A DNS parametric study of streamwise‐aligned rectangular ridges is carried out in a fully developed turbulent channel flow with constant flow rate at $Reb = 18000$. The simulations were carried out systematically varying the ridge height h, width w and structural wavelength $S$. The ridges generate a strong large‐scale secondary motion, which is measured in terms of the integral swirl strength. Of the presented cases, the configuration with the ridge height$ h = 0.1 \delta$, $S/w = 4$, $ S = 1 \delta$ produces the strongest secondary flow of $4.5\%$ $U_b$. The varying flow topology is discussed as a result of the varying ridge dimensions.