الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 133 |  |  |
| | | from | 133 | | |
Abstract
An experimental and numerical study are performed to investigate the effect of free stream turbulence on turbulent boundary layer drag reduction over flat and riblet surfaces. The riblets have been machined with symmetrical, sharp, stream-wise triangular-shaped grooves having a peak-to-valley height (h) and peak-to-peak width (s). The data includes mean velocity, turbulence intensities, momentum thickness, and skin friction. Three test plates, one smooth and two with triangular profile riblets of aspect ratio, h/s = 0.7, and 1.3 with fixed riblet spacing (s= 0.6 mm), have been used in this comparative study. Three screens of different sizes were used to change the turbulence intensity from 1.3 % to 32 %. Any screen was inserted at a distance 1050 mm upstream the front edge of the test plate. The Reynolds number based on the free stream velocity, kinematic viscosity, 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+ = SuT/v, of 13.7 to 36.4. The measurements were made in a suction type, low-speed wind tunnel using a Dantec 56C01 contant temperature anemometer (CTA) with a Dantec 55P04 hot single-wire probe. The numerical investigation has been formulated using a finite k-є-E turbulence model. The results show that the drag increases with increasing the free stream turbulence. The drag reduction for the riblet (h/s=0.7) occurred whenever riblet spacing in wall units is in the range of 0 < s+< 30. Whereas for the riblet (h/s=0.7), drag reduction occurred whenever riblet spacing in wall units is in the range of 0 < s+ < 23. For grooved plates at the lowest turbulence intensity, 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 intensity 0.025 compared with the smooth surface at the lowest turbulence intensity. Also, the results suggest that the effectiveness of the riblets with turbulence promoters increased compared with the smooth surface with turbulence promoters.