Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two-dimensional curved channel. A rib was placed on either an inner- or outer-wall of the straight part nearby curved-channel inlet for controlling the flow and thermal fields. Outer-wall was heated at the constant temperature and inner-wall was cooled similarly. In the simulation, fully developed flow and temperature computed by the straight channel was given to the inlet of curved channel domain. The frictional Reynolds number was assigned at 150, and the Prandtl number was given 0.71. When a rib was attached to the inner-wall, heat transfer in the upstream region of curved channel was enhanced effectively. On the other hand, placing a rib on the outer-wall caused an enhancement of heat transfer in the relatively downstream region of curved part, suggesting active heat transport of the large-scale vortices. Consequently, the inner-rib case affected more greatly on heat transfer than the outer-rib case. Mean velocity vector and power spectrum analysis implied that development of coherent structures near the outer-wall were controlled by a rib. It was found that inner-rib restrained large-scale vortex, while outer-rib promoted growth and power of large-scale vortex.

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Direct numerical simulation was performed for a spatially advancing turbulent flow in a two-dimensional curved channel. Fully developed turbulence generated by straight channel simulation was used as the inlet value of curved part simulation. The radius ratio of the curved part, α, was set 0.92, the same as Kobayashi et al.'s experiment. However, the frictional Reynolds number, Re_τ0, was assigned 150, which was roughly quarter the experiment and allowed direct simulation of all the essential scale of turbulence. Computation was made for three cases changing domain size, and mainly discussed was the results from the computational volume extending 150° with spanwise length of 7.2 times channel half width. In this case, total number of 512×61×128 grid points was allocated. Numerically solved mean velocity showed trends consistent with the experiment in spite of the Reynolds number difference. Mean velocity normalized by the local frictional velocity indicated upward and downward offset of the logarithmic region, respectively, in the inner and outer side of the channel, but simulation showed stronger laminarization near the inner wall due to the low Reynolds number effect. Numerical data of mean velocity field illustrated that characteristic behavior of ejection from the outer wall and streamwise vortices appeared with advancement of large scale vortices. Power spectrum analysis implied that coherent structures near the outer wall were related to the birth and formation of large scale vortices.

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Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two-dimensional curved channel, where one wall was heated at the constant temperature and another wall was cooled similarly. In the simulation, fully developed flow and temperature computed by the straight-channel driver was given to the inlet of curved channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, thermal features are mainly targeted in the present paper. Turbulent heat flux showed trends consistent with growing process of large-scale vortices. In the curved part, the wall-normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large-scale vortices. In the inner side of the same part, temperature fluctuation was abnormally large when compared with modest fluctuation of wall-normal velocity. This was caused by the combined effect of the large-scale motion of the vortices and the monotonous variation of the mean temperature; in such a temperature distribution, large-scale ejection of the hot fluid near the outer, which is transported into the near inner-wall region, should made large impact on thermal boundary layer near the inner wall. Wave number decomposition was made for various statistics, and such effort showed that contribution of the large-scale vortex to the total turbulent heat flux normal to the wall was roughly 80 percent at the channel center of 130° downstream from the curved-channel inlet.

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