A direct numerical simulation is performed for a turbulent concentric annular pipe flow. The transverse curvature effects on near-wall turbulent structures are elucidated for two radius ratios ($R_1$/$R_2$ =0.1 and 0.5). The Reynolds number based on the bulk velocity ($U_m$) and the hydraulic diameter ($D_h$) is $Re_{D_h}$ = 8900. A fully-implicit decoupling method is employed to simulate the flow. This method is validated by testing a direct numerical simulation of turbulent pipe flow. Lower-order and higher-order statistics are obtained to analyze the near-wall turbulent structures in the inner wall and in the outer wall. The Reynolds stress budgets are computed to confirm the results of lower-order statistics. The quadrant analysis of Reynolds shear stress is made to obtain the detailed information on the contribution to turbulence production(consumption) from various flow events. Probability density functions of the inclination angles of projected vorticity vectors are investigated to analyze the transverse curvature effects on the orientation of the vorticity field. The present numerical results show that the turbulent structures near the outer wall are more activated than those in the inner wall.
