An experimental study was conducted to examine the formation of secondary circulation in a grid-mixing box and to determine its effect on turbulence. This apparatus has been used extensively to study turbulence and mixing in a variety of geophysical contexts, and it is commonly assumed that turbulence is nearly isotropic and horizontally homogenous and that it is a zero-mean shear flow. Exceptions to these assumptions, however, have been reported, where a secondary flow pattern has been observed consisting of two roughly symmetric large-scale circulations with upward flow in the center of the box and downward return flow along the sides. These secondary flows appear to be associated with different grid oscillation conditions and box and grid geometries, and criteria have been proposed to describe conditions when secondary flow may be expected. Experiments were conducted for three different combinations of the grid oscillation stroke and frequency, while maintaining a near constant grid Reynolds number, to examine the formation and strength of the secondary flow and its effect on the magnitude and distribution of turbulence within the box. Velocity characteristics were obtained by particle image velocimetry (PIV). Results show that (1) secondary circulations were present for all combinations of the grid oscillation conditions; (2) as stroke length increased, the intensity of the secondary circulation and the contribution of these motions to total kinetic energy increased; and (3) the presence of secondary circulation results in greater overall mixing and turbulent transport in the region close to the grid. These insights are expected to be relevant to a wide range of mixing box applications.

中文翻译：

---

混合箱内的二次循环及其对湍流的影响

进行了一项实验研究，以检查网格混合箱中二次循环的形成并确定其对湍流的影响。该装置已被广泛用于研究各种地球物理环境中的湍流和混合，并且通常假设湍流几乎是各向同性和水平均匀的，并且是零均值剪切流。然而，已经报告了这些假设的例外情况，其中观察到二次流动模式由两个大致对称的大型循环组成，在盒子的中心向上流动，沿着侧面向下回流。这些二次流似乎与不同的网格振荡条件以及箱体和网格几何形状有关，并且已经提出了标准来描述可能出现二次流的条件。对网格振荡冲程和频率的三种不同组合进行了实验，同时保持网格雷诺数接近恒定，以检查二次流的形成和强度及其对箱内湍流大小和分布的影响。通过粒子图像测速法 (PIV) 获得速度特性。结果表明：（1）电网振荡条件的所有组合都存在二次环流；(2) 随着行程长度的增加，二次循环的强度和这些运动对总动能的贡献增加；(3) 二次环流的存在导致靠近网格的区域有更大的整体混合和湍流输运。