Coherent structures in streamwise rotating channel flow

Direct numerical simulation and large-eddy simulation are employed to study the effects of streamwise rotation on turbulent channel flow, especially the coherent structures. Two Reynolds numbers ( Re = U(m)h/nu = 2800 and 7000) with a wide range of rotation numbers ( 0 <= Ro = 2 ohm h/U-m <= 7.5) are considered, where ohm denotes the angular velocity of the system and U-m and h are the bulk velocity and one-half of the channel height, respectively. The demand on the streamwise computational domain size increases as the rotation number increases. Flow statistics are presented and discussed. Among the near-wall quasi-streamwise vortices rotating in opposite directions, the rotation of the channel tends to promote the cyclones (vortices rotating in the same direction as the channel) and suppress the anti-cyclones, which is visualized by the conditional-sampling method. Elongated large-scale vortices, which typically form in rotating homogeneous turbulence, are observed through the conditional-sampling method. Similar to the columnar vortices in homogeneous turbulence, the elongated large-scale vortices are dominated by cyclones at Ro similar to 1, while the dominance becomes less apparent as the rotation number further increases. When rotation is suddenly imposed to fully developed turbulent flow, the integral scale along the rotating axis increases linearly at a growth rate proportional to the rotation number. What is interesting is that the cyclones of the large-scale vortices are not completely elongated along the rotating axis but slightly tilted to the spanwise direction. The cyclones prefer to develop in the lower and upper half channels, while the anti-cyclones, if they exist, occur in the central region of the channel. This preference of the large-scale vortices in different regions contributes to the secondary flow. It is revealed that the mean shear of the channel flow may have effects on the tilting of the large-scale vortical structures and the preference of their distributions in the channel. Published under license by AIP Publishing.