Statistical quantity in turbulence and turbulence structure were investigated by direct numerical simulation on turbulent flow triggered by different mechanisms in a vertical channel. The statistical results for forced convection

combined convection and natural convection include mean streamwise velocity

mean temperature

intensity of fluctuating velocity and temperature

Reynolds stress

and turbulent structure. The results show that: Compared with forced convection

for combined convection

buoyancy leads to the higher mean velocity and lower intensity of velocity fluctuation near hot wall

but the lower mean velocity and higher intensity of velocity fluctuation near the cold wall; buoyancy results in the stronger temperature fluctuation near the wall regions and the weaker temperature fluctuation in the central part of the channel. Compared with forced convection and combined convection

for the natural convection

the profile of the mean velocity is antisymmetric to the center line

and both the largest velocity fluctuation and the smallest temperature fluctuation occur in the central part of the channel; the maximum value of Reynolds stress also locates in the central part of the channel

and negative Reynolds stresses are near the walls.