Influence of PIV interrogation area on turbulent statistics up to 4th order moments in smooth and rough wall turbulent flows

The paper presents measurements performed over a smooth surface, a transitionally rough surface generated by wire mesh and a fully rough surface that comprised transverse square ribs in a channel. The flow was seeded using fluorescent particles and the high quality vectors obtained using particle image velocimetry (PIV) technique were used to compute the mean velocity, turbulent statistics up to the fourth order moments as well as the production and dissipation terms in the transport equation for the turbulent kinetic energy. By using different interrogation areas and overlaps to process the PIV images, the interrogation area sizes in wall units varied from 2.9 to 52.4. In the first part of the paper, these interrogation areas were used to study the spatial resolution effects on the mean and higher order moments over the smooth and rough surfaces. For the second part of the paper, classical scaling and the scaling proposed by George and Castillo [W.K. George, L. Castillo, Zero-pressure-gradient turbulent boundary layer, Appl. Mech. Rev. 50 (12) (1997) 689–729] and Zagarola and Smits [M.V. Zagarola, A.J. Smits, Mean-flow scaling of turbulent pipe flow, J. Fluid Mech. 373 (1998) 33–79] were used to document the salient effects of surface roughness on the flow characteristics. The results show spatial resolution effects on some of the turbulent statistics, and it is evident that these effects are more pronounced over the rough surfaces than on the smooth surface. The smaller interrogation area sizes used in this study are adequate to reasonably evaluate all the statistics and production term but not the dissipation rate. It was observed that the power law proposed by George and Castillo [W.K. George, L. Castillo, Zero-pressure-gradient turbulent boundary layer, Appl. Mech. Rev. 50 (12) (1997) 689–729] is better than the log law for modeling low Reynolds number turbulent flows over rough surfaces. The results also support the premise that the mechanisms for turbulence production and transport close to the ribs are markedly different from those for a smooth surface.