TED-AJ03-550 TURBULENT HEAT TRANSFER IN CHANNEL FLOW WITH NON-ORTHOGONAL SYSTEM ROTATION

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Direct numerical simulations of a fully developed turbulent channel flow with combined system rotations have been performed. A pseudo-spectral method is used to solve the unsteady Navier-Stokes equation and the heat convection equation in a reference frame rotating with the system. The Reynolds number, based on the χ-direction bulk mean velocity and the half-width of the channel, and the Prandtl number are set to 4560 and 0.71,respectively. In a channel with combined spanwise and streamwise system rotations, two cases are considered. In the first case, the absolute rotation number is kept constant for three values, 5,7.5 and 11,and the angle between the rotating axis and the positive χ-direction is increased from 0 to 90°(case I). In the other case, the spanwise rotation number is set to 2.5 and the streamwise rotation number is increased to 15 (case II). Also considered in a channel with combined wall-normal and spanwise rotations are two cases : a constant wall-normal rotation number 0.04 with the spanwise rotation number increased to 15 (case III) and a constant spanwise rotation number 2.5 with the wall-normal rotation number increased to 0.04 (case IV). The results of the present simulation for a spanwise rotating channel and for a streamwise rotating channel are compared with those by Kristoffersen et al. (1993) and by Elsamni (2001), respectively. Good agreements between these results validate the present simulations. As the angle is increased in case I, the Nusselt number has a similar tendency to that in the single spanwise rotating channel with increasing the spanwise rotation number. On the suction side, however, the Nusselt number has some local maxima at 45°for all the three absolute rotation numbers considered presently. The ratio of the heat flux on the wall to the total friction also reaches a local maximum at this angle. The similar tendencies of the mean properties and the temperature statistics to those in the single spanwise rotating channel reveal the dominant effect of the spanwise rotation. In case II, the increment of the streamwise rotation number enhances the heat transfer on both walls, especially for the streamwise rotation numbers larger than the twice spanwise rotation number. This tendency is different from that in the flow field, where the turbulence is mainly enhanced along the suction side. The changes of most statistics in case III are quite similar to those in the single spanwise rotating channel. The Nusselt number, however, is comparatively larger than that in the single spanwise rotating channel, especially for the spanwise rotation number larger than 7.5 on the pressure side. If the spanwise rotation number is kept constant and just the wall-normal rotation number is increased (case IV), an increasingly larger and asymmetric spanwise mean velocity is induced. The changes of the streamwise mean velocity and the mean temperature are comparatively small. The wall-normal rotation enhances the Nusselt number mainly along the suction side. The visualization of the temperature fluctuation in χ-z planes reveals the existence of streaks between high and low temperatures in the near wall region, similar to the streaky structures of the streamwise velocity fluctuation. The spanwise rotation increases the number and intensity of these streaks on the pressure side, but reduces on the suction side. In case II, the increasing of the streamwise rotation number enhances these streaky structures on both walls, but more evidently on the suction side. These structures drift towards positive or negative z direction, coinciding with the sign of the spanwise mean velocity on the two walls. In case IV, these streaky structures are only enhanced slightly, but the tilting direction is changed to the positive z direction.

TED-AJ03-550 TURBULENT HEAT TRANSFER IN CHANNEL FLOW WITH NON-ORTHOGONAL SYSTEM ROTATION

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