The flow in a U-turn duct with a 180-degree curved section is a typical model of various applications such as the bend and return channel in multi-stage radial turbo-machines. The flow exhibits complex patterns including the separation on the inner wall, highly sheared flow in the curved and downstream sections, and recovery of deficit flow to uniform flow. Most of current researches focus on the simplified models of two-dimensional plane duct or three-dimensional duct with constant cross-section, while in realistic circumstances the duct can be varying in the shape and area of the cross-section to meet the design requirement. In this work, we performed a numerical investigation on the turbulent flow in a U-turn duct with spanwise diverging at Reynolds number Re = 10⁶. The effect of diverging, as represented by the height of the duct, on the formation of secondary flow, flow separation and recovery is explored, and the three-dimensionality of the separated flow is quantitatively assessed. Numerical results reveal that with stronger spanwise diverging, the flow separates earlier from the inner wall and the size of the separated vortex is larger; the primary characteristics of the secondary flow also change significantly. The three-dimensional separated flow substantially affects the flow in the downstream straight duct that both streamwise and radial velocities exhibit fluctuating patterns as a result of the spatially non-uniform separation. The characteristics of boundary layer flow on all solid walls are also analyzed and discussed regarding the pressure field and local shearing.

具有 180 度弯曲截面的 U 形转弯管道中的流动是各种应用的典型模型，例如多级径向涡轮机中的弯曲和回流通道。流动表现出复杂的模式，包括内壁分离，弯曲部分和下游部分的高剪切流，以及逆流恢复为均匀流。目前的研究大多集中在二维平面风管或等截面三维风管的简化模型上，而在现实情况下，风管可以通过改变截面的形状和面积来满足设计要求。 . 在这项工作中，我们对在雷诺数Re  = 10 ⁶处展向发散的 U 形转弯管道中的湍流进行了数值研究. 探索了以管道高度为代表的分流对二次流形成、流分离和恢复的影响，并定量评估了分离流的三维度。数值结果表明，展向发散越大，流动越早与内壁分离，分离涡的大小越大；二次流的主要特性也发生了显着变化。三维分离流显着影响下游直管中的流动，由于空间不均匀分离，流向和径向速度都表现出波动模式。还分析和讨论了所有实体壁上边界层流动的压力场和局部剪切特性。