AN EXPERIMENTAL INVESTIGATION OF THE INFLUENCE OF A DRAG REDUCING, LONGITUDINALLY ALIGNED, TRIANGULAR RIBLET SURFACE ON THE VELOCITY AND STREAMWISE VORTICITY FIELDS OF A ZERO-PRESSURE GRADIENT TURBULENT BOUNDARY LAYER

It is generally accepted that the dominant flow structures in the wall region of bounded turbulent shear flows are some form of counter-rotating vortices which play a major role in the production of turbulence and Reynolds stress. The present study was conducted to investigate the effect of a drag reducing surface modification on the structure of a zero-pressure gradient turbulent boundary layer. The surface modification consisted of symmetric, longitudinally aligned, triangular riblets with height and peak to peak spacing of 11 and 22 viscous lengths respectively. A single-sensor and a four-sensor hotwire probe were used to measure the velocity components and streamwise vorticity in the boundary layer over a reference smooth surface and the riblet surface in a low-speed wind tunnel at the Reynolds number of Re(,(theta)) = 3605 (Re(,x) = 1.8 x 10('6)). Variable Interval Time Averaging (VITA) was used to detect "bursts" and compare the frequency of their occurrence over the two surfaces. Quadrant technique was used to detect "ejection" and "sweep" events for the purpose of conditional sampling analysis.; Results indicate a reduction of about 40% in local shear stress over the valleys of the riblet surface and an increase of only about 10% over the peaks as compared to over the smooth surface. While the bursting frequency, as measured by VITA, is almost unchanged due to the riblets, the streamwise turbulence intensity profiles indicate reduction of up to 20% near the wall by the riblets. No other consistent indication of the riblets effect on the flow structure can be inferrred from comparison of the velocity and streamwise vorticity statistics and the conditional averages associated with the hypothesized "coherent structures." It is conjectured that the secondary vortices created at the riblet peaks help to retain the low-speed fluid within the riblets--immune from being upwardly ejected by the counter-rotating vortices--resulting in reduction of normal momentum transfer. The fluid within the grooves form flow regions dominated by viscous forces where the velocity gradients normal to the walls are substantially lower compared to the smooth plate, resulting in the reduction of the total skin friction drag over the riblet surface.