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

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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