BEHAVIOR OF THE REYNOLDS STRESS ON ROUGH WALLS

Quadrant analysis is used to study the contributions, associated to the four quadrants of the (u, nu) plane, to the production of the turbulent shear stress on rough walls. The measurements are described for a fully developed turbulent flow between two rough plates with varying the parameter lambda(z) (span/height ratio of roughness elements). The application of this technique indicates that the (Q2) events (ejections) and (Q4) events (sweeps) cause an intense production of the Reynolds stress - rhounuBAR as compared with the (Q1) and (Q3) quadrants. The (Q2) contribution to the Reynolds stress depends on the geometry factor lambda(z). Variation of the parameter lambda(z) affects the distributions of S2,H = (unuBAR)Q2, H/unuBAR and DELTAS(H) = (unuBAR)Q4 - (unuBAR)Q2/unuBAR for H less-than-or-equal-to 3, where H is a particular threshold. Comparison with boundary layer flow shows that the region 0.2 less-than-or-equal-to y/h less-than-or-equal-to 0.7 is characterized by a Reynolds stress production, independent of the flow nature. The third moment of the longitudinal velocity fluctuations is found to be sensitive to the surface roughness.