DIRECT NUMERICAL SIMULATION OF HEAT TRANSFER MECHANISM IN SEPARATED FLOW

采用直接数值模拟的方法，研究了表面贴装矩形块体周围分离和再附着流动中形成的大尺度涡旋结构及其与传热机理的关系。采用有限差分法计算了三维非定常Navier-Stokes方程和能量方程。采用对流项空间导数的五阶迎风差分对流动特性进行了细致的模拟。所得结果总结如下：1。对于单个块体，随着雷诺数的增加，其周围形成的马蹄涡数由2个增加到6个。在两个直列街区的情况下，再循环区域扩展了两个街区之间的整个空间，以获得小街区空间。进一步发现，即使在很宽的空间内，在第二块周围也不会形成马蹄形涡。在一个非常小的横向空间内，两个横向块周围的流动与单个矩形块周围的流动相似，就像在它们周围形成一个大的单一马蹄形漩涡一样。随着空间的增大，每个块体周围形成两个马蹄形涡，它们之间的相互作用严重，两个块体之间的流动非常复杂。在交错三排块体的情况下，第一和第二块体的正面和侧面附近的流动与单个块体相似。马蹄形涡仅在Re= 500.5时在第三块周围形成。当雷诺数增加到1000时，流动的不稳定性变得严重，并伴随着从块体上脱落的大规模旋涡。上面简要描述的这些流动结构使块体周围的传热特性发生了很大的变化。

Direct numerical simulation methodology has been employed to clarify the large scale vortex structure formed in the separated and reattached flow around the surface-mounted rectangular blocks and its correlation with the heat transfer mechanisms. The finite difference method is used to calculate the three-dimensional unsteady Navier-Stokes equations and the energy equation. The fifth-order upwind difference for the space derivative of convection terms is adopted to simulate minutely the detailed flow characteristics. Results obtained are summarized as follows.1. As for the single block, the number of horseshoe vortices formed around it increases from two to six with an increase of Reynolds number.2. In the case of two in-line blocks, the recirculation region extends the whole space between two blocks for small block space. Further, it is found that the horseshoe vortex is not formed around the second block even for a very wide space.3. The flow around two transversal blocks becomes similar to that around a single rectangular block at a very small transversal space, as illustrating a large single horseshoe vortex around them. As the space increases, two horseshoe vortices are formed around each block, they severely interact, and the flow between two blocks is very complicated.4. In the case of staggered three-row blocks, the flow in the neighborhood of the front and side surfaces of the first and second blocks is similar to that of the single block. The horseshoe vortex is formed around the third block only at Re= 500.5. As the Reynolds number increases up to 1000, the unsteadiness of flow becomes severe, as accompanying the shedding of large scale vortices from the blocks.6. These flow structures described briefly above bring about great variations of the heat transfer characteristics around the blocks.