Experimental and numerical studies are performed to investigate the effect of free-stream turbulence on turbulent boundary layer drag reduction over plates with stream-wise micro-grooves. The data includes mean velocity, turbulence intensities, momentum thickness, and skin friction. Riblet or grooved plates of various riblet aspect ratios (h/s) have been machined with symmetrical, sharp, stream-wise triangular-shaped grooves having a peak-to-valley height (h) and peak-to-peak width (s). Three test plates, one smooth and two with triangular profile grooves (riblets) of h/s = 0.7, and 1.3 aspect ratio with fixed riblet spacing (s = 0.6 mm), have been used in this comparative study. Three screens of different sizes inserted upstream the test plates (with and without grooves) were used to study the effect of the free-stream turbulence. The turbulence intensities level varied from 1.3% to 3.2% by the screens. The Reynolds number based on the free-stream velocity and the stream-wise length, ranged from 1.4 x 106 to 4.3 x 106 covering a range of riblet spacing in
wall units (s+), = st.1,/v, of 13.7 to 36.4. The measurements were made in a suction type, low-speed open-return wind tunnel using a Dantec 56C01 constant temperature anemometer (CTA) with a Dantec 55PO4 hot single-wire probe. The numerical investigation has been formulated using a finite difference method and k-r, turbulence model. The results show that the drag increases with increasing the free-stream turbulence. The data show that the drag reduction occurred whenever riblet spacing in wall units in range of 0 < s+ < 25. For grooved plates without screens, a maximum drag reduction of 9% occurred at s+ = 13.7. The maximum drag reduction reached 6.3% in the case of grooved plate with h/s = 1.3 with turbulence promoter compared with the smooth surface without turbulence promoters. Also, the results suggest increased the effectiveness of the riblets with turbulence promoters compared with the smooth surface with turbulence promoters.