Abstract: A numerical simulation method is used to study the effect of groove length on backward-facing step flows at the Reynolds numbers of 6500 and 9000, focusing on the distributions of flow velocity and turbulent kinetic energy and the variations in time-average reattachment distance in the conditions of different groove lengths. The results show that (1) this distance is reduced by the groove, but it is less affected by groove length and tends to a constant value at the length no less than 7 times the step height. This constant distance is 12.0% shorter than that of no groove. (2) At the step section, the distribution of turbulent kinetic energy varies with groove length, and turbulence intensity is increased. With the groove length no less than 7 times the step height, turbulent kinetic energy features an S-shaped distribution, and it peaks at a position of roughly 0.3 times the section height above the step; with a shorter groove length, it peaks at roughly 0.1 times the section height first and then drops to the minimum at roughly 0.6 times the section height. (3) Using the scale factor, an approximate reattachment distance of backward steps, well correlated with the time-average reattachment distance, is calculated to explain the mechanism of turbulence affecting the flows. An increase in turbulence promotes the sinking of the free shear layer and shortens the time for the shear layer to reattach to the downstream wall, thus shortening the time-average reattachment distance.

Key words: groove, backward-facing step flow, time-averaged reattachment distance, turbulent energy, velocity