能量急剧减少。尽管如此，向平衡光滑壁水平的回归是缓慢的，并且最终是不完整的，因为湍流级联的调整相当缓慢。在各种模拟中，无量纲粗糙度高度$k^{+}$的范围为42 ~ 254，光滑壁面处的摩擦速度雷诺数$Re_{\unicode[STIX]{x1D70F}S}$的范围为284 ~ 1160。
High-fidelity simulations of turbulent flow through a channel with a rough wall, followed by a smooth wall, demonstrate a high degree of non-equilibrium within the recovery region. In fact, the recovery of all the flow statistics studied is incomplete by the streamwise exit of the computational domain. Above a thin wall layer, turbulence intensities significantly higher than fully developed, smooth-wall levels persist in the developing region. Within the thin wall layer, the profile shapes for turbulence stresses recover very quickly and wall-normal locations of characteristic peaks are established. However, even in this thin layer, complete recovery of magnitudes of turbulence stresses is exceptionally slow. A similar initially swift but eventually incomplete and slow relaxation behaviour is also shown by the skin friction. Between the turbulence shear and streamwise stresses, the turbulence shear stress shows a comparatively quick rate of recovery above a thin wall layer. Over the developing smooth wall, the balance is not merely between fluxes due to pressure and shear stresses. Strong momentum fluxes, which are directly influenced by the upstream roughness size, contribute significantly to this balance. Approximate curve fits estimate the streamwise distance required by the outer peaks of Reynolds stresses to attain near-fully-developed levels at approximately $20\unicode[STIX]{x1D6FF}{-}25\unicode[STIX]{x1D6FF}$ , with $\unicode[STIX]{x1D6FF}$ being the channel half height. An even longer distance, of more than $50\unicode[STIX]{x1D6FF}$ , might be needed by the mean velocity to approach near-fully-developed magnitudes. Visualizations and correlations show that large-scale eddies that are created above the roughness persist downstream, and sporadically perturb the elongated streaks. These streaks of alternating high and low momentum appear almost instantly after the roughness is removed. The mean flow does not re-establish an equilibrium log layer within the computational domain, and the velocity deficit created by the roughness continues throughout the domain. On the step change in roughness, near the wall, profiles for turbulence kinetic energy dissipation rate, $\unicode[STIX]{x1D716}$ , and energy spectra indicate a sharp reduction in energy at small scales. Despite this, reversion towards equilibrium smooth-wall levels is slow, and ultimately incomplete, due to a rather slow adjustment of the turbulence cascade. The non-dimensional roughness height, $k^{+}$ ranges from 42 to 254 and the friction velocity Reynolds number at the smooth wall, $Re_{\unicode[STIX]{x1D70F}S}$ , ranges from 284 to 1160 in the various simulations.