Secondary Currents and Turbulent Motions in Open-channel Flows over Streamwise Ridges

Particle image velocimetry (PIV) was used to study open-channel flows over streamwise ridges. The spanwise spacing (?) between the ridges was systematically varied while keeping all other experimental parameters the same (0.4≤?/?≤4, ? is flow depth). It was found that the ridges generated secondary currents (SCs) with sizes scaling with ? for ?/?≤2 and with ? for ?/?>2 which completely suppressed very-large-scale motions (VLSMs). For sufficiently large ?/?>2 (e.g., ?/?=4), the scale separation between ? and ? provides space for the emergence of additional (weaker) SC cells. Near the SC cell centres, a new spectral feature, termed here as “secondary current instability” (SCI), is identified for the first time. SCs represent an additional momentum transfer mechanism which is quantified by dispersive stresses. The data show that the dispersive stress distributions collapse as functions of (?−?)/? (? is vertical coordinates and ? is a constant that aligns the SC cell centre elevations), attaining a maximum value of 0.3?∗2 at the SC cell centre elevation. Ultimately, SCs are found to reduce turbulent kinetic energy and to define the shape of the mean velocity profile away from the bed and the water surface. These findings are discussed in light of friction factor measurement previously carried out by the authors.