DNS of Large Scale Vortices and Heat Transfer Mechanism in a Three-Dimensional Unsteady Separated Flow

采用直接数值方法结合有限差分方法，对表面贴装矩形块对、错开表面贴装矩形块、后向台阶和突扩矩形通道的三维非定常分离流动和换热进行了数值模拟。1.表面贴装矩形块的流道发现，本文的数值结果较好地模拟了马蹄形旋涡和回流分离流等实际情况。阐明了局部换热特性和平均换热特性，并研究了它们与流动结构的关系。2.交错表面安装的矩形块绕第一排和第二排块的流动与绕单排块的流动相似，并形成了马蹄形旋涡。对于第三排块，在Re=400之前没有形成马蹄形涡旋，但在Re=500处最终形成了马蹄形涡旋。温度场受流动结构的影响较大，t…附近的温度场受影响较大更多的Hird-row区块与第一和第二行区块周围的区块有很大的不同。对于所考察的所有倾角，流动在Re≦5500时是稳定的，但在Re=700时变得不稳定。在α为30°~75°的范围内，在较宽的雷诺数范围内，α对再附着长度和最大努塞尔数的影响相对较小。4.矩形突扩流道随着流道长宽比AR的减小，对称流动维持到较高的雷诺数。随着AR和Re的增大，Nusselt数在侧壁附近达到最大值，尤其是在上壁处。流动的三维性和局部Nusselt数随Re的增加而加剧，导致流道中心区的Coanda效应减弱。5.突扩流道当Re=200时，流动是稳定的，且沿y轴和z轴对称。Re=300处的流动是稳定的，但在y轴上是不对称的。最大Nusselt数是在远离通道中心处获得的。在≧400时，流动变得不稳定。当Re增加到1000时，侧壁附近约x/H=30的上游区域形成并脱落了纵向涡。详细研究了通道扩张率ER对流动和换热特性的影响。结果表明，在Re≧为70 0、6 0 0和5 0 0时，ER=1.5、2.0和3.0的流动变得不稳定。即使在非定常流动状态下，ER=2.0时的时间平均再附着长度和中心线上的流向平均速度也与前人的实验结果吻合得很好。纵向旋涡在两侧壁面附近形成，大小尺度的旋涡向下游脱落。这些旋涡对流动和换热特性有很大影响。对于非定常流动，流动的复杂性随着电流变比的减小而增加。即使在大台阶展宽比为36.0的情况下，流动的三维性也很严重，对换热的影响很大，特别是在非定常流动状态下。较少

Direct numerical methodology along with the finite difference method clarified the three-dimensional unsteady separated flow and heat transfer around the pair of surface-mounted rectangular blocks, the staggered surface-mounted rectangular blocks, the backward-facing steps and the rectangular channels with sudden expansion.1.Pair of surface-mounted rectangular blocksIt is found that the present numerical results well simulate the observed ones such as horseshoe vortices and the recirculating separated flow. Local and mean heat transfer characteristics are clarified and their correlations with the flow structure are investigated.2.Staggered surface-mounted rectangular blocksThe flow around the first- and second-row blocks is similar to that around the single block, and horseshoe vortices are formed. For the third-row block, a horseshoe vortex is not formed up to Re=400 but is finally formed at Re=500. The temperature field is greatly affected by the flow structure, and that around the t … More hird-row block is considerably different from those around the first- and second- row blocks. The mean Nusselt numbers averaged on each surface and also on the five surfaces are presented as a function of the Reynolds number.3.Two-dimensional inclined downward stepFor all the inclination angles examined, the flow is steady at Re≦5500 but becomes unsteady at Re=700. In a range of α from 30°to 75°, effects of α upon the reattachment length and the maximum Nusselt number are relatively small in wide range of the Reynolds number.4.Rectangular channel with sudden expansionAs the aspect ratio AR of channel decreases, the symmetric flow is maintained to higher Reynolds number. The maximum Nusselt number is reached near the side wall with increases of AR and Re, especially on the upper wall. Three-dimensionality of flow and local Nusselt number becomes severe with Re, resulting in a decrease of Coanda effect in the central region of the channel5.Sudden-expansion channelAt Re=200, the flow is steady and symmetric as to the y-and z-axes. The flow at Re=300 is steady but asymmetric as to the y-axis. The maximum Nusselt number is attained away from the channel center. The flow becomes unsteady at ≧400. As Re increases to 1000, the longitudinal vortices are formed and shed in the upstream region of about x/H=30 in the neighborhood of the side walls6.Downward stepsThree-dimensional separated flow and heat transfer around three downward steps are clarified. Minutely investigated are effects of the channel expansion ratio ER upon the flow and heat transfer characteristics. It is found the flow for ER=1.5, 2.0 and 3.0 becomes unsteady at Re≧700, 600 and 500, respectively. The present time averaged reattachment length and the streamwise mean velocity on the center line for ER=2.0 are in excellent agreement with the previous experimental ones even in the unsteady flow state. The longitudinal vortices are formed in the neighborhood of two side walls and the vortices having large and small scales are shed to the downstream. These vortices influences largely upon the flow and heat transfer features. The complexity of flow increases with a decrease of ER for the unsteady state. Even in the case of large step aspect ratio of 36.0, the three-dimensionality of the flow is severe and its effects upon the heat transfer are great, especially in the unsteady flow state. Less