Abstract: Different cases of upstream flow were investigated and the occurring conditions of separation, separation degree, and attachment in the frontal face of the cube were analyzed. Based on the comparison of the experimental data, occurring conditions of backflow and attachment, which were analyzed using standard k-ε model, the attachment length Z_R are directly related with the upstream flow velocity u, the equation is u= y^1/m.When m is decreases, the Z_R increases. When m is larger than 7, there is a strong separation above the top of the cube, but no attachment, Z_R is over 1.0.

Key words: turbulent, upstream flow velocity, separation, attachment