DIRECT NUMERICAL SIMULATION OF HEAT TRANSFER MECHANISM IN SEPARATED FLOW

采用直接数值模拟方法，研究了表面贴装矩形块分离再附流中形成的大尺度旋涡结构及其与换热机理的关系。采用有限差分法对三维非定常N-S方程和能量方程进行了数值计算。采用对流项空间导数的五阶迎风差分方法对流动特性进行了详细的数值模拟。所获得的结果总结如下。对于单个块体，随着雷诺数的增大，其周围形成的马蹄形涡数从2个增加到6个。在两个串联区块的情况下，回流区延伸两个区块之间的整个空间以获得较小的区块空间。进一步研究发现，即使在很宽的空间范围内，马蹄形涡旋也不会在第二块周围形成。在很小的横向空间内，绕两个横向块体的流动变得类似于绕单个矩形块体的流动，因为它们周围有一个很大的单一马蹄形旋涡。随着空间的增大，每个块体周围形成两个马蹄形旋涡，相互作用严重，两个块体之间的流动非常复杂。在交错的三排块的情况下，第一和第二块的正面和侧面附近的流动与单个块的流动相似。马蹄形涡旋仅在Re=500.5的第三个块体周围形成。当雷诺数增加到1000时，流动的不稳定性变得更加严重，同时伴随着大尺度涡的脱落。上面简要描述的这些流动结构导致了块周围的换热特性的巨大变化。

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.